Basic Organic Chemistry: Resonance, Electronic Effects, Aromaticity and Reactive Intermediates

9.1Delocalised bonding and resonance

When a π bond, a lone pair, an empty p orbital or an unpaired electron sits next to another π system, the p orbitals overlap side by side and the electrons spread over three or more atoms. This is delocalisation. A single Lewis structure cannot show it, so we draw several resonance structures (contributors) that differ only in the position of π electrons and lone pairs; the real molecule is one resonance hybrid, lower in energy than any contributor. The extra stability is the resonance energy.

Rules for drawing contributors

• Only electrons move; nuclei stay where they are. Every contributor has the same number of paired and unpaired electrons.
• Only atoms in one plane with parallel p orbitals can share electrons. A group twisted out of the plane loses its resonance.
• Second-row atoms cannot exceed an octet.

Which contributor counts most

1. Complete octets on every atom beat everything else. CH₂=O⁺–CH₃ (all octets) outweighs ⁺CH₂–O–CH₃ (carbon with a sextet).
2. Fewer formal charges (more covalent bonds) is better; charge separation costs energy.
3. A negative charge on the more electronegative atom (and positive on the less) is better: in an enolate the form with O⁻ dominates.
4. Like charges far apart; opposite charges close together.
5. Equivalent contributors (carboxylate, benzene, allyl) give the largest resonance energy.

Evidence: heats of hydrogenation

Adding H₂ to a C=C releases heat. The more stable the alkene or diene, the less heat it gives out.

• Cyclohexene: about 120 kJ mol⁻¹. Benzene, if it had three isolated C=C, would release about 360; it actually releases about 208. The difference, about 150 kJ mol⁻¹ (36 kcal), is the resonance energy of benzene.
• Conjugated dienes release less than isolated dienes (1,3-pentadiene ≈ 226 against 1,4-pentadiene ≈ 254 kJ mol⁻¹).
• More alkyl groups on the C=C lower the heat (hyperconjugation, 9.2): tetrasubstituted < trisubstituted < disubstituted < monosubstituted; trans < cis.
• A cyclic monoene takes one H₂; a diene takes two, so its value is roughly double. Compare per C=C only when the number of C=C is the same.

Steric inhibition of resonance

Resonance needs coplanarity. Bulky ortho groups twist –NMe₂, –NO₂ or –COOH out of the ring plane and cut off its resonance with the ring. So 2,6-disubstituted N,N-dimethylanilines are more basic than expected (the lone pair is no longer delocalised), and plain PhNMe₂ is the most resonance-stabilised of the series.

π molecular orbitals of conjugated systems

• n p orbitals give n π MOs. ψ_k has k − 1 nodes; energy rises with the number of nodes.
• Butadiene (4 π e⁻): ψ₁ and ψ₂ are filled. HOMO = ψ₂ (one node, between C2 and C3; C1, C2 in one phase and C3, C4 in the other). LUMO = ψ₃ (two nodes).
• Allyl system: ψ₁ (bonding), ψ₂ (non-bonding, node through the central carbon), ψ₃ (antibonding). The allyl cation, radical and anion put 2, 3 and 4 electrons into these, so the charge or odd electron sits on C1 and C3 only.

9.2Hyperconjugation

A C–H (or C–C) σ bond on the carbon next to an empty p orbital, a π bond or a radical centre can overlap sideways with it and share its electrons. This is hyperconjugation (σ–π conjugation, or “no-bond resonance”, because the contributors show H⁺ with no bond to carbon).

• Number of hyperconjugative structures = number of α-C–H bonds, that is, H atoms on carbons directly attached to the cationic, radical or sp² carbon. H on the charged carbon itself does not count, nor do H further away. (CH₃)₃C⁺ has 9; (CH₃)₂CH⁺ has 6; CH₃CH₂⁺ has 3; CH₃–C⁺H–C(CH₃)₃ has only 3, because the quaternary carbon carries no H.
• It explains: stability of carbocations and radicals 3° > 2° > 1° > methyl; greater stability of more substituted alkenes; the slightly shorter C–C single bond next to a C=C (propene 1.50 Å); the small dipole moment of propene; and the Baker–Nathan order (CH₃ > CH₃CH₂ > (CH₃)₂CH > (CH₃)₃C) seen in some reactions, opposite to the inductive order.
• σ(C–H) → σ*(C–X) donation (the same idea with an empty σ* orbital) gives the gauche effect in 1,2-difluoroethane (see Unit 10).

9.3Inductive and mesomeric effects

Inductive effect (I)

A permanent polarisation passed along σ bonds by an electronegative or electropositive group. It falls off quickly with distance (almost nil after three bonds).

• −I (withdraw): –NO₂ > –N⁺R₃ > –CN > –COOH > –F > –Cl > –Br > –I > –OCH₃ > –OH > –C₆H₅.
• +I (donate): –O⁻ > –COO⁻ > –C(CH₃)₃ > –CH(CH₃)₂ > –CH₂CH₃ > –CH₃.

Mesomeric (resonance) effect (M or R)

Transfer of π or lone-pair electrons through a conjugated system. It acts only at conjugated positions (ortho and para in a benzene ring) but over long distances, and it usually outweighs the inductive effect.

• +M (lone pair next to the π system): –O⁻ > –NH₂ > –NR₂ > –OH > –OR > –NHCOR > halogens.
• −M (π bond to an electronegative atom): –NO₂ > –CN > –CHO > –COR > –COOH > –COOR.
• Halogens are −I and +M; −I wins for reactivity, but +M decides ortho/para orientation.
• Hammett σ_p sums up both effects at the para position: –NO₂ 0.78, –CN 0.66, –COCH₃ 0.50, –Cl 0.23, –H 0, –CH₃ −0.17, –OCH₃ −0.27, –OH −0.37, –NH₂ −0.66. Positive = withdrawing.

Dipole moments and molecular polarity

• Bond dipoles are vectors. For two dipoles μ₁ and μ₂ at angle θ: . 1 D = 3.336 × 10⁻³⁰ C m.
• One polar bond always makes a molecule polar. Two or more polar bonds may cancel in a symmetric shape (CO₂, BF₃, CCl₄, trans-1,2-dichloroethene, p-dichlorobenzene: zero). A molecule with only non-polar bonds is non-polar.
• Dichlorobenzenes: ortho (60°) ≈ 2.5 D > meta (120°) ≈ 1.5 D > para = 0. Chlorobenzene ≈ 1.6–1.7 D. In p-BrC₆H₄Cl two nearly equal dipoles almost cancel (≈ 0.1–0.2 D).
• Where a +M group and a −M group sit para to each other (p-nitroaniline), the dipoles add and the moment is large.