Alkynes have two $$\\pi$$ bonds, allowing for two sequential additions.

Question 1

Which statement best explains why alkynes undergo two addition reactions with HX or X$$_2$$ while alkenes undergo only one?

Accepted Answer

Alkynes have two $$\pi$$ bonds, allowing for two sequential additions.

Question 2

A student wants to prepare a terminal alkyne from an alkene. Which sequence of reagents would be most appropriate?

Accepted Answer

$$X_2$$ followed by excess strong base

Question 3

Which product is formed when 1-butyne reacts with excess Br$$_2$$?

Accepted Answer

$$1,1,2,2-$$Tetrabromobutane

Question 4

Why is the mechanism for HX addition to alkynes less well understood than for alkenes?

Accepted Answer

sp carbocations formed during the reaction are unstable

Question 5

Which statement about hydration of internal alkynes is correct?

Accepted Answer

Markovnikov's rule does not apply because both carbons are equally substituted

Question 6

During acid-catalyzed hydration of a terminal alkyne, what is the final major product after tautomerization?

Accepted Answer

An aldehyde

Question 7

Which catalyst is used to selectively hydrogenate an alkyne to a cis-alkene?

Accepted Answer

Lindlar catalyst

Question 8

What is the role of NaNH$$_2 in $$the alkylation of terminal alkynes?

Accepted Answer

It deprotonates the terminal alkyne to form an acetylide anion

Question 9

Which of the following best describes the product of hydroboration-oxidation of a terminal alkyne?

Accepted Answer

An aldehyde after tautomerization

Question 10

A chemist wants to synthesize a trans-alkene from an alkyne. Which reagents should be used?

Accepted Answer

Na or Li in liquid NH$$_3$$

Question 11

Which property of terminal alkynes allows them to undergo alkylation reactions not possible for internal alkynes?

Accepted Answer

Presence of an acidic C-H bond

Question 12

In click chemistry, what is the key feature that makes the reaction between alkynes and azides useful for biological labeling?

Accepted Answer

Alkynes and azides are unreactive under biological conditions but react together with a catalyst

Question 13

Which of the following is true about the hydrogenation of alkynes using Pd/C?

Accepted Answer

The reaction is not chemoselective and will reduce the alkyne all the way to an alkane

Question 14

Which intermediate is formed during the dissolving metal reduction of alkynes?

Accepted Answer

A vinyl radical

Question 15

What is the pKa of the terminal alkyne C-H bond, and why is this significant in organic synthesis?

Accepted Answer

$$24$$, allowing deprotonation with strong bases like NH$$_2^-$$

Question 16

Which of the following is a correct retrosynthetic step for preparing an alkyne from an alkene?

Accepted Answer

Convert the alkene to an alkyl dihalide, then treat with excess strong base

Question 17

Which statement about tautomerization of vinyl alcohols is correct?

Accepted Answer

It is an equilibrium reaction favoring the carbonyl compound

Question 18

A student performs hydroboration-oxidation on an internal alkyne. What is the expected major product after tautomerization?

Accepted Answer

A ketone

Question 19

Which of the following best describes the difference in hydration reactions between terminal and internal alkynes?

Accepted Answer

Terminal alkynes follow Markovnikov's rule, internal alkynes do not

Ochem Ch 9 Alkyne Reactions

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