Organic Chemistry Some Basic Principles Chapter 12 PDF

Organic Chemistry Some Basic Principles Chapter 12 Enrichment Cluster 6 CHEMISTRY OF CARBON COMPOUNS Learning objectives : The student will be able to Name the compounds according to IUPAC sy...

Organic Chemistry Some Basic Principles Chapter 12 Enrichment Cluster 6 CHEMISTRY OF CARBON COMPOUNS Learning objectives : The student will be able to Name the compounds according to IUPAC system of nomenclature and also derive their structures from the given names can write isomers of the given copounds understand the concept of organic reaction mechanism explain the influence of electronic displacements on structure and reactivity of organic compounds recognise the types of organic reactions. Flow Chart: 12.1. Organic Chemistry Organic chemistry is the branch of chemistry that deals with the study of hydrocarbons and their derivatives. Carbon is able to form millions of compounds because of its tetra valency and catenation property. 12.2.Functional Group: The functional group is an atom or group of atoms which determines the chemical properties of the organic compound. The examples are hydroxyl group (—OH), aldehyde group (—CHO) and carboxylic acid group (—COOH) etc. 12.3. Homologous Series A homologous series may be defined as a series of organic compounds having the same functional group, similar chemical properties and the successive members differ from each other in molecular formula by —CH2 units and molecular mass difference 14 gm. The members of a homologous series can be represented by same general molecular formula. 12.4. Nomenclature of Organic Compounds 12.4.1.Common name (Common system): Before the IUPAC system of nomenclature, organic compounds were named after the sources of origin. for example, urea was so named because it was obtained from the urine of mammals. Formic acid was so named since it was extracted from red ants called formica. Some of the common names of the compounds. HCHO Formaldehyde. CH3CHO Acetaldehyde CH3COOH Acetic acid C6H5OCH3 Anisole  CHCl3 Chloroform C6H5NH2 Aniline 12.4.2. I UP AC (International Union of Pure and Applied Chemistry) System. According to IUPAC system, the name of an organic compound contains three parts: (i) word root, (ii) suffix, (iii) prefix. (i) Word root: Word root represents the number of carbon atoms present in the parent carbon chain, which is the longest possible chain of carbon atoms. (ii) Suffix: Suffix are of two types, primary suffix, secondary suffix. (a) Primary Suffix: It indicates saturation or unsaturation in the compound. (b) Secondary Suffix: Secondary suffix is used to represent the functional group. (iii) Prefix: Prefix is a part of IUPAC name which appears before the word root. Prefix are of two types: (a) Primary prefix: For example, primary prefix cyclo is used to differentiate cyclic compounds. (b) Secondary prefix: Some functional groups are considered as substituents and denoted by secondary prefixes. For example: Substituted Group Suffix prefix. —F - Fluro — Cl - Chloro — Br - Bromo > CO one oxo — NO2 - Nitro — CH3 - Methyl — OCH3 - Methoxy -OH ol hydroxy -CHO al formyl -COOH oic acid carboxy -NH2 amine amino 12.4.3.How to name organic compounds using the IUPAC rules In order to name organic compounds you must first memorize a few basic names. These names are listed within the discussion of naming alkanes. In general, the base part of the name reflects the number of carbons in what you have assigned to be the parent chain. The suffix of the name reflects the type(s) of functional group(s) present on (or within) the parent chain. Other groups which are attached to the parent chain are called substituents. Substituents: In hydrocarbons generally substituents are alkyl groups. Alkyl group has one hydrogen less than parent hydrocarbon. Alkyl group name is obtained by changing the suffix –ane with –yl For ex : -CH3 ----- Methyl -C2H5 -----Ethyl -C3H7------ Propyl Word Roots: tells us about the number of carbon atoms in the parent chain. Number of Carbons Name 1 meth 2 eth 3 prop 4 but 5 pent 6 hex 7 hept 8 oct 9 non 10 dec 11 undec 12 dodec  There are a few common branched substituents which you should memorize. These are shown below. 12.4.4.Rules for naming hydrocarbons: Here is a simple list of rules to follow. Some examples are given at the end of the list. 1. Identify the longest carbon chain. This chain is called the parent chain. 