NCERT Class 12 Chemistry Part 2 PDF

CONTENTS FOREWORD iii PREFACE V Unit 10 Haloalkanes and Haloarenes 281 10.1 Classification 282 10.2...

CONTENTS FOREWORD iii PREFACE V Unit 10 Haloalkanes and Haloarenes 281 10.1 Classification 282 10.2 Nomenclature 283 10.3 Nature of C–X Bond 285 10.4 Methods of Preparation 286 10.5 Physical Properties 289 10.6 Chemical Reactions 291 10.7 Polyhalogen Compounds 308 Unit 11 Alcohols, Phenols and Ethers 315 11.1 Classification 316 11.2 Nomenclature 317 11.3 Structures of Functional Groups 320 11.4 Alcohols and Phenols 321 11.5 Some Commercially Important Alcohols 336 11.6 Ethers 337 Unit 12 Aldehydes, Ketones and Carboxylic Acids 349 12.1 Nomenclature and Structure of Carbonyl Group 350 12.2 Preparation of Aldehydes and Ketones 353 12.3 Physical Properties 357 12.4 Chemical Reactions 358 12.5 Uses of Aldehydes and Ketones 365 12.6 Nomenclature and Structure of Carboxyl Group 366 12.7 Methods of Preparation of Carboxylic Acids 367 12.8 Physical Properties 371 12.9 Chemical Reactions 371 12.10 Uses of Carboxylic Acids 376 Unit 13 Amines 381 13.1 Structure of Amines 381 13.2 Classification 382 13.3 Nomenclature 382 13.4 Preparation of Amines 384 13.5 Physical Properties 387 13.6 Chemical Reactions 388 13.7 Method of Preparation of Diazonium Salts 396 13.8 Physical Properties 397 13.9 Chemical Reactions 397 13.10 Importance of Diazonium Salts in Synthesis of 398 Aromatic Compounds Unit 14 Biomolecules 403 14.1 Carbohydrates 403 14.2 Proteins 412 14.3 Enzymes 417 14.4 Vitamins 417 14.5 Nucleic Acids 419 Unit 15 Polymers 425 15.1 Classification of Polymers 426 15.2 Types of Polymerisation 428 15.3 Molecular Mass of Polymers 435 15.4 Biodegradable Polymers 435 15.5 Polymers of Commercial Importance 436 Unit 16 Chemistry in Everyday Life 439 16.1 Drugs and their Classification 439 16.2 Drug-Target Interaction 440 16.3 Therapeutic Action of Different Classes of Drugs 443 16.4 Chemicals in Food 449 16.5 Cleansing Agents 450 Answers to Some Questions in Exercises 456 Index 461 xii  INDEX Terms Page No. Terms Page No. Achiral 297 Baeyers' reagent 362 Acidity of alcohols 327 Bakelite 428, 432 Acidity of phenols 328 Barbiturates 445 Active site 440 Benzylic alcohols 317 Acylation 392 Benzylic halides 282, 295 Addition polymers 427 Biodegradable polymers 435 Adduct 323 Biomolecules 403 Alcohols 315, 317, 321 Branched chain polymers 426 Aldehydes 349, 350, 352 Broad spectrum antibiotics 447 Aldol condensation 363 Buna - N 428, 435 Aldol reaction 363 Buna - S 427 Aldopentose 412 Cannizzaro reaction 364 Alkanamines 382, 390 Carbocation 295, 300 Alkenes 288 Carbohydrates 403 Alkyl halides 281, 282 Carboxylic acids 349, 366 Alkylation 392 Carbylamine reaction 393 Alkylbenzenes 368 Catalytic action of enzymes 440 Alkynes 354 Cationic detergents 452 Allosteric site 441 Cellulose 411 Allylic alcohols 316 Chain initiating step 429 Allylic halides 282, 295 Chain propagating step 429 Ambident nucleophiles 292 Chain terminating step 429 Amines 381 Chemical messengers 442 Amino acids 412 Chemotherapy 439 Ammonolysis 384 Chirality 296, 297 Amylopectin 410 Cleansing agents 450 Amylose 410 Clemmensen reduction 360 Analgesics 444 Coagulation 417 Anhydrides 369 Competitive inhibitors 441 Animal starch 411 Condensation polymers 427 Anionic detergents 452 Copolymerisation 433 Anomers 408 Copolymers 427 Antacids 443 Cross aldol condensation 364 Antibiotics 445 Cross linked polymers 426 Antidepressant drugs 444 Cumene 324 Antifertility drugs 448 Cyclic structure 407 Antihistamines 443 DDT 309 Antimicrobial drugs 446 Dehydrogenation 331 Antipyretic 445 Denaturation 336 Antiseptics 446, 448 Denaturation of protein 416 Aromatic ring 317 Deoxyribonucleic acid 419 Artificial sweetening agents 449 Deoxyribose 412 Aryl halides 283 Detergents 450 Arylamines 383, 391 Dextrorotatory 296 Aspirin 445 Diazonium salt 287, 288 Asymmetric carbon 297 Diazonium salts 396 Azo dyes 400 Diazotisation 396 Bactericidal 447 Disaccharides 404, 409 Bacteriostatic 447 Disinfectants 446, 448 461 Index C:\Chemistry-12\Index.pmd 28.02.07  Terms Page No. Terms Page No. Drug - enzyme interaction 441 Hinsberg's reagent 393 Drug - target interaction 440 Histamines 443 Drugs 439 Hoffmann bromamide reaction 386 Elastomers 427 Hydroboration 322 Electron donating group 372 Hyperacidity 443 Electron withdrawing group 372 Intermolecular bonding 333 Electrophilic aromatic substitution 333, 341 Intramolecular bonding 333 Electrophilic substitution 287, 305 Inversion of configuration 293 Electrostatic forces 415 Invert sugar 409 Elimination reaction 291 Ketones 349, 352, 353 Emulsifiers 449 Kolbe electrolysis 375 Enantiomers 296, 298 Kolbe's reaction 334 Environmental pollution 454 Lactose 410 Enzyme inhibitors 441 Laevorotatory 296 Enzymes 417 Laundry soaps 451 Esterification 329 Lewis bases 399 Esters 322 Limited spectrum antibiotics 447 Etard reaction 355 Linear polymers 426 Ethers 315, 317, 319 Low density polythene 429 Fat soluble vitamins 418 Lucas test 330 Fatty acids 366 Maltase 417 Fehling's test 361 Maltose 409 Fibres 428 Markovnikov's rule 321, 322 Fibrous proteins 414 Medicated soaps 451 Finkelstein reaction 289 Medicines 439 Fittig reaction 307 Melamine - formaldehyde polymer 431 Free radical 286 Messenger - RNA 421 Free radical mechanism 429 Molecular asymmetry 296 Freon refrigerant 309 Molecular targets 440 Friedel-Crafts reaction 305, 356 Monosaccharides 404 Fructose 408 Narrow spectrum antibiotics 447 Furanose 408 Natural polymers 426 Gabriel phthalimide synthesis 386 Natural rubber 433 Gatterman - Koch reaction 355 Neoprene 428, 434 Gatterman reaction 397 Network polymers 426 Geminal halides 283, 284 Nitration 395 Globular proteins 415 Nomenclature 283 Gluconic acid 405 Non-biodegradable 454 Glucose 405 Non-ionic detergents 452 Glyceraldehyde 406 Non-narcotic analgesics 445 Glycogen 411 Novolac 431 Glycosidic linkage 409, 410 Nucleic acids 419 Grignard reagent 301 Nucleophilic substitution 291 Haloalkane 281, 291 Nucleosides 420 Haloarene 281, 324 Nucleotides 419 Halogenation 334, 341 Nylon 6 431 Haworth structures 408 Nylon 6, 6 425, 427, 431 Hell - Volhard Zelinsky reaction 375 Oligosaccharides 404 Hemiacetal 359 Optical isomerism 296 Heterocyclic compounds 419 Optically inactive 299 High density polythene 430 Organo-metallic compounds 301 Chemistry 462 C:\Chemistry-12\Index.pmd 28.02.07  Terms Page No. Terms Page No. 3 Oxidoreductase 417 Sp hybridised 381 Ozonolysis 353 Starch 405 Peptide bond 414 Stephen reaction 354 Peptide linkage 414 Stereo centre 297 PHBV 435 Structure - basicity relationship 390 Phenols 315, 318 Structure of proteins 414 Polarity 358 Substitution nucleophilic bimolecular 293 Polyacrylonitrile 430 Substitution nucleophilic unimolecular 294 Polyamides 431 Sucrose 405, 409 Polyesters 431 Sulphonation 395 Polyhydric compounds 316 Swarts reaction 289 Polymerisation 425 Sweeteners 449 Polymers 425 Synthetic detergents 451 Polysaccharides 404, 410 Synthetic polymers 426 Polythene 427, 429 Synthetic rubber 434 Preservatives 449, 450 Teflon 430 Propellants 308 Terylene 428 Proteins 412 Thermoplastic polymers 428 Protic solvents 295 Thermosetting polymers 428 Pyranose structure 408 Toilet soaps 451 Racemic mixture 298 Tollens' test 361 Racemisation 296 Tranquilizers 444 Receptors 