Electronic Effects and Reaction Intermediates: Quick Revision

A brief note on Electronic Effects and Reaction Intermediates. Read it before a test. Try each question at the end before you open its answer.

Breaking bonds

• Homolytic fission: each atom keeps one electron. It gives free radicals, with half-headed arrows.
• Heterolytic fission: one atom takes both electrons. It gives a cation and an anion, with a full-headed arrow.

Intermediates

• Free radical: neutral, one unpaired electron. Stability: tertiary, secondary, primary, methyl.
• Carbocation: six electrons, flat. Same order of stability, helped by alkyl groups and hyperconjugation.
• Carbanion: lone pair, pyramidal. The order of stability is reversed: methyl is the best.

Electron shifts

• Inductive effect: permanent, along sigma bonds, fades with distance. $-I$ groups pull. $+I$ groups, such as alkyl groups, push.
• Resonance: electrons spread over several atoms. The hybrid is more stable than any structure. $+R$ groups give. $-R$ groups take.
• Electromeric effect: a temporary shift of a pi pair, under attack.
• Hyperconjugation: sigma electrons of an alpha C-H bond spread out. More alpha hydrogens, more stability.

Reagents and reactions

• Nucleophile: electron-rich, gives a pair. Electrophile: electron-poor, takes a pair.
• Substitution replaces. Addition joins and uses up a pi bond. Elimination removes and makes a pi bond. Rearrangement moves atoms inside one molecule.

A worked example

Compare the isobutyl cation, $(\text{CH}_3)_2\text{CHCH}_2^{+}$, with the tert-butyl cation.

1. Isobutyl. The positive carbon touches one carbon, which is a $\text{CH}$ with one hydrogen. So there is $1$ helpful bond.
2. Tert-butyl. The positive carbon touches three $\text{CH}_3$ groups, so there are $9$ helpful bonds.
3. Meaning. The tert-butyl cation is far more stable. An isobutyl cation tends to change into it by a hydrogen shift.

Practice with new numbers

A program to try

The program counts helpful bonds for three cations. It also shows how a pull fades when each bond passes on half of it.

Mistakes to avoid

• Reversing the carbanion order. Alkyl groups hurt a carbanion.
• Moving atoms in resonance. Only electrons move.
• A permanent electromeric effect. It needs a reagent.
• Mixing up the reagents. A nucleophile gives a pair. An electrophile takes one.
• Counting the wrong hydrogens. Alpha hydrogens are on the carbons next to the charged or double-bonded carbon.

Check yourself

1. Which kind of curved arrow shows one electron moving?

2. Which is more stable, a primary or a tertiary carbocation?

3. Is $-\text{OCH}_3$ a $+R$ or a $-R$ group?

4. Is $\text{BF}_3$ a nucleophile or an electrophile?

5. Which type of reaction turns one molecule into an isomer?