Problem 114 - A Complete Resource Book in Chemistry for JEE Main

Question

In order to obtain trans Me-CH=CH-Me from \(\mathrm{MeC}=\mathrm{CMe}\), the reagent(s) to be used would includes (a) \(\mathrm{LiAlH}_{4}\) in \(\mathrm{EtO}_{2}\) (b) \(\mathrm{H}_{2} / \mathrm{Pd}, \mathrm{BaSO}_{4}\) (c) \(\mathrm{H}_{2}^{2} / \mathrm{Pd}, \mathrm{Pb}(\mathrm{OAc})_{2}\) (d) sodium in liquid ammonia or sodium in \(\mathrm{EtOH}\)

Step-by-Step Solution

Answer

Use sodium in liquid ammonia or sodium in \(EtOH\) (option d).

Step 1: Understand the Reaction Context

The goal is to convert a cis-alkene to a trans-alkene. The molecule given is MeC=CMe, which is a cis isomer (same substituents on the same side). To obtain trans Me-CH=CH-Me, we want the substituents on opposite sides.

Step 2: Review Possible Reagents for Isomerization

Consider the reagents given:- (a) \(LiAlH_4\) in \(EtO_2\) is a strong reducing agent not typically used for double bond isomerization.- (b) \(H_2 / Pd, BaSO_4\) is used for partial hydrogenation of alkynes to cis-alkenes. It will not isomerize alkenes.- (c) \(H_2 / Pd, Pb(OAc)_2\) is another form of selective hydrogenation which generally affects alkynes.- (d) Sodium in liquid ammonia is known as the Birch reduction but also can isomerize alkenes.

Step 3: Select the Correct Reagent

Since we are looking for a method to convert the cis to a trans alkene, option (d) using sodium in liquid ammonia (or \(EtOH\)) is a classical method known to isomerize alkenes by facilitating electron transfer reactions that convert cis to trans double bonds.

Key Concepts

cis-trans isomerization

Cis-trans isomerization is a chemical process that alters the spatial arrangement of atoms around a double bond, transforming cis isomers to trans isomers or vice versa. In a cis isomer, the substituents adjacent to a double bond are on the same side, while in a trans isomer, they're on opposite sides.
The conversion between these isomeric forms is significant because often, one isomer can have different physical and chemical properties than the other. For example, the trans isomer typically has a lower boiling point and is more stable due to reduced steric strain.

• Achieving cis-trans isomerization typically involves the use of a reagent or a catalyst that can affect the π-bond electrons in the double bond.
• Certain conditions like stereo-specific catalysts, light, or heat, are typically applied.

This concept is crucial in organic chemistry for the design and synthesis of molecules with desired properties.

alkene isomerization

Alkene isomerization involves the rearrangement of carbon atoms around the carbon-carbon double bond in alkenes, typically converting one type of geometric isomer into another. This process does not change the molecular formula of the compound but alters its geometry.
There are catalysts and reagents that can be specifically used for alkene isomerization:

• Sodium in liquid ammonia serves this purpose well. It initiates an electron transfer process that temporarily removes the π-electrons, allowing free rotation around the formerly rigid double bond.
• Alkene isomerization is less energy-intensive than full hydrogenation and is used when the desired product configuration can't be easily achieved through other methods.

Understanding the conditions that favor alkene isomerization helps chemists synthesize more complex molecules efficiently, which is crucial in pharmaceuticals and materials science.

reduction reactions

Reduction reactions in the context of alkenes typically involve the gain of electrons by the alkene molecule. These reactions often use metal catalysts or hydride donors to achieve this electron transfer.
In organic chemistry, reduction is essential for transforming functional groups and changing oxidation states. However, not all reducing agents are suitable for alkene isomerization as they might reduce the double bond completely.

• An important distinction is made between reductions that simply alter the electron arrangement and those that add hydrogen directly to the compound.
• Reagents like sodium in liquid ammonia are notable because, while they typically serve in reductions, they can also facilitate alkene isomerization based on specific reaction conditions.

Understanding these reactions and choosing the right reagent conditions is crucial for achieving the desired transformation in synthesis pathways.

hydrogenation processes

Hydrogenation processes involve the addition of hydrogen to compounds, specifically unsaturated hydrocarbons like alkenes and alkynes. This is commonly used to reduce or "saturate" these molecules by converting double or triple bonds into single bonds.
While hydrogenation can change the saturation of a hydrocarbon, it's not usually employed for geometric isomerization of alkenes due to its indiscriminate addition of hydrogen.

• Partial hydrogenation, such as using H2 with a poisoned catalyst like Pd/BaSO4, selectively hydrogenates without fully saturating the molecule. However, it typically results in a cis-alkene, not a trans, making it unsuitable for isomerization needs.
• Complete hydrogenation fully saturates the molecule, eliminating any isomeric distinctions, and is usually reserved for cases where full reduction is desired.

In synthetic chemistry, these processes must be selected with care to ensure the desired product configuration is produced without unintended reactions.