Question 1

What happens when bromine adds to an alkyne?

Accepted Answer

Bromine undergoes electrophilic addition across the π bonds of alkynes. The first equivalent of bromine forms a trans-dihaloalkene as the major product through anti addition, where both halogen atoms attach on opposite sides. A second equivalent of bromine can add to the resulting alkene, forming a tetrahaloalkane as the final product.

Question 2

How does the halonium ion intermediate form during alkyne halogenation?

Accepted Answer

A π bond from the alkyne acts as a nucleophile and attacks the electrophilic center on the polarized halogen molecule. This displaces the halide ion and forms a cyclic halonium ion intermediate—a three-membered ring with a double bond. The halide ion then attacks from the backside, opening the ring and forming the trans-dihaloalkene product.

Question 3

Why are alkynes less reactive than alkenes toward halogenation?

Accepted Answer

Alkynes are less reactive for two reasons. First, sp-hybridized carbons in triple bonds hold π electrons more tightly than sp2-hybridized carbons in double bonds, making electrons less available for nucleophilic attack. Second, the halonium ion from alkynes is a strained three-membered ring with a 120° bond angle forced into a triangle, making it highly unstable and difficult to form.

Question 4

What is the difference between the first and second halogenation steps in alkynes?

Accepted Answer

The first halogenation adds one equivalent of halogen across a π bond of the alkyne, forming a trans-dihaloalkene through anti addition via a halonium ion intermediate. The second halogenation adds another equivalent across the remaining π bond of the alkene product, also proceeding through a bridged halonium ion to yield the tetrahalide.

Question 5

What bonds are broken and formed during alkyne halogenation?

Accepted Answer

For each mole of halogen added, one π bond is broken and two new σ bonds are formed. In the first addition, the C≡C triple bond is partially broken and two C-halogen σ bonds form, creating a C=C double bond. In the second addition, the remaining C=C π bond breaks and two more C-halogen σ bonds form.

Question 6

What is the stereochemistry of halogenation products from alkynes?

Accepted Answer

Halogenation of alkynes produces trans-dihaloalkenes as the major product through anti addition, where both halogen atoms are positioned on opposite sides of the double bond. The cis isomer forms as a minor product. This stereospecific outcome results from backside attack of the halide ion on the halonium ion intermediate.

Question 7

How does alkyne halogenation compare to other electrophilic addition reactions?

Accepted Answer

Alkyne halogenation follows the same electrophilic addition mechanism as other reactions on multiple bonds, where π electrons attack electrophilic centers. However, alkynes have two π bonds allowing sequential addition of two halogen equivalents, whereas alkenes typically undergo single additions. Related transformations include electrophilic addition to alkynes hydrohalogenation and reduction pathways.

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