Alkynes

Introduction

alkynes 1

Alkynes are compounds which have got a triple bond, and the Carbons adjacent to the triple bond would thus be sp hybridised, meaning that these would have an s orbital and a p orbital hybridised together with two p orbitals left unhybridised. The chemistry is similar to that of alkenes and they react more or less the same, but some reactions can be stopped so that only one addition is allowed so that an alkene would be formed.

Preparation

Alkynes can be prepared from both halogenoalkanes and alcohols, keeping in mind that due to the fact that a triple bond is needed these must be dihalogenoalkanes and diols. Due to the fact that diols are normally unstable and perform dehydration to produce the carbonyl group preparation of alkenes would be from dihalogenoalkanes.

alkynes 2

This method can produce any alkyne, but the starting product might be difficult to acquire and thus this would produce an expensive product.

Formation of ethyne

Ethyne can be formed by elimination reactions such those used in the formation of an alkyne, but it can be produced by the hydrolysis of Calcium Carbide (CaC2)

alkynes 3

Hydration

The hydration of an alkyne always gives a ketone, except if the alkyne is ethyne where the final product would be an aldehyde. The reaction is shown below:

alkynes 4

where there is an addition of water as a normal addition on a π bond and then there is enol tautomerisation. Enol tautomerisation is a structural isomeration from where an alkene with an alcohol group attached to an α Carbon would produce a ketone due to the fact that ketones are normally more stable. This process is acid hydrolysed.

Hydrogenation

Hydrogenation of an alkyne is a controlled process and thus it can be stopped either in the formation of an alkene or else in the formation of an alkane, although it must be noted that this would be misusing alkynes, due to the difficulty of producing alkynes in the first place.

The reaction is the same as for hydrogenation of alkanes, and it must be noted that the conditions used are 150oC with Nickel as a catalyst while the different condition would be the concentration of Hydrogen in the reaction chamber. If the ratio of Hydrogen to alkyne is 1:1 then an alkene would be formed if the ratio is 2:1 or more then the alkane would be formed.

Test for terminal alkynes

Advertisements