91Ó°ÊÓ

Question: Draw the products formed in each reaction and indicate stereochemistry.

a.

b.

c.

d.

Question: What reactions are needed to convert alcohol A into either alkyne B or alkyne C?

Question: Identify the lettered compounds in the following reaction schemes. Each reaction sequence was used in the synthesis of a natural product.

a.

b.

Question: Treatment of 2,2-dibromobutane with two equivalents of strong base affords but-1-yne and but-2-yne, as well as a small amount of buta-1,2-diene. Draw a mechanism showing how each compound is formed. Which alkyne should be the major product?

Question: Explain the following statement. Although $\mathbf{HC}\mathbf{≡}{\mathbf{C}}^{\mathbf{-}}$is more stable than${\mathbf{H}}_{\mathbf{2}}\mathbf{C}\mathbf{=}{\mathbf{C}}^{\mathbf{-}}$,$\mathbf{HC}\mathbf{≡}{\mathbf{C}}^{\mathbf{+}}$is less stable than ${\mathbf{H}}_{\mathbf{2}}\mathbf{C}\mathbf{=}{\mathbf{C}}^{\mathbf{+}}$.

Question: Tautomerization in base resembles tautomerization in acid, but deprotonation precedes protonation in the two-step mechanism. (a) Draw a stepwise mechanism for the following tautomerization. (b) Then, draw a stepwise mechanism for the reverse reaction, the conversion of the keto form to the enol.

Question: Draw a stepwise mechanism for each reaction.

a.

b.

Question: From what you have learned about enols and the hydration of alkynes, predict what product is formed by the acid-catalyzed hydration of ${\mathbf{CH}}_{\mathbf{3}}{\mathbf{CH}}_{\mathbf{2}}{\mathbf{CH}}_{\mathbf{2}}\mathbf{C}\mathbf{≡}{\mathbf{COCH}}_{\mathbf{3}}$ . Draw a stepwise mechanism that illustrates how it is formed.

Question: What steps are needed to prepare phenylacetylene, ${\mathbf{C}}_{\mathbf{6}}{\mathbf{H}}_{\mathbf{5}}\mathbf{C}\mathbf{≡}\mathbf{CH}$, from each compound:

$$\begin{gathered} a.{\mathbf{C}}_{\mathbf{6}}{\mathbf{H}}_{\mathbf{5}}{\mathbf{CH}}_{\mathbf{2}}{\mathbf{CHBr}}_{\mathbf{2}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}} \\ \mathbf{b}\mathbf{.}\mathbf{}\mathbf{}{\mathbf{C}}_{\mathbf{6}}{\mathbf{H}}_{\mathbf{5}}{\mathbf{CHBrCH}}_{\mathbf{3}} \\ \mathbf{c}\mathbf{.}\mathbf{}\mathbf{}{\mathbf{C}}_{\mathbf{6}}{\mathbf{H}}_{\mathbf{5}}{\mathbf{CH}}_{\mathbf{2}}{\mathbf{CH}}_{\mathbf{2}}{\mathbf{OH}}_{\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}} \end{gathered}$$

Question: The alkyl halides must be methyl or$\mathbf{1}\mathbf{°}$

a.

b.

c.