9.3 Reactions of Alkynes: Addition of HX and X2

Addition of HX and X₂ to Alkynes

Electrophilic Addition to Alkynes

Alkynes undergo electrophilic addition reactions much like alkenes do, but with a key difference: the triple bond contains two pi bonds, so alkynes can react once to give a substituted alkene or twice to give a fully saturated product. This section focuses on the first addition.

The general process has two steps:

1. Step 1: The electrophile (a proton from HX, or a halogen from $X_2$) adds to one of the triple-bond carbons. This breaks one pi bond and generates a vinylic carbocation intermediate.

   • Regioselectivity is determined by which carbon forms the more stable carbocation (Markovnikov's rule).

2. Step 2: A nucleophile (the halide ion $X^-$) attacks the vinylic carbocation to give the product, a vinyl halide or dihaloalkene.

   • Because the vinylic carbocation is planar at the positively charged carbon, the nucleophile can approach from either face, which affects stereochemistry.

Mechanism of HX Addition (Hydrohalogenation)

The mechanism closely parallels HX addition to alkenes, but the carbocation intermediate is different.

1. The proton ($H^+$) from HX adds to one of the alkyne carbons. Following Markovnikov's rule, it adds to the carbon bearing more hydrogens (the less substituted carbon), so the positive charge ends up on the more substituted carbon.
2. The halide ion ($X^-$) attacks the resulting vinylic carbocation to form a vinyl halide.

Vinylic carbocation stability is worth understanding clearly. A vinylic carbocation has its positive charge on an $sp$-hybridized carbon (before rehybridizing to $sp^2$). The stability order is:

Vinylic carbocations are more stable than primary alkyl carbocations. This extra stability comes partly from the fact that the empty p orbital on the cationic carbon can overlap with the adjacent pi system, providing some stabilization. However, they're still less stable than secondary or tertiary carbocations, which is why these additions are slower than comparable alkene reactions.

Products of HX and X₂ Reactions

HX addition (hydrohalogenation):

• Regiochemistry: Markovnikov addition. The halogen ends up on the more substituted carbon of the original triple bond, and the hydrogen ends up on the less substituted carbon.
• Stereochemistry: The product is a mixture of E and Z isomers. The planar vinylic carbocation allows nucleophilic attack from either face, so you don't get a single stereoisomer.
• Product type: A vinyl halide (haloalkene). With excess HX, a second addition can occur to give a geminal dihalide (both halogens on the same carbon), again following Markovnikov's rule.

$X_2$ addition (halogenation):

• Stereochemistry: Anti addition. The two halogen atoms add to opposite faces of the triple bond. This occurs through a cyclic halonium ion intermediate (similar to alkene halogenation), not through an open carbocation.
• Product type: A vicinal dihalide with the halogens on adjacent carbons and trans (anti) to each other. With excess $X_2$, a second addition can produce a tetrahalide.

Hybridization Changes During Alkyne Reactions

When one equivalent of HX or $X_2$ adds across the triple bond:

• The starting alkyne carbons are $sp$ hybridized (linear geometry, 180° bond angles).
• The product alkene carbons are $sp^2$ hybridized (trigonal planar geometry, ~120° bond angles).

This shift from $sp$ to $sp^2$ means the molecule goes from linear to having a bent shape around those carbons. The remaining pi bond in the product is why the vinyl halide can still undergo a second addition reaction if excess reagent is present.