JEE Chemistry Organic Chemistry Some Basic Principles and Techniques Notes Set 19

Electronic Effects: Overview

The way electrons are distributed in a molecule decides its stability and reactivity. Shifts of electrons are called electronic effects. Permanent effects are the inductive effect (I), the mesomeric or resonance effect (M) and hyperconjugation. The electromeric effect (E) is temporary and appears only when an attacking reagent approaches.

Inductive Effect

Two carbon atoms joined to each other share their bonding pair equally. When one carbon carries an electronegative atom such as Cl, the sigma bonding pair is pulled towards that atom, a small positive charge appears on the attached carbon, and this in turn polarises the next bond, and so on. This transmission of polarity through a chain of sigma bonds is the inductive effect.
Key points: it is permanent, it involves only sigma electrons, electrons never leave their orbitals, and it dies out quickly with distance (almost nil after the third carbon).
A group that draws electrons away from the chain shows -I effect and a group that pushes electrons into it shows +I effect. Hydrogen is the reference (zero effect).

Resonance and Conjugation

When a single Lewis structure cannot describe a molecule, several structures that differ only in the placement of electrons are written. They are the resonating (contributing, canonical) structures. The real molecule is their weighted average, the resonance hybrid, which is more stable than any one contributor. The contributors are imaginary; only the hybrid exists.
A group is in conjugation with an unsaturated system when it is attached to a multiple bond through a single bond and carries a pi bond, a charge, an odd electron or a lone pair. Typical cases: C=C-C=C, +C-C=C, :Cl-C=C, \(\bullet\)C-C=C, and \(^-\)C-C=C.

Rules for Writing Resonating Structures

1. Every structure must be a proper Lewis structure (for example no carbon with five bonds, no second-period atom with more than eight electrons).
2. Nuclei must not move; only pi electrons, lone pairs and charges shift.
3. All atoms taking part must lie in one plane so the p orbitals overlap sideways.
4. The number of unpaired electrons (and paired electrons) is the same in every structure, and the total charge is conserved.
5. Delocalisation lowers energy, so the hybrid is more stable than any contributor.
6. Contributors are not equally important; the more stable a contributor, the more it resembles the hybrid.

Stability of Resonating Structures

Prefer a structure in which (a) all atoms have a complete octet; (b) there is no charge separation (neutral beats charged); (c) there are more covalent bonds; (d) a negative charge sits on the more electronegative atom and a positive charge on the less electronegative atom. Structures with like charges on adjacent atoms are very unstable.
Resonance energy is the difference in energy between the most stable contributor and the hybrid; the larger it is, the more stable the molecule. For benzene it is about 36 kcal/mol.

Mesomeric Effect

The mesomeric (resonance) effect is the permanent transfer of pi electrons or lone pairs through a conjugated system. A group that donates electrons into the system shows +M effect, and one that withdraws electrons shows -M effect.

+M groups on a benzene ring increase its electron density (activating groups); -M groups decrease it (deactivating groups). If a +M and a -M group are meta to each other they cannot conjugate directly. When a group has a lone pair or a pi bond competing for delocalisation, only one pi bond or one lone pair actually participates.
The relative electron density of an aromatic ring rises with the +M strength of the substituent, for example C\(_6\)H\(_5\)NH\(_2\) > C\(_6\)H\(_5\)NHCH\(_3\) > C\(_6\)H\(_5\)NHCOCH\(_3\) > C\(_6\)H\(_5\)OCOCH\(_3\).

Steric Inhibition of Resonance (SIR)

If bulky groups at the positions next to a substituent force that group out of the plane of the ring, its p orbitals no longer overlap properly with the ring and its resonance is reduced. The SIR effect increases with the size of the group. For ortho-halobenzoic acids the SIR order is I > Br > Cl > F.

Hyperconjugation

Hyperconjugation is the delocalisation of the sigma electrons of a C-H bond on a carbon attached to a double bond, an empty p orbital (carbocation), a half-filled p orbital (radical) or a benzene ring. It is also called no-bond resonance because the contributing structures have no bond between H and C. The necessary condition is at least one hydrogen on the alpha carbon (the carbon directly attached to the unsaturated centre).
Number of hyperconjugation structures = number of alpha hydrogens. Examples: propene 3, 2-methylbut-2-ene 9, tert-butyl cation 9, toluene 3.
Consequences: more alkylated alkenes are more stable and have lower heat of hydrogenation; the C-C single bond next to a double bond is shortened and the C=C bond is slightly lengthened; the C-H bond is lengthened; dipole moment increases; carbocations and radicals are stabilised. In an alkyl group attached to benzene the effect falls in the order CH\(_3\) > CH\(_3\)CH\(_2\) > (CH\(_3\))\(_2\)CH > (CH\(_3\))\(_3\)C (Baker-Nathan order).

