Question

Show how you would make 3,3 -dibromohexane from ethyl bromide and acetylene. Assume you have access to any inorganic reagents.

Short answer

Form acetylene from ethyl bromide, add ethyl bromide to acetylene to form 3-bromohex-1-yne, and then brominate the compound to get 3,3-dibromohexane.

Step-by-step solution

Formation of Acetylene from Ethane

To form acetylene from ethane, perform a dehydrohalogenation reaction. Use potassium hydroxide (KOH) in ethanol. This is carried out by adding KOH to ethyl bromide in ethanol, which results in the elimination of HBr and formation of acetylene (C₂H₂).

Addition of Ethyl Bromide to Acetylene

React acetylene with ethyl bromide (CH₃CH₂Br) in the presence of sodium amide (NaNH₂) as a catalyst. The acetylene will undergo nucleophilic substitution to form the intermediate 3-bromohex-1-yne.

Bromination of 3-bromohex-1-yne

Add bromine (Br₂) to the 3-bromohex-1-yne. The addition of bromine will result in bromination at the triple bond, converting it to a dibromo compound, yielding 3,3-dibromohexane.

Key concepts

dehydrohalogenation

Dehydrohalogenation is a chemical reaction that involves the removal of a hydrogen halide (such as HBr) from an organic molecule. In this context, it is the first step in converting ethyl bromide (C₂H₅Br) to acetylene (C₂H₂).

• This process typically uses a strong base, such as potassium hydroxide (KOH), in a solvent like ethanol.
• The reaction works by the base abstracting a hydrogen atom from the molecule, while the leaving group—bromine in this case—is expelled.
• The end result is the formation of a carbon-carbon triple bond, producing acetylene (C₂H₂).

It's important to understand that KOH not only helps in removing the hydrogen atom but also stabilizes the reaction environment, ensuring better yields of acetylene. This reaction is fundamental in organic chemistry because it sets the stage for subsequent reactions in complex molecule synthesis.

nucleophilic substitution

Nucleophilic substitution plays a crucial role in forming 3-bromohex-1-yne. Here, acetylene reacts with ethyl bromide in the presence of sodium amide (NaNH₂).

• Acetylene acts as a nucleophile, a species that donates an electron pair to form a chemical bond.
• Ethyl bromide, on the other hand, is the electrophile, a molecule that accepts the electron pair.

In this reaction, sodium amide (NaNH₂) serves as both a base and a catalyst, promoting the interaction between acetylene and ethyl bromide. The acetylene molecule donates its electron pair to carbon bonded to bromine, resulting in the displacement of bromine. This substitution yields an intermediate compound known as 3-bromohex-1-yne. This intermediate is essential for the final bromination step.

bromination

Bromination is the final step to synthesize 3,3-dibromohexane from 3-bromohex-1-yne. In this step, bromine (Br₂) is added to the triple bond of 3-bromohex-1-yne.

• When 3-bromohex-1-yne reacts with bromine, the bromine molecules attack the triple bond.
• This reaction converts the triple bond into a double bond initially and then into single bonds, where each carbon atom gets a bromine atom attached to it.

The bromination process is quite effective and leads to the formation of 3,3-dibromohexane as the final product. This transformation is crucial because the addition of bromine across the triple bond effectively saturates it, resulting in a stable dibromo compound. This method is widely used in organic chemistry for the synthesis of various brominated compounds.