440 Transfer - RNA 421 Reducing sugars 404 Transparent soaps 451 Reimer - Tiemann reaction 335 Trisaccharides 404 Resins 428, 436 van der Waal forces 290 Ribose 412 Vasodilator 443 Ribosomal - RNA 421 Vicinal halides 283, 284 Ring substitution 376 Vinylic alcohol 317 Rochelle salt 361 Vinylic halides 283 Rosenmund reduction 354 Vitamins 417, 418 Rubber 433 Vulcanisation 434 Saccharic acid 406 Water soluble vitamins 418 Salvarsan 446 Williamson synthesis 337 Sandmayer's reaction 287, 397 Wolff - Kishner reduction 361 Saponification 450 Wurtz reaction 302 Scouring soaps 451 Wurtz-Fittig reaction 307 Semi - synthetic polymers 426 Ziegler - Natta catalyst 430 Shaving soaps 451 Zwitter ion 414 Soaps 450 463 Index C:\Chemistry-12\Index.pmd 28.02.07  Unit U it Objectives Ha looa 10 Haloalkanes Haloalkane lloalkanesness aand kane aallka nd After studying this Unit, you will be able to ⑨ name haloalkanes and haloarenes Haloar H a lloar oa renes Haloarenes Haloaren es according to the IUPAC system of nomenclature from their given structures; Halogenated compounds persist in the environment due to their ⑨ describe the reactions involved in resistance to breakdown by soil bacteria. the preparation of haloalkanes and haloarenes and understand various reactions that they The replacement of hydrogen atom(s) in a undergo; hydrocarbon, aliphatic or aromatic, by halogen ⑨ correlate the structures of atom(s) results in the formation of alkyl halide haloalkanes and haloarenes with (haloalkane) and aryl halide (haloarene), respectively. various types of reactions; Haloalkanes contain halogen atom(s) attached to the ⑨ use stereochemistry as a tool for sp3 hybridised carbon atom of an alkyl group whereas understanding the reaction haloarenes contain halogen atom(s) attached to sp2 mechanism; hybridised carbon atom(s) of an aryl group. Many ⑨ appreciate the applications of organo-metallic compounds; halogen containing organic compounds occur in nature and some of these are clinically useful. These ⑨ highlight the environmental effects of polyhalogen compounds. classes of compounds find wide applications in industry as well as in day-to-day life. They are used as solvents for relatively non-polar compounds and as starting materials for the synthesis of wide range of organic compounds. Chlorine containing antibiotic, chloramphenicol, produced by soil microorganisms is very effective for the treatment of typhoid fever. Our body produces iodine containing hormone, thyroxine, the deficiency of which causes a disease called goiter. Synthetic halogen compounds, viz. chloroquine is used for the treatment of malaria; halothane is used as an anaesthetic during surgery. Certain fully fluorinated compounds are being considered as potential blood substitutes in surgery. In this Unit, you will study the important methods of preparation, physical and chemical properties and uses of organohalogen compounds.  10.11 Classification l at Haloalkanes and haloarenes may be classified as follows: 10.1.1 On the These may be classified as mono, di, or polyhalogen (tri-,tetra-, etc.) Basis of compounds depending on whether they contain one, two or more halogen Number of atoms in their structures. For example, Halogen Atoms Monohalocompounds may further be classified according to the hybridisation of the carbon atom to which the halogen is bonded, as discussed below. 10.1.2 Compounds This class includes Containing (a) Alkyl halides or haloalkanes (R—X) sp3 C—X Bond (X= F, In alkyl halides, the halogen atom is bonded to an alkyl group (R). Cl, Br, I) They form a homologous series represented by CnH2n+1X. They are further classified as primary, secondary or tertiary according to the nature of carbon to which halogen is attached. (b) Allylic halides These are the compounds in which the halogen atom is bonded to an sp3-hybridised carbon atom next to carbon-carbon double bond (C=C) i.e. to an allylic carbon. (c) Benzylic halides These are the compounds in which the halogen atom is bonded to an sp3-hybridised carbon atom next to an aromatic ring. Chemistry 282 C:\Chemistry-12\Unit-10.pmd 28.02.07 10.1.3 Compounds This class includes: Containing (a) Vinylic halides sp2 C—X Bond These are the compounds in which the halogen atom is bonded to an sp2-hybridised carbon atom of a carbon-carbon double bond (C = C). (b) Aryl halides These are the compounds in which the halogen atom is bonded to the sp2-hybridised carbon atom of an aromatic ring. 10.2 o l tu Nomenclature Having learnt the classification of halogenated compounds, let us now learn how these are named. The common names of alkyl halides are derived by naming the alkyl group followed by the halide. Alkyl halides are named as halosubstituted hydrocarbons in the IUPAC system of nomenclature. Haloarenes are the common as well as IUPAC names of aryl halides. For dihalogen derivatives, the prefixes o-, m-, p- are used in common system but in IUPAC system, the numerals 1,2; 1,3 and 1,4 are used. The dihaloalkanes having the same type of halogen atoms are named as alkylidene or alkylene dihalides. The dihalo-compounds having same type of halogen atoms are further classified as geminal halides (halogen atoms are present on the same carbon atom) and vicinal halides (halogen atoms are present on the adjacent carbon atoms). In common name system, gem-dihalides are named as alkylidene halides and vic-dihalides 283 Haloalkanes and Haloarenes C:\Chemistry-12\Unit-10.pmd 28.02.07  are named as alkylene dihalides. In IUPAC system, they are named as dihaloalkanes. Some common examples of halocompounds are mentioned in Table 10.1. Table 10.1: Common and IUPAC names of some Halides Structure Common name IUPAC name CH3CH2CH(Cl)CH3 sec-Butyl chloride 2-Chlorobutane (CH3)3CCH2Br neo-Pentyl bromide 1-Bromo-2,2-dimethylpropane (CH3)3CBr tert-Butyl bromide 2-Bromo-2-methylpropane CH2 = CHCl Vinyl chloride Chloroethene CH2 = CHCH2Br Allyl bromide 3-Bromopropene o-Chlorotoluene 1-Chloro-2-methylbenzene or 2-Chlorotoluene Benzyl chloride Chlorophenylmethane CH2Cl2 Methylene chloride Dichloromethane CHCl3 Chloroform Trichloromethane CHBr3 Bromoform Tribromomethane CCl4 Carbon tetrachloride Tetrachloromethane CH3CH2CH2F n-Propyl fluoride 1-Fluoropropane E Example 0 1 Draw the structures of all the eight structural isomers that have the p 10.1 molecular formula C5H11Br. Name each isomer according to IUPAC system and classify them as primary, secondary or tertiary bromide. S l uti Solution CH3CH2CH2CH2CH2Br 1-Bromopentane (1 ) o CH3CH2CH2CH(Br)CH3 2-Bromopentane(2o) CH3CH2CH(Br)CH2CH3 3-Bromopentane (2o) (CH3)2CHCH2CH2Br 1-Bromo-3-methylbutane (1o) Chemistry 284 C:\Chemistry-12\Unit-10.pmd 28.02.07  o (CH3)2CHCHBrCH3 2-Bromo-3-methylbutane(2 ) o (CH3)2CBrCH2CH3 2-Bromo-2-methylbutane (3 ) o CH3CH2CH(CH3)CH2Br 1-Bromo-2-methylbutane(1 ) o (CH3)3CCH2Br 1-Bromo-2,2-dimethylpropane (1 ) Write IUPAC names of the following: e 10.2 Example x 1. (i) 4-Bromopent-2-ene (ii) 3-Bromo-2-methylbut-1-ene o Solution n (iii) 4-Bromo-3-methylpent-2-ene (iv) 1-Bromo-2-methylbut-2-ene (v) 1-Bromobut-2-ene (vi) 3-Bromo-2-methylpropene t t Question Intext Q si 10.1 Write structures of the following compounds: (i) 2-Chloro-3-methylpentane (ii) 1-Chloro-4-ethylcyclohexane (iii) 4-tert. Butyl-3-iodoheptane (iv) 1,4-Dibromobut-2-ene (v) 1-Bromo-4-sec. butyl-2-methylbenzene. 