9. Alkenes and Alkynes

What is the product of the reaction of an ester with excess acetylide ion followed by the addition of pyridinium chloride?

Explain why ethyne should be alkylated before, rather than after, nucleophilic addition.

Suggest an acetylide ion and a carbonyl that might be used to make the following products.

(a) oct-4-yn-3-ol

Suggest an acetylide ion and a carbonyl that might be used to make the following products.

(b) 2,6-dimethylhept-3-yn-2-ol

Suggest an acetylide ion and a carbonyl that might be used to make the following products.

(c) 5-phenylhex-2-yn-1-ol

Show the products of the following acetylide alkylation reactions. [Make sure your product has the correct number of carbons.]

(d)

The introduction of elimination reactions provides a second way to synthesize alkynes in a two-step process starting with an alkene. Suggest a mechanism for both steps of this process.

When the reaction scheme in Assessment 12.63 is done on a monosubstituted alkene, at least three equivalents of base are needed. Reacting the product of step 2 with D–Cl (D is an isotope of H) incorporates deuterium at the terminal carbon. Explain these two observations.

Calculate K_eq for the following acid–base reactions. Which is the best base to use to deprotonate acetylene and make the acetylide anion?

(a) HO^- + H―C ≡ C―H ⇌ ^-C ≡ C―H + H₂O

Calculate for the following acid–base reactions. Which is the best base to use to deprotonate acetylene and make the acetylide anion?

(b) H₂N^- + H―C ≡ C―H ⇌ ^-C ≡ C―H + H₃N

Show the products of the following acetylide alkylation reactions. [Make sure your product has the correct number of carbons.]

(b)

Calculate for the following acid–base reactions. Which is the best base to use to deprotonate acetylene and make the acetylide anion?

(c)

Suggest reagents you might use to generate the product from the given reactant.

(b)

A chemist is planning to synthesize 3-octyne by adding 1-bromobutane to the product obtained from the reaction of 1-butyne with sodium amide. Unfortunately, however, he forgot to order 1-butyne. How else can he prepare 3-octyne?

Draw the product of each of the following reactions:

1.