Question 1

Which statement best explains why alkynes generally have higher boiling points than their corresponding alkanes and alkenes?

Accepted Answer

The linear geometry of alkynes allows for stronger intermolecular interactions.

Question 2

How many degrees of unsaturation does a molecule of 1-hexyne ($$\mathrm{C_6H_{10}}) $$possess?

Accepted Answer

$$2$$

Question 3

Which of the following best describes the molecular orbital arrangement in alkynes?

Accepted Answer

One sigma bond and two orthogonal pi bonds, with the pi bonds perpendicular to each other.

Question 4

Which reagent and conditions would you use to selectively reduce an alkyne to a cis-alkene?

Accepted Answer

$$\mathrm{H_2$}$$, Lindlar catalyst$

Question 5

Why are terminal alkynes more acidic than alkenes and alkanes?

Accepted Answer

The sp-hybridized carbon in alkynes has more s character, stabilizing the conjugate base.

Question 6

When naming a molecule that contains both an alkene and an alkyne (an enyne), which functional group gets priority in numbering?

Accepted Answer

The lowest overall numbers for both the alkene and alkyne are prioritized.

Question 7

Which of the following statements about the stability of alkenes and alkynes is correct?

Accepted Answer

Alkynes are generally less stable than alkenes.

Question 8

A student wants to synthesize an internal unsymmetrical alkyne using acetylene and two different alkyl halides. Which sequence of steps is correct?

Accepted Answer

React acetylene with one equivalent of base and one alkyl halide, then repeat with a different alkyl halide.

Question 9

Which of the following best explains why alkynes do not have cis/trans (E/Z) designations?

Accepted Answer

Alkynes are linear due to their $$\mathrm{sp}$$ hybridization, so substituents cannot be on the same or opposite sides.

Question 10

Which of the following is a correct application of the Index of Hydrogen Deficiency (IHD) formula for $$\mathrm{C_5H_6}$$?

Accepted Answer

$$\mathrm{IHD} = \frac{5 - 6/2 + 0/2 + 1}{1} = 3$$

Question 11

Which of the following reactions will produce a trans-alkene from an alkyne?

Accepted Answer

Reduction with $$\mathrm{Na} in $$liquid $$\mathrm{NH_3$}$$

Question 12

Which statement best describes the nucleophilicity of alkynes compared to alkenes?

Accepted Answer

Alkynes are more nucleophilic because their orthogonal pi bonds form a cylinder of electron density.

Question 13

A chemist wants to synthesize 2-butyne from acetylene. Which sequence of reagents should be used?

Accepted Answer

1. $$\mathrm{$$NaNH_2$$}$$, 2. ethyl bromide, 3. $$\mathrm{$$NaNH_2$$}$$, 4. methyl bromide

Question 14

Which of the following is a common mistake when predicting the product of an acetylide alkylation?

Accepted Answer

Failing to draw a new line to represent the new C-C bond formed.

Question 15

Which catalyst is considered 'poisoned' and is used to stop the hydrogenation of alkynes at the alkene stage?

Accepted Answer

Lindlar catalyst (Pd, CaCO$$_3$$, Pb(OAc)$$_2)$$

Question 16

Why are trans-alkenes more stable than cis-alkenes?

Accepted Answer

Trans-alkenes have less steric hindrance between substituents.

Question 17

Which of the following is true about the bond length in alkynes compared to alkanes and alkenes?

Accepted Answer

Alkynes have shorter C-C bonds due to increased s character.

Question 18

Which of the following is a real-world application of alkynes in medicinal chemistry?

Accepted Answer

Alkynes are used as anti-cancer agents due to their unique orbital shape.

Alkynes: Electrophilic Addition, Redox Reactions, and Properties (CHEM-201 Lecture 14)

Study Guide - Practice Questions