1 0. 3 Nature r of Since halogen atoms are more electronegative than carbon, the carbon- halogen bond of alkyl halide is polarised; the carbon atom bears a C-X BoBond partial positive charge whereas the halogen atom bears a partial negative charge. Since the size of halogen atom increases as we go down the group in the periodic table, fluorine atom is the smallest and iodine atom, the largest. Consequently the carbon-halogen bond length also increases from C—F to C—I. Some typical bond lengths, bond enthalpies and dipole moments are given in Table 10.2. 285 Haloalkanes and Haloarenes C:\Chemistry-12\Unit-10.pmd 28.02.07  Table 10.2: Carbon-Halogen (C—X) Bond Lengths, Bond Enthalpies and Dipole Moments Bond Bond length/pm C-X Bond enthalpies/ kJmol-1 Dipole moment/Debye CH3–F 139 452 1.847 CH3– Cl 178 351 1.860 CH3–Br 193 293 1.830 CH3–I 214 234 1.636 4 Methods 10.4 e h d of 10.4.1 From Alcohols Preparation r aarr i Alkyl halides are best prepared from alcohols, which are easily accessible. The hydroxyl group of an alcohol is replaced by halogen on reaction with concentrated halogen acids, phosphorus halides or thionyl chloride. Thionyl chloride is preferred because the other two products are escapable gases. Hence the reaction gives pure alkyl halides. Phosphorus tribromide and triiodide are usually generated in situ (produced in the reaction mixture) by the reaction of red phosphorus with bromine and iodine respectively. The preparation of alkyl chloride is carried out either by passing dry hydrogen chloride gas through a solution of alcohol or by heating a solution of alcohol in concentrated aqueous acid. The reactions of primary and secondary alcohols with HX require the presence of a catalyst, ZnCl2. With tertiary alcohols, the reaction is conducted by simply shaking with concentrated HCl at room temperature. Constant boiling with HBr (48%) is used for preparing alkyl bromide. Good yields of R—I may be obtained by heating alcohols with sodium or potassium iodide in 95% phosphoric acid. The order of reactivity of alcohols with a given haloacid is 3 >2 >1. The above method is not applicable for the preparation of aryl halides because the carbon-oxygen bond in phenols has a partial double bond character and is difficult to break being stronger than a single bond (Unit 11, Class XI). 10.4.2 From (a) By free radical halogenation Hydrocarbons Free radical chlorination or bromination of alkanes gives a complex Chemistry 286 C:\Chemistry-12\Unit-10.pmd 28.02.07  mixture of isomeric mono- and polyhaloalkanes, which is difficult to separate as pure compounds. Consequently, the yield of any one compound is low (Unit 13, Class XI). Identify all the possible monochloro structural isomers expected to be x m e 10.3 Example 03 formed on free radical monochlorination of (CH3)2CHCH2CH3. In the given molecule, there are four different types of hydrogen atoms. o n Solution Replacement of these hydrogen atoms will give the following (CH3)2CHCH2CH2Cl (CH3)2CHCH(Cl)CH3 (CH3)2C(Cl)CH2CH3 CH3CH(CH2Cl)CH2CH3 (b) By electrophilic substitution Aryl chlorides and bromides can be easily prepared by electrophilic substitution of arenes with chlorine and bromine respectively in the presence of Lewis acid catalysts like iron or iron(III) chloride. The ortho and para isomers can be easily separated due to large difference in their melting points. Reactions with iodine are reversible in nature and require the presence of an oxidising agent (HNO3, HIO4) to oxidise the HI formed during iodination. Fluoro compounds are not prepared by this method due to high reactivity of fluorine. (c) Sandmeyer’s reaction When a primary aromatic amine, dissolved or suspended in cold aqueous mineral acid, is treated with sodium nitrite, a diazonium salt is formed (Unit 13, Class XII). Mixing the solution of freshly prepared diazonium salt with cuprous chloride or cuprous bromide results in the replacement of the diazonium group by –Cl or –Br. 287 Haloalkanes and Haloarenes C:\Chemistry-12\Unit-10.pmd 28.02.07  Replacement of the diazonium group by iodine does not require the presence of cuprous halide and is done simply by shaking the diazonium salt with potassium iodide. (d) From alkenes (i) Addition of hydrogen halides: An alkene is converted to corresponding alkyl halide by reaction with hydrogen chloride, hydrogen bromide or hydrogen iodide. Propene yields two products, however only one predominates as per Markovnikov’s rule. (Unit 13, Class XI) (ii) Addition of halogens: In the laboratory, addition of bromine in CCl4 to an alkene resulting in discharge of reddish brown colour of bromine constitutes an important method for the detection of double bond in a molecule. The addition results in the synthesis of vic-dibromides, which are colourless (Unit 13, Class XI). mp e 10.4 Example 04 Write the products of the following reactions: Solution o o Chemistry 288 C:\Chemistry-12\Unit-10.pmd 28.02.07 10.4.3 Halogen Alkyl iodides are often prepared by the reaction of alkyl chlorides/ Exchange bromides with NaI in dry acetone. This reaction is known as Finkelstein reaction. NaCl or NaBr thus formed is precipitated in dry acetone. It facilitates the forward reaction according to Le Chatelier’s Principle. The synthesis of alkyl fluorides is best accomplished by heating an alkyl chloride/bromide in the presence of a metallic fluoride such as AgF, Hg2F2, CoF2 or SbF3. The reaction is termed as Swarts reaction. I Intext ttii s Questions 10.2 Why is sulphuric acid not used during the reaction of alcohols with KI? 10.3 Write structures of different dihalogen derivatives of propane. 10.4 Among the isomeric alkanes of molecular formula C5H12, identify the one that on photochemical chlorination yields (i) A single monochloride. (ii) Three isomeric monochlorides. (iii) Four isomeric monochlorides. 10.5 Draw the structures of major monohalo products in each of the following reactions: 10.55 Physical h s ic Alkyl halides are colourless when pure. However, bromides and iodides develop colour when exposed to light. Many volatile halogen compounds r r ie Properties have sweet smell. 289 Haloalkanes and Haloarenes C:\Chemistry-12\Unit-10.pmd 28.02.07  Melting and boiling points Methyl chloride, methyl bromide, ethyl chloride and some chlorofluoromethanes are gases at room temperature. Higher members are liquids or solids. As we have already learnt, molecules of organic halogen compounds are generally polar. Due to greater polarity as well as higher molecular mass as compared to the parent hydrocarbon, the intermolecular forces of attraction (dipole-dipole and van der Waals) are stronger in the halogen derivatives. That is why the boiling points of chlorides, bromides and iodides are considerably higher than those of the hydrocarbons of comparable molecular mass. The attractions get stronger as the molecules get bigger in size and have more electrons. The pattern of variation of boiling points of different halides is depicted in Fig. 10.1. For the same alkyl group, the boiling points of alkyl halides decrease in the order: RI> RBr> RCl> RF. This is because with the increase in size and mass of halogen atom, the magnitude of van der Waal forces increases. Fig. 10.1: Comparison of boiling