$$\mathrm{cis\text{-}2\text{-}butene}$$

Question 1

Which of the following statements best explains why terminal alkynes are more acidic than alkenes and alkanes?

Accepted Answer

The sp-hybridized carbon in alkynes has 50% s-character, pulling electron density closer and stabilizing the negative charge on the conjugate base.

Question 2

Which product is formed when 2-butyne is partially hydrogenated using Lindlar’s catalyst?

Accepted Answer

$$\mathrm{cis	ext{-}2	ext{-}butene}$$

Question 3

Which of the following best describes the geometry around the carbon atoms in an alkyne?

Accepted Answer

Linear, with bond angles of $$180^\circ$$

Question 4

A chemist wants to synthesize a terminal alkyne from a vicinal dibromide. Which sequence of reagents would be most appropriate?

Accepted Answer

Br$$_2 in $$CCl$$_4$$, then 2 equivalents NaNH$$_2$$

Question 5

Which of the following statements about the reduction of alkynes with sodium in liquid ammonia is correct?

Accepted Answer

It produces a trans-alkene via anti addition of hydrogen.

Question 6

Which of the following is a correct statement about the stability of alkynes?

Accepted Answer

Internal alkynes are more stable than terminal alkynes due to hyperconjugation.

Question 7

What is the major product when 1-hexyne undergoes hydroboration-oxidation?

Accepted Answer

$$\mathrm{hexanal}$$

Question 8

Which of the following best describes the mechanism of halogenation of alkynes?

Accepted Answer

Anti addition via a cyclic bromonium ion intermediate

Question 9

A terminal alkyne reacts with $$\mathrm{CH_3MgBr}. $$What is the expected product?

Accepted Answer

An alkynyl magnesium bromide and $$\mathrm{CH_3H}$$

Question 10

Which of the following is true regarding the Markovnikov hydration of terminal alkynes?

Accepted Answer

It produces an enol intermediate that tautomerizes to a ketone.

Question 11

Which of the following best explains why alkynes do not have cis-trans isomers?

Accepted Answer

The triple bond is linear and does not allow for restricted rotation.

Question 12

Which of the following sequences would best convert 1,2-dibromopropane to propyne?

Accepted Answer

2 equivalents of NaNH$$_2$$

Question 13

What is the expected product when 1-butyne is treated with excess HBr?

Accepted Answer

$$\mathrm{2,2	ext{-}dibromobutane}$$

Question 14

Which of the following best describes the product of complete hydrogenation of 2-pentyne?

Accepted Answer

$$\mathrm{n	ext{-}pentane}$$

Question 15

Which of the following statements about the heat of hydrogenation of alkynes is correct?

Accepted Answer

The heat of hydrogenation of an alkyne is greater than the sum of two alkene double bonds due to electron repulsion.

Question 16

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

Accepted Answer

An aldehyde via anti-Markovnikov addition

Question 17

Which of the following is a correct statement about the mechanism of Na/NH$$_3$$ reduction of alkynes?

Accepted Answer

The reaction proceeds via two one-electron transfers and two protonations, resulting in a trans-alkene.

Question 18

Which of the following is the correct order of acidity (most to least acidic) for the following compounds: ethyne, ethene, ethane?

Accepted Answer

$$\mathrm{ethyne \u003e ethene \u003e ethane}$$

Question 19

Which of the following best explains why internal alkynes are more stable than terminal alkynes?

Accepted Answer

Internal alkynes benefit from hyperconjugation from adjacent alkyl groups.

Alkynes: Structure, Properties, Preparation, and Reactions

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