2. Identify all the substituents (groups attached to parent chain). 3. Give numbers to carbon atoms in such a way so that the carbon atom having substituents should get minimum number 4. If two or more side chains are in equivalent positions, assign the lowest number to the one which will come first in the name. 5. If the same substituent occurs more than once, the location of each point on which the substituent occurs is given. In addition, the number of times the substituent group occurs is indicated by a prefix (di, tri, tetra, etc.). 6. If there are two or more different substituents are present then the substituent are listed in alphabetical order using the base name (ignore the prefixes). 7. If chains of equal length are competing for selection as the parent chain, then the choice goes in series to: a) the chain which has the greatest number of side chains. b) the chain whose substituents have the lowest- numbers. c) the chain having the greatest number of carbon atoms in the smaller side chain. d) the chain having the least branched side chains.  Here are some examples: 12.4.5.Naming of Compounds Containing Functional Groups: The longest chain of carbon atoms containing the functional group is numbered in such a manner that the functional group is attached at the carbon atoms possessing lowest possible number in the chain. In case of poly functional compounds, one of the functional group is selected as principal functional group and the compound is named on that basis of priority of the functional group. The choice of principal functional group is made on the basis of order of preference. The order of decreasing priority for the functional group is 12.4.6.Naming of compounds with functional groups: 12.4.7.Naming of aromatic compounds: 1. The simple aromatic ring containing six carbon atoms is considered benzene. 2. All other six-membered substituted structures are considered derivatives of benzene. 3. The six carbon atoms of the benzene ring is given as numbers from number 1 to 6. 4. The 1,4 position is called as para position. 5. The 2,6 position is called as ortho position. 6. The 3,5 position is called a meta position. 7. In the common name system the words ortho, para and meta are used. 8. In the IUPAC name system numbers 1 to 6 to which substituents are attached is used. 9. The substituents are numbered according to the priority order. 10. Example is shown in the diagram. 12.5. Isomerism When there are two or more compounds possessing the same molecular formula but different structural formula and different physical properties, the phenomenon is called isomerism. Such compounds are called isomers. It is of two types: (1) Structural Isomerism (2) Stereoisomerism (1) Structural Isomerism: Structural isomerism is shown by compounds having the same molecular formula but different structural formulae differing in the arrangement of atoms. Structural isomers are  a. Chain isomers: Compounds having same molecular formula but different skeleton structure are called chain isomers. b. Position isomers: Compounds having same molecular formula but the position of the substituent is different is called positional isomers. c. Functional isomers: Compounds having same molecular formula but different functional group is different is called functional isomer. d. Metamers: Compounds having same molecular formula but the distribution of alkyl groups on both sides of the functional group are different. (2) Stereoisomerism: When isomerism is caused by the different arrangements of atoms or groups in space, the phenomenon is called stereoisomerism. The stereo isomers have same structural formula but differ in arrangement of atoms in space. Stereoisomerism is of two types: (i) Geometrical or Cis-Trans Isomerism (ii) Optical Isomerism Try yourself: 1. The I.U.P.A.C. name of CH3COCH(CH3)2 is (a) 3-methyl-2-butanone (b) Isopropyl methyl ketone (c) 2-methyl-3-butanone (d) 4-methyl isopropyl ketone 2. The IUPAC name of CH3CHO is: (a) Acetaldehyde (b) Methylaldehyde (c) Formyl chloride (d) Ethanal 3. The structure of 4-Methylpent-2-en-1-ol is: (a) CH3CH2CH=CHCH2OH (b) (CH3)2C=CHCH2CH2OH (c) (CH3)2CHCH=CHCH2OH (d) CH3CH(OH)CH-CH=C(CH3)2 4.What is the correct IUPAC name of (a) 