points of some alkyl halides The boiling points of isomeric haloalkanes decrease with increase in branching (Unit 12, Class XI). For example, 2-bromo-2-methylpropane has the lowest boiling point among the three isomers. Boiling points of isomeric dihalobenzenes are very nearly the same. However, the para-isomers are high melting as compared to their ortho- and meta-isomers. It is due to symmetry of para-isomers that fits in crystal lattice better as compared to ortho- and meta-isomers. Chemistry 290 C:\Chemistry-12\Unit-10.pmd 28.02.07  Density Bromo, iodo and polychloro derivatives of hydrocarbons are heavier than water. The density increases with increase in number of carbon atoms, halogen atoms and atomic mass of the halogen atoms (Table 10.3). Table 10.3: Density of some Haloalkanes Compound Density (g/mL) Compound Density (g/mL) n–C3H7Cl 0.89 CH2Cl2 1.336 n–C3H7Br 1.335 CHCl3 1.489 n-C3H7I 1.747 CCl4 1.595 Solubility The haloalkanes are only very slightly soluble in water. In order for a haloalkane to dissolve in water, energy is required to overcome the attractions between the haloalkane molecules and break the hydrogen bonds between water molecules. Less energy is released when new attractions are set up between the haloalkane and the water molecules as these are not as strong as the original hydrogen bonds in water. As a result, the solubility of haloalkanes in water is low. However, haloalkanes tend to dissolve in organic solvents because the new intermolecular attractions between haloalkanes and solvent molecules have much the same strength as the ones being broken in the separate haloalkane and solvent molecules. Intext IInn x Question e t oonn 10.6 Arrange each set of compounds in order of increasing boiling points. (i) Bromomethane, Bromoform, Chloromethane, Dibromomethane. (ii) 1-Chloropropane, Isopropyl chloride, 1-Chlorobutane. em l 10.6 Chemical 10.6.1 Reactions of Haloalkanes a Reactions The reactions of haloalkanes may be divided into the following categories: (i) Nucleophilic substitution (ii) Elimination reactions (iii) Reaction with metals. (i) Nucleophilic substitution reactions In this type of reaction, a nucleophile reacts with haloalkane (the substrate) having a partial positive charge on the carbon atom bonded 291 Haloalkanes and Haloarenes C:\Chemistry-12\Unit-10.pmd 28.02.07  to halogen. A substitution reaction takes place and halogen atom, called leaving group departs as halide ion. Since the substitution reaction is initiated by a nucleophile, it is called nucleophilic substitution reaction. It is one of the most useful classes of organic reactions of alkyl halides in which halogen is bonded to sp3 hybridised carbon. The products formed by the reaction of haloalkanes with some common nucleophiles are given in Table 10.4. Table 10.4: Nucleophilic Substitution of Alkyl Halides (R–X) Reagent Nucleophile Substitution Class of main (Nu–) product R–Nu product NaOH (KOH) HO– ROH Alcohol H2O H2O ROH Alcohol NaOR✂ R O– ROR✂ Ether NaI I – R—I Alkyl iodide NH3 NH3 RNH2 Primary amine R✂NH2 R✂NH2 RNHR✂ Sec. amine R✂R✂✂NH R✂R✂✂H RNR✂R✂✂ Tert. amine KCN RCN Nitrile (cyanide) AgCN Ag-CN: RNC Isonitrile (isocyanide) KNO2 O=N—O R—O—N=O Alkyl nitrite AgNO2 Ag—Ö—N=O R—NO2 Nitroalkane R✂COOAg R✂COO – R✂COOR Ester LiAlH4 H RH Hydrocarbon R✂ – M + R✂ – RR✂ Alkane Groups like cyanides and nitrites possess two nucleophilic centres and are called ambident nucleophiles. Actually cyanide group is a hybrid of two contributing structures and therefore can act as a nucleophile in two different ways [✁C✄N ☎ :C=N✁], i.e., linking through carbon atom resulting in alkyl cyanides and through nitrogen atom leading to isocyanides. Similarly nitrite ion also represents an ambident ✆✆ nucleophile with two different points of linkage [–O— ◆ =O]. The linkage through oxygen results in alkyl nitrites while through nitrogen atom, it leads to nitroalkanes. Chemistry 292 C:\Chemistry-12\Unit-10.pmd 28.02.07  Haloalkanes react with KCN to form alkyl cyanides as main product mp e 110.5 Example while AgCN forms isocyanides as the chief product. Explain. KCN is predominantly ionic and provides cyanide ions in solution. Solution S llutio uti o Although both carbon and nitrogen atoms are in a position to donate electron pairs, the attack takes place mainly through carbon atom and not through nitrogen atom since C—C bond is more stable than C—N bond. However, AgCN is mainly covalent in nature and nitrogen is free to donate electron pair forming isocyanide as the main product. Mechanism: This reaction has been found to proceed by two different mechanims which are described below: (a) Substitution nucleophilic bimolecular (SN2) The reaction between CH3Cl and hydroxide ion to yield methanol and chloride ion follows a second order kinetics, i.e., the rate depends upon the concentration of both the reactants. As you have already learnt in Section 12.3.2 of Class XI, the solid wedge represents the bond coming out of the paper, dashed line going down the paper and a straight line representing bond in the plane of the paper. This can be represented diagrammatically as shown in Fig. 10.2. Fig. 10.2: Red dot represents the incoming hydroxide ion and green dot represents the outgoing halide ion It depicts a bimolecular nucleophilic displacement (SN2) reaction; the incoming nucleophile interacts with alkyl halide causing the carbon- halide bond to break while forming a new carbon-OH bond. These two In the year 1937, processes take place simultaneously in a single step and no intermediate Edward Davies Hughes and Sir Christopher is formed. As the reaction progresses and the bond between the Ingold proposed a nucleophile and the carbon atom starts forming, the bond between mechanism for an SN2 carbon atom and leaving group weakens. As this happens, the reaction. configuration of carbon atom under attack inverts in much the same way as an umbrella is turned inside out when caught in a strong wind, while the leaving group is pushed away. This process is called as inversion of configuration. In the transition state, the carbon atom is simultaneously bonded to incoming nucleophile and the outgoing leaving 293 Haloalkanes and Haloarenes C:\Chemistry-12\Unit-10.pmd 28.02.07  group and such structures are unstable and cannot be isolated. This is because the carbon atom in the transition state is simultaneously bonded to five atoms and therefore is unstable. Since this reaction requires the approach of the nucleophile to the carbon bearing the leaving group, the presence of bulky substituents Hughes worked under Ingold and earned a on or near the carbon atom have a dramatic inhibiting effect. Of the D.Sc. degree from the simple alkyl halides, methyl halides react most rapidly in SN2 reactions University of London. because there are only three small hydrogen atoms. Tertiary halides are the least reactive because bulky groups hinder the approaching nucleophiles. Thus the order of reactivity followed is: Primary halide > Secondary halide > Tertiary halide. Fig.10.3: Steric effects in SN2 reaction. The relative rate of SN2 reaction is given in parenthesis (b) Substitution nucleophilic unimolecular (SN1) SN1 reactions are generally carried out in polar protic solvents (like water, alcohol, acetic acid, etc.). The