Question

The compound but- 2 -ene exists as two isomers, but when both isomers are hydrogenated, the products are identical. Explain.

Short answer

Both cis and trans isomers of but-2-ene become butane upon hydrogenation because they both have the same carbon skeleton and the double bond, which is broken and saturated with hydrogen, resulting in the same molecular formula and structure, C4H10.

Step-by-step solution

Identify Isomers of But-2-ene

But-2-ene has two isomers based on the position of the double bond and the arrangement of the substituents around this double bond: cis-but-2-ene, where the two methyl (CH3) groups are on the same side of the double bond, and trans-but-2-ene, where the methyl groups are on opposite sides.

Hydrogenation of But-2-ene

The hydrogenation process involves adding hydrogen (H2) across the double bond, converting it into a single bond. In both isomers, the double bond is located between the second and third carbon atoms, so the addition of hydrogen to either form of but-2-ene will result in the same product.

Formation of the Same Product

Upon hydrogenation, both the cis and trans isomers lose their double bond and the methyl groups are no longer constrained in their spatial arrangement relative to each other. Both isomers yield butane (C4H10) as the product, which has a straight chain structure with only single bonds between carbon atoms.

Key concepts

Stereochemistry

Understanding the stereochemistry of molecules is fundamental in organic chemistry. Stereochemistry refers to the study of the spatial arrangement of atoms within molecules and how this arrangement affects their chemical behavior and properties. In the case of but-2-ene, the presence of a double bond between carbons creates the possibility for two distinct geometric isomers: cis-but-2-ene and trans-but-2-ene.

The cis-isomer has both methyl groups on the same side of the double bond, making it slightly less stable due to steric hindrance, which refers to the repulsion between the electron clouds of the adjacent methyl groups. Conversely, the trans-isomer, with methyl groups on opposite sides, is generally more stable and has a lower energy state because the steric hindrance is minimized. Despite these differences, the core structure remains the same, which plays a significant role when these isomers undergo chemical reactions, such as hydrogenation.

Organic Chemistry

Organic chemistry is the branch of chemistry that deals with carbon-containing compounds and their reactions. But-2-ene, for instance, is an alkene because of the carbon-carbon double bond presence. Alkenes are reactive due to this double bond and can participate in various addition reactions, one of them being hydrogenation.

When studying organic chemistry, it's essential to recognize functional groups, like the C=C double bond, and understand how these groups dictate the molecule's reactivity. In hydrogenation, the addition of hydrogen (H2) across the double bond is a crucial transformation that converts unsaturated hydrocarbons, such as but-2-ene, into saturated ones, like butane.

Chemical Reactions

In the context of but-2-ene isomers and hydrogenation, it's important to grasp the nature of chemical reactions. Hydrogenation is a type of addition reaction where hydrogen is added to a double bond, converting an unsaturated compound to a saturated one. This reaction is typically catalyzed by metals such as platinum, palladium, or nickel.

During the hydrogenation of but-2-ene, whether cis or trans, hydrogen atoms are added to the carbons involved in the double bond, effectively saturating the molecule and yielding butane. The process does not discriminate between the two isomers of but-2-ene, as the final product is the same regardless of the original configuration of the double bond. This leads to an interesting point in stereochemistry: while the starting materials may be different, the products of certain reactions can be indistinguishable.

Isomerization

Isomerization is a reaction where a molecule is transformed into another molecule with the same molecular formula but a different structural arrangement. However, during hydrogenation, isomerization plays no role because the end product has no room for isomeric variation; the double bond, which allows for cis and trans isomers of but-2-ene, is eliminated.

Isomerization is distinct from hydrogenation in that no atoms are added or removed; it is purely a reorganization of the molecule's structure. Isomers like cis and trans but-2-ene are stereoisomers, which differ only in the arrangement of atoms in space and not in the connectivity of the atoms. Upon hydrogenation, the need for this distinction vanishes, as the four carbon atoms in butane are connected in a straight chain with single bonds only, eliminating the possibility of stereoisomerism.