4-methoxy-2-nitrobenzaldehyde (b) 4-formyl-3-nitro anisole (c) 4-methoxy-6-nitrobenzaldehyde (d) 2-formyl-5-methoxy nitrobenzene 5. The I.U.P.A.C. name of (a) 3-Methyl cyclohexene (b) 1-methyl cylohex-2-ene. (c) 6-methyl cyclohexene (d) 1-methyl cyclohex5-ene. Answers : A 2 D 3. C 4. A 5. B 12.6.Fundamental Concepts in Organic Reaction Mechanism Fission of a covalent bond: A covalent bond can undergo Fission in two ways: (i) By Homolytic Fission or Homolysis (ii) By Heterolytic Fission or Heterolysis 12.6.1.HomolyticFission: The bond between the atoms is broken in such away so that the two electrons in the bond formation are equally distributed between the atoms.Electrically neutral species having a single electron are formed. They are called free radicals. Three types of free are there. They are primary,secondary and tertiary. The stability of the free radicals is in the fillowing order. Lessr the number of alkyl groups more is the electron density on the carbon atom and less is the stability of free radical 12.6.2.Heterolytic Fission: In this process one of the atoms in the bond formation are more electronegative than the other and acquires both the bonding electrons when the bond is broken. If B is more electronegative than A which thereby acquires both the bonding electrons and becomes negatively charged. The products of heterolytic fission are ions. Try yourself: 1. For the following bond cleavages, use curved-arrows to show the electron flow and classify each as homolysis or heterolysis. Identify reactive intermediate produced as free radical, carbocation and carbanion. Answers:\ a. Free radical b. Carbanion c. Carbocation d. phenyl carbocation. 12.6.3.Reaction Intermediates: Heterolytic and homolytic bond fission results in the formation of short-lived fragments called reaction intermediates.Important reaction intermediates are carbonium ions, carbanions, carbon free radicals. a.Carbonium Ions (carbocations): Carbon atoms which contain a positive charge are called carbonium ions or carbocations. They are formed by heterolytic bond fission. where Z is more electronegative than carbon. Tertiary carbonium ion is more stable than a secondary, which in turn is more stable than a primary because of +I effect associated with alkyl group. Greater the number of alkyl groups, less is the charge on the carbon atom and more is the stability. b.Carbanion: Carbon atom which contains a negative charge is called carbanion. They are also formed by heterolytic bond fission. Where Z is less electronegative than carbon. A primary carbanion is more stable than a secondary, which in turn is more stable than a tertiary. Because of +I effect associated with alkyl group more negative charge exists on carbon atom with more number of alkyl groups. Greater the charge, less is the stability of the ion. c.Free Radicals: Carbon atom with single electron is called free radical. It is formed in homolytic fission of covalent bond between two atoms. This type of fission happens when the electronegativity difference between the atoms is less. Three types of free radicals are there. They are primary,secondary and tertiary. The stability of the free radicals is in the fillowing order. Greater the number electron donating alkyl groups more is the stability of free radical. 12.6.4. Attacking reagents: Electrophile: It is positively charged or neutral species which is electron deficient, e.g., Cl+ , H2O+, CH3+ , NH4+, AICl3 Nucleophile: It is negatively charged or neutral species with lone pair of electrons e.g., (HO –), Cyanide CN-, H2O: R3N, R2NH etc. 12.6.5.Electron Displacement Effects in Covalent Bonds: Electronic displacements in covalent bonds occur due to the presence of an atom or group of different electronegativity or under the influence of some outside attaching group. These lead to a number of effects which are as follows: (i) Inductive effect (ii) Elecromeric effect (iii) Resonance or Mesomeric effect (iv) Hyperconjugation effect. a.Inductive Effect: It involves displacement of electrons. The electrons which form a covalent bond are seldom shared equally between the two atoms. Due to different electronegativity electrons are displaced towards the more electronegative atom. This introduces a certain degree of polarity in the bond. The more electronegative atom acquires a small negative charge (δ –). The less electronegative atom acquires a small positive charge (δ+). Consider the carbon-chlorine bond As chlorine is more electronegative, it will become negatively charged with respect to the carbon atom. Structure I- indicates the relative charges on the two atoms. Structure II- indicates the direction in which the electrons are drawn. Atoms or groups which lose electrons towards a carbon atom are said to have a +I effect. Those atoms or groups which draw electrons away from a carbon atom are said to have a -I Effect. Some common atoms or groups which cause +I or -I effects are shown below: The inductive effect of C3 upon C2 is significantly less than the effect of the chlorine atom on C. It’s effect is almost nill after fourth carbon atom. Affects of Inductive effect:1. Electron withdrawing groups(-I) increase the acidic character of the compound. 2. Electron releasing groups(+I) increase the basic character of the compound. b.Resonance: A number of organic compounds cannot be accurately represented by one structure. A single structure can not explain all the properties of the molecule. Two or more than two structures are required to explain all the properties of the molecules. The different structures are called canonical structures and the phenomena is called resonance. For example, benzene is ordinarily represented as Carbon-carbon double bond length = 1.34 A Carbon-carbon single bond length = 1.54A. But it has been determined experimentally that all carbon-carbon bonds in benzene are identical and have same bond length (1.39A). Thus the structure of benzene cannot be represented by single structure. It can be represented equally well by energetically similar structures I and II. The two structures are called resonance structures. Actual structure of benzene is resonance hybrid of structures I and II. Another example of resonance is provided by nitro methane (CH3N02) which can be represented by two Lewis structures. The actual structure of nitro methane is a resonance hybrid of the two canonical forms I and II. Resonance energy: The difference in the energy between the most stable contributing structure for a compound and its resonance hybrid is called as resonance energy or resonance stabilization energy. Resonance Effect: The polarity produced in the molecule by the interaction of two π-bonds or between a π-bond and a lone pair of electrons present on an adjacent atom. There are two types of resonance or mesomeric effects designated as R or M effect. Positive Resonance Effect (+R effect): Those atoms which lose electrons towards a carbon atom are said to have a +M effect or +R effect. For example: —Cl, —Br, —I, —NH2, —NR2, —OH, —OCH3 Negative Resonance Effect (-R effect): Those atoms or groups which draw electrons away from a carbon atom are said to have a -M effect or -R effect. For example: c.Electromeric Effect (E Effect): Pie bond is shifted towards one of the atoms on the demand of attacking reagent is known as electromeric effect. The curved arrow shows the displacement of the electron pair. The atom A has lost its share in the electron pair and B has gained this share. Therefore A acquires a positive charge and B a negative charge. It is represented by E and shifting of electrons is shown by a curved arrow Types of electromeric effects: (i)Positive Electromeric effect (+E) :Shifting of ԯ electrons and attack of attacking reagent takes place on the same atom. (ii)Negative Electrmeric effect (-E) : Shifting of ԯ electrons and attack of attacking reagent takes place on different atoms (d) Hyper conjugation or No Bond Resonance: When the alkyl group is attached to an unsaturated system such as CH3CH2+ the behavior of electron movement can be explained with Hyperconjugation. The structures that are arrived at by shifting of the bonding electrons from an adjacent C —H bond to the electron deficient carbon atom. In this way, the positive charge originally on carbon atom is dispersed to the hydrogen. This way of electron release by assuming no bond character in the adjacent C—H bond is called No-Bond Resonance or Hyper conjugation.  