Basic Principles of Organic Chemistry PDF

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2017-18 100 & op kers Class 11 T By E ran culty -JE Fa r IIT enior emie. S fP r es o titut Ins CHEMISTRY FOR JEE MAIN & ADVANCED SECOND EDITION Exhaustive Theory (Now Revised) Formula Sheet 9000+ Problems based on latest JEE pattern 2500 + 1000 (New) Problems of previous 35 years of AIEEE (JEE Main) and IIT-JEE (JEE Adv) 5000+Illustrations and Solved Examples Detailed Solutions of all problems available Plancess Concepts Topic Covered Tips & Tricks, Facts, Notes, Misconceptions, Key Take Aways, Problem Solving Tactics Basic Principles of Organic Chemistry PlancEssential Questions recommended for revision  8. BASIC PRINCIPLES OF ORGANIC CHEMISTRY 1. INTRODUCTION What is organic chemistry? Organic chemistry is the study of most carbon compounds with the exception of a few (e.g., CO2 and carbonate salts). While inorganic chemistry deals with the study of all other compounds. 2. PROPERTIES OF ORGANIC COMPOUNDS In general, organic compounds. (a) Are far more in number than inorganic compounds. This is due to the catenation property of the C atom. Carbon has the ability to form bonds with almost every other element (Other than the noble gases), forming long chains as well as ring compounds. Moreover, C compounds exist as many isomers. (b) React more slowly and require higher temperatures for reactions to take place. (c) Are less stable and sometimes decompose on heating to compounds of lower energy levels. (d) Undergo more complex reactions and produce more side reaction products. (e) Are largely insoluble in water. (f) Have generally lower melting and boiling points. (g) Are classified into families of compounds such as carboxylic acids, which have similar reactive groups and chemical properties. (h) Are mostly obtained from animals or plants as opposed to the mineral origin of inorganic compounds. 3. CLASSIFICATION OF ORGANIC COMPOUNDS They are classified follows: Classification of organic compounds is basically based on the functional group. The chemical properties of compound depends on the properties of the functional group present in it. The rest of the molecule simply affects the physical properties, e.g., m.p., b.p., density etc. and has very little effect on its chemical properties. 8. 2 | Basic Principles of Organic Chemistr y Organic compounds Open chain or acrylic or Closed chain or aliphatic compounds, alicyclic or cyclic or e.g. CH4 (Methane) ring compounds C2H6(Ethane) Straight chain Branched chain CH3CH2CH3 CH3 CH CH3 (Propane) CH3 CH3 C H (Isobutane) O (Ethanol) Homocyclic or Heterocyclic compounds carbocyclic compounds ( ( Contains hetero ( Containing rings of entirely C atom ( atom in ring e.g. N, O, S, P, etc Alicyclic Aromatic N S O compounds compounds Pyridine Thiophene Furan O Benzenoid Non-benzenoid (THF) (Cyclopropane) compounds compounds Tetrahydrofuran (Benzene) (Cyclobutane) (Azulene) CH3 (Toluene) O (Cyclohexane) (Tropolone) (Naphthalene) Flowchart 8.1: Classification of organic compound Homologous Series Organic compounds containing one particular characteristic group or functional group constitute a homologous series, e.g., alkanes, alkenes, haloalkanes, alkanols, alkanals, alkanones, alkanoic acids amines etc. 4. NOMENCLATURE OF ORGANIC COMPOUNDS 4.1 Trivial or Common Names In the earlier days, because of the absence of IUPAC names, the names of the compounds were dependent on the source from which the compound was obtained. Even today, in spite of IUPAC nomenclature some of the common names are still at use. In some case, where the IUPAC name is very tedious we prefer to use common names, for example lactic acid, sucrose etc.  Chem i str y | 8.3 Table 8.1: Common names of organic compounds, their sources, and structures S.No Common name Source Structure 1. Formic acid Formica (red ant) HCOOH 2. Acetic acid Acetum (vinegar) MeCOOH 3. Propionic acid Portopion (first fat) MeCH2COOH 4. Butyric acid Butyrum (butter) MeCH2CH2COOH 5. Valeric acid Valerian (shrub) Me(CH2)3COOH 6. Caproic acid Caper (Goat) Me(CH2)4COOH 7. Urea Urine NH2CONH2 8. Malic acid Malum (apple) CH2COOH | CH(OH)COOH 9. Methyl alcohol Methu hule (Mehtu-wine, hule = wood) MeOH 4.2 IUPAC Names The IUPAC nomenclature of organic compounds is a systematic method of naming organic compounds as recommended by the International Union of Pure and Applied Chemistry (IUPAC). This system uses substitutive nomenclature, which is based on the principal group, and principal chain. The IUPAC rules for the naming of alkanes from the basis of the substitutive nomenclature of most other compounds - IUPAC name Prefix Word root Suffix  Basic units of name  Denotes the no. of Primary prefix Secondary prefix Primarysuffix Secondary suffix  Distinguish from  Added before word root C atoms in the longest  Added to primary suffix  Added to word root acyclic compounds primary prefix chain such as C1-Meth,  Indicates the nature of the  Indicates whether the C C2-Eth, C3-Prop, C4-But, functional group, e.g., CH2 (carboxylic compounds) chain is saturated or  C5-Pent, etc alcohol (—OH), Treated as substituents and unsaturated CH2 CH2 aldehyde (—CHO), not functional groups Saturated  Added in alphabetical CH2OH CH2 CH2 (for single bond) order –F(Fluoro),–NO (Nitroso),etc. Eth+ane +ol —ane Cyclo + Pent + ane Word primary suffix root Primary Word Primary Unsaturated : secondary suffix prefix root suffix (for one double bond) —ene (for two double bond) —diene (for one triple bond) —yne (for two triple bonds) —diyne Br Br A complete IUPAC name consists of Sec. prefix 1 Pr. prefix 1 Word root 1 Pr. suffix 1 Sec. suffix 4-Bromo Cyclo Hex an(e) 1-ol Flowchart 8.2: IUPAC nomenclature of organic compound 8. 4 | Basic Principles of Organic Chemistr y 4.3 IUPAC and Common Names of Some Functional Groups and Classes of Organic Compounds 4.3.1 IUPAC Rules for Saturated Hydrocarbons 1. Alkanes: General formula: CnH2n+2 IUPAC group suffix: - ane 43 2 1 E.g. (H3CCH2 CH2CH3) 3 1 or 4 2 (Me(CH2)2Me) Butane (IUPAC name)  suffix = –ene  2. Alkenes: General Formula: CnH2n ⇒    Alkane –ane alkene   +ene  Functional group structure: C=C E.g. 1 2 3 i. (CH2 = CHCH2CH3 ) or 4 ( 4 2 3 1 ( But-1-ene (IUPAC)  suffix = –yne  3. Alkyne: General formula: CnH2n – 2  –ane   Alkane → Alkyne   + yne  Functional group structure: (– C ≡ C – ) E.g. i. HC ≡ CH or H – ≡ – H Ethyne Acetylene (IUPAC) (Common name) 4. Halides: General formula: CnH2n+1 X (X = F, Cl, Br, I) (RX) suffix = Halo Functional group structure – X 3 1 3 1 4 3 2 1 4 4 E.g. CH3CH 2CH2CH2Cl or Me 2 Cl or 2 Cl (1-Chlorobutane) (1-Chloere butane) 5. Alcohols: General formula: CnH2n+1 OH (R – OH) IUPAC suffix: – ol Common name = Alcohol IUPAC prefix: -Hydroxy. Functional group structure: – OH Example IUPAC name Common name i. CH3OH Methanol Methyl alcohol or Carbinol or Zerone  Chem i str y | 8.5 6. Carboxylic acids: General formula: CnH2n+1 COOH (R – COOH) IUPAC suffix: -oic acid Common name = Acid IUPAC perfix : - Carboxy –e Alkan e +oic acid Alkanol Functional group structure: (–COOH) Example IUPAC name Common name O Methanoic acid Formic acid HCOOH or H OH 7. Aldehydes: General formula: Cn2n+1. CHO (R – CHO) IUPAC suffix: - al Common name = Aldehyde IUPAC prefix: Formyl or oxo- –e Alkane  → Alkanal + al  H   |  Functional group structure: (–CHO) or  − C =O       Example IUPAC name Common name (derived from acid) O Methanal Formaldehyde HCHO or H H R 8. Ketones: General formula: C=O R IUPAC suffix: - one Common name; Ketone IUPAC prefix; – oxo, –e Alkan e Alkanone + one Functional group structure: C=O Example IUPAC name Common name O O Propan-2-one Acetone CH3COCH3 or Me Me or 9. Nitriles: General formula: CnH2n+1 CN (R – C ≡ N) IUPAC suffix: nitrile Common name: Cyanide IUPAC prefix: Cyano Functional group structure: ( – C ≡ N) 8. 