Conversion Reactions (I) Problem 3

Description:

Problem

What reactions will bring about the following conversions?

1. Ethanol to but − 1 − yne

2. Benzene to biphenyl

3. Bromomethane to propanone

Solution

1. Ethanol to but − 1 − yne

• Write the structure of the reactant and product.

  CH₃CH₂OH → CH₃CH₂C ≡ CH

• Look at the difference in the number of carbon atoms in the reactants and products.

• Product has double the number of carbon atoms as compared to starting material: Wurtz Reaction.

• For wurtz reaction, starting reactant should be an alkyl halide.

• Ethanol is converted to ethyl iodide by reacting it with red P/I₂; 2 molecules of alkyl iodide reacts with Na/ether to give butane which on reduction gives but − 1 − yne.

2. Benzene to biphenyl

• The product has double the number of carbon atoms as compared to the reactant.

• Fittig Reaction (aromatic systems).

• Benzene is converted to bromobenzene and 2 molecules of bromobenzene react with Na/ether to give biphenyl.

3. Bromomethane to propanone

CH₃Br → CH₃COCH₃

• Reagent: KCN is employed whenever we need to add 1 carbon atom to the product.

• Methyl bromide is converted to methyl cyanide which on reacting with methyl magnesium bromide (grignard reagent), gives an intermediate which on hydrolysis gives acetone as the final product.

• Here, CH₃⁻ acts as a nucleophile and attacks the methyl cyanide.