Question

The following functional group interchange is a useful synthesis of aldehydes.

(a)What reagents were used in this chapter for this transformation? Give an example to illustrate this method.

(b)This functional group interchange can also be accomplished using the following sequence.

Propose mechanisms from these steps.

(c) Explain why a nucleophilic reagent such as ethoxide adds to an alkyne more easily than it adds to an alkene.

Short answer

(a)

(b)

(c)

Step-by-step solution

Synthesis of aldehydes

Aldehydes can be generated using numerous methods.Alcohols get subjected to oxidation to generate an aldehyde. The ozonolysis of alkenes followed by thecombination of the obtained products with zinc dust and water generates an alcohol.

Conversion of alkynes to aldehydes

Aldehydes are created by the Anti Markonikov addition of a hydroxyl group to an alkyne. A tautomerization reaction happens due to the incorporation of the hydroxyl group to an alkyne generating a carbonyl compound.

Step 3:Transformation of alkynes to aldehydes

(a)Hydroboration of a terminal alkyne with disiamylborane (Sia₂ BH)followed by the oxidation with hydrogen peroxide in the presence of a base results in an aldehyde. The reaction can be given as:

(b)The mechanism of the reaction can be given as:

The ethoxide ion strikes the electrophilic carbon atom of the alkyne group and generates a carbanion. The base abstracts a proton forming a trans-alkene. The double bond combines with an acid to form a carbocation. The water molecule functions as a nucleophile and strikes the carbocation to form an intermediate.

Another water molecule then abstracts a proton forming a hemiacetal. Protonation of the ether oxygen results in a good leaving group. The leaving group (ethanol) expels and leads to a resonance stabilized intermediate. The desired product is generated by the loss of hydroxyl proton.

The reaction can be given as:

(c)The closer the electrons are to the nucleus, the more stable is the molecule. An s orbital is near to the nucleus than the p orbital as the p orbitals are situated away from the nucleus. An sp²carbanion is more stable than the carbanion. The sp²carbanion has 33% s character and the electron pair is near to the positive nucleus than in the sp³ carbanion which has 25% s character. The sp²carbanion is easier to form due to its relative stability.

Formation of sp²and sp³carbanion