6 | Basic Principles of Organic Chemistr y Example IUPAC name Common name 2 1 2 1 1 Methyl cyanide CH3 CN or Me– C ≡ N or (2 – C ≡ N)  Ethane nitrile  or  Acetonitrile  10. Ethers: General formula: (R – O – R’) IUPAC suffix: -- Common name: (Ether) IUPAC prefix: alkoxy (smaller chain) alkane (larger chain) Functional group structure: (R – O – R’) Example IUPAC name Common name CH3 – O – CH3 or (CH3)2O or Me2O Methoxy methane Dimethyl ether 11. Esters: General formula: IUPAC suffix: -oate IUPAC prefix: alkoxy carbonyl Functional group structure: (–COOR) or Example IUPAC name Common name HCOOCH3 or Methyl methanoate Methyl formate O 12. Acyl halides: General formula: (R – C – X) (X = F, Cl, Br, I) IUPAC suffix: -oyl halide IUPAC prefix: halocarbonyl –e Alkane  → Alkanoyl halide oyl halide O Functional group structure: ( – C – X) Example IUPAC name Common name (derived from acid) CH3COCl or Ethanoyl chloride Formyl chloride  Chem i str y | 8.7 13. Amides: General formula: IUPAC suffix: amide IUPAC prefix: Carbamyl –e Alkan e Alkanomide + amide Functional group structure: Example IUPAC name Common name Methanamide Formamide 14. Anhydrides: General formula: IUPAC suffix: – oic anhydride IUPAC prefix: Acetoxy or acytyloxy or –acid Alkanoic acid Alkanoic anhydride +anhydride Functional group structure: (–COOCO–) or Example IUPAC name Common name (derived from acid) HCOOCH or (HCO2) or Methanoic anhydride Formic anhydride 15. Acid hydrazides: General formula: IUPAC suffix – hydrazide IUPAC prefix: -- –ic acid Alkanoic acid  → Alkanohydrazide +hydrazide Functional group structure: (–CONHNH2 ) or Example IUPAC name Common name (derived from acid) HCONHNH2 or Methanohydrazide Formyl hydrazide 8. 8 | Basic Principles of Organic Chemistr y      O   O  ||  ||     ⊕ Θ 16. Acid azides: General formula:  R – C – N3  or R – C – N= N= N   –ic acid IUPAC suffix: azide Alkanoic acid → Alkanoazide + azide    O  Functional group structure: (–CON3) or  ||   ⊕ Θ –C – N= N= N    Example IUPAC O name Common name HCON3 or H N3 Methanoazide Formyl azide 17. Thioalcohols or Thiols or Mercaptans: General formula: (RSH) IUPAC suffix: thiol IUPAC prefix: mercapto –e Functional group structure: (–SH) Alkan e Alkan thiol +thiol Example IUPAC name Common name (derived from acid) CH3SH or MeSH or Methanthiol Methyl thioalcohol or Methylmercaptan 18. Thioethers: General formula: (R – S – R) IUPAC suffix: thioether IUPAC prefix: -- Example IUPAC name Common name (derived from acid) CH3SCH3 ore MeSMe or Me2S Methyl thio ether or Dimethyl sulphide (Methyl thio) Methane 19. Amines: General formula: RHN2 RNHR R–N–R [R4N ]X ⊕ Θ 1º 2º 4ºsalt R 3º IUPAC suffix: amine IUPAC prefix: amino –e Alkan e Alkanamine +amine Functional group structure: –NH2, (1º), NH(2º), N – (3º)  Chem i str y | 8.9 Example IUPAC name Common name CH3CH2NH2 or EtNH2 or Me NH2 or NH2 Ethan amine Ethyl amine  ⊕    20. Nitro compounds: General formula: (RNO2) or  R – N = O  or  R – N = O   |     ↓   OΘ   O        IUPAC suffix: -- IUPAC prefix: nitro Functional group structure: (–NO2) Example IUPAC name CH3NO2 Nitro methane 21. Alkyl nitrites: General formula: (R – O – N = O) IUPAC suffix: nitrite Functional group structure: (–O–N=O) Example IUPAC Name i. CH3 – ONO or Me – O – N = O Methyl nitrite ii. CH3CH2CH2ONO or Pr – O – N = O Propyl nitrite 22. Alkyl isocyanides or Isonitriles: General formula: R – N ≡ C or ( R – N C) ⊕ Θ According to an IUPAC recommendation the substituent – NC is termed as carbylamino. Thus, CH3NC is carbylamino methane and so on. However, this name is not in use. For naming isocyanides, iso is prefixed to the name of the corresponding cyano/nitrile compound. In another mode the suffix carbylamine is added to the name of the alkyl group. Example Common name CH3NC Methyl isocyanide or Acetoisonitrile or Methyl carbylamine 23. Sulphonic acids: General formula: (R – SO3H) IUPAC suffix: sulphonic acid IUPAC prefix: sulpho    O   ||  Functional group structure: (R – SO3H) or  – S – OH   ||     O    Example IUPAC name O || CH3SO3H or Me – S – OH Methyl sulphonic acid || O 8. 1 0 | Basic Principles of Organic Chemistr y 24. Imines: General formula: RCXH = NH IUPAC suffix: imine IUPAC prefix: None –e Alkan e Alkanal imine +al imine Functional group structure: (–CH = NH) Example IUPAC name Common name HCH = NH Methanalimine Formaldimine 25. Cyclic ethers: General formula: O atom ring IUPAC suffix: -- IUPAC prefix: epoxy Example IUPAC name Common name O Oxirane or 1,2-epoxy ethane Ethylene oxide 1 2 4.3.2 IUPAC Rules for Saturated Hydrocarbons (a) The longest possible chain (parent chain) is selected. The chain should be continuous. (b) C atoms which are not included in this chain are considered substituents (side chain) (c) In case of two equal chains having the same length, the one with the larger number of side chains or alkyl groups in selected. (d) Numbering of C atoms in the parent chain starts from that end where the substituent acquires the lowest position numbers or locant. (e) Lowest sum rule: In case of two or more substituents, numbering is done is such a way that the sum of position number substituent or location is the lowest (f) Position and substituent name are separated with a case (-) (g) In case of more than one substituent, they are prefixed by their respective locants in alphabetical order. 4.3.3 IUPAC Rules for Unsaturated Hydrocarbons (a) All the rules of alkanes are also applicable there, (b) The parent or the longest chain is selected irrespective of the = or σ bonds. (c) The numbering is done from the end which is nearer to the = bond, and according to the lowest sum of locant rule. (d) The numbering or sum rule will follow the alphabetical order of the substituent. 4.3.4 IUPAC Rules for Functional Groups While numbering the longest chain, the function group should acquire the lowest number followed by other substituent and the family of multiple bonds even if it violates the lowest sum rule.  