Reactions Of Acetylide Ions: Mechanisms And Predicting The Products

Acetylide ion is created by the deprotonation of terminal acetylene (triple bond at the end of the chain) by a very strong base such as sodium amide NaNH2.

How to make acetylide ions
How to make acetylide ions

Once an acetylide ion is created, it can act as a nucleophile and react with primary or methyl alkyl halides (SN2) as well as aldehydes and ketones.

Let's take a more in depth look.


Acetylide ion with primary or methyl alkyl halides

acetylide ion reaction with alkyl halide
acetylide ion reaction with alkyl halide

Acetylide ion with Aldehydes and Ketones.

acetylide ion reaction with ketone or aldehyde
acetylide ion reaction with ketone or aldehyde

The carbon that has the negative charge at the end of triple bond attacks C=O pushing electrons up to the oxygen in the first step. In the second step, O- gets protonated by H3O+ to become OH (alcohol).


Mayya's trick to predict the product: turn =O into OH and attach negatively charged carbon from the triple bond to the C with OH (don't forget to draw the R groups attached to the carbon with OH).


Oftentimes, a problem will give a terminal alkyne that is reacting with NaNH2 in the first step followed by the reaction with ketone or aldehyde and H3O+ in the last step.

Example:

acetylide ion reaction with PhCHO
acetylide ion reaction with PhCHO

In this example we first deprotonate terminal alkyne ( 3-methyl but-1-yne) using NaNH2. Next, the negatively charged carbon attacks C=O pushing electrons up to the oxygen. Finally, O- is protonated with H3O+ yielding the final product.


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