reaction between tert-butyl bromide and hydroxide ion yields tert-butyl alcohol and follows the first order kinetics, i.e., the rate of reaction depends upon the concentration of only one reactant, which is tert- butyl bromide. It occurs in two steps. In step I, the polarised C—Br bond undergoes slow cleavage to produce a carbocation and a bromide ion. The carbocation thus formed is then attacked by nucleophile in step II to complete the substitution reaction. Chemistry 294 C:\Chemistry-12\Unit-10.pmd 28.02.07  Step I is the slowest and reversible. It involves the C–Br bond breaking for which the energy is obtained through solvation of halide ion with the proton of protic solvent. Since the rate of reaction depends upon the slowest step, the rate of reaction depends only on the concentration of alkyl halide and not on the concentration of hydroxide ion. Further, greater the stability of carbocation, greater will be its ease of formation from alkyl halide and faster will be the rate of reaction. In case of alkyl halides, 30 alkyl halides undergo SN1 reaction 0 very fast because of the high stability of 3 carbocations. We can sum up the order of reactivity of alkyl halides towards S N1 and SN2 reactions as follows: For the same reasons, allylic and benzylic halides show high reactivity towards the SN1 reaction. The carbocation thus formed gets stabilised through resonance (Unit 12, Class XI) as shown below. For a given alkyl group, the reactivity of the halide, R-X, follows the same order in both the mechanisms R–I> R–Br>R–Cl>>R–F. In the following pairs of halogen compounds, which would undergo xa Example 06 10.6 SN2 reaction faster? It is primary halide and therefore undergoes SN2 S ut o Solution reaction faster. As iodine is a better leaving group because of its large size, it will be released at a faster rate in the presence of incoming nucleophile. Predict the order of reactivity of the following compounds in SN1 and E l 10.7 Example 07 SN2 reactions: (i) The four isomeric bromobutanes (ii) C6H5CH2Br, C6H5CH(C6H5)Br, C6H5CH(CH3)Br, C6H5C(CH3)(C6H5)Br 295 Haloalkanes and Haloarenes C:\Chemistry-12\Unit-10.pmd 28.02.07  S l ti (i) CH3CH2CH2CH2Br < (CH3)2CHCH2Br < CH3CH2CH(Br)CH3 < (CH3)3CBr (SN1) Solution CH3CH2CH2CH2Br > (CH3)2CHCH2Br > CH3CH2CH(Br)CH3 > (CH3)3CBr (SN2) Of the two primary bromides, the carbocation intermediate derived from (CH3)2CHCH2Br is more stable than derived from CH3CH2CH2CH2Br because of greater electron donating inductive effect of (CH3)2CH- group. Therefore, (CH3)2CHCH2Br is more reactive than CH3CH2CH2CH2Br in SN1 reactions. CH3CH2CH(Br)CH3 is a secondary bromide and (CH3)3CBr is a tertiary bromide. Hence the above order is followed in SN1. The reactivity in SN2 reactions follows the reverse order as the steric hinderance around the electrophilic carbon increases in that order. (ii) C6H5C(CH3)(C6H5)Br > C6H5CH(C6H5)Br > C6H5CH(CH3)Br > C6H5CH2Br (SN1) C6H5C(CH3)(C6H5)Br < C6H5CH(C6H5)Br < C6H5CH(CH3)Br < C6H5CH2Br (SN2) Of the two secondary bromides, the carbocation intermediate obtained from C6H5CH(C6H5)Br is more stable than obtained from C6H5CH(CH3)Br because it is stabilised by two phenyl groups due to resonance. Therefore, the former bromide is more reactive than the latter in SN1 reactions. A phenyl group is bulkier than a methyl group. Therefore, C6H5CH(C6H5)Br is less reactive than C6H5CH(CH3)Br in SN2 reactions. (c) Stereochemical aspects of nucleophilic substitution reactions A SN2 reaction proceeds with complete stereochemical inversion while a SN1 reaction proceeds with racemisation. In order to understand this concept, we need to learn some basic stereochemical principles and notations (optical activity, chirality, retention, inversion, racemisation, etc.). (i) Plane polarised light and optical activity: Certain compounds rotate the plane polarised light (produced by passing ordinary William Nicol (1768- light through Nicol prism) when it is passed through their 1851) developed the first solutions. Such compounds are called optically active prism that produced plane polarised light. compounds. The angle by which the plane polarised light is rotated is measured by an instrument called polarimeter. If the compound rotates the plane polarised light to the right, i.e., clockwise direction, it is called dextrorotatory (Greek for right rotating) or the d-form and is indicated by placing a positive (+) sign before the degree of rotation. If the light is rotated towards left (anticlockwise direction), the compound is said to be laevo- rotatory or the l-form and a negative (–) sign is placed before the degree of rotation. Such (+) and (–) isomers of a compound are called optical isomers and the phenomenon is termed as optical isomerism. (ii) Molecular asymmetry, chirality and enantiomers: The observation of Louis Pasteur (1848) that crystals of certain compounds exist in the form of mirror images laid the foundation of modern stereochemistry. He demonstrated that aqueous solutions of both types of crystals showed optical rotation, equal in magnitude (for solution of equal concentration) but opposite in direction. He believed that this difference in Chemistry 296 C:\Chemistry-12\Unit-10.pmd 28.02.07  optical activity was associated with the three dimensional arrangements of atoms (configurations) in two types of crystals. Jacobus Hendricus Dutch scientist, J. Van’t Hoff and French scientist, C. Le Bel in Van’t Hoff (1852-1911) the same year (1874), independently argued that the spatial received the first Nobel arrangement of four groups (valencies) around a central carbon Prize in Chemistry in is tetrahedral and if all the substituents attached to that carbon 1901 for his work on are different, such a carbon is called asymmetric carbon or solutions. stereocentre. The resulting molecule would lack symmetry and is referred to as asymmetric molecule. The asymmetry of the molecule is responsible for the optical activity in such organic compounds. The symmetry and asymmetry are also observed in many day to day objects: a sphere, a cube, a cone, are all identical to their mirror images and can be superimposed. However, many objects are non superimposable on their mirror images. For example, your left and right hand look similar but if you put your left hand on your right hand, they do not coincide. The objects which are non- superimposable on their mirror image (like a pair of hands) are said to be chiral and this property is known as chirality. While the objects, which are, superimposable on their mirror images are called achiral. The above test of molecular chirality can be applied to organic molecules by constructing models and its mirror images or by drawing three dimensional structures and attempting to superimpose them in our minds. There are other aids, however, that can assist us in recognising chiral molecules. One such aid is the presence of a single asymmetric carbon atom. Let us consider Fig 10.4: Some common examples of chiral and two simple molecules propan-2-ol and achiral objects butan-2-ol and their mirror images. As you can see very clearly, propan-2-ol does not contain an asymmetric carbon, as all the four groups attached to the tetrahedral carbon are not different. Thus it is an achiral molecule. 