Chem i str y | 8.11 E.g. 5 6 C == O (Functional O CH == CH2 1 2 || 3 4| group) acquires the CH3—C—CH—CH—CH2—CH3 lowest number CH3 | (Substituent methyl CH3 CH == CH2( == bond) 4.3.5 IUPAC Rules for Chain Terminating Functional Groups (-CHO, -COOH, -CONH2, -COCl) These chain terminating groups are included in the numbering, starting from the end where it acquires the lowest number followed by other substituent’s in alphabetical order. E.g. Substituent (ethyl) 4 3 2 CH3—CH2—CH—CH2—CH3 | 2-Ethyl butan-1-oic 1 COOH acid Functional group Me O 2 | 1 || 3 CH3—C—C—OC2H5 1 2 Ethyl 2-methyl-2-(3-nitro phenyl) propanoate. 3 NO2 4.3.6 IUPAC Rules for Polyfunctional Compounds In case of polyfunctional compounds, one of the functional groups is chosen as the principal functional group and the compound is named on that basis. The remaining functional group which are subordinate functional groups, are named as substituents using the appropriate prefixes. The decreasing order of priority of some functional groups is –COOH > – SO3H > – COOR (ester) > – COCl (acylhalide) > – CONH2 (amide) > – C ≡ N (nitriles) > – CH = O (aldehyde)> C = O (keto) > – OH (alcohol) > – NH2 (amine) > C = C (alkene) > – C ≡ C – (alkyne) The – R (alkyl group), Ph or C6H5 –(phenyl), halogens (F, Cl, Br, I) – NO2 alkoxy (–OR). Etc., are always prefix substituents. Thus, a compound containing both an alcohol and a keto group is named hydroxyl alkanone since the keto group is preferred to the hydroxyl group. PLANCESS CONCEPTS When the names of two or more substituents are composed of Identical Words The priority of citation is given to the substituent which has the first cited point of difference with in the complex substituent. e.g. 8. 1 2 | Basic Principles of Organic Chemistr y PLANCESS CONCEPTS Poly-Functional Compounds containing more than two like-functional groups According to the latest convention (1993 recommendation for IUPAC nomenclature), if an unbranded carbon chain is directly linked to more than two like-functional groups, then the organic compound is named as a derivative of the parent alkane which does not include the carbon atoms of the functional groups. E.g. C≡N | 3 1 2 i. N ≡≡ C—CH2—CH—CH2—C ≡≡ N Propane-1,2,3-tricarbonitrile (Not 3-cyanopentane-1, 5-dinitrilic) COOH 1 2 3| 4 5 ii. HOOC—CH2—CH—CH2—CH2—CH2—COOH Pentane-1,3,5-tricarboxylic acid (Not 4-carboxyheptane-1,7-dioic acid) When both double and triple Bonds are present in the compound In such cases, their locants are written immediately before their respective suffixes and the terminal ‘e’ from the suffix ‘ene’ is dropped while writing their complete names. It may be emphasized here that such unsaturated compounds are always named as derivatives of alkyne rather than alkene. E.g. 5 4 3 2 1 i. CH3— CH == CH—C ≡≡ CH Pent+3-en(e)+1-yne = Pent-3-en-1-yne (Formerly 3-Penten-1-yne) 5 4 3 2 1 ii. HC ≡≡ C—CH2—CH == CH2 Pent-1-en(e) + 4-yne = Pent 1-en-4-yne (Formerly 1-Penten-4-yne) When two or more prefixes consist of identical Roman letters (words): The priority for citation is given to that group which contains the lowest locant at the first point of difference. e.g. Cl 2 1| 1 2 CH3—CH CH2—CH2Cl 1 2 3 |4 5| 6 7 8 9 i. CH3—CH2—CH2—CH—CH—CH2—CH2—CH2—CH3 4-(1-chloroethyl)-5-(2-chloroethyl) nonane Cl 2 2 3 1 2 4 ii. 1 CH2—CH2 1 Cl 1-(2-chlorophenyl)-2-(4-chlorophenyl) ethane  Chem i str y | 8.13 PLANCESS CONCEPTS When the Organic molecular contains more than one similar complex substituents In such case, the numerical prefixes, such as di, tri, tetra etc,. are replaced by bis, tris, tetrakis, etc., respectively. E.g. Cl | Cl CH—C—Cl 1 | 2 Cl 3 4 Cl 1,1,1-trichloro-2,2-bis(4-chlorophenyl)ethane When all the three like groups are not directly linked to the unbranched carbon chain The two like groups are included in the parent chain while the third group which forms the side chain is considered a substituent group. Vaibhav Krishnan (JEE 2010, AIR 44) Illustration 1: Write the IUPAC name of the following compound: (JEE ADVANCED) CH3 CH3 | | CH3CH2CH—CH2 CH—CH2CH2CH3 | | CH3—CH2CH2CH2CH2CH2—CH—CH2—CH—CH2CH2CH2CH3 Sol: In case of a complex substituent and other substituents, the complex substituent begins with the first letter of its complete name. In case of two same complex substituents, one with the lowest positional number or locant is named first. This called priority citation. CH3 CH3 |2 1 4 1| 3 2 3 4 ← Lowest locant number CH3CH2CH—CH2 CH—CH2CH2CH3 13 12 8 7 | 6 5| 4 3 2 1 Lowest locant number 11 10 9 CH3—CH2CH2CH2CH2CH2—CH—CH2—CH—CH2CH2CH2CH3 ←(Numbering should be from this end and not from the other end) The IUPAC name of this compound is 5-(1-methyl butyl)-7-(2-methyl butyl) tridecane Priority of citation (5 < 7, 1 < 2); Locant 1 comes before 2. 4.3.7 IUPAC Rules for Alicyclic Compounds 1. IUPAC suffix: ane, ene, yne IUPAC prefix: cyclo I. II. III. 8. 1 4 | Basic Principles of Organic Chemistr y Two or more alkyl groups or other substituent’s are present in the ring, their positions are indicated, e.g., 1,2,3 …., etc. The substituent which comes first in the alphabetical order is give n the lowest number, as per the lowest sum rule, e.g. 2. (a) If the ring contains equal or more number of C atoms than the alkyl groups attached to it, is named as an alkyl cycloalkane. Me Me 2 1 Et 2 1 (I) (II) 4 3 1- Ethyl-2-2methyl Me cyclopentane 1- Methyl-4-propylcyloexane (b)  If the ring contains lesser number of C atoms than the alkyl groups attached to it, is named cycloalkyl alkane, e.g., 2 1 5 Me Me 1 3 5 Me 4 5 3 Me 1 Me 2 III. 3 4 2 4 II. I. (Same number of C atoms (Ring contains more C (Ring contains less C in ring and side chain) atoms than side chain) atoms than side chain) Pentyl cylopentane 2-Cyclopropyl pentane 2-Pemtyl cyclohexane (Alkly cylcloalkane) (Cycloalkyl alkane) (Alkly cylcloalkane) (c)  If the side chain constrains a functional group or a multiple bond, then the alicyclic ring is considered substituent irrespective of the size of the ring, e.g., 3 1 2 2 Me 3 1 4 O I. II. 3-Cyclobutyl 4-Cyclopentyl but prop-1-ene -3-en-2-one 4.3.8 IUPAC Names of Bicyclo Compounds Compounds with two fused cycloalkane rings are called bicyclo compounds. They are cyclo alkanes having two or more atoms in common. The prefix bicycle is followed by the name of the alkane whose number of C atoms is equal to number of C atoms in the two rings. The bracketed numbers show the number of C atoms (except bridge-head position C atoms) in each bridge and they are cited in decreasing order.  