Orbital Concept of Hyper conjugation It involves delocalisation of sigma electrons of C—H bond of an alkyl group which is attached directly to an atom of unsaturated system or to an atom with an unshared p-orbital. Let us consider CH3CH2+(ethyl cation) in which the positively charged carbon atom has an empty p-orbital. One of the C—H bonds of the methyl group can align in the plane of this empty p- orbital and electron constituting the C—H bond in plane with this p-orbital can then be delocalized into the empty p-orbital as in Fig. In general, greater the number of alkyl groups attached to a positively charged carbon atom, the greater is the hyper conjugation. 12.7.Types of Organic Reactions. There are different types of organic reactions that can take place. They are as follows: 1) Substitution reactions 2) Elimination reactions 3) Addition reactions Let us study each of these reactions in detail, to understand more about them. 1) Substitution Reactions In a substitution reaction, generally, one atom or a group of atoms take place of another atom or a group of atoms which leads to the formation of an altogether new substance. Nucleophilic substitution takes place in alky halides. CH3Cl + OH- —-> CH3OH+ Cl- Also, a strong base such as NaOH has to be in dilute form because suppose if the base is of higher concentration, there are chances of de hydro halogenation taking place in the reaction. And, the solution needs to be in an aqueous state such as water for the reaction to take place. Electrophilic substitution tales palce on aromatic compounds. Free radical substitution takes place in alkanes, CH4 + Cl2 → CH3Cl + HCl 2) Elimination Reactions There are certain reactions which involve the elimination and removal of the adjacent atoms. After this multiple bonds are simultaneously formed and there is a release of small molecule as products. One of the examples of a typical elimination reaction is the conversion of ethyl chloride to ethylene. CH3CH2Cl → CH2= CH2 + HCl In the above reaction, the eliminated molecule is HCl, which can form out of the combination of H+ from the carbon atom which is on the left side and Cl– from the carbon atom which is on the right side. In this type of reactions saturated compound becomes unsaturated. 3) Addition Reactions An addition reaction is simply just the opposite of an elimination reaction. In an addition reaction, the components or molecules are added to the carbon-carbon multiple bonds and form a single molecule. In the reaction given below when HCl is added to ethylene, it will give us ethylene chloride. In this type of reaction unsaturated compound becomes saturated compound. HCl + CH2 = CH2 → CH3CH2Cl Try yourself. 1. The following reaction is (a) Elimination reaction (b) Substitution reaction (c) Free radical reaction (b) Addition reaction 2. Which of the following is an electrophile? (a) H2O (b) NH3 (c) AlCl3 (d) C2H5NH2 3. (a) resonating structure (b) Tautomers (c) Geometrical isomers (d) Optical isomers. 4. CH3CH2Cl undergoes homolytic fission to produce (a) CH3CH2 & Cl (b) CH3CH+2 & Cl– (c) CH3CH+2 & Cl (d) CH3CH2 & Cl– 5. The addition of carbonyl compound to HCN is an example of (a) Nucleophilic substitution (b) Electrophilic addition (c) Nucleophilic addition (d) Electrophilic substitution  Answers : 1. B 2. C 3. A 4. A 5. C Summary: 1. Carbon is able to form millions of compounds because of its catenation and tetra valency. 2. International union of pure and applied chemists framed certain rules for naming of organic compounds. 3. Isomers are the compounds having same molecular formula but different structural or spatial arrangements. 4. Isomers have same chemical properties but different physical properties. 5. Bond fission is the breakage of earlier bonds. 6. Bond fission can be homolytic or heterolytic fission. 7. Free radicals are formed because of homolytic fission 8. Carbocations or carbo anions are formed because of heterolytic fission. 9. Free radicals. Carbocations or carbo anions are called reaction intermediates. 10. Electrophile is electron deficient and Nucleophile is electron rich. 11. Electrophile and Nucleophile are attacking reagents. 12. Inducing charge into a long carbon chain is called Inductive effect. +I effect increase the basic character of the compound and – I effect increases acidic character of the compound. 13. Shifting of Pie bond on the demand of attacking reagent towards one of the atom is called electromeric effect. It can be of +E effect or -E effect. 