Question

How would you synthesize \((E)\) - and \((Z)\) -3-heptene from acetylene and any other chemicals?

Short answer

(E)-3-heptene can be synthesized from acetylene by shooting chain extension through coupling reactions, forming a double bond via dissolving metal reduction. (Z)-3-heptene synthesis is similar, but the double bond formation step must employ a Lindlar's catalyst.

Step-by-step solution

Extension of the Carbon Chain

Alkyne acetylene needs to be extended to a 7-carbon chain. This could be achieved by undergoing a sequence of coupling reactions with an appropriate alkyl halide using sodium amide (NaNH2) as a base in liquid ammonia. The process aims for a C-C bond formation.

Establishment of the Double Bond

After achieving a 7-carbon chain, a double bond needs to be introduced at the 3rd carbon. This could be achieved by partial reduction of the alkyne to an alkene by using the Lindlar's catalyst which just reduces the triple bond to a double bond, not a single bond.

Stereochemistry Control

E/Z isomerism refers to the stereochemistry of double bonds. In general, Lindlar's catalyst gives the (Z)-alkene and dissolving metal reduction gives the (E)-alkene. Therefore, (E)-3-heptene would be generated from dissolving metal reduction while (Z)-3-heptene would be synthesized using Lindlar's catalyst. This step includes the portion of controlling for E/Z stereochemistry.

Key concepts

Alkyne Synthesis

In organic chemistry, synthesizing alkynes is a fundamental process that involves building carbon-carbon triple bonds. One common method of synthesis starts with smaller molecules, such as acetylene, and involves extending the carbon chain to form more complex structures.
A typical approach involves using a base like sodium amide (NaNH\(_2\)) in liquid ammonia to facilitate the coupling of acetylene with alkyl halides. This reaction helps to form new carbon-carbon bonds, effectively elongating the initial carbon chain. As acetylene features a highly reactive triple bond, its reactions must be carefully controlled to achieve the desired number of carbon atoms.
By choosing appropriate alkyl halides, one can sequentially add carbon units, thereby incrementing the length of the hydrocarbon chain. This process is repeated until the desired molecular size is achieved, as seen in the synthesis of 3-heptene from acetylene.

Stereochemistry

Stereochemistry focuses on the spatial arrangement of atoms within molecules. This concept is crucial in organic chemistry, especially when dealing with compounds that possess double bonds, like alkenes, which exhibit E/Z isomerism.
E/Z isomerism arises due to the restricted rotation around doubly bonded carbons. The different spatial arrangements can lead to significant variations in the chemical properties and biological activities of these isomers. It is defined by the priority of groups attached to the double-bonded carbons. The (Z)-isomer (from the German 'zusammen') has higher priority groups on the same side, while the (E)-isomer (from 'entgegen') has them on opposite sides.
In practical terms, controlling the formation of E/Z isomers during synthesis is critical. Special reagents or catalysts, such as Lindlar's catalyst, are used to selectively generate the (Z)-isomer, while dissolving metal reductions are effective for producing the (E)-isomer.

Reduction Reactions

Reduction reactions are a key aspect of transforming alkynes into alkenes. These reactions involve the addition of hydrogen atoms to unsaturated bonds, effectively reducing the degree of unsaturation in a molecule.
Partial reduction of an alkyne to an alkene can be achieved using specific catalysts or reaction conditions. Lindlar's catalyst, which consists of palladium deposited on calcium carbonate and treated with lead, is widely employed for such reductions. It allows the transformation of alkynes to alkenes without fully saturating the triple bond, thus maintaining the lesser degree of unsaturation.
In contrast, dissolving metal reduction, usually involving sodium or lithium in liquid ammonia, efficiently reduces alkynes to (E)-alkenes. This method involves single-electron transfers and proton balance to convert triple bonds to double bonds selectively. Both of these reduction routes provide chemists with powerful tools to control the overall structure and stereochemistry of the synthesized compounds.