JEE Chemistry Organic Chemistry Some Basic Principles and Techniques Notes Set 11

What a Reaction Mechanism Is

A reaction mechanism is the step-by-step account of how bonds break and form as reactants become products, including the intermediates formed on the way. Three ideas are needed: how a bond breaks, what kind of reagent attacks, and which intermediate results.

Types of Bond Cleavage

Homolytic fission: the shared pair is split equally, one electron to each atom, giving free radicals. It occurs in non-polar solvents, at high temperature or in light, between atoms of similar electronegativity.
Heterolytic fission: both electrons go to the more electronegative atom, giving a cation and an anion (ions). It is common in polar solvents and between atoms of different electronegativity, for example \( \ce{CH3-Cl -> CH3+ + Cl-} \).

Attacking Reagents

Nucleophiles are electron-pair donors (Lewis bases) that attack electron-poor centres: negative ions such as \( \ce{OH-} \), \( \ce{CN-} \), \( \ce{NH2-} \) and neutral molecules with a lone pair such as \( \ce{NH3} \), \( \ce{H2O} \), \( \ce{ROH} \).
Electrophiles are electron-pair acceptors (Lewis acids) that attack electron-rich centres: cations such as \( \ce{H+} \), \( \ce{NO2+} \), \( \ce{R+} \), and neutral electron-deficient species such as \( \ce{BF3} \), \( \ce{AlCl3} \), \( \ce{SO3} \) and carbenes like \( \ce{:CCl2} \).

Reaction Intermediates

Alkyl groups stabilise cations and radicals (by +I effect and hyperconjugation) and destabilise carbanions. Groups that can delocalise the charge or electron by resonance stabilise all three.

Inductive Effect

The inductive effect is the permanent polarisation of a sigma bond by electronegative or electropositive atoms, passed along a chain of sigma bonds and fading quickly: it is felt strongly on the next carbon and hardly beyond the third.
Groups that withdraw electrons show a −I effect: \( \ce{-NH3+} \) > \( \ce{-NO2} \) > \( \ce{-CN} \) > \( \ce{-COOH} \) > \( \ce{-F} \) > \( \ce{-Cl} \) > \( \ce{-Br} \) > \( \ce{-I} \) > \( \ce{-OH} \) > \( \ce{-C6H5} \). Groups that release electrons show a +I effect: tertiary alkyl > secondary alkyl > primary alkyl > \( \ce{-CH3} \).
Consequences: acid strength rises with −I groups near the COOH (trifluoroacetic acid > trichloroacetic acid > chloroacetic acid > acetic acid) and falls with +I groups (formic acid > acetic acid); the effect weakens as the substituent moves away from the carboxyl group. The reverse holds for bases: +I groups make amines more basic.

Resonance (Mesomeric) Effect

When a molecule cannot be shown by a single Lewis structure, it is a hybrid of several resonance structures that differ only in the position of electrons. The hybrid is more stable than any single structure, and the extra stability is the resonance energy. Equal bond lengths in the carboxylate ion and the shorter C–Cl bond of vinyl chloride are signs of resonance.
+M (+R) groups push lone-pair or π electrons into the system: \( \ce{-OH} \), \( \ce{-OR} \), \( \ce{-NH2} \), \( \ce{-NR2} \), halogens. −M (−R) groups withdraw π electrons: \( \ce{-NO2} \), \( \ce{-CN} \), \( \ce{-CHO} \), the C=O group, \( \ce{-COOH} \), \( \ce{-SO3H} \).
Rules for comparing resonance structures: structures with more covalent bonds and full octets are more stable; structures with charges are less stable than neutral ones; separated charges are less stable than adjacent opposite charges that can neutralise; negative charge is best on the more electronegative atom.
A +M group at the ortho or para position stabilises a carbocation (for example the 4-methoxybenzyl cation) and destabilises a phenoxide or carbanion; a −M group does the opposite. At the meta position only the inductive effect operates.

Effects on Acidity and Basicity

A stronger acid has a more stable conjugate base. Phenol is more acidic than alcohols because the phenoxide ion is resonance-stabilised. In substituted phenols and benzoic acids, −M and −I groups (nitro, cyano, halogen) increase acidity, and +M and +I groups (methoxy, methyl) decrease it. For example 2,4,6-trinitrophenol (picric acid) has pKₐ about 0.4, phenol about 10.
Typical order of acidity: carboxylic acids > phenols > water > alcohols > ethyne > ethene > ethane. Hydrocarbon acidity follows s-character: sp > sp² > sp³.
A stronger base has a more available lone pair. Alkyl amines are more basic than ammonia; aryl amines are less basic than alkyl amines because the lone pair is delocalised into the ring; −M or −I groups on the ring decrease basicity and +I groups increase it. In water, the order for methyl amines is secondary > primary > tertiary > ammonia, because of solvation. Benzylamine behaves as an alkylamine.
Pyrrole is essentially non-basic because the nitrogen lone pair is part of the aromatic sextet; pyridine, with the lone pair in an sp² orbital outside the ring, is a weak base; piperidine is strongly basic.

Aromaticity

A species is aromatic if it is planar, cyclic and fully conjugated and has \( 4n+2 \) π electrons (Hückel's rule). It is anti-aromatic if it has \( 4n \) π electrons in such a ring, and is non-aromatic if it lacks planarity or full conjugation.
Aromatic examples: benzene, naphthalene, pyridine, pyrrole, thiophene, the cyclopropenyl cation, the cyclopentadienyl anion and the tropylium (cycloheptatrienyl) cation. Anti-aromatic: cyclobutadiene, the cyclopentadienyl cation and the cyclopropenyl anion. This is why cyclopentadiene is an unusually strong C–H acid and the tropylium ion is an unusually stable cation.

Hyperconjugation

Hyperconjugation (no-bond resonance) is the delocalisation of electrons of a C–H sigma bond on a carbon next to an unsaturated carbon, a carbocation centre or a radical centre. The number of such α C–H bonds measures the effect.
Consequences: the order of carbocation and radical stability (3° > 2° > 1° > methyl); the greater stability of more substituted alkenes (2-methylbut-2-ene > pent-2-ene > pent-1-ene, and trans > cis); a shorter C–C single bond next to a double bond; and the unusual dipole moment of propene.

Electromeric Effect

The electromeric effect is the temporary, complete shift of a pair of π electrons to one of the doubly bonded atoms, brought about by an attacking reagent and reversed when the reagent is removed. In a carbonyl group the π pair moves to oxygen when a nucleophile attacks, and in an alkene it moves towards the carbon that better stabilises the charge. It is a +E effect when the electrons move towards the attacking reagent and −E when they move away.

Tautomerism

Tautomers are structural isomers in rapid equilibrium that differ in the position of a hydrogen and a double bond. The commonest case is keto-enol tautomerism, which needs a hydrogen on the carbon next to the carbonyl group (an α-hydrogen). Compounds without α-hydrogen, such as 2,2-dimethylpropanal or benzaldehyde, cannot show it.
Enol content: simple ketones and aldehydes have a tiny enol content, but 1,3-dicarbonyl compounds are enolised appreciably because the enol is conjugated and held by an intramolecular hydrogen bond (pentane-2,4-dione is mostly enol). Cyclohexa-2,4-dienone is almost entirely phenol, because the enol is aromatic.
Nitroalkanes with an α-hydrogen show nitro–aci-nitro tautomerism; the aci form is acidic and dissolves in alkali, so nitroethane and 2-nitropropane dissolve in NaOH but nitrobenzene does not. Urea, amides and HCN/HNC are further examples of tautomeric or related systems.