Chemistry · Ch 13 — Hydrocarbons

Preparation

13.4.3

Preparation

9.4.3 Preparation of Alkynes

Alkynes, particularly ethyne (acetylene), are prepared on both industrial and laboratory scales. The textbook describes two principal methods.

1. From Calcium Carbide (Industrial Method)

This is the most common industrial route for producing ethyne. The process involves three distinct chemical steps, starting from naturally occurring limestone.

Step 1: Obtaining Quick Lime from Limestone

Limestone (calcium carbonate, $CaCO_3$ ) is heated strongly in a lime kiln. This thermal decomposition reaction produces calcium oxide (quick lime) and carbon dioxide gas.

CaCO_3 \xrightarrow{\Delta} CaO + CO_2

Step 2: Preparing Calcium Carbide

The quick lime obtained above is then heated with coke (a form of carbon) in an electric furnace at a very high temperature (around 2000°C). This is a reduction reaction where carbon reduces calcium oxide to calcium carbide, and carbon monoxide is released as a by-product.

CaO + 3C \xrightarrow{\text{high temperature}} CaC_2 + CO

The product, calcium carbide ( $CaC_2$ ), is a greyish-black solid.

Step 3: Producing Ethyne from Calcium Carbide

Calcium carbide reacts vigorously with water. This hydrolysis reaction yields ethyne gas and calcium hydroxide (slaked lime).

CaC_2 + 2H_2O \longrightarrow Ca(OH)_2 + C_2H_2

Watch out

The reaction of calcium carbide with water is highly exothermic. The heat released can cause the ethyne gas to ignite if not properly controlled. In industrial practice, water is added slowly to the carbide, or the reaction is carried out in a controlled atmosphere.

Note

The ethyne produced by this method often contains impurities such as phosphine ( $PH_3$ ) and hydrogen sulphide ( $H_2S$ ), which arise from impurities in the calcium carbide. These impurities give the gas a characteristic unpleasant odour. For many applications, the gas is purified by passing it through an acidified solution of copper sulphate ( $CuSO_4$ ).

2. From Vicinal Dihalides (Laboratory Method)

This method is a two-step dehydrohalogenation process. A vicinal dihalide is a compound that has two halogen atoms (like chlorine or bromine) on adjacent carbon atoms.

Step 1: Formation of an Alkenyl Halide

A vicinal dihalide is treated with an alcoholic solution of potassium hydroxide ( $KOH$ ). This reagent acts as a strong base and causes the elimination of one molecule of hydrogen halide ( $HX$ ). This reaction is called dehydrohalogenation. The product is an alkenyl halide (also known as a vinyl halide), which contains a carbon-carbon double bond and a halogen atom.

For example, starting with 1,2-dibromoethane:

CH_2Br-CH_2Br \xrightarrow[\text{heat}]{\text{alc. KOH}} CH_2=CHBr + HBr

The general reaction can be written as:

R-CHX-CH_2X \xrightarrow[\text{heat}]{\text{alc. KOH}} R-CH=CHX + HX

Step 2: Formation of the Alkyne

The alkenyl halide formed in the first step is then treated with a very strong base, sodamide ( $NaNH_2$ ). This base is strong enough to eliminate a second molecule of hydrogen halide, this time from the alkenyl halide. The result is the formation of a carbon-carbon triple bond, yielding the alkyne.

Continuing the example with bromoethene:

CH_2=CHBr \xrightarrow[\text{heat}]{NaNH_2} HC \equiv CH + HBr

The overall reaction from the vicinal dihalide to the alkyne is:

$R-CHX-CH_2X \xrightarrow[\text{heat}]{\text{1. alc. KOH, 2. NaNH}_2} R-C \equiv CH + 2HX$ …

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