Alkynes - Definition, Structure, Preparation, Properties

Alkynes are unsaturated hydrocarbons containing at least one carbon–carbon triple bond (C≡C). They have the general formula C_nH_2n-2. In alkynes, the carbon atoms involved in the triple bond are sp-hybridised and exhibit linear geometry with a bond angle of 180°. Due to the presence of multiple bonds, alkynes are more reactive than alkanes and undergo mainly addition reactions.

Nomenclature of Alkynes

Alkynes are named according to the IUPAC system by considering the presence of a carbon–carbon triple bond in the parent chain.

1. Choose the longest carbon chain containing the triple bond.
2. Number the chain from the end nearest to the C≡C bond to give it the lowest number.
3. Replace the suffix “-ane” with “-yne”.
4. Indicate the position of the triple bond by the number of the carbon atom where it begins.
5. Name and number substituents as prefixes along with their positions.
6. For more than one triple bond, use suffixes like -diyne, -triyne and mention their positions.
7. If both double and triple bonds are present, give priority to the double bond in numbering if there is a tie.

Isomerism in Alkynes

Alkynes show structural isomerism due to different arrangements of carbon atoms and position of the triple bond.

• Chain Isomerism: Chain isomerism occurs in alkynes with five or more carbon atoms owing to various carbon chain configurations.
• Position isomerism: In this case, the isomers differ in terms of the triple bond's location. The two isomers of butyne:

Physical Properties of Alkynes

Alkynes are non-polar hydrocarbons, so their physical properties are mainly governed by weak intermolecular forces.

• Physical state: Lower alkynes such as ethyne and propyne are gases. As the number of carbon atoms increases, they become liquids and then solids. This happens because intermolecular forces increase with size.
• Solubility: Alkynes are insoluble in water because they are non-polar, while water is polar. However, they dissolve readily in organic solvents like ether, benzene, and carbon tetrachloride.
• Boiling and melting points: Boiling and melting points increase with increase in molecular mass due to stronger van der Waals forces. Also, for isomers, straight-chain alkynes generally have higher boiling points than branched ones because of better surface area for intermolecular attraction.
• Density: Alkynes are lighter than water but their density increases with increase in molecular mass.
• Odour and colour: Alkynes are generally colourless and have a slightly characteristic odour.

Chemical Properties of Alkynes

Alkynes are reactive due to the presence of a carbon–carbon triple bond (two π bonds). They mainly undergo addition reactions.

1. Addition of Hydrogen (Hydrogenation)

Alkynes add hydrogen in the presence of catalysts (Ni, Pd, Pt) to form alkenes and then alkanes. Partial hydrogenation can give alkenes.

2. Addition of Halogens (Halogenation)

Alkynes react with halogens (Cl₂, Br₂) to form dihaloalkenes and then tetra haloalkanes.

3. Addition of Hydrogen Halides (Hydrohalogenation)

Addition of HX (HCl, HBr, HI) follows Markovnikov Rule, where hydrogen attaches to the carbon with more hydrogens.

4. Addition of Water (Hydration)

In the presence of acid and catalyst (HgSO₄), alkynes form enols which rearrange to carbonyl compounds (aldehydes/ketones).

5. Oxidation

Alkynes are oxidised by KMnO₄ or ozone leading to formation of diketones or carboxylic acids depending on conditions.

6. Acidic Character of Alkynes

Terminal alkynes (–C≡CH) are weakly acidic and can form salts (acetylides) with strong bases or metals like sodium.

Preparation of Alkynes

Alkynes are generally prepared by elimination reactions of dihalides or from simple inorganic compounds.

1. From vicinal dihalides (double dehydrohalogenation)

Vicinal dihalides have halogen atoms on adjacent carbon atoms. On heating with alcoholic KOH or strong bases, two molecules of HX are eliminated in two steps, first forming an alkene and then an alkyne.

2. From geminal dihalides

Geminal dihalides have both halogens on the same carbon atom. They also undergo double dehydrohalogenation to form alkynes.

3. Preparation of ethyne from calcium carbide

Ethyne is prepared by reacting calcium carbide with water.

4. From terminal alkynes (formation of higher alkynes)

Terminal alkynes (–C≡CH) are slightly acidic. They react with strong bases like sodium amide (NaNH₂) to form acetylide ions, which can further react with alkyl halides to form higher alkynes. This method is useful for chain extension (increasing carbon length)

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