JEE Chemistry Organic Chemistry Some Basic Principles and Techniques Notes Set 15

Electron Displacement in Organic Molecules

Electrons in a covalent molecule can shift because of a substituent or because an attacking reagent approaches. The main effects are the inductive effect, resonance (mesomeric) effect, hyperconjugation and the electromeric effect. The first four are permanent, while the electromeric effect appears only in the presence of a reagent.

Inductive Effect

When a sigma bond joins atoms of different electronegativity, the shared pair is pulled towards the more electronegative atom, and the neighbouring sigma bonds become polarised too. This transmission through sigma bonds is the inductive effect.
1. It is permanent, acts through sigma bonds only and fades rapidly, being negligible beyond about three carbons.
2. C–H is taken as the zero reference.
3. –I groups (electron withdrawing): NO2 > CN > COOH > F > Cl > Br > I > OH > C6H5 > H.
4. +I groups (electron donating): alkyl groups, with (CH3)3C– > (CH3)2CH– > CH3CH2– > CH3–.
Uses: more –I groups make an acid stronger (O2N–CH2COOH > F–CH2COOH > CH3COOH), more +I groups make an amine more basic, +I stabilises carbocations and radicals, –I stabilises carbanions, and a larger –I group on carbon gives a larger dipole moment.

Resonance

When a molecule or ion cannot be described by a single Lewis structure, it is represented by two or more canonical structures. The real molecule is a resonance hybrid, more stable than any one contributor. The stability gained is the resonance energy.
1. Conditions: planar arrangement, atoms sp or sp2 hybridised, parallel p orbitals and a conjugated system (pi bond with pi bond, with a lone pair, with a charge or with an odd electron).
2. Rules: only pi electrons and lone pairs move, atoms never move, the number of paired (and unpaired) electrons stays the same, the octet of second-period atoms is not exceeded.
3. A canonical form contributes more when it has more covalent bonds, complete octets, little charge separation and the negative charge on the more electronegative atom.
4. Equivalent contributors, as in carboxylate ions, benzene and the tropylium cation, give extra stability.

Mesomeric Effect

The mesomeric effect is the permanent transfer of pi electrons or lone pair electrons through a conjugated system.
1. +M groups donate a lone pair or pi electrons: –O⁻ > –NH2 > –OH > –OR > –NHCOCH3 > –Cl, and the phenyl ring or alkene in some cases.
2. –M groups withdraw pi electrons through a multiple bond to an electronegative atom: NO2 > CN > CHO > COCH3 > COOH.
3. +M groups increase electron density at the ortho and para positions of benzene and activate it. –M groups reduce it and direct the attack to the meta position.
4. In halobenzenes the –I effect outweighs +M, so the ring is deactivated, yet substitution still goes ortho or para.

Hyperconjugation

Hyperconjugation is the delocalisation of the electrons of an alpha C–H sigma bond into an adjacent pi bond, empty p orbital or half-filled p orbital. It is also called no-bond resonance.
1. The number of hyperconjugation forms equals the number of alpha hydrogens: ethyl cation 3, isopropyl 6, tert-butyl 9.
2. It explains why more substituted alkenes and carbocations are more stable, the shorter C–C single bond in propene, and the order of stability of free radicals.
3. A carbon with no alpha hydrogen, such as a quaternary carbon, gives no hyperconjugation.

Aromaticity

A compound is aromatic when it is cyclic, planar, completely conjugated and has (4n + 2) pi electrons (Huckel rule). Compounds with 4n pi electrons in such a ring are antiaromatic and very unstable.
1. Aromatic: benzene (6), cyclopentadienyl anion (6), tropylium cation (6), cyclopropenyl cation (2), furan, pyrrole and thiophene (6), azulene (10).
2. Antiaromatic: cyclobutadiene (4), cyclopentadienyl cation (4), cyclopentadienone.
3. Cyclooctatetraene is non-planar and non-aromatic.
4. All C–C bonds in a symmetrical aromatic ring have the same length, for example 1.397 Å in benzene.

Reaction Intermediates

1. Free radical: 7 electrons, formed by homolytic cleavage; stability 3° > 2° > 1° > methyl and benzyl and allyl are the most stable.
2. Carbocation: 6 electrons, sp2 and planar; stability: tropylium, trityl > benzyl, allyl > 3° > 2° > 1° > methyl. An adjacent oxygen or nitrogen lone pair, or cyclopropyl groups, stabilise a cation greatly. Cations rearrange to more stable ones by hydride or alkyl shifts.
3. Carbanion: 8 electrons, pyramidal sp3 (or planar when delocalised); stability depends on s-character (sp > sp2 > sp3), resonance and –I groups; alkyl groups destabilise it: methyl > 1° > 2° > 3°.
4. Heterolysis gives a carbocation and a carbanion; the most polar bond breaks most easily.

Tautomerism

Tautomers are structural isomers in rapid equilibrium that differ in the position of a proton and a double bond. The common case is keto–enol tautomerism, which needs an alpha hydrogen.
1. Simple aldehydes and ketones exist almost wholly as the keto form, but 1,3-dicarbonyl compounds have a high enol content because the enol is conjugated and hydrogen bonded.
2. If the enol is aromatic, as with cyclohexa-2,5-dienone giving phenol, the enol form is overwhelmingly preferred.
3. Compounds with no alpha hydrogen, such as benzaldehyde or triphenylacetaldehyde, show no keto–enol tautomerism.
4. Nitroalkanes with an alpha hydrogen show nitro and aci-nitro tautomerism.

Acidic and Basic Strength

Acid strength is decided by the stability of the conjugate base: anything that spreads or neutralises the negative charge raises acidity.
1. –I and –M groups increase acidity, and +I and +M groups decrease it. The closer the withdrawing group is to the acidic site, the stronger its effect.
2. Typical orders: CF3COOH > CCl3COOH > CH3COOH; sulphonic acids > carboxylic acids > phenols > water > alcohols > terminal alkynes; among alcohols, methanol is the most acidic and tertiary alcohols the least.
3. Phenol is acidic because its anion is stabilised by resonance; nitro groups at the ortho and para positions increase its acidity strongly, while methyl groups decrease it.
4. Hybridisation matters for carbon acids: C–H acidity increases in the order sp3 < sp2 < sp.
5. Basic strength follows the availability of the lone pair: +I groups raise it and –I, –M groups or delocalisation of the lone pair lower it. Guanidine is a very strong base because its protonated form is stabilised by resonance over three nitrogen atoms.