7. Substitution Reactions

Crown ethers are able to solvate cations based on their size. Specifically, 15-crown-5 forms stable complexes with sodium. How would the addition of a crown ether change the rate of an S_N2 reaction?

Cardura, a drug used to treat hypertension, is synthesized as shown here.

b. Show the mechanism for conversion of A to B. Which is formed more rapidly, A or B?

Propose a mechanism for the following reaction that explains why the configuration of the asymmetric center in the reactant is retained in the product:

The S_N2 reaction is the concerted, backside displacement of a good leaving group by a nucleophile. Why do nucleophiles attack from the back in S_N2 reactions?

For each pair, choose the haloalkane that would react most quickly in an S_N2 reaction.

(a)

Predict the product(s) that would result when molecules (a)–(p) are allowed to react under the following conditions: (iii) SOCl₂ , NEt₃ (iv) 1. TsCl, Et₃N 2. NaCN; If no reaction occurs, write 'no reaction.'

(f)

Practice your electron-pushing skills by drawing a mechanism for the following S_N2 reactions.

(c)

For each pair, choose the nucleophile that would react most quickly in an S_N2 reaction (assume H₂O is the solvent).

(c)

For each pair, choose the haloalkane that would react most quickly in an S_N2 reaction.

(c)

In which solvent—ethanol or diethyl ether—would the equilibrium for the following S_N2 reaction lie farther to the right?

How will the rate of each of the following S_N2 reactions change if it is carried out in a more polar solvent?

b.

How will the rate of each of the following S_N2 reactions change if it is carried out in a more polar solvent?

c.

Show all the products, including their configurations, that are obtained from the above reaction.

You were told in [SECTION 7.11] that is best to use a methyl halide or a primary alkyl halide for the reaction of an acetylide ion with an alkyl halide. Explain why this is so.

Would you expect acetate ion (CH₃CO₂⁻) to be a better nucleophile in an S_N2 reaction with an alkyl halide carried out in methanol or in dimethyl sulfoxide?