14. The group which is present on benzene ring directs the incoming group either at o and p positions or at o position dur resonance effect. 15. Hyperconjugation effect is a permanent effect in which localization of σ electrons of C-H bond of an alkyl group directly attached to an atom of the unsaturated system or to an atom with an unshared p orbital takes place. 16. Saturated compounds undergo substitution reaction in which an atom or group of atoms are substituted with other atoms or groups. 17. Unsaturated compounds undergo addition reactions in which two raecatnt molecules are added up with each other to give a single compound. 18. Saturated compounds are converted into unsaturated compounds in elimination raections. LINKS : 1. CBSE Class 11 Chemistry || Organic Chemistry Part-1 || Full Chapter || By Shiksha House Terminal questions: Multiple Choice Questions  1. The large number of organic compounds is due to (a) the valency of carbon (b) a small size of carbon (c) a special property of carbon known as catenation 2. The IUPAC name of (a) 1, 2-dichloropropane (b) 3, 3-dichloropropane (c) 1, 1-dichloropropane (d) dichloropropane 3. The IUPAC name of (a) 2-methyl butanal (b) butan-2-aldehyde (c) 2-ethylpropanal (d) 3-methyl isobutraldehyde 4. The bond that undergoes heterolytic cleavage most readily is (a) C-C (b) C-O (c) C-H (d) O-H 5. The reaction (a)carbocation formation (b) free-radical mechanism (c) carbanion formation (d) none of these 6. The hybridization state of a carbocation is (a) sp4 (b) sp3 (c) sp2 (d) sp 7. Which of the following are electrophiles? (a) Dimethyl sulphide (b) Bromides (c) Carbon dioxide (d) Ammonia 8. Which of the following compounds will exhibit cis-trans isomerism? (a) 2-Butene (b) 2-Butyne (c) 1-Butene (d) 2-Butanol 9. Fill in the blanks 1. Electrophilic reagents are ……………….. & ……………….. deficient species. 2. Carbocation is formed by a group of carbon & other atoms having only ……………….. electrons. 3. Free radicals are formed due to ……………….. bond fission. 4. Mesomeric effect is observed only in the molecules having -------- Answer the following: 1. Write the hybridized state of C atoms in the following CH2 = CH – C-N 2. Explain why (CH3)3C+ is more stable than CH 3C+H2 3.Which of the two O2NCH2CH2 - or CH3CH2O – is expected to be more stable and why ? 4.Arrange the following in increasing Order of Stability ; (CH3 )3C + , CH3CH2CH2CH2+ , CH3CH2C+HCH3, CH3C H2+. 5.Draw the Structures of the following compounds. A) Hex-3-enoic acid b) 2-chloro-2- methylbutan-1-ol. 6.Explain Inductive effect with example. 7.Give the number of Sigma and pi bonds in the following molecules a) CH3NO2 b) HCONHCH3. 8.Write the condensed and expanded formula of 2,2,4-Trimethylpentane. 9.Account for the following. Alcohols are weaker acids than Water, Why ? 10.Which bond is more polar in the following pairs of molecules. a) H3C-H, H3CBr b) H3C-NH2, H3C-OH. 11.Define Isomerism. Explain position Isomerism and Functional Isomerism with examples. 12.Define functional groups. Write the general formula of Carboxylic acids ,acid chlorides.  13.Write a short note on Resonance effect. Draw the resonating structuresof phenol. 14.Explain hyperconjugation effect. How does hyperconjugation effect explain the stability of alkenes? 15.Explain the terms inductive effect and electromeric effect with examples. 16. Arrange the following compounds in the increasing order of their acidic and basic strength : (a) ClCH2COOH, Cl3CCOOH and Cl2CHCOOH. (b) CH3NH2 , CH3NHCH3 , (CH3 )3N, C6H5NHCH3. 17. Which electron displacement effect explain the following correct orders of acidity of the carboxylic acids? (a) Cl3CCOOH > Cl2CHCOOH > ClCH2 COOH (b) CH3CH2COOH > (CH3)2 CHCOOH > (CH3)3CCOOH 18.What do you mean by electrometric effect? Explain with example. 19.Draw the resonance structures for phenol. 20.Explain hyperconjugation effect. How does hyperconjugation effect explain the stability of alkenes 21. Give equation for the following: (i) Electrophilic Substitution (ii) Nucleophilic Substitution 22. What are electrophiles? Explain electrophile substitution reaction with the help of example. 23.What are addition reactions? In which type of compounds addition reactions takes place? 24. What are attacking reagents? Give examples. 25. What do you mean by bond fission? What are its types? What are the intermediates formed in the bond fission?