Chem i str y | 8.15 E.g. (a) 1 2 1 Bridge-head (A) (B) 2 position 3 (i) Number of C atoms in ring A = 3 (ii) Number of C atoms in ring B = 2 (iii) Number of C atoms between bridge-head position = 0 (iv) Total C atoms = 3 (in ring A) + 2(in ring B) + 2 (Bridge-head position) = 7 2 1 7 3 (Numbering starts from the bridge-head (A) (B) to the larger ring and then back to the 4 6 5 smaller ring.) 4.3.9 IUPAC Names of Tricyclo Compounds Compounds with three fused rings are called tricyclo compounds. The prefix tricyclo is followed by the name of alkane whose number of C atoms is equal to the number of C atoms in the rings. E.g. 7 1 Bridge - head 6 2 Position 4 3 5 Tricyclo [2.2.1.0] heptane The bracketed numbers show the number of C atoms (except the bridge-head position) in each bridge and they are cited in decreasing order. 4.3.10 IUPAC Names of Spiranes Spiranes are polycyclics that share only one C atom. In substituted spiranes, the numbering is started next to the fused C atom in the lower-member ring. E.g. 7 8 5 4 1 6 1 3 6 5 4 2 2 7 8 3 Spiro [2.5] octane Spiro [3, 4] octane  8. 1 6 | Basic Principles of Organic Chemistr y 4.3.11 IUPAC Name of Aromatic Compounds (a) No specific rules are required to name aromatic compounds. However, they are named substituted benzene, e.g. Me Me Me I. II. Methyl beznene Isoperopylbenznen or (Toluene) 2-Phenyl propane or Cumene (b) When larger and complex groups are attached to the benzene ring, the molecule is named as an alkane, alkene, etc., and benzene as Me side3chain derivatives, 1 Me abbreviated as, Ph –, or C6H5 – Ph – or φ. When the benzene ring contains some substituent’s, it is2abbreviated side as Ar –. ring Me Me Me 2-Phenyl propanoic II. acid Methyl beznene Isoperopylbenznen or (Toluene) 2-Phenyl propane or Cumene 3 1 Me Me 4.4 Writing the2Structural side Formula from the Given IUPAC Name The IUPACring name of an organic compound consists of the following parts: a. Root word b. 1º suffix c. 2º suffix d. 1º prefix e. 2º prefix 2-Phenyl propanoic (a) Root word indicates the longest chain thus, first locate the longest chain from the root word. Write the acid number of C atoms in a straight chain or in zigzag manner (for bond line structure) and then number them from any end. (b) 1º suffix (-ane, -ene, or -yne) indicates the nature of the chain. Put the multiple bonds at proper places in the chain. (c) 2º suffix indicates the principal functional group. Put it at a proper place in the chain. (d) Prefixes are the substituents or secondary functional groups. Put them at a proper place with the help of locants. (e) Add H atoms to satisfy valences of each C atom if stick formula is used. If the structure is written bond line, then there is no need of adding H atoms. PLANCESS CONCEPTS If more than one alicyclic ring is attached to a single chain, then the compound is named as cycloalkyl alkane (i.e., derivative of alkane) irrespective of the number of C atoms in the ring or the chain, e.g., Dicyclobutyl methane  Chem i str y | 8.17 PLANCESS CONCEPTS If double or triple (multiple) bonds and some other substituents are present in the ring. The numbering is done in such a way that the multiple bond gets the lowest number, e.g. Me Me NO2 6 3 3 5 1 Me 4 Me 4 2 2 (I) (not) (II) 4 2 1 1 5 5 Me 1 6 6 1,6-Dimethyl cyclohex-1-en 2,3-Dimethyl cyclohex-1-ene 1-Methyl-3-nitro (correct) (incorrect) cyclohex-1-ene Lowest number for Me substituent and also for double bond. although it violates lowest sum rule If the ring contains a multiple bond and the side chain contains functional group, then the ring is considered the substituent and the compound is named a derivative of the side chain, e.g., Me 4 2 Me 5 3 1 COOH OH 4 3 1 2 1 2 6 1 2 I. 3 I. 5 3 3–(Cyclohex-2-enyl) 4 butan-1-ol Me 4-(4-Methyl cyclohex-3-enyl) Pentanoic acid If the ring and side chain both contain functional groups, then (i) If the side chain constrains higher priority of of functional group then the compound is named the derivative of the side chain 4 2 1 1 3 COOH 3 2 (Priority of COOH>OH) OH 4-(3-Hydroxy cyclopent-1-enyl) but-3-en-1-oic acid (ii) If the ring contains higher priority of functional group, then the compound is named the derivative of the alicyclic ring, e.g., 1 O OH 2 NH – Et 1 2 3 4 2-(4-Amino ethyl-2-hydroxy butlyl) cyclohexan-1-one 8. 1 8 | Basic Principles of Organic Chemistr y PLANCESS CONCEPTS If both the side chain and the alicyclic ring contain the same functional group, then it is of two types. (i) If the number of C atoms of the alicyclic ring is equal or greater than that of the side chain, then it is named the derivative of the alicyclic ring. e.g., O 1 3 1 2 4 2 Me O 2-(2-Oxobutyl)cyclohexan-1-one (ii) If the number of C atoms of the side chain is greater than that of the alicyclic ring, then it is named the derivative of the side chain. e.g., CHO 2 8 6 4 2 1 1 7 5 3 CHO 8-(2-Formyl cyclohexyl ) oct-6-en-1-al If an alicyclic ring is directly attached to the benzene ring, it is named the derivative of benzene. e.g., 5 6 6 5 1 4 4 1 3 3 2 2 Cyclopentyl benzene Et NO 2 1-(3-Ethyl cyclohexyl)- 3-nitro benzene Naming of cyclic ethers The IUPAC names for cyclic ether (CH2)nO, where n = 2,3,4,5 and 6. O O O O O a. b. c. d. e. Oxirane Oxetane Oxolane Oxane Oxepane H O1 2 CH3 4 1O 4 2 3 h. Et Ph g. Cl 3 O1 i. f. 3 2 2 1 O 3 4 Cl H Cl Oxetane Phenyloxirane 3,3-Dichloro-2-methyl trans-2-chloro-4-ethyloxane oxetane Mono-substituted benzene compounds : According to IUPAC nomenclature, the substituent is placed as prefix and benzene as suffix. However, common names (written in bracket) of many substituted compound are commonly used, e.g- Toluene, Phenol etc.  Chem i str y | 8.19 PLANCESS CONCEPTS If the benzene ring is disubstituted, the substituents are located at the lowest number. In the trivial system of nomenclature, the terms ortho (o), meta (m) and para (p) are used as prefixes to indicate the relative positions 1,2 - ; 1,3 -, and 1,4- respectively, e.g. _ Cl Cl 1 1 2 Cl 2 I. II. 3 Cl 1,2-Dichloro benzene 1,3-Dichloro benzene (o-Dichloro benzene) (m-Dichloro benzene) If the benzene ring is tri- or higher substituted, then the compounds are named by identifying the substituent position on the ring by following the lowest locant sum rule. The substituent of the base compound is given the number 1 and then the direction of the numbering is selected such that the next substituent gets the lowest number. The substituent’s are written in the name in alphabetical order, e.g., 1 Br 4 Br I. not O2N 4 3 2 NO2 O2N 1 2 3 NO (1 -Bromo-2,4-dinitro benzne) (Lowest sum = 1 + 2 + 4 =7) (not 4- Bromo- 1,3 -dinitro benzne) (Highest sum = 4 + 1 + 3 = 8) When a benzene ring is attached to an alkane with a functional group, it is considered as a substituent instead of a parent. The name for benzene as substituent is phenyl (C6H5–) also abbreviated as Ph, e.g., 1 2 3 Me abbreviated as Ph, e.g., I. OH 1-Phenyl propan-2-ol Nikhil Khandelwal (JEE 2010, AIR 443) Illustration 2: Give the IUPAC name of the following compounds:  (JEE MAIN) H3C CH2CHO I. I I. i. Cyclohexylcyclohexane Sol: ii. i. Cyclohexylcyclohexane ethanal 2-(2-Methylcylobut-1-enyl) ii. 2-(2-Methylcylobut-1-enyl) ethanal Illustration 3: Write the IUPAC Name for  (JEE MAIN) Me 5 6 7 4 8 3 1 9 2 Me Me 2 5 1 9 6 7 3 4 8 8  Me 5 6 7 4 8 8. 