Understanding the Reaction: CH₂=C=O to C₄H₈O

This reaction involves ketene (CH₂=C=O) undergoing a two-step process to form a compound with the molecular formula C₄H₈O. Let's break it down step by step.

Step 1: Reaction with Bromine (Br₂)

Ketene has a cumulated diene system. The first step is electrophilic addition of bromine (Br₂) across the C=C bond. This results in the formation of an alpha-bromo acyl bromide.

The reaction is: $CH 2 = C = O + {Br}_{2} \to BrCH 2 - C (= O) - Br$
The product is Bromoacetyl bromide.

Step 2: Reaction with 2 equivalents of CH₃MgBr (Grignard Reagent)

The bromoacetyl bromide from Step 1 has two highly electrophilic centers: the carbonyl carbon (C=O) and the carbon attached to bromine (C-Br). A Grignard reagent (CH₃MgBr) is a strong nucleophile and will attack both of these sites.

First equivalent: The Grignard reagent attacks the carbonyl carbon, leading to nucleophilic addition. This forms a bromomagnesium alkoxide intermediate. $BrCH 2 - C (= O) - Br + {CH}_{3} MgBr \to BrCH 2 - C ({CH}_{3}) (OMgBr) - Br$

Second equivalent: The second equivalent of CH₃MgBr performs a nucleophilic substitution on the carbon-bromine (C-Br) bond. This is possible because the Grignard reagent can act as a source of a methyl carbanion (CH₃^-), displacing bromide ion (Br^-) and forming a new C-C bond. The final product after aqueous workup is a ketone, 2-butanone (CH₃COCH₂CH₃), which has the molecular formula C₄H₈O. $BrCH 2 - C ({CH}_{3}) (OMgBr) - Br + {CH}_{3} MgBr \to {CH}_{3} CH 2 - C (= O) - {CH}_{3}$

Final Answer

The final product is 2-butanone (Methyl ethyl ketone). Its structure is: ${CH}_{3} - C (= O) - {CH}_{2} - {CH}_{3}$
This corresponds to the third image option.

Related Topics & Formulae

1. Properties of Ketene: Ketene (CH₂=C=O) is a highly reactive molecule due to its strained cumulated double bonds. It readily undergoes addition reactions with electrophiles like halogens (X₂).

2. Grignard Reactions: Grignard reagents (R-MgX) are organometallic compounds that are strong nucleophiles. They are commonly used for:

Nucleophilic addition to carbonyl groups (aldehydes, ketones, esters) to form alcohols.
Nucleophilic substitution on alkyl halides (especially for primary halides).

General formula for formation:

R - X + Mg \to R - Mg - X

3. Nucleophilic Substitution: A fundamental reaction where a nucleophile replaces a leaving group in a molecule. The mechanism can be S_N1 (unimolecular) or S_N2 (bimolecular). In this case, the Grignard reagent facilitates an S_N2-type displacement on the alkyl bromide.