Question

How does a terminal alkyne react in a Grignard reaction, will the carbonyl carbon attack the terminal carbon?

Answer 1

Hint: As we know that the Grignard reagent is an organometallic compound where in aldehydes or ketone, alkyl, allyl, vinyl or aryl magnesium halides is added to the carbonyl group. But it does not react with all the alkynes because not all the alkynes contain an acidic hydrogen.

Complete step-by-step answer:As we know that the Grignard reagent reacts only with terminal alkynes because the terminal alkynes contains an acidic hydrogen which would be removed by the Grignard reagent’s alkyl group resulting in the formation different compounds. So, we can say that the terminal alkynes are less stable than the internal alkynes because in internal alkynes the carbon bond is connected with more than one carbon thereby providing the compound a more stability than terminal ones. Let us taken an example of terminal alkyne like acetylene where acetylene is reacting with the Grignard reagent in the presence of anhydrous $E{t_2}O$ which can be shown as below:
$HC \equiv CH + C{H_3}MgI\xrightarrow{{anhy.E{t_2}O}}HC \equiv C - MgI + C{H_4}$
Here, the carbonyl carbon of the Grignard reagent actually attacked the terminal carbon and extracted one hydrogen from there thereby resulting in the product formation. We can show this as below:
$HC \equiv C - H + C{H_3} - MgI \to HC \equiv C - MgI + C{H_4}$

Additional information: Grignard reactions are generally reversible when involved with allylic reagents and the Grignard reagent generally reacts with the carbonyl group of aldehyde or ketone and result in the formation of alcohols.

Note: Always remember that the Grignard reagent is one of the strongest nucleophiles but it is also the strongest base and thus it is considered as essentially an anionic alkane. Also, the terminal alkynes are generally less reactive due to the compact carbon-carbon electron cloud and thus with the Grignard reagent it undergoes an acid-base reaction. Grignard reagent is also used for the synthesis of carbon-carbon double bonds.