2 0 | 3Basic Principles of Organic Chemistr y 1 9 2 Sol: Me Me 2 5 1 9 6 7 3 4 8 8 4 6 3 1 7 9 5 2 Wrong numbering Correct numbering since the position since the position of the substituent is at C-9 of the substituent is at the lowest number ,i.e., at C-7 Me 8 2 1 1 7 Me 3 7 2 6 5 5 3 4 6 4 IUPAC name: 7-Methyl bicycle [4.3.0] nonane Numbering from the longest bridge-head (i.e., from the larger ring) to the next longest bridge-head (i.e., to the smaller ring.) Illustration 4: Write the IUPAC name: (JEE ADVANCED) (CH2)9CH3 I. CH3(CH2)9 (CH2)9CH3 II. OH Sol: (i) 1,3,5-Tris(decyl)cyclohexane ; (ii) Cyclohex-2-en-1-ol 5. GENERAL ORGANIC CHEMISTRY 5.1 Basics of GOC 5.1.1 Theory of Development of Quantum Mechanics The Quantum theory was developed by Erwin Schrödinger. He worked on a mathematical model for the motion of electrons based on wave functions. This whole model was based on the fact that electron have a dual nature i.e., they show properties of both particles as well as waves. This theory led to the idea of atomic orbitals. Atomic orbital: Due to the dual nature of electrons, the Schrodinger wave equation came up. However, the wave equation fails to tell us exactly where the electron is at any particular moment, or the speed with which it is moving. All it tells us is the probability of finding the electron at any particular place. The region in space where the electron is most likely to be, is known as an orbital. These orbitals are of different kinds, and are hence dispersed about the nucleus in specific ways. The particular shape of orbital that an electron occupies, depends: upon the energy of the electron.  Chem i str y | 8.21 By knowing the shapes of these orbitals and there dispositions with respect to each other, we can be more precise in conveniently explaining the arrangement in the space of the atoms forming the nucleus and as a result, determine its physical and chemical behaviors. 5.1.2 Covalent Bonding Covalent bonds, make up compounds of carbon. This bond is of chief importance in the study of organic chemistry. Overlap Theory: According to this theory, for a covalent bond to the formed, the atoms must be located sufficiently close together so that an orbital of one atom overlaps with the other. Each orbital must contain single (unpaired) electrons. When this happens, single bond orbitals are occupied by both electrons. The two electrons that occupy the orbital must have opposite spins i.e. it must be paired. This arrangement contains less energy and hence is more stable. E.g. F atom Valence shell contains 7 electrons 1S 2S 2P The two F-atom some together and overlap through their p-orbital - - - - - - - BeCl2 molecule - - - pz pz F2 Molecule F-atom F-atom BeC2 molecule 1s 2s BeCl2: The electronic configuration of Be atom can be represented as Be atom in order to take part in covalent bonding must have single electron orbitals. Electronic Configuration of Be atoms just about to get bonded to chlorine atoms. 1s 2s 2p This leads to the idea that Be forms two different kinds of overlaps with every chlorine atom i.e. one Be-Cl bond s-p overlap and other Be-Cl bond p-p overlap which would result in two different types of Be-Cl bonds having different bond energies and bond lengths. But experiments have shown that both the Be-Cl bonds in BeCl2 are identical. So the theory of overlap is not applicable everywhere Even if you consider the molecule of CH4. Here, the central atom is carbon. The electronic configuration of carbon is C → 1s2 2s2 2p1x 2p1y C  1s 2s 2px2py2pz Just before combining with the 4H–atoms the electronic configuration of carbon becomes C  1s’ 2s’ 2p’x2p’y2p’z Here again we will find that according to the theory of overlap, there are 3 p-s overlaps and one s-s overlap meaning that the bonds are not identical, but experiments have shown beyond doubt that the 4 C–H bonds are all equivalent. Hence we apply the concept of hybridization.  8. 2 2 | Basic Principles of Organic Chemistr y Hybridization: It is the process of mixing up of non-degenerate atomic orbitals of the atom to form degenerate orbitals called hybrid orbitals each having the greatest degree of directionality. Hybrid orbitals: sp Be 1s 2s 2p Ground sate 1s 2s 2p Excited state sp hybridization 2-sp hybrid orbitals O 80 Cl Be Cl Cl Cl Hybrid orbitals: Sp2 B 1s 2s 2px 2py 2pz Ground sate 1s 2s 2px 2py 2pz Excited state 2 sp hybridization 2 3-sp hybrid orbitals BF3 is trigonal planar R F B-F Bf4 lence is plane triponal F Hybrid Orbital –sp3 C 1s 2s 2p1 2p2 Ground sate 3 4sp -hyrid orbitals 1s 2s 2p1 2p2 2p3 Excited H state 109.5O 3 sp hybridization 1.10AO C H H H  Chem i str y | 8.23 Table 8.2: Shape and geometry of the compound depending upon the hybridization S. No. Hybridization Number of lone pair Geometry Shape and example 1. sp 0 Linear O 180 BeF2 BeF2 O Be O CH  CH 2. sp3 0 Trigonal Planar Angular or bond BF3, CH2=CH2 3. sp3 0 Tetrahedral O Tetrahedral CH4CH2H6 X O O O 4. sp3 1 Tetrahedral Pyramidal X NH3, RNH2 O O H 5. sp3 2 Tetrahedral Angular or X bend O O H2O 5.1.3 Polarity in Molecule Each time a covalent bonds is formed between the same atoms, then the electrons are shared equally between the two atoms forming the bond e.g. F2, H2, etc. However, when the covalent bond is formed between two dissimilar atoms then there is an unequal sharing of electrons resulting in the electron of the covalent bond being drawn closer to the more electronegative atom, resulting in a bond dipole. e.g. HCl, HBr etc. The polarized covalent bond due to the difference of electro negativity may be shown as δ+ δ– δ+ δ– H– F | → H– C l | → H–F H–Cl The polarity in a bond arises from the difference in electronegativity of the atoms participating in the bond formation. The greater the difference in the electronegativity between the atoms bonded, the greater will be the polarity of the bond. Electronegativity order of some elements is below: F > O > Cl ~ N > Br > C ~ S > I > P ~ H > Si > Al > Mg > Li > Na > K 4.0 3.5 3.0 3.0 2.8 2.5 2.5 2.4 2.1 2.1 1.8 1.5 1.2 1.0 0.9 0.8 Electronegativity of carbon and hydrogen are close enough, hence C-H bonds do not have much polarity. H–O H–F H – Cl H–N Even C – X, C – O and C – N bonds are also polar Dipole moment = charge × distance Bond polarity contributes greatly to the physical and chemical properties of molecules. 8. 2 4 | Basic Principles of Organic Chemistr y Dipole Moments of Covalent Molecules (a) For a distant molecule with different atoms, the level dipole is also the dipole moment. H F H Cl H Br H I  = 1.98D  = 1.03D  = 078D  = 038D (b) For diatomic molecules with the same atoms there is no bond dipole e.g., H—H and I — I (c) The overall dipole moment of a molecule containing more than two atoms is the vector sum of the individual bond dipole moments. A molecule may contain polar bonds but has no overall dipole moment if the shape of the molecule is such that the individual bond moments cancel out.    