Chapter 17, Problem 17.21P

Interpretation Introduction

Interpretation:

The synthesis of $\text{oct-4-ene}$ using butanal as the only source of carbon is to be proposed.

Concept introduction:

Retrosynthetic analysis involves considering the reaction sequence in reverse. The analysis starts with the product, determining the precursor that can produce it when the reaction is carried out in the forward direction. The retrosynthesis step, called a transform, does not consider a specific reaction or reaction conditions for the reaction in the forward direction. It is a proposed undoing of a single reaction and dissects the target into a simpler precursor. This process is continued till an easily available or easily synthesized precursor is reached.

The synthesis in the forward direction is then determined on the basis of the structural differences and the functional groups to be incorporated or modified. The specific reaction and reaction conditions are determined at this stage.

Aldehydes can be reduced to the corresponding primary alcohols by ${\text{NaBH}}_{\text{4}}$. Alcohols can be converted to the corresponding alkyl bromides with ${\text{PBr}}_{\text{3}}$. This has the advantage of converting a poor leaving group, OH, to a good leaving group, Br.

Alkyl halides can be converted to Wittig reagents (ylides) by treating them with triphenylphosphine $\text{P}{\left({\text{C}}_{\text{6}}{\text{H}}_{\text{5}}\right)}_{\text{3}}$ in an ${\text{S}}_{\text{N}}\text{2}$ reaction, followed by a strong base like butyllithium.

Wittig reagents are good nucleophiles and can add to the carbonyl carbon of aldehydes and ketones. The reaction of a Wittig reagent with aldehyde leads to the formation of an alkene.