JEE Chemistry Organic Chemistry Some Basic Principles and Techniques Notes Set 02

Attacking Reagents

An electrophile is electron-deficient and accepts an electron pair (H\(^+\), NO\(_2^+\), SO\(_3\), BF\(_3\), carbocations). A nucleophile has a lone pair or negative charge and donates it (OH\(^-\), CN\(^-\), NH\(_3\), carbanions).

Reaction Intermediates

A carbocation has a sextet and is planar (sp\(^2\)); a carbanion has a lone pair and is pyramidal; a free radical has an unpaired electron. Carbocation stability rises with alkyl substitution (hyperconjugation and +I effect) and with resonance: benzyl and allyl cations are stable and p-methoxybenzyl is even more so. Carbanion stability rises with s-character and with electron-withdrawing groups: sp > sp\(^2\) > sp\(^3\), and primary > secondary > tertiary.

Inductive Effect

Permanent polarisation of a sigma bond transmitted along a chain, decreasing rapidly with distance. -I groups include -NO\(_2\), -CN, -F, -Cl, -OH and -NH\(_3^+\); +I groups are alkyl groups. -I groups increase the acidity of carboxylic acids (CCl\(_3\)COOH is stronger than CH\(_3\)COOH), while +I groups decrease it.

Resonance and Mesomeric Effect

Delocalisation of pi electrons or lone pairs lowers energy. +M groups (-OH, -OR, -NH\(_2\), -Cl) donate electrons into the pi system; -M groups (-NO\(_2\), -CHO, -COOH, -CN) withdraw electrons. Resonance explains the acidity of phenol, the low basicity of aniline, the short C-Cl bond and low dipole moment of vinyl chloride, and the stability of the benzyl cation. Structures with complete octets and without charge separation contribute most.

Hyperconjugation

Delocalisation of electrons of a C-H sigma bond next to a pi bond, empty p orbital or radical centre. The more alpha hydrogens, the greater the stability: tertiary > secondary > primary carbocations, and the stability of alkenes increases with alkyl substitution.

Aromaticity

A compound is aromatic if it is planar, cyclic, fully conjugated and has (4n + 2) pi electrons (Huckel's rule). Benzene, naphthalene, pyridine, the cyclopropenyl cation, the cyclopentadienyl anion and the tropylium cation are aromatic. Cyclobutadiene (4 pi electrons) is antiaromatic.

Acidity and Basicity Trends

Acid strength increases with stability of the conjugate base: HCOOH > CH\(_3\)COOH; phenols > alcohols; nitrophenols > phenol > cresols; ortho and para nitro groups are most effective. Amines: alkylamines are stronger bases than ammonia; aryl amines are weaker because the lone pair is delocalised; benzylamine is stronger than aniline; electron-withdrawing groups such as -NO\(_2\) reduce basicity.