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9.4: Preparation of Alkynes: Alkylation Reaction

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Organic Chemistry

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Preparation of Alkynes: Alkylation Reaction

9.4: Preparation of Alkynes: Alkylation Reaction


Alkylation of terminal alkynes with primary alkyl halides in the presence of a strong base like sodium amide is one of the common methods for the synthesis of longer carbon-chain alkynes. For example, treatment of 1-propyne with sodium amide followed by reaction with ethyl bromide yields 2-pentyne.


The reaction takes place in two steps:

1. The first step is the deprotonation of the terminal alkyne by the strong base forming an acetylide ion.


2. The second step is a nucleophilic substitution reaction in which the acetylide ion reacts with the alkyl halide to form a new C–C bond.


Reaction Mechanism —SN2 pathway

Acetylide ions are strong bases and can also function as good nucleophiles. The substitution reaction follows an SN2 pathway where the nucleophilic acetylide ion attacks the electrophilic carbon of the primary alkyl halide from the backside.


Since this is a concerted process, the nucleophilic attack and the departure of the leaving group takes place simultaneously to form the product with an inverted stereochemistry at the reaction center.

The reaction works best with unhindered alkyl halides like methyl halide and other primary halides. With secondary and tertiary alkyl halides, acetylide ions act as strong bases, in which case E2 elimination is the preferred pathway.

Application in organic synthesis

Alkylation of simple terminal alkynes is a useful method to extend the length of the carbon chain. Acetylene, a terminal alkyne, can be converted to a higher internal alkyne by repeated alkylation as shown below:



Alkynes Alkylation Reaction Primary Alkyl Halides Strong Base Sodium Amide Carbon-chain Alkynes 1-propyne Ethyl Bromide Deprotonation Acetylide Ion Nucleophilic Substitution Reaction SN2 Pathway Concerted Process Inverted Stereochemistry Unhindered Alkyl Halides Methyl Halide Primary Halides Secondary Alkyl Halides Tertiary Alkyl Halides E2 Elimination Organic Synthesis

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