O C O Carbon dioxide Bond dipoles cancel  = 0  Cl Carbon tetrachloride C =0  Cl  Cl Cl  Rotate to 180 o 5.1.4 Molecular Interactions It is found that covalent compounds exist as solids, liquids and gases. So what forces hold neutral molecules together? Like interionic forces, these forces seem to be, electrostatic in nature, involving the attraction of +ve charge for negative charge (a) Dipole-Dipole interactions (b) Vander waal’s forces. Dipole-Dipole Interactions (a) This exists mainly in polar molecules. Here there is attraction of the positive end of one polar molecule for the negative end of another polar molecule. In acetaldehyde the relatively –ve CH3   CH3   C O C O H H As a result of dipole-dipole interactions the molecules are generally near to each other more strongly than all the non-polar molecules of comparable molecular mass. (b) H–bonding [king of dipole-dipole interaction]. Here the H-atom seems to act as a bridge between two electronegative atoms, holding one by a covalent bond and the other by purely an electrostatic force. F F F F F F H O N H O O H O Cl H O S H O N N N N N covalent bond electrostatics force Also the strength of hydrogen bonding order is F H>O H >>>> N H.  Chem i str y | 8.25 5.1.5 Non-Polar Forces It has been found that even non- polar molecules solidify and hence there must be some forces which exist in order for this to happen. Such attractive forces are called Van der Waal forces. Quantum mechanics accounts for the existence of these forces, as it states that the average distribution of charge about e.g., CCI4 molecule is symmetrical, so there is no net dipole moment. However, electrons move about, so at any instant the distributions become distorted leading to a small dipole. This momentary dipole induces another small dipole moment in another molecule and so on and so forth to the neighbouring molecules. Though the momentary dipole and induced dipoles are constantly changing, the net result is the attraction between the two molecules. These Van der Waals forces have a very short range, they act only between the portions of different molecules that are in close contact in between the surfaces of molecules. (a) Van der Waals forces are directly proportional to molecular mass. (b) Van der Waals forces are directly proportional to surface area. The molecular forces of attraction are very useful in comparing the rates of evaporation, vapour pressures, boiling points, melting points, viscosity, etc. PLANCESS CONCEPTS (a) Hybridization: Some special case of hybridization are – (i) Carbanion is sp3 hybridized. (ii) Carbocation is sp2 hybridized. (iii) CH3 Radical is sp2 hybridized while CF3 is sp3 hybridized. Electronegativity of fluorine is responsible for the latter case. (iv) Triplet carbine is sp hybridized while singlet carbene is sp2 hybridized. (a) Polarity: (i) Dipole moment = q x d (ii) Polarity determines many physical factors like intermolecular interaction, boiling and melting points, solubility. (iii) Greater the polarity, greater is the intermolecular interaction. (iv) Greater the polarity, higher are the boiling and melting points. (v) Polar molecules are soluble in polar solvent while non-polar molecules are soluble in non-polar solvent. (a) Molecular interactions: (i) Dipole-dipole interactions – attraction between two polar molecules. (ii) Van der Waals forces – increases with increase in molecular weights. (iii) Hydrogen bonding – occurs with hydrogen attached with fluorine, nitrogen and oxygen only. (iv) Magnitude of molecular interaction – H bonding > Dipole – dipole interactions > Van der waals forces. Saurabh Gupta (JEE 2010, AIR 443) 8. 2 6 | Basic Principles of Organic Chemistr y Illustration 5: Explain why μ of NH3 > NF3?  (JEE MAIN) Sol: Explain this question by taking into account the direction of contribution of N-H and N-F bond and the lone pair electrons. In NH3, the net moment of (N – H) bonds and the contribution from the LP eletrons (lone pair electrons) are in the same direction and are additive. The net moment of the (N – F) bonds opposes the dipole effect of the LP electrons in the NH3 and the resultant is less µ. So µ of NH3 > NF3. i. N N H H H F F F 3 3 (sp H.O.) (sp H.O.) (a) (b) Illustration 6: Explain why μ of CH3Cl > CH3F > CH3Br > CH3I  (JEE MAIN) Sol: The electro-negativities of halogens decrease from F to I so µ of HF > HCl > HBr > HI, but µ of CH3F is smaller than CH3Cl due to shorter (C – F) bond distance, although EN of F is greater than that of Cl. Illustration 7: Explain why CO2 has dipole moment zero whereas for SO2 its non-zero? (JEE ADVANCED) Sol: In CO2, C is sp hybridized and linear. The dipole moments of (C – O) are equal and in opposite directions and cancel each other. Hence, µ is zero. O=C =O In SO2 , S is sp2 hybridized having one LP on S atom. The (O – S – O) bond angle is nearly 120º; (S – O) bond moment does not cancel and shows a net resultant µ. S Net moment O O Illustration 8: Explain why the lone pairs of electrons has no effect on the μ of PH3.The bond angle in PH3 is 92º  (JEE ADVANCED) Sol: The 92º bond angle suggests that P uses three p atomic orbitals in forming bonds with H, with one LP e in 3s atomic orbital, i.e., P in PH3 is sp2 hybridized (unlike NH3, in which N is sp3 hybridized)  Therefore, due to the presence of LP e s in 3s atomic orbital of P, which is spherical symmetrical, the polarity of the  molecule is not affected enough to affect the polarity of the molecule, the e ‘s must be in a directional orbital. Moreover, EN of P and H are nearly same, so PH3 molecule is almost nonpolar, Illustration 9: (a) Describe heterolytic (polar) bond cleavage of: ⊕Θ i. Agl, ii. NBF3 iii. [Cu(OH2)4]Θ (b) Name the reverse of heterolytic cleavage. (c) Describe hemolytic bond cleavage of CH3 – CO – CO – CH3. (d) Compare the relative energies of singlet and triplet carbenes.  