Addition Reactions of Alkynes

Expert reviewed 23 November 2024 4 minute read


Introduction

Alkynes are unsaturated hydrocarbons containing a carbon-carbon triple bond. Their unique electronic structure makes them particularly reactive, allowing them to undergo various addition reactions. These molecules can participate in two consecutive addition reactions due to the presence of two π-bonds, making them more reactive than alkenes.

Understanding Triple Bonds

The triple bond in alkynes consists of one σ-bond and two π-bonds. This electronic configuration makes alkynes excellent candidates for addition reactions, where both π-bonds can be broken sequentially to form new single bonds.

Types of Addition Reactions

1. Hydrohalogenation (Addition of HX)

When alkynes react with hydrogen halides (HX), they undergo a two-step addition process:

  • First addition: RCCH+HXRCX=CH2\text{RC}\equiv\text{CH} + \text{HX} \rightarrow \text{RCX=CH}_2

  • Second addition: RCX=CH2+HXRCX2CH3\text{RCX=CH}_2 + \text{HX} \rightarrow \text{RCX}_2\text{CH}_3

The addition follows Markovnikov's rule, where the halogen (X) bonds to the carbon with more alkyl substituents.

2. Halogenation (Addition of X₂)

Halogenation involves the addition of halogens (X₂) to alkynes, producing tetrahalogenated alkanes:

RCCH+2X2RCX2CHX2\text{RC}\equiv\text{CH} + 2\text{X}_2 \rightarrow \text{RCX}_2\text{CHX}_2

This reaction proceeds through a vicinal dihalogenated alkene intermediate.

3. Hydrogenation (Addition of H₂)

Hydrogenation of alkynes occurs in two steps using a palladium catalyst (Pd/C):

  • Formation of alkene: RCCH+H2Pd/CRCH=CH2\text{RC}\equiv\text{CH} + \text{H}_2 \stackrel{\text{Pd/C}}{\longrightarrow} \text{RCH=CH}_2

  • Formation of alkane: RCH=CH2+H2Pd/CRCH2CH3\text{RCH=CH}_2 + \text{H}_2 \stackrel{\text{Pd/C}}{\longrightarrow} \text{RCH}_2\text{CH}_3

4. Hydration (Addition of H₂O)

Hydration of alkynes yields carbonyl compounds (aldehydes or ketones) rather than alcohols:

RCCH+H2OH+,Hg2+RC(O)CH3\text{RC}\equiv\text{CH} + \text{H}_2\text{O} \stackrel{\text{H}^+, \text{Hg}^{2+}}{\longrightarrow} \text{RC(O)CH}_3

This reaction follows Markovnikov's rule and requires a mercury(II) catalyst.

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