8.8 Unimolecular Nucleophilic Substitution S N 1

Transcription

1 8.8 Unimolecular Nucleophilic Substitution S N 1

2 A question. Tertiary alkyl halides are very unreactive in substitutions that proceed by the S N 2 mechanism. Do they undergo nucleophilic substitution at all? Yes. But by a mechanism different from S N 2. The most common examples are seen in solvolysis reactions.

3 Example of a solvolysis. ydrolysis of tert-butyl bromide. Br : + : O: O + Br :

4 Example of a solvolysis. ydrolysis of tert-butyl bromide. Br : O + + : O: Br : + + O: : Br :

5 Example of a solvolysis. ydrolysis of tert-butyl bromide. Br : + : O: This is the nucleophilic substitution stage of the reaction; the one with which we are concerned. + + O: : Br :

6 Example of a solvolysis. ydrolysis of tert-butyl bromide. Br : + : O: The reaction rate is independent of the concentration of the nucleophile and follows a first-order rate law. rate = k[( 3 ) 3 Br] + + O: : Br :

7 Example of a solvolysis. ydrolysis of tert-butyl bromide. Br : + : O: The mechanism of this step is not S N 2. It is called S N 1 and begins with ionization of ( ) 3 Br. + O: + : Br :

8 Kinetics and Mechanism rate = k[alkyl halide] First-order kinetics implies a unimolecular rate-determining step. Proposed mechanism is called S N 1, which stands for substitution nucleophilic unimolecular

9 Br : Mechanism unimolecular slow : Br :

10 3 + : O: Mechanism bimolecular fast + O:

11 carbocation formation R + carbocation capture proton transfer RX + RO 2 RO

12 haracteristics of the S N 1 mechanism first order kinetics: rate = k[rx] unimolecular rate-determining step carbocation intermediate rate follows carbocation stability rearrangements sometimes observed reaction is not stereospecific much racemization in reactions of optically active alkyl halides

13 8.9 arbocation Stability and S N 1 Reaction Rates

14 Electronic Effects Govern S N 1 Rates The rate of nucleophilic substitution by the S N 1 mechanism is governed by electronic effects. arbocation formation is rate-determining. The more stable the carbocation, the faster its rate of formation, and the greater the rate of unimolecular nucleophilic substitution.

15 Table 8.5 Reactivity toward substitution by the S N 1 mechanism RBr solvolysis in aqueous formic acid Alkyl bromide lass Relative rate Br 2 Br ( ) 2 Br ( ) 3 Br Methyl 1 Primary 2 Secondary 43 Tertiary 100,000,000

16 Decreasing S N 1 Reactivity ( ) 3 Br ( ) 2 Br 2 Br Br

17 8.10 Stereochemistry of S N 1 Reactions

18 Generalization Nucleophilic substitutions that exhibit first-order kinetic behavior are not stereospecific.

19 Stereochemistry of an S N 1 Reaction R-( )-2-Bromooctane Br ( 2 ) 5 O 3 2 O O ( 2 ) 5 (S)-(+)-2-Octanol (83%) ( 2 ) 5 (R)-( )-2-Octanol (17%)

20 Figure Ionization step gives carbocation; three bonds to stereogenic center become coplanar

21 Figure Leaving group shields one face of carbocation; nucleophile attacks faster at opposite face.

22 More than 50% Less than 50% +

23 8.11 arbocation Rearrangements in S N 1 Reactions

24 Because. carbocations are intermediates in S N 1 reactions, rearrangements are possible.

25 Example 2 O 2 Br O (93%)

26 Example 2 Br O (93%) 2 O + +

27 8.12 Solvent Effects

28 In general. S N 1 Reaction Rates Increase in Polar Solvents

29 S N Table Reactivity versus Solvent Polarity Solvent Dielectric constant Relative rate acetic acid 6 1 methanol 33 4 formic acid 58 5,000 water ,000

30 transition state stabilized by polar solvent δ+ R X δ R + energy of RX not much affected by polarity of solvent RX

31 transition state stabilized by polar solvent δ+ R X δ activation energy decreases; rate increases R + energy of RX not much affected by polarity of solvent RX

32 In general. S N 2 Reaction Rates Increase in Polar Aprotic Solvents An aprotic solvent is one that does not have an O group.

33 S N Table Reactivity versus Type of Solvent Br + N 3 Solvent O 2 O DMSO DMF Acetonitrile Type Relative rate polar protic 1 polar protic 7 polar aprotic 1300 polar aprotic 2800 polar aprotic 5000

34 Mechanism Summary S N 1 and S N 2

35 When. primary alkyl halides undergo nucleophilic substitution, they always react by the S N 2 mechanism tertiary alkyl halides undergo nucleophilic substitution, they always react by the S N 1 mechanism secondary alkyl halides undergo nucleophilic substitution, they react by the S N 1 mechanism in the presence of a weak nucleophile (solvolysis) S N 2 mechanism in the presence of a good nucleophile