Addition Reaction of Alkynes: Hydrogenation, Halogenation, Hydration Reaction

Edited By Team Careers360 | Updated on Feb 03, 2024 01:00 PM IST

Alkynes are organic molecules and also an unsaturated hydrocarbon which contains at least one carbon-carbon triple bond and the triple bond is also known as an “acetylic bond”. The old name of alkynes is “acetones”. Alkynes have very similar reactivity as alkenes and alkynes undergo additional reactions like hydrogenation and hydrohalogenation. Alkynes are hydrophobic similar to the other hydrocarbons. The triple bond between the carbons is very strong with a bond strength of 839 KJ/mol. In this one sigma and two pi bonds are present so a total of three bonds. Here one sigma bond is formed by the overlap of sp orbital from each atom. And the two pi bonds are formed by the overlap of the p orbital on each atom. This results in the formation of an alkyne group.

This Story also Contains

Hydrogenation reaction (Addition of Alkynes with Hydrogens)
Halogenation reaction (Addition of Alkynes with Halogens)
Hydration reaction (Addition of Alkynes with Water)
Hydrohalogenation reaction (Addition of Alkynes with Hydrogen Hallides)

Hydrogenation reaction (Addition of Alkynes with Hydrogens)

Alkynes reaction with hydrogen is also known as “hydrogenation”. Here alkynes react with di hydrogen in the presence of Platinum or Palladium or Nickel catalysts to form alkenes. The alkenes formed after this reaction further react with di hydrogen and form alkanes. In this process at first, the triple bond in alkynes is converted to a double bond and then it is converted to single bond with the formation of alkanes as the end product in this reaction. Here the complete reaction is very smooth and slowing down of the reaction in the intermediate state is not possible. More alkenes can be produced by the use of poisoned catalysts like Lindlar catalyst. It consists of Palladium coated with quinoline and absorbed on calcium carbonate. This process of hydrogenation can also be done using sodium in liquid ammonium at very low temperatures. This reaction is a chemical reaction but not a catalytic reaction and hydrogens reach the atoms in different directions. Finally, this leads to the formation of trans-alkenes.

$CH_3 -C \equiv \ CH + H_2 \overset{Pt/Pd/Ni}{\rightarrow} CH_3-CH=CH_2\overset{H_2}{\rightarrow} CH_3-CH_2-CH_3$

(Propyne) (Propene) (Propane)

Halogenation reaction (Addition of Alkynes with Halogens)

Alkynes reaction with halogens is also known as “halogenation”. Here alkynes react with halogens like Chlorine, Fluorine and Bromine to form halogenated alkenes and this further reacts with halogens and form substituted alkanes. As a result of this reaction, the reddish orange coloured solution of carbon tetrachloride and bromine gets decolourized. This reaction can be stopped in the intermediary stage by lowering the temperatures to below 0 degrees centigrade. The end product of this reaction is tetra haloalkane. This reaction of halogens with the alkynes is done in a stepwise fashion and leads to the formation of alkanes. Thai reaction can also be used as a test for unsaturation.

$\\CH_3 -C \equiv \ CH + Br-Br\rightarrow CH_3-CBr=CHBr\overset{Br_2}{\rightarrow} CH_3-CBr_2-CHBr_2$

(Propyne) (1,2-Dibromopropene) (1,1,2,2 Tetrabromopropane)

Hydration reaction (Addition of Alkynes with Water)

Alkynes reaction with water is also known as “hydration”. Alkynes do not react with water under general conditions. These react with water only in the presence of sulphuric acid and mercuric sulphate. This reaction is processed at a temperature of 333K(Kelvins). The end products of these reactions are carbonyl compounds. The reaction of water with terminal alkynes leads to the formation of aldehydes and when water reacts with non-terminal alkynes results in the formation of ketones. These two products are formed by the rearrangement of vinyl alcohol which is also known as “enol”. This reaction of alkynes with water can be done in different steps with the addition of other components in the intermediary stage of the reaction.

$\\\mathrm{CH}_3-\mathrm{C} \equiv \mathrm{CH}+\mathrm{H}-\mathrm{OH} \frac{\mathrm{Hg}^{2+} / \mathrm{H}^{+}}{333 \mathrm{~K}} CH_3-COH=CH_2\overset{Isomerisation}{\rightarrow}CH_3COCH_3$

(Propyne) (Propanone)

Hydrohalogenation reaction (Addition of Alkynes with Hydrogen Hallides)

Gem halides are produced when alkynes (compounds with three bonds) are combined with hydrogen halide. Gem halides are molecules in which two halogen atoms are joined to the same carbon atom, creating these substances.

$H-C\equiv \ C-H + H-Br \rightarrow CH_2=CH-Br \rightarrow CHBr_2 - CH_3$

(Ethyne) (Hydrogen Bromide) (1,1 - Dibromoethane)

Frequently Asked Questions (FAQs)

1. What are the addition reactions and elimination reactions?

Addition reaction mainly occurs when two compounds are combined to form a single end product. In these reactions, unsaturated compounds are converted into saturated compounds by a series of steps. In elimination reactions, one product is broken down into two different products. In these reactions, unsaturated compounds are converted into saturated compounds.

2. What are the uses of an additional reaction?

Addition reaction is used in various applications like in the synthesis of all types of natural products and drugs. These are also used in industrial applications and also in the preparation of other materials mainly in the field of medicine and nature.

3. Why do alkynes show these addition reactions?

Alkynes show these reactions because they consist of loosely held pi bonds. There are two pi bonds between the two carbon atoms which are loosely coupled. Therefore alkynes react with water, hydrogen and halogens due to the presence of a triple bond in it.

4. What is the difference between alkenes and alkynes?

Alkynes have one or more triple bonds but the alkenes have one or more double bonds. Alkynes show an addition reaction due to the presence of unsaturated bonds. Alkynes support various properties over alkenes such as polymerisation, oligomerization and also semiconducting properties and these are better reactors when compared to alkenes.

5. What is the reaction of alkyne with hydrogen halide?

In this reaction, hydrogen halides are reacted with the alkyne and form gem halides .Gem halides are defined as the halides in which the two halogens are attached to the same carbon atom.