Problem 85 - Chemistry and Chemical Reactivity

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.

Step-by-Step Solution

Answer

The product is butane, which is achiral. An isomer of butane is isobutane.

Step 1: Understanding the Reactants

We have the molecule cis-2-butene reacting with hydrogen ( H_{2} ) in the presence of a catalyst. Cis-2-butene is an alkene with the chemical formula C_{4}H_{8} where the double bond is between the second and third carbon atoms, and the larger substituents (the methyl groups) are on the same side of the double bond.

Step 2: Identifying the Reaction Type

The reaction of an alkene with hydrogen in the presence of a catalyst is called hydrogenation. The double bond in the alkene is converted into a single bond by the addition of hydrogen atoms.

Step 3: Determining the Reaction Product

In the hydrogenation of cis-2-butene , the product is butane. This is achieved by adding hydrogen atoms to the carbons which were previously connected by a double bond.

Step 4: Assessing Chirality

Butane has the structure CH_{3}-CH_{2}-CH_{2}-CH_{3} . For a molecule to be chiral, it must have a carbon atom with four different substituents. In butane, no carbon atom fulfills this criterion, so the molecule is achiral.

Step 5: Drawing an Isomer of the Product

An isomer of butane is isobutane (or 2-methylpropane). It has the same chemical formula, C_{4}H_{10} , but a different structure: (CH_{3})_{2}CHCH_{3} , where three methyl groups are attached to a central carbon.

Key Concepts

Cis-2-butene

Cis-2-butene is a type of alkene, and alkenes are hydrocarbons that contain one or more carbon-carbon double bonds. In cis-2-butene, the double bond is located between the second and third carbon atoms. The distinguishing feature of cis-2-butene is that its substituents on either side of the double bond are positioned on the same side. This same-sided arrangement is what "cis" refers to in its name.

Understanding the positioning

• The double bond restricts rotation, making the positioning of the substituents significant.
• In cis-2-butene, two hydrogen atoms are on one side of the double bond, and two methyl groups are on the other side.

The double bond and the arrangement of cis-2-butene make it less stable than its trans counterpart. However, this configuration plays an essential role in reactions like hydrogenation, where the double bond can be broken to form a more stable single-bonded product.

Chiral Molecules

Chirality is a property of a molecule having a non-superimposable mirror image, much like our left and right hands. In chemistry, for a molecule to be chiral, it must have at least one carbon atom (known as a chiral center) with four different groups attached to it.

Characteristics of chiral molecules:

• They have a chiral center, which is absent in compounds like butane.
• Chiral molecules can rotate plane-polarized light, creating optical activity.

Despite being interesting and crucial in many biochemical applications, the product of the hydrogenation of cis-2-butene—being butane—is achiral. None of its carbon atoms have four different substituents, making it impossible for butane to exhibit chirality. This lack of chirality is an important characteristic to identify in hydrogenation reactions involving simple alkenes.

Alkene Isomerization

Isomerization is a process whereby one molecule is transformed into another molecule that has the same atoms, but in a different arrangement. In alkenes, isomers can exist as structural or stereoisomers.

Cis and trans isomerization

• Alkene isomers often differ by the positioning of substituents around the double bond, resulting in cis and trans forms.
• For example, trans-2-butene is the isomer of cis-2-butene, where the substituents are on opposite sides of the double bond.

Although hydrogenation of cis-2-butene to butane does not involve isomerization, understanding isomerization is vital in reactions because it affects chemical properties. Moreover, breaking and reforming bonds can lead to different structural arrangements, affecting both physical and chemical characteristics of the product. This knowledge provides insight into why different configurations exist and how they contribute to the reactivity and stability of molecules in reactions.