Unimolecular Nucleophilic Substitution S N 1 A question
Unimolecular Nucleophilic Substitution S N 1
A question. . . Tertiary alkyl halides are very unreactive in substitutions that proceed by the SN 2 mechanism. Do they undergo nucleophilic substitution at all? Yes. But by a mechanism different from SN 2. The most common examples are seen in solvolysis reactions.
The Sn 1 Reaction
Example of a solvolysis. Hydrolysis of tert-butyl bromide. CH 3 C . . : Br. . H + : O: H CH 3 C CH 3 . . OH. . + H . . Br : . .
Kinetics and Mechanism rate = k [alkyl halide] First-order kinetics implies a unimolecular rate-determining step. Mechanism is called SN 1, which stands for substitution nucleophilic unimolecular (1)
CH 3 Mechanism . . : Br. . C CH 3 unimolecular slow H 3 C + C CH 3 + . . – : Br : . .
Mechanism H 3 C CH 3 + C H : O: CH 3 H bimolecular fast CH 3 C CH 3 + H O: H
carbocation formation R+ carbocation capture proton transfer RX + ROH 2 ROH
Characteristics of the SN 1 mechanism • First order kinetics: rate = k [RX] unimolecular rate-determining step • Carbocation intermediate rate follows carbocation stability rearrangements are observed • Reaction is not stereospecific: racemization in reactions of optically active alkyl halides
Reaction Coordinate Diagram for an SN 1 Reaction
Carbocation Stability and SN 1 Reaction Rates
Electronic Effects Govern SN 1 Rates The rate of nucleophilic substitution by the SN 1 mechanism is governed by electronic effects. Carbocation formation is rate-determining. The more stable the carbocation, the faster its rate of formation, and the greater the rate of unimolecular nucleophilic substitution.
Reactivity toward substitution by the SN 1 mechanism RBr solvolysis in aqueous formic acid Alkyl bromide Class CH 3 Br Methyl CH 3 CH 2 Br. Primary (CH 3)2 CHBr Relative rate 1 2 Secondary (CH 3)3 CBr Tertiary 43 100, 000
Decreasing SN 1 Reactivity (CH 3)3 CBr (CH 3)2 CHBr CH 3 CH 2 Br CH 3 Br
Stereochemistry of SN 1 Reactions
Generalization Nucleophilic substitutions that exhibit first-order kinetic behavior are not stereospecific.
Stereochemistry of an SN 1 Reaction CH 3 H C R-(–)-2 -Bromooctane Br CH 3(CH 2)5 H HO CH 3 C (CH 2)5 CH 3 (S)-(+)-2 -Octanol (83%) CH 3 H 2 O H C OH CH 3(CH 2)5 (R)-(–)-2 -Octanol (17%)
The carbocation reaction intermediate leads to the formation of two stereoisomeric products
Step 1 + Ionization step gives carbocation; three bonds to stereogenic center become coplanar
Step 2 + Leaving group shields one face of carbocation; nucleophile attacks faster at opposite face.
+ More than 50% Less than 50%
Carbocation Rearrangements in SN 1 Reactions
Because. . . carbocations are intermediates in SN 1 reactions, rearrangements are possible.
Carbocations
Carbocations rearrange to the more stable form(s)
Carbocation Rearrangement Mechanism What is the starting carbocation: 1 o, 2 o or 3 o? What is the rearranged carbocation: 1 o, 2 o or 3 o?
Carbocation Rearrangement
Example CH 3 C CHCH 3 H Br H 2 O CH 3 C OH CH 2 CH 3 (93%)
Example CH 3 C CHCH 3 H Br CH 3 C CH 2 CH 3 OH (93%) H 2 O CH 3 C H CH 3 CHCH 3 + CH 3 C + CHCH 3 H
Mechanism Summary SN 1 and SN 2
When. . . primary alkyl halides undergo nucleophilic substitution, they always react by the SN 2 mechanism tertiary alkyl halides undergo nucleophilic substitution, they always react by the SN 1 mechanism secondary alkyl halides undergo nucleophilic substitution, they react by the SN 1 mechanism in the presence of a weak nucleophile (solvolysis) SN 2 mechanism in the presence of a good nucleophile
Putting things together
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