Question

Consider the reaction of cis-2-butene with \(\mathrm{H}_{2}\) (in the presence of a catalyst). (a) Draw the structure, and give the name of the reaction product. Is this reaction product chiral? (b) Draw an isomer of the reaction product.

Short answer

The reaction product is butane, and it is not chiral. An isomer is 2-methylpropane.

Step-by-step solution

Introduction to the Reaction

The reaction involves the hydrogenation of cis-2-butene. In presence of a catalyst (like a metal catalyst such as Pt, Pd, or Ni), cis-2-butene reacts with hydrogen ( H_2) to produce the saturated alkane, butane.

Drawing and Identifying the Reaction Product

In the hydrogenation of cis-2-butene, each carbon in the double bond gains a hydrogen atom, resulting in butane: \(CH_3-CH_2-CH_2-CH_3\). This product is called butane, which is an open chain, saturated hydrocarbon (alkane).

Determining Chirality

A molecule is chiral if it has a carbon atom bonded to four different groups with no internal plane of symmetry. Since butane has symmetrical structures and does not have a chiral carbon, it is not chiral.

Drawing and Naming an Isomer

One isomer of butane is 2-methylpropane (also known as isobutane). The structure of 2-methylpropane is \(\text{CH}_3-\text{CH(\text{CH}_3)-CH}_3\). This isomer has the same molecular formula, \(C_4H_{10}\), as butane but a different arrangement of atoms.

Key concepts

cis-2-butene

When we discuss cis-2-butene, we are referring to one geometric form of butene, which is a type of alkene, boasting a double bond. In cis-2-butene, the two methyl groups (\(\text{CH}_3 \)) are located on the same side of the double bond. This cis configuration gives the molecule certain physical properties like boiling points different from its trans counterpart. Cis-2-butene plays a vital role in organic chemistry reactions.

In hydrogenation reactions, like the one in our exercise, these double bonds are broken, allowing hydrogen atoms to attach to the carbons. This reaction requires a catalyst, such as platinum (Pt) or palladium (Pd), to proceed efficiently. Understanding the structure and the arrangement of molecules like cis-2-butene can help predict the outcomes of their chemical reactions.

chiral molecules

Chirality is an incredibly important concept in chemistry, especially in stereochemistry. A molecule is considered chiral if it cannot be superimposed on its mirror image, much like how your left and right hands are. This property occurs when a carbon atom is attached to four different groups, resulting in non-superimposable mirror images called enantiomers.

However, in the case of our reaction product, butane, chirality does not apply. Butane lacks this chiral center, as its structure is symmetrical and each carbon atom does not bind to four distinct groups. Thus, the product of our hydrogenation reaction of cis-2-butene is not chiral, meaning it doesn’t possess enantiomers.

saturated hydrocarbons

Saturated hydrocarbons, often referred to as alkanes, are compounds composed entirely of single bonds between carbon and hydrogen atoms. This lack of double or triple bonds distinguishes them from alkenes and alkynes, which have these multiple bonds. Saturated hydrocarbons are known for their relative stability because of the strong carbon-hydrogen single bonds.

In the hydrogenation reaction described, cis-2-butene is converted into butane, a saturated hydrocarbon. This transformation involves the addition of hydrogen atoms to "saturate" the former double bond. As a result, butane is an example of a saturated hydrocarbon, characterized by having more hydrogen atoms compared to its unsaturated precursor.

isomers

Isomers are fascinating entities in chemistry exhibiting the same molecular formula but different structural configurations. This simple variation can dramatically influence their chemical properties and physical behavior. There are various types of isomers, such as structural isomers, which differ in the connectivity of their atoms, and stereoisomers which differ in the spatial arrangement.

In our exercise, we explore isomers through butane, the product of the hydrogenation reaction. 2-Methylpropane, also called isobutane, is a structural isomer of butane. Both have the molecular formula \( \text{C}_4\text{H}_{10} \), but their internal arrangement of atoms is different. While butane is a straight chain, isobutane has a branched structure, impacting its boiling point, melting point, and how it reacts in certain chemical processes.