297 Haloalkanes and Haloarenes C:\Chemistry-12\Unit-10.pmd 28.02.07  Butan-2-ol has four different groups attached to the tetrahedral carbon and as expected is chiral. Some common examples of chiral molecules such as 2-chlorobutane, 2, 3-dihyroxypropanal, (OHC–CHOH–CH2OH), bromochloro-iodomethane (BrClCHI), 2-bromopropanoic acid (H3C–CHBr–COOH), etc. Fig. 10.5: A chiral molecule The stereoisomers related to each other as non- and its mirror image superimposable mirror images are called enantiomers (Fig. 10.5). Enantiomers possess identical physical properties namely, melting point, boiling point, solubility, refractive index, etc. They only differ with respect to the rotation of plane polarised light. If one of the enantiomer is dextro rotatory, the other will be laevo rotatory. However, the sign of optical rotation is not necessarily related to the absolute configuration of the molecule. A mixture containing two enantiomers in equal proportions will have zero optical rotation, as the rotation due to one isomer will be cancelled by the rotation due to the other isomer. Such a mixture is known as racemic mixture or racemic modification. A racemic mixture is represented by prefixing dl or (➧) before the name, for example (➧) butan-2-ol. The process of conversion of enantiomer into a racemic mixture is known as racemisation. Example E a l 10.8. Identify chiral and achiral molecules in each of the following pair of compounds. (Wedge and Dash representations according to Class XI, Fig 12.1). Chemistry 298 C:\Chemistry-12\Unit-10.pmd 28.02.07  Solution S ut ut o (iii) Retention: Retention of configuration is the preservation of integrity of the spatial arrangement of bonds to an asymmetric centre during a chemical reaction or transformation. It is also the configurational correlation when a chemical species XCabc is converted into the chemical species YCabc having the same relative configuration. In general, if during a reaction, no bond to the stereocentre is broken, the product will have the same general configuration of groups around the stereocentre as that of reactant. Such a reaction is said to proceed with retention of the configuration. Consider as an example, the reaction that takes place when (–)-2-methylbutan-1-ol is heated with concentrated hydrochloric acid. (iv) Inversion, retention and racemisation: There are three outcomes for a reaction at an asymmetric carbon atom. Consider the replacement of a group X by Y in the following reaction; If (A) is the only compound obtained, the process is called retention of configuration. If (B) is the only compound obtained, the process is called inversion of configuration. If a 50:50 mixture of the above two is obtained then the process is called racemisation and the product is optically inactive, as one isomer will rotate light in the direction opposite to another. 299 Haloalkanes and Haloarenes C:\Chemistry-12\Unit-10.pmd 28.02.07  Now let us have a fresh look at SN1 and SN2 mechanisms by taking examples of optically active alkyl halides. In case of optically active alkyl halides, the product formed as a result of SN2 mechanism has the inverted configuration as compared to the reactant. This is because the nucleophile attaches itself on the side opposite to the one where the halogen atom is present. When (–)-2-bromooctane is allowed to react with sodium hydroxide, (+)-octan-2-ol is formed with the –OH group occupying the position opposite to what bromide had occupied. Thus, SN2 reactions of optically active halides are accompanied by inversion of configuration. In case of optically active alkyl halides, S N1 reactions are accompanied by racemisation. Can you think of the reason why it 2 happens? Actually the carbocation formed in the slow step being sp hybridised is planar (achiral). The attack of the nucleophile may be accomplished from either side resulting in a mixture of products, one having the same configuration (the –OH attaching on the same position as halide ion) and the other having opposite configuration (the –OH attaching on the side opposite to halide ion). This may be illustrated by hydrolysis of optically active 2-bromobutane, which results in the formation of (➧ )-butan-2-ol. 2. Elimination reactions When a haloalkane with ✆-hydrogen atom is heated with alcoholic solution of potassium hydroxide, there is elimination of hydrogen atom from ✆-carbon and a halogen atom from the ✝-carbon atom. As a result, an alkene is formed as a product. Since ✆-hydrogen atom is involved in elimination, it is often called ✆ -elimination. Chemistry 300 C:\Chemistry-12\Unit-10.pmd 28.02.07  If there is possibility of formation of more than one alkene due to the availability of more than one ✝-hydrogen atoms, usually one alkene is formed as the major product. These form part of a pattern first observed by Russian chemist, Alexander Zaitsev (also pronounced as Saytzeff) who in 1875 formulated a rule which can be summarised as “in dehydrohalogenation reactions, the preferred product is that alkene which has the greater number of alkyl groups attached to the doubly bonded carbon atoms.” Thus, 2-bromopentane gives pent-2-ene as the major product. l i ai Elimination r u substitution versus t A chemical reaction is the result of competition; it is a race that is won by the fastest runner. A collection of molecules tend to do, by and large, what is easiest for them. An alkyl halide with -hydrogen atoms when reacted with a base or a nucleophile has two competing routes: substitution (SN1 and SN2) and elimination. Which route will be taken up depends upon the nature of alkyl halide, strength and size of base/nucleophile and reaction conditions. Thus, a bulkier nucleophile will prefer to act as a base and abstracts a proton rather than approach a tetravalent carbon atom (steric reasons) and vice versa. Similarly, a primary alkyl halide will prefer a SN2 reaction, a secondary halide- SN2 or elimination depending upon the strength of base/nucleophile and a tertiary halide- SN1 or elimination depending upon the stability of carbocation or the more substituted alkene. 3. Reaction with metals Most organic chlorides, bromides and iodides react with certain metals to give compounds containing carbon-metal bonds. Such compounds are known as organo-metallic compounds. An important class of organo-metallic compounds discovered by Victor Grignard in 1900 is alkyl magnesium halide, RMgX, referred as Grignard Reagents. These reagents are obtained by the reaction of haloalkanes with magnesium metal in dry ether. 