Chem i str y | 8.27 (e) Of X2C : (singlet) and X2Cl :(triplet), which is stable ? (f) Of F3C :, Cl2C:, Br2C :, I2C : (singlet), which is more stable ? (g) Compare and explain the difference in the IE and EA of +CH3. (JEE ADVANCED) + − Sol: (a) (i) Ag − I → Ag + I (More EN atoms acquire negative charge.) ⊕Θ (ii) H3 NBF3 → H3N : BF3 (Bonded atoms with formal charges give uncharged products.) (iii) [Cu(OH2 )4 ]2+ → Cu2+ + 4H2O (b) Coordinates covalent bonding. (c) H3C − CO − CO − CH3 → 2CH3CO ( A radical) (d) Triplet carbene has lower energy because with two e−’s in different orbitals there is less electrostatic repulsion than when both are in the same orbital. (e) X2C: Singlet is more stable, because of the lone pair of electrons on X which can overlap laterally with the empty orbital. (f) F3C is the most stable singlet, since F and C are in the same period of the periodic table and are about the same size permitting a more efficient overlap (2p(F) –2p(C) ore pπ – pπ bond). Moreover, (F – C) bond length is the shortest bond length and provides a more extensive lateral overlap. (g) The EA is less than IE. When – CH3 gains an e− to become carbanion, C acquires a stable octet of e−s. When it loses an e− , it becomes unstable with only 6 e− s. 6. ELECTRONIC EFFECTS 6.1 Inductive Effect The Inductive effect is an electronic effect due to the polarization of σ bonds within a molecule or ion. This is typically due to an electronegativity difference between the atoms at either end of the bond. The more electronegative atom pulls the electrons in the bond towards itself creating some bond polarity for example the O-H and C-Cl bonds in the following examples: - + + - CH3 O H CH3 Cl The inductive effect is divided into two types depending on their strength of electron withdrawing or electron releasing nature with respect to hydrogen. (a) Negative inductive effect (-I): The electron withdrawing nature of groups or atoms is called the negative inductive effect. It is indicated by -I. Following are the examples of groups in the decreasing order of their -I effect: NH3 + > NO2 > CN > SO3H > CHO > CO > COOH >COCl> CONH2 > F >Cl> Br > I > OH > OR > NH2 > C6H5 > H (b) Positive inductive effect (+I): It refers to the electron releasing nature of the groups or atoms and is denoted by +I. Following are the examples of groups in the decreasing order of their +I effect. C(CH3)3 > CH(CH3)2 > CH2CH3 > CH3 > H 8. 2 8 | Basic Principles of Organic Chemistr y Why do alkyl groups show a positive inductive effect? Though the C-H bond is practically considered as non-polar, there is partial positive charge on hydrogen atom and partial negative charge on carbon atom. Therefore each hydrogen atom acts as electron donating group. This cumulative donation turns the alkyl moiety into an electron donating group. 6.1.1 Applications of Inductive Effect (a) Stability of Carbonium Ions: The stability of carbonium ions increases with the increase in the number of alkyl groups due to their +I effect. The alkyl groups release electrons to carbon, bearing a positive charge and thus stabilizes the ion. The order of stability of carbonium ions is: CH3 CH3 H H + + + + H3C C > H3C C > H3C C >H C CH3 H H H o o o 3 2 1 Methyl (b) Stability of Free Radicals: In the same way the stability of free radicals increases with increase in the number of alkyl groups. Thus the stability of different free radicals is: CH3 CH3 H H H3C C > H3C C > H3C C >H C CH3 H H H o 3 2o 1o Methyl (c) Stability of Carbanions: However the stability of carbanions decreases with increase in the number of alkyl groups since the electron donating alkyl groups destabilize the carbanions by increasing the electron density. Thus the order of stability of carbanions is: H CH3 CH3 CH3 H C - >H C - > H3C C - > H3C C- H H H CH3 o o o Methyl 1 2 3 (d) Acidic Strength of Carboxylic Acids and Phenols: The electron withdrawing groups (-I) decrease the negative charge on the carboxylate ion by stabilizing it. Hence the acidic strength increases when -I groups are present. However the +I groups decrease the acidic strength. E.g. (i) The acidic strength increases with increase in the number of electron withdrawing Fluorine atoms as shown below. CH3COOH < CH2FCOOH < CHF2COOH < CF3COOH (ii) Formic acid is a stronger acid than acetic acid since the –CH3 group destabilizes the carboxylate ion. On the same lines, the acidic strength of phenols increases when -I groups are present on the ring. E.g. p-nitrophenol is a stronger acid than phenol since the -NO2 group is a -I group and withdraws electron density. Whereas the para-cresol is a weaker acid than phenol since the -CH3 group shows a positive (+I) inductive effect. Therefore the decreasing order of acidic strength is: OH OH OH > > NO2 CH3 p-nitrophenol phenol p-cresol  Chem i str y | 8.29 (e) Basic strength of amines: The electron donating groups like alkyl groups increase the basic strength of amines whereas the electron with drawing groups like aryl groups decrease the basic nature. Therefore alkyl amines are stronger Lewis bases than ammonia, whereas aryl amines are weaker than ammonia. Thus the order of basic strength of alkyl and aryl amines with respect to ammonia is: CH3NH2 > NH3 > C6H5NH2 (f) Reactivity of Carbonyl Compounds: The +I groups increase the electron density at the carbonyl carbon. Hence their reactivity towards nucleophiles decreases. Thus, formaldehyde is more reactive than acetaldehyde and acetone towards nucleophilic addition reactions. Thus the order of reactivity follows: O O O H C H > H2C C H > H3C C CH3 Formaldehyde Acetaldehyde Acetone 6.2 Electromeric Effect A molecular polarizing effect occurring by an intermolecular electron displacement (sometimes called the ‘conjugative mechanism’ and, previously, the ‘tautomeric mechanism’) characterized by the substitution of one electron pair for another within the same atomic octet of electrons. It can be indicated by curved arrows symbolizing the displacement of electron pairs, as in: R2N C=C C =O which represents the hypothetical electron shift - 6.3 Mesomeric Effect The Mesomeric effect (on reaction rates, ionization equilibria, etc.) is attributed to a substituent due to the overlap of its p- or π-orbitals with the p- or π-orbitals of the rest of the molecular entity. Delocalization is thereby introduced or extended, and electronic charge may flow to or from the substituent. The effect is symbolized by M. Strictly understood, the mesomeric effect operates in the ground electronic state of the molecule. When the molecule undergoes electronic excitation or its energy is increased on the way to the transition state of a chemical reaction, the mesomeric effect may be enhanced by the electromeric effect, but this term is not much used, and the mesmeric and electromeric effects tend to be assumed to be taken in the term resonance effect of a substituent. Mesomeric effect is divided into 2 parts on basis of withdrawal or donation of electrons. Negative resonance or mesomeric effect (-M or -R): It is shown by substituents or groups that withdraw electrons by the delocalization mechanism from rest of the molecule and are denoted by -M or -R. The electron density on rest of the molecular entity is decreased due to this effect. E.g. -NO2, Carbonyl group (C=O), -C≡N, -COOH, -SO3H etc. Positive resonance or mesomeric effect (+M or +R): The groups show a positive mesomeric effect when they release electrons to the rest of the molecule by delocalization. These groups are denoted by +M or +R. Due to this effect, the electron density on rest of the molecular entity is increased. E.g. -OH, -OR, -SH, -SR, -NH2, -NR2 etc. Applications of Resonance Effect (Or) Mesomeric Effect (a) The negative resonance effect (-R or -M) of the carbonyl group is shown below. It withdraws electrons by delocalization of π electrons and reduces the electron density particularly on 3rd carbon. O O H2C = CH C CH3  H2C+ CH = C CH3 8. 