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Concept 03 / 08Organic Chemistry - Basic Principles and Techniques

Electronic Effects: Inductive, Resonance, Electromeric Effect and Hyperconjugation

Permanent and temporary electron displacements in covalent bonds: inductive effect (+I and -I groups), resonance (mesomeric) effect with +R and -R groups and rules for resonance structures, electromeric effect, hyperconjugation, and their use in explaining acidity, basicity and stability of intermediates and alkenes.

Figure 1
Inductive effect (I). A permanent displacement of σ\sigma electrons along a chain due to an electronegativity difference. It weakens rapidly with distance (negligible beyond about three bonds). −I-I groups (electron-withdrawing): −NO2>−CN>−F>−COOH>−Cl>−Br>−I>−OH>−OR>−C6H5\mathrm{-NO_{2} > -CN > -F > -COOH > -Cl > -Br > -I > -OH > -OR > -C_{6}H_{5}}. +I+I groups (electron-releasing): alkyl groups, (CH3)3C−>(CH3)2CH−>CH3CH2−>CH3−\mathrm{(CH_{3})_{3}C- > (CH_{3})_{2}CH- > CH_{3}CH_{2}- > CH_{3}-}; also −O−\mathrm{-O^{-}} and −COO−\mathrm{-COO^{-}}. Application: electron-withdrawing groups stabilise a carboxylate anion and increase acid strength (see figure). Chloroacetic acid (pKa\text{p}K_{a} 2.862.86) is stronger than acetic acid (4.764.76); the effect is cumulative (trichloroacetic, 0.650.65) and decreases as the halogen moves away from COOH. Alkyl groups (+I+I) make formic acid (3.753.75) stronger than acetic acid. Resonance (mesomeric) effect (R or M). When a molecule can be represented by two or more Lewis structures differing only in the placement of π\pi electrons or lone pairs, the true structure is a resonance hybrid, more stable than any contributor (resonance energy; about 150 kJ mol−1150\,\mathrm{kJ\,mol^{-1}} for benzene). Rules for contributors: same positions of all nuclei; same number of unpaired electrons; more covalent bonds, complete octets, less charge separation and negative charge on the more electronegative atom make a structure contribute more; equivalent structures contribute equally. +R+R groups donate electrons to a conjugated system through a lone pair: −OH\mathrm{-OH}, −OR\mathrm{-OR}, −NH2\mathrm{-NH_{2}}, −NHCOCH3\mathrm{-NHCOCH_{3}}, −X\mathrm{-X}. −R-R groups withdraw electrons through a π\pi bond to an electronegative atom: −NO2\mathrm{-NO_{2}}, −CHO\mathrm{-CHO}, >C=O\mathrm{>C=O}, −COOH\mathrm{-COOH}, −CN\mathrm{-CN}. Applications: phenol (pKa\text{p}K_{a} 1010) is far more acidic than ethanol (about 1616) because the phenoxide ion is resonance-stabilised; a −R-R nitro group at ortho or para makes phenol still more acidic (p-nitrophenol 7.17.1). Aniline is a much weaker base than methylamine, because its lone pair is delocalised into the ring. Carboxylate ions are resonance-stabilised with two equal C-O bonds. Electromeric effect (E). A temporary, complete shift of a π\pi electron pair to one atom of a multiple bond, only in the presence of an attacking reagent, and reversed when the reagent is removed. +E+E: electrons move towards the atom to which the reagent attaches (typical with electrophiles on alkenes); −E-E: electrons move away from the attacking site (nucleophilic attack on C=O). When inductive and electromeric effects oppose, the electromeric effect usually dominates. Hyperconjugation (no-bond resonance, Baker-Nathan effect). Delocalisation of σ\sigma electrons of a C-H bond on a carbon next to an unsaturated system or a carbocation into the adjacent empty or π\pi orbital. The more α\alpha-hydrogens, the greater the stabilisation. Carbocations: (CH3)3C+\mathrm{(CH_{3})_{3}C^{+}} (99 α\alpha-H) > (CH3)2CH+\mathrm{(CH_{3})_{2}CH^{+}} (66) > CH3CH2+\mathrm{CH_{3}CH_{2}^{+}} (33) > CH3+\mathrm{CH_{3}^{+}}. Alkenes: more substituted alkenes are more stable (lower heats of hydrogenation): (CH3)2C=C(CH3)2\mathrm{(CH_{3})_{2}C=C(CH_{3})_{2}} (1212 α\alpha-H) is the most stable; trans-but-2-ene > but-1-ene. Key Takeaways: - Inductive: permanent, through σ\sigma bonds, falls off with distance. - Resonance: through π\pi systems; +R+R (OH, NH2\mathrm{NH_{2}}, OR, X) and −R-R (NO2\mathrm{NO_{2}}, CN, CHO, COOH). - Electromeric: temporary, needs an attacking reagent. - Hyperconjugation: more α\alpha-H means more stable carbocations, radicals and alkenes. - Pitfall: halogens are −I-I but +R+R; on a benzene ring the −I-I effect wins for reactivity, while +R+R directs ortho/para. Brain-Teaser 1: Arrange in increasing acid strength: CH3COOH\mathrm{CH_{3}COOH}, FCH2COOH\mathrm{FCH_{2}COOH}, ClCH2COOH\mathrm{ClCH_{2}COOH}, CH3CH2COOH\mathrm{CH_{3}CH_{2}COOH}. Solution: CH3CH2COOH<CH3COOH<ClCH2COOH<FCH2COOH\mathrm{CH_{3}CH_{2}COOH} < \mathrm{CH_{3}COOH} < \mathrm{ClCH_{2}COOH} < \mathrm{FCH_{2}COOH}. The ethyl group is more +I+I than methyl (destabilises the anion); F is more −I-I than Cl. Brain-Teaser 2: Why is p-nitrophenol more acidic than m-nitrophenol, while both are more acidic than phenol? Solution: In both, the −I-I effect of NO2\mathrm{NO_{2}} stabilises the phenoxide. In the para isomer, the negative charge can also be delocalised onto the nitro oxygens through resonance (−R-R), which is impossible from the meta position. So para (pKa\text{p}K_{a} 7.17.1) > meta (8.38.3) > phenol (10.010.0). Brain-Teaser 3: Which is more stable, 2-methylbut-2-ene or 2-methylbut-1-ene? Justify by counting α\alpha-hydrogens. Solution: 2-methylbut-2-ene, (CH3)2C=CHCH3\mathrm{(CH_{3})_{2}C=CHCH_{3}}, has 99 α\alpha-hydrogens (three methyls). 2-methylbut-1-ene, CH2=C(CH3)CH2CH3\mathrm{CH_{2}=C(CH_{3})CH_{2}CH_{3}}, has 55 (33 on methyl, 22 on the ethyl CH2\mathrm{CH_{2}}). More hyperconjugation makes the trisubstituted alkene more stable, the major product of eliminations (Saytzeff rule).

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