301 Haloalkanes and Haloarenes C:\Chemistry-12\Unit-10.pmd 28.02.07  Victor Grignard had a strange start in academic life for a chemist - he took a maths degree. When he eventually switched to chemistry, it was not to the mathematical province of physical chemistry but to organic chemistry. While attempting to find an efficient catalyst for the process of methylation, he noted that Zn in diethyl ether had been used for this purpose and wondered whether the Mg/ether combination might be successful. Grignard reagents were first reported in 1900 and Grignard used this work for his doctoral thesis in 1901. In 1910, Grignard obtained a professorship at the University of Nancy and in 1912, he was awarded the Nobel prize for Chemistry which he shared with Paul Sabatier who had made advances in nickel catalysed hydrogenation. In the Grignard reagent, the carbon-magnesium bond is covalent but highly polar, with carbon pulling electrons from electropositive magnesium; the magnesium halogen bond is essentially ionic. Grignard reagents are highly reactive and react with any source of proton to give hydrocarbons. Even water, alcohols, amines are sufficiently acidic to convert them to corresponding hydrocarbons. It is therefore necessary to avoid even traces of moisture from a Grignard reagent. On the other hand, this could be considered as one of the methods for converting halides to hydrocarbons. Wurtz reaction Alkyl halides react with sodium in dry ether to give hydrocarbons containing double the number of carbon atoms present in the halide. This reaction is known as Wurtz reaction. (Unit 13, Class XI). 10.6.2 Reactions of 1. Nucleophilic substitution Haloarenes Aryl halides are extremely less reactive towards nucleophilic substitution reactions due to the following reasons: (i) Resonance effect : In haloarenes, the electron pairs on halogen atom are in conjugation with ✞-electrons of the ring and the following resonating structures are possible. C—Cl bond acquires a partial double bond character due to resonance. As a result, the bond cleavage in haloarene is difficult than haloalkane and therefore, they are less reactive towards nucleophilic substitution reaction. Chemistry 302 C:\Chemistry-12\Unit-10.pmd 28.02.07  (ii) Difference in hybridisation of carbon atom in C—X bond: In haloalkane, the carbon atom attached to halogen is sp3 hybridised while in case of haloarene, the carbon atom attached to halogen is sp2-hybridised. The sp2 hybridised carbon with a greater s-character is more electronegative and can hold the electron pair of C—X bond more tightly than sp3-hybridised carbon in haloalkane with less s-chararcter. Thus, C—Cl bond length in haloalkane is 177pm while in haloarene is 169 pm. Since it is difficult to break a shorter bond than a longer bond, therefore, haloarenes are less reactive than haloalkanes towards nucleophilic substitution reaction. (iii) Instability of phenyl cation: In case of haloarenes, the phenyl cation formed as a result of self-ionisation will not be stabilised by resonance and therefore, SN1 mechanism is ruled out. (iv) Because of the possible repulsion, it is less likely for the electron rich nucleophile to approach electron rich arenes. Replacement by hydroxyl group Chlorobenzene can be converted into phenol by heating in aqueous sodium hydroxide solution at a temperature of 623K and a pressure of 300 atmospheres. The presence of an electron withdrawing group (-NO2) at ortho- and para-positions increases the reactivity of haloarenes. 303 Haloalkanes and Haloarenes C:\Chemistry-12\Unit-10.pmd 28.02.07  The effect is pronounced when (-NO2) group is introduced at ortho- and para- positions. However, no effect on reactivity of haloarenes is observed by the presence of electron withdrawing group at meta-position. Mechanism of the reaction is as depicted: Can you think why does NO2 group show its effect only at ortho- and para- positions and not at meta- position? As shown, the presence of nitro group at ortho- and para-positions withdraws the electron density from the benzene ring and thus facilitates the attack of the nucleophile on haloarene. The carbanion thus formed is stabilised through resonance. The negative charge appeared at ortho- and para- positions with respect to the halogen substituent is stabilised by –NO2 group while in case of meta-nitrobenzene, none of the resonating structures bear the negative charge on carbon atom bearing the –NO2 group. Therefore, the presence of nitro group at meta- position does not stabilise the negative charge and no effect on reactivity is observed by the presence of –NO2 group at meta-position. Chemistry 304 C:\Chemistry-12\Unit-10.pmd 28.02.07  2. Electrophilic substitution reactions Haloarenes undergo the usual electrophilic reactions of the benzene ring such as halogenation, nitration, sulphonation and Friedel-Crafts reactions. Halogen atom besides being slightly deactivating is o, p- directing; therefore, further substitution occurs at ortho- and para- positions with respect to the halogen atom. The o, p-directing influence of halogen atom can be easily understood if we consider the resonating structures of halobenzene as shown: Due to resonance, the electron density increases more at ortho- and para-positions than at meta-positions. Further, the halogen atom because of its –I effect has some tendency to withdraw electrons from the benzene ring. As a result, the ring gets somewhat deactivated as compared to benzene and hence the electrophilic substitution reactions in haloarenes occur slowly and require more drastic conditions as compared to those in benzene. (i) Halogenation (ii) Nitration (iii) Sulphonation 305 Haloalkanes and Haloarenes C:\Chemistry-12\Unit-10.pmd 28.02.07  (iv) Friedel-Crafts reaction xa l 10.9 Example 0 9 Although chlorine is an electron withdrawing group, yet it is ortho-, para- directing in electrophilic aromatic substitution reactions. Why? o o Chlorine withdraws electrons through inductive effect and releases Solution electrons through resonance. Through inductive effect, chlorine destabilises the intermediate carbocation formed during the electrophilic substitution. Through resonance, halogen tends to stabilise the carbocation and the effect is more pronounced at ortho- and para- positions. The inductive effect is stronger than resonance and causes net electron withdrawal and thus causes net deactivation. The resonance effect tends to oppose the inductive effect for the attack at ortho- and para- positions and hence makes the deactivation less for ortho- and para- attack. Reactivity is thus controlled by the stronger inductive effect and orientation is controlled by resonance effect. Chemistry 306 C:\Chemistry-12\Unit-10.pmd 28.02.07  3. Reaction with metals Wurtz-Fittig reaction A mixture of an alkyl halide and aryl halide gives an alkylarene when treated with sodium in dry ether and is called Wurtz-Fittig reaction. Fittig reaction Aryl halides also give analogous compounds when treated with sodium in dry ether, in which two aryl groups are joined together. It is called Fittig reaction. I e Questions Intext e to s 10.7 Which alkyl halide from the following pairs would you expect to react more rapidly by an SN2 mechanism? Explain your answer. 10.8 In the following pairs of halogen compounds, which compound undergoes faster SN1 reaction? 1 10.9 Identify A, B, C, D, E, R and R in the following: 307 Haloalkanes and Haloarenes C:\Chemistry-12\Unit-10.pmd 28.02.07  10.7 0. Polyhalogen yh loge Carbon compounds containing more than one halogen atom are usually referred to as polyhalogen compounds. Many of these compounds are om ou Compounds useful in industry and agriculture. Some polyhalogen compounds are described in this section. 