Question

Draw structures for the products expected from the following reactions. Show configurations where significant. a. b. cis-2-butene \(\frac{\mathrm{D}_{2}, \mathrm{Pt}}{25^{\circ}}\) c. \(\mathrm{CH}_{2}=\mathrm{CHCOCH}_{3} \stackrel{\mathrm{H}_{2}, \mathrm{Pt}}{{ }_{25^{\circ}}}\) d. 1 -penten-3-yne \(\stackrel{\mathrm{H}_{2}, \mathrm{Pd}-\mathrm{Pb}}{\longrightarrow}\) e. \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{C} \equiv \mathrm{CC}_{6} \mathrm{H}_{5} \stackrel{\mathrm{H}_{2}, \mathrm{Pd}-\mathrm{Pb}}{\longrightarrow}\) f. 1,3 -dimethylcyclopentene \(\stackrel{\mathrm{H}_{2}, \mathrm{Pt}}{25^{\circ}}\)

Short answer

The products are: b) 1,2-dideuteriobutane, c) butan-2-one, d) cis-1-penten-3-ene, e) cis-stilbene, f) 1,3-dimethylcyclopentane.

Step-by-step solution

Hydrogenation of alkenes

For reaction (b), we have cis-2-butene being hydrogenated using deuterium gas (D₂) over a platinum catalyst at 25°C. Hydrogenation is the addition of hydrogen (or in this case, deuterium) across the double bond of an alkene, turning it into an alkane. Since deuterium is used in place of hydrogen, we will replace the double bond in cis-2-butene with a single bond, and the deuteriums will add to each carbon atom previously double-bonded. The product is butane with each double-bonded carbon now attached to a deuterium (1,2-dideuteriobutane).

Hydrogenation of conjugated ketone

For reaction (c), the substrate is methyl vinyl ketone (CH₂=CHCOCH₃). It undergoes hydrogenation over a platinum catalyst. The double bond between the carbons in the alkene will be reduced (saturated with hydrogens), resulting in the conversion of the alkene to an alkane, yielding butan-2-one.

Partial hydrogenation with Lindlar's catalyst

For reaction (d), 1-penten-3-yne is undergoing hydrogenation with H₂ in the presence of a poisoned catalyst (Pd-Pb). Lindlar’s catalyst allows only partial hydrogenation, converting an alkyne to a cis-alkene without further reduction to an alkane. The product will be cis-1-penten-3-ene.

Partial hydrogenation of diethylphenylacetylene

In reaction (e), diphenylacetylene is hydrogenated in the presence of a Lindlar catalyst (Pd-Pb), similar to the last step. This partial hydrogenation results in the conversion of the triple bond in the alkyne to a double bond in an alkene, retaining the cis configuration to form cis-stilbene.

Hydrogenation of cyclic alkene

For reaction (f), 1,3-dimethylcyclopentene is subjected to complete hydrogenation over platinum. The double bond in the cycloalkene is reduced, turning 1,3-dimethylcyclopentene into 1,3-dimethylcyclopentane, with all bonds now single between the carbons.

Key concepts

Alkene Hydrogenation

Alkene hydrogenation is a reaction in which hydrogen is added across the double bond of an alkene. This process transforms alkenes into alkanes. The double bond opens up, allowing hydrogen atoms to attach to each of the previously double-bonded carbon atoms. This reaction is typically facilitated by a metal catalyst like platinum, which assists in breaking the double bonds and enabling the addition of hydrogen.

Key characteristics of alkene hydrogenation include:

• It is a reduction process, as it involves the gain of hydrogen atoms.
• A metal catalyst such as platinum or nickel is commonly used to speed up the reaction.
• The reaction often occurs at lower temperatures to manage the addition of hydrogen carefully without over-reduction.

For example, when methyl vinyl ketone is hydrogenated using a platinum catalyst, the result is its conversion into butan-2-one by reducing the carbon-carbon double bond to a single bond, saturating it with hydrogens.

Lindlar Catalyst

Lindlar Catalyst is a specially designed catalyst to enable selective hydrogenation of alkynes to alkenes, producing cis isomers specifically. This catalyst is essentially palladium on calcium carbonate which is "poisoned" by lead acetate and quinoline. The poisoning of the catalyst is crucial as it prevents complete hydrogenation to alkanes.

The application of Lindlar Catalyst results in several benefits:

• Alkynes are converted to cis-alkenes exclusively without further reduction.
• The specificity for producing cis isomers is vital in stereochemistry, controlling the spatial arrangement of atoms.
• This partial hydrogenation process is essential for synthesizing certain chemicals requiring specific molecular configurations.

A good example is the conversion of 1-penten-3-yne into cis-1-penten-3-ene when treated with Lindlar's catalyst. The reaction stops at the alkene stage, forming the desirable cis configuration.

Deuterium Substitution

Deuterium is an isotope of hydrogen, denoted as D, and it replaces hydrogen in certain chemical reactions. This replacement is known as deuterium substitution. In chemical synthesis, deuterium can be utilized to study reaction mechanisms and the behavior of molecules, as it behaves similarly to hydrogen but is twice as heavy.

Here's how deuterium substitution impacts reactions:

• It offers insight into reaction pathways, helping chemists understand the order of steps in a synthesis.
• In hydrogenation reactions involving deuterium, it provides labeled compounds useful for spectroscopic analysis.
• When used in pharmaceuticals, deuterium can sometimes enhance drug stability or modify drug metabolism.

In the context of organic chemistry exercises, deuterium substitution is exemplified by converting cis-2-butene into 1,2-dideuteriobutane through deuterium gas and a platinum catalyst. Here, each carbon in the double bond bonds with deuterium instead of hydrogen.

Cis and Trans Isomers

Isomerism is a fascinating concept in organic chemistry, where molecules with the same formula have different arrangements of atoms in space. Cis and trans isomers are types of geometric isomers. In cis isomers, substituents are on the same side of the double bond, whereas, in trans isomers, they are on opposing sides.

Understanding the formation of these isomers is essential:

• Cis isomers often result from partial hydrogenation using Lindlar's catalyst, as they maintain the layout of substituents on one side.
• Trans isomers are less common in reactions using Lindlar's catalyst but can form through other selective hydrogenation methods.
• The distinct spatial arrangement in cis and trans isomers can significantly affect the physical properties and reactivity of the compounds.

In practice, synthesizing cis-stilbene from diphenylacetylene using Lindlar's catalyst is a classic example, confirming the precision in obtaining specific isomeric forms.

Platinum Catalyst

A platinum catalyst is a widely used catalyst in various organic reactions due to its high efficiency. In hydrogenation reactions, platinum assists in adding hydrogen to the unsaturated bonds, converting alkenes and alkynes into their saturated forms.

Key roles of platinum catalysts include:

• Facilitating the reduction of unsaturated hydrocarbons by lowering the activation energy needed for the reaction.
• Producing an even distribution of hydrogen atoms across carbon atoms in alkenes, leading to uniform alkane products.
• Platinum catalysts are robust, supporting numerous cycles of reactions without significant loss of activity.

For instance, using platinum to fully hydrogenate 1,3-dimethylcyclopentene into 1,3-dimethylcyclopentane exemplifies its application, where the cyclic structure transitions to a completely saturated form.