Nucleophilic substitution reactions of alkyl halides (ch.9)

What is a substitution reaction?

A reaction where one atom/group (leaving group) is replaced by another atom/group (nucleophile).

What is a leaving group?

The atom/group that departs with its electron pair.

What is a nucleophile?

A species that donates an electron pair to form a new bond with carbon.

: At what carbon hybridization does nucleophilic substitution occur?

Only at sp³ carbons.(Not at sp² or sp hybridized carbons.)

What does SN2 stand for?

Substitution, Nucleophilic, Bimolecular.

Is the rate-determining step unimolecular or bimolecular in SN2?

Bimolecular — depends on both nucleophile and substrate.

SN2 rate equation?

Rate=k[R-LG][Nu−]

How many steps does an SN2 reaction occur in?

One step (concerted mechanism).

What is formed in the SN2 transition state?

Partial bond formation to nucleophile and partial bond breaking to leaving group.

What causes inversion of configuration ("backside attack")?

Nucleophile attacks from the side opposite the leaving group.

What does SN1 stand for?

Substitution, Nucleophilic, Unimolecular.

Is the rate-determining step unimolecular or bimolecular?

Unimolecular — depends only on the substrate.

SN1 rate equation?

Rate=k[R-LG]

How many steps does SN1 occur in?

Two steps:

1. Leaving group leaves → carbocation forms

2. Nucleophile attacks carbocation

What key intermediate is formed in SN1?

A carbocation.

Does SN1 cause retention, inversion, or racemization?

Racemization (mixture of both stereoisomers).

What type of carbon favors SN2?

Methyl > Primary > Secondary (steric hindrance must be low).

What type of carbon favors SN1?

Tertiary > Secondary (carbocation stability required).

Which mechanism prefers strong nucleophiles?

SN2.

Which mechanism prefers polar protic solvents?

SN1 (stabilizes carbocation & leaving group).

Which mechanism prefers polar aprotic solvents?

SN2 (leaves nucleophile unsolvated & strong).

What causes SN1 reactions to speed up?

Better leaving groups and more stable carbocations.

What causes SN2 reactions to slow down?

Steric hindrance around the reactive carbon.