10.7.1 Dichloro- Dichloromethane is widely used as a solvent as a paint remover, as a methane propellant in aerosols, and as a process solvent in the manufacture of (Methylene drugs. It is also used as a metal cleaning and finishing solvent. chloride) Methylene chloride harms the human central nervous system. Exposure to lower levels of methylene chloride in air can lead to slightly impaired hearing and vision. Higher levels of methylene chloride in air cause dizziness, nausea, tingling and numbness in the fingers and toes. In humans, direct skin contact with methylene chloride causes intense burning and mild redness of the skin. Direct contact with the eyes can burn the cornea. 10.7.2 Trichloro- Chemically, chloroform is employed as a solvent for fats, alkaloids, methane iodine and other substances. The major use of chloroform today is in (Chloroform) the production of the freon refrigerant R-22. It was once used as a general anaesthetic in surgery but has been replaced by less toxic, safer anaesthetics, such as ether. As might be expected from its use as an anaesthetic, inhaling chloroform vapours depresses the central nervous system. Breathing about 900 parts of chloroform per million parts of air (900 parts per million) for a short time can cause dizziness, fatigue, and headache. Chronic chloroform exposure may cause damage to the liver (where chloroform is metabolised to phosgene) and to the kidneys, and some people develop sores when the skin is immersed in chloroform. Chloroform is slowly oxidised by air in the presence of light to an extremely poisonous gas, carbonyl chloride, also known as phosgene. It is therefore stored in closed dark coloured bottles completely filled so that air is kept out. 10.7.3 Triiodo- It was used earlier as an antiseptic but the antiseptic properties are methane due to the liberation of free iodine and not due to iodoform itself. Due (Iodoform) to its objectionable smell, it has been replaced by other formulations containing iodine. 10.7.4 Tetrachlo- It is produced in large quantities for use in the manufacture of romethane refrigerants and propellants for aerosol cans. It is also used as (Carbon feedstock in the synthesis of chlorofluorocarbons and other chemicals, tetrachloride) pharmaceutical manufacturing, and general solvent use. Until the mid 1960s, it was also widely used as a cleaning fluid, both in industry, as a degreasing agent, and in the home, as a spot remover and as fire extinguisher. There is some evidence that exposure to carbon tetrachloride causes liver cancer in humans. The most common effects are dizziness, light headedness, nausea and vomiting, which can cause Chemistry 308 C:\Chemistry-12\Unit-10.pmd 28.02.07  permanent damage to nerve cells. In severe cases, these effects can lead rapidly to stupor, coma, unconsciousness or death. Exposure to CCl4 can make the heart beat irregularly or stop. The chemical may irritate the eyes on contact. When carbon tetrachloride is released into the air, it rises to the atmosphere and depletes the ozone layer. Depletion of the ozone layer is believed to increase human exposure to ultraviolet rays, leading to increased skin cancer, eye diseases and disorders, and possible disruption of the immune system. 10.7.5 Freons The chlorofluorocarbon compounds of methane and ethane are collectively known as freons. They are extremely stable, unreactive, non-toxic, non-corrosive and easily liquefiable gases. Freon 12 (CCl2F2) is one of the most common freons in industrial use. It is manufactured from tetrachloromethane by Swarts reaction. These are usually produced for aerosol propellants, refrigeration and air conditioning purposes. By 1974, total freon production in the world was about 2 billion pounds annually. Most freon, even that used in refrigeration, eventually makes its way into the atmosphere where it diffuses unchanged into the stratosphere. In stratosphere, freon is able to initiate radical chain reactions that can upset the natural ozone balance (Unit 14, Class XI). 10.7.6 p,p’-Dichlo- DDT, the first chlorinated organic insecticides, was originally prepared rodiphenyl- in 1873, but it was not until 1939 that Paul Muller of Geigy trichloro- Pharmaceuticals in Switzerland discovered the effectiveness of DDT as ethane(DDT) an insecticide. Paul Muller was awarded the Nobel Prize in Medicine and Physiology in 1948 for this discovery. The use of DDT increased enormously on a worldwide basis after World War II, primarily because of its effectiveness against the mosquito that spreads malaria and lice that carry typhus. However, problems related to extensive use of DDT began to appear in the late 1940s. Many species of insects developed resistance to DDT, and it was also discovered to have a high toxicity towards fish. The chemical stability of DDT and its fat solubility compounded the problem. DDT is not metabolised very rapidly by animals; instead, it is deposited and stored in the fatty tissues. If ingestion continues at a steady rate, DDT builds up within the animal over time. The use of DDT was banned in the United States in 1973, although it is still in use in some other parts of the world. 309 Haloalkanes and Haloarenes C:\Chemistry-12\Unit-10.pmd 28.02.07  u ary Summary Alkyl/ Aryl halides may be classified as mono, di, or polyhalogen (tri-, tetra-, etc.) compounds depending on whether they contain one, two or more halogen atoms in their structures. Since halogen atoms are more electronegative than carbon, the carbon- halogen bond of alkyl halide is polarised; the carbon atom bears a partial positive charge, and the halogen atom bears a partial negative charge. Alkyl halides are prepared by the free radical halogenation of alkanes, addition of halogen acids to alkenes, replacement of –OH group of alcohols with halogens using phosphorus halides, thionyl chloride or halogen acids. Aryl halides are prepared by electrophilic substitution to arenes. Fluorides and iodides are best prepared by halogen exchange method. The boiling points of organohalogen compounds are comparatively higher than the corresponding hydrocarbons because of strong dipole-dipole and van der Waals forces of attraction. These are slightly soluble in water but completely soluble in organic solvents. The polarity of carbon-halogen bond of alkyl halides is responsible for their nucleophilic substitution, elimination and their reaction with metal atoms to form organometallic compounds. Nucleophilic substitution reactions are categorised into SN1 and SN2 on the basis of their kinetic properties. Chirality has a profound role in understanding the reaction mechanisms of SN1 and SN2 reactions. SN2 reactions of chiral alkyl halides are characterised by the inversion of configuration while SN1 reactions are characterised by racemisation. A number of polyhalogen compounds e.g., dichloromethane, chloroform, iodoform, carbon tetrachloride, freon and DDT have many industrial applications. However, some of these compounds cannot be easily decomposed and even cause depletion of ozone layer and are proving environmental hazards. Exercises 10.1 Name the following halides according to IUPAC system and classify them as alkyl, allyl, benzyl (primary, secondary, tertiary), vinyl or aryl halides: (i) (CH3)2CHCH(Cl)CH3 (ii) CH3CH2CH(CH3)CH(C2H5)Cl (iii) CH3CH2C(CH3)2CH2I (iv) (CH3)3CCH2CH(Br)C6H5 (v) CH3CH(CH3)CH(Br)CH3 (vi) CH3C(C2H5)2CH2Br (vii) CH3C(Cl)(C2H5)CH2CH3 (viii) CH3CH=C(Cl)CH2CH(CH3)2 (ix) CH3CH=CHC(Br)(CH3)2 (x) p-ClC6H4CH2CH(CH3)2 (xi) m-ClCH2C6H4CH2C(CH3)3 (xii) o-Br-C6H4CH(CH3)CH2CH3 10.2 Give the IUPAC names of the fo