Electromeric Effect

When a reagent attacks a compound with a double or triple bond, the pi electron pair is completely transferred to one atom. This temporary effect is the electromeric effect. In the +E effect the pair moves to the atom that the reagent attacks (for example H\(^+\) adding to an alkene). In the -E effect it moves away from the attacked atom (for example CN\(^-\) adding to carbonyl carbon so that the pair goes to oxygen). When inductive and electromeric effects oppose each other, the electromeric effect wins.

Aromaticity

A compound is aromatic if it is cyclic, planar, fully conjugated (every ring atom sp\(^2\) or with a p orbital) and has (4n + 2) pi electrons (2, 6, 10, 14, ...), Huckel's rule. With 4n pi electrons the planar cyclic system is antiaromatic and very unstable (cyclobutadiene). If the ring is non-planar or not fully conjugated it is non-aromatic. For similar systems the stability order is aromatic > non-aromatic > antiaromatic.
Counting electrons: a double bond gives 2 pi electrons; a lone pair on a ring atom that is in conjugation gives 2 (pyrrole N, furan O); an empty p orbital gives 0 (cyclopropenyl cation has 2 pi electrons, tropylium cation 6); a pyridine nitrogen lone pair is in the plane and does not count.
Aromatic compounds are carbon rich and burn with a sooty flame; they prefer substitution to addition.

Reaction Intermediates

Short-lived, high-energy species formed after bond breaking and before bond formation are reaction intermediates. A covalent bond breaks by homolysis (each atom keeps one electron; radicals form) or by heterolysis (one atom takes both electrons; ions form). Heterolysis needs a polar bond and is helped by greater electronegativity difference.

Stability of Intermediates

Carbocation: stabilised by +I, +M, hyperconjugation and resonance. Order: Ph\(_3\)C\(^+\) > Ph\(_2\)CH\(^+\) > PhCH\(_2^+\) > allyl > 3\(^\circ\) > 2\(^\circ\) > 1\(^\circ\) > CH\(_3^+\) > vinyl. Carbocations rearrange to more stable ones by 1,2 shift of hydride, alkyl or aryl, ring expansion of strained rings, or ring contraction.
Free radical: stabilised by resonance, hyperconjugation and +I. Order: benzyl, allyl > 3\(^\circ\) > 2\(^\circ\) > 1\(^\circ\) > methyl. Radicals are formed by light, heat or peroxides and usually do not rearrange.
Carbanion: stabilised by -I, -M and delocalisation and by greater s character (sp > sp\(^2\) > sp\(^3\)). Alkyl groups destabilise it: CH\(_3^-\) > 1\(^\circ\) > 2\(^\circ\) > 3\(^\circ\). Carbanions are Lewis bases and generally do not rearrange.

Carbenes, Nitrenes and Benzyne

A carbene is a neutral divalent carbon species with a sextet, made for example from chloroform with a base (:CCl\(_2\)), from CH\(_2\)I\(_2\) with Zn, or from diazomethane by light. Singlet carbene is sp\(^2\), bent and diamagnetic and reacts stereospecifically; triplet carbene is nearly linear (sp), paramagnetic (a diradical) and is the ground state. A nitrene (R-N:) is the nitrogen analogue with an incomplete octet. In benzyne the extra pi bond lies in the plane of the ring, formed by weak overlap of two sp\(^2\) orbitals, so it cannot take part in the ring resonance.

Applications and Worked Examples

Example 1. Compare the stability of 2-methylbut-2-ene and propene.
2-Methylbut-2-ene has 9 alpha hydrogens and propene 3, so the former has more hyperconjugative structures, is more stable and has the lower heat of hydrogenation.

Example 2. Rank the carbanions CH\(_3^-\), CH\(_3\)CH\(_2^-\), (CH\(_3\))\(_2\)CH\(^-\) in decreasing stability.
Alkyl groups push electrons towards the carbon that already holds extra negative charge, so stability falls with alkyl substitution: CH\(_3^-\) > CH\(_3\)CH\(_2^-\) > (CH\(_3\))\(_2\)CH\(^-\).

Example 3. Is cyclopentadienyl anion aromatic?
It is cyclic, planar and fully conjugated, with 4 pi electrons from two double bonds plus 2 from the lone pair on the carbanion carbon, giving 6 (= 4n + 2 with n = 1). It is aromatic, which is why cyclopentadiene (pKa about 16) is unusually acidic for a hydrocarbon.

Example 4. Rearrange (CH\(_3\))\(_2\)CH-CH\(^+\)-CH\(_3\).
A hydride shift from the tertiary carbon to the positive secondary carbon gives the more stable tertiary cation (CH\(_3\))\(_2\)C\(^+\)-CH\(_2\)-CH\(_3\).