Question

Describe how the following compounds can be prepared from compounds containing no more than six carbons. (You can also use triphenylphosphine.)

Short answer

The formation of compound (a) is done by reacting 2-bromo-3-methylbutane (alkyl halide) with triphenylphosphine (C₆H₅)₃ P by S_N2 mechanism. Itresults in the formation of alkyl phosphonium salt, which on deprotonation with strong base n-butyllithium,forms ylide(Wittig reagent).

Wittig reagent is formed upon reacting with cyclohexanone (carbonyl compound) formed the desired product (3-methylbutan-2-ylidene)cyclohexane.

The formation of (3-methylbutan-2-ylidene) cyclohexane is shown below.

(3-methylbutan-2-ylidene)cyclohexane formation

1. 
2. 
3. 
4. 

Step-by-step solution

Preparation of (3-methylbutan-2-ylidene) cyclohexane

The formation of compound (a) is done by reacting 2-bromo-3-methylbutane (alkyl halide) with triphenylphosphine (C₆H₅)₃ P by S_N2 mechanism. Itresults in the formation of alkyl phosphonium salt, which on deprotonation with strong base n-butyllithium,forms ylide(Wittig reagent).

Wittig reagent is formed upon reacting with cyclohexanone (carbonyl compound) formed the desired product (3-methylbutan-2-ylidene)cyclohexane.

The formation of (3-methylbutan-2-ylidene) cyclohexane is shown below.

(3-methylbutan-2-ylidene)cyclohexane formation

Preparation of octa-3-yne

The formation of compound (b) is done by reacting 1-hexyne (terminal alkyne) with the strong base (NH₂). It results in the formation of alkynide, which on reaction with ethyl bromide (alkyl halide) through nucleophilic substitution reaction,forms the desired product octa-3-yne.

The formation of octa-3-yneis shown below.

Octa-3-yne formation

Preparation of (E)-2-ethylhex-2-enal

The formation of the compound (d) is done by reacting two moles of butanal (aldehyde having alpha-hydrogen)with nucleophile hydroxide ion (OH^-), which forms beta-hydroxy aldehyde2-ethyl-3-hydroxyhexanal (aldol)through aldol condensation.

Beta-hydroxy aldehyde upon dehydration,removes the water molecule from the β-hydroxy aldehyde to form an α,β-unsaturated aldehyde, thusforming the desired product, (E)-2-ethylhex-2-enal.

The formation of (E)-2-ethylhex-2-enal is shown below.

Formation of (E)-2-ethylhex-2-enal

Preparation of (2R,3S)-2butyl-3-propyloxirane

Formation of compound (d) is done by hydrogenation of octa-3-yne with the help of lindlar catalyst. Itresults in the formation of cis alkene (Z)-non-4-ene, which reacts with reagent meta-chloroperbenzoic acid(MCPBA), and as a result, the stereochemistry of the product is retained. This leads to formation of (2R,3S)-2butyl-3-propyloxirane (cyclic ether).

The formation of (2R,3S)-2butyl-3-propyloxirane is shown below.

Formation of (2R,3S)-2butyl-3-propyloxirane