3 0 | Basic Principles of Organic Chemistr y (b) The negative mesomeric effect (-R or -M) shown by the cyanide group in acrylonitrile is illustrated below. The electron density on the third carbon decreases due to delocalization of π electrons towards cyanide group. (+) H2C CH C N  H2C CH C N Because of negative resonance effect, the above compounds act as good acceptors. (c) The nitro group, -NO2, in nitrobenzene shows -M effect due to the delocalization of conjugated π electrons as shown below. Note that the electron density on the benzene ring is decreased particularly on ortho and para positions. O- O O- O- O- O- O- O- + N N N N + + + I II III II This is the reason for why nitro group deactivates the benzene ring towards the electrophilic substitution reaction. (d) In phenol, the -OH group shows +M effect due to the delocalization of a lone pair on the oxygen atom towards the ring. OH +OH +OH +OH - - - Thus the electron density on the benzene ring is increased particularly on ortho and para positions. Hence phenol is more reactive towards electrophilic substitution reactions. The substitution is favoured more at ortho and para positions. (e) The -NH2 group in aniline also exhibits +R effect. It releases electrons towards the benzene ring through delocalization. As a result, the electron density on the benzene ring increases particularly at the ortho and para positions. Thus, aniline activates the ring towards electrophilic substitution. NH+2 NH+2 NH+2 NH+2 - - - It is also worth mentioning that the electron density on nitrogen in aniline decreases due to the delocalization which is the reason for its less basic strength when compared to ammonia and alkyl amines. Inductive Effect Vs Resonance Effect In most cases, the resonance effect is stronger and outweighs inductive effect. For example, the -OH and -NH2 groups withdraw electrons by the inductive effect (-I). However they also release electrons by delocalization of lone pairs (+R effect). Since the resonance effect is stronger than the inductive effect the net result is of the electron releasing to rest of the molecule. This is clearly observed in phenol and aniline, which are more reactive than benzene towards electrophilic substitution reactions.  Chem i str y | 8.31 X X+ X+ X+ - - - Whereas the inductive effect is stronger than the resonance effect in case of halogen atoms. These are electronegative and hence exhibit -I effect. However, at the same time they also release electrons by the delocalization (+R effect) of the lone pair. This is evident in the case of the reactivity of halobenzenes, which are less reactive than benzene towards electrophilic substitution due to -I effect of halogens. However, it is interesting to note that the substitution is directed at ortho and para positions rather than meta position. It can be ascribed to the fact that the electron density is increased at ortho and para positions due to +R effect of halogens as shown below. 6.4 Hyper Conjugation The displacement of σ-electrons towards the multiple bond occurs when there are hydrogens on the α-carbon (which is adjacent to the multiple bond). This results in the polarization of the multiple bond. In the formalism that separates bonds into σ and π types, hyper conjugation is the interaction of σ-bonds (e.g. C–H, C–C, etc.) with a π network. This conjugation between electrons of single (H-C) bond with multiple bonds is called hyperconjugation. This occurs when the sigma (s) electrons of the H-C bond that is attached to an unsaturated system, such as double bond or a benzene ring, enter into conjugation with the unsaturated system. This interaction is customarily illustrated by contributing structures, e.g. for toluene (below), sometimes said to be an example of ‘heterovalent’ or ‘sacrificial hyper conjugation’, so named because the contributing structure contains one two-electron bond less than the normal Lewis formula for toluene: H H C H C H H H At present, there is no evidence for sacrificial hyper conjugation in neutral hydrocarbons. The concept of hyper conjugation is also applied to carbonium ions and radicals, where the interaction is now between σ-bonds and an unfilled or partially filled π- or p-orbital. A contributing structure illustrating this for the tert-butylcation is: CH3 H CH3 H + C C H C C H CH3 H CH3 H This latter example is sometimes called an example of ‘isovalent hyper-conjugation’ (the contributing structure containing the same number of two-electron bonds as the normal Lewis formula). Both structures shown on the right hand side are also examples of ‘double bond-no-bond resonance’. The interaction between filled π- or p- orbitals and adjacent antibonding σ* orbitals is referred to as ‘negative hyperconjugation’, as for example in the fluoroethyl anion: H F H F - C C H C C H H H H H 8. 3 2 | Basic Principles of Organic Chemistr y Consequences and Applications of Hyperconjugation (a) Stability of alkenes: A general rule is that, the stability of alkenes increases with increase in the number of alkyl groups (containing hydrogens) on the double bond. It is due to the increase in the number of contributing no bond resonance structures. For example, 2-butene is more stable than 1-butene. This is because in 2-butene, there are six hydrogens involved in hyperconjugation whereas there are only two hydrogens involved in case of 1-butene. Hence the contributing structures in 2-butene are more and is more stable than 1-butene. 2 hydrogens 6 hydrogens H3C – CH 2 H H3C CH 2 C=C C=C H H H H 1-butene 2-butene The increasing order of stability of alkenes with increases in the number of methyl groups on the double bond is depicted below. H H H3C H H3C H H3C CH 3 H3C CH 3 C=C < C=C < C=C < C=C < C=C H H H H H3C H H3C H H3C CH 3 This order is supported by the heat of hydrogenation data of these alkenes. The values of heats of hydrogenation decrease with the increase in the stability of alkenes. Also the heats of formation of more substituted alkenes are higher than expected. However it is important to note that the alkyl groups attached to the double bond must contain at least one hydrogen atom for hyperconjugation. For example, in case of the following alkene containing a tert-butyl group on doubly bonded carbon, the hyperconjugation is not possible. H C H H C H3C – C – CH3 CH3 No H atoms on a carbon Hence no hyperconjugation. It is also important to note that the effect of hyperconjugation is stronger than the inductive effect. For example, the positive inductive effect of ethyl group is stronger than that of methyl group. Hence, based on