Chapter 8, Problem 25P

How many alkenes yield 2,2,3,4,4-pentamethylpentane on catalytic hydrogenation?

How many yield 2,3-dimethylbutane?

How many yield methylcyclobutane?

Interpretation Introduction

Interpretation:

The number of alkenes which are required to produce the given alkanes on catalytic hydrogenation is to be determined.

Concept introduction:

On catalytic hydrogenation, alkenes get converted to the corresponding alkanes with the same number of carbon atoms.

In hydrogenation reaction, one hydrogen atom gets attached to each of the double bonded carbon atoms.

To accelerate the rate of hydrogenation, the metal catalyst provides an alternative pathway involving low activation energy steps.

The alkane that is formed contains two hydrogen atoms more than the corresponding alkene and these two hydrogen atoms are added on the adjacent carbon atoms in the alkene.

Answer to Problem 25P

Solution:

a) Only one alkene can produce $\text{2,2,3,4,4-pentamethylpentane}$ upon catalytic hydrogenation.

b) Two alkenes can produce $\text{2,3-dimethylbutane}$ upon catalytic hydrogenation.

c) Three alkenes can produce methylcyclobutane upon catalytic hydrogenation.

Explanation of Solution

a) The structure of $\text{2,2,3,4,4-pentamethylpentane}$ is shown below:

Hydrogenation of alkenes adds one hydrogen atom on each carbon atom of the double bonded carbon atoms. So in the alkane, these two carbon atoms (previously double bonded) should be attached to at least one hydrogen atom each.

In $\text{2,2,3,4,4-pentamethylpentane}$, the $\text{C3}$ carbon atom is attached to a hydrogen atom. It has an adjacent methyl group. So in the structure of the alkene, the double bond must be present between these two carbon atoms.

The alkane structure is symmetric, and carbon atoms $\text{2 and 4}$ do not have any hydrogen atoms attached. Thus, there is no other pair of adjacent carbon atoms having one hydrogen atom attached to each. This means there cannot be any other alkene structure which would produce $\text{2,2,3,4,4-pentamethylpentane}$ upon catalytic hydrogenation.

b) The structure of $\text{2,3-dimethylbutane}$ is shown below:

The hydrogen of alkenes adds one hydrogen atom on each carbon atom of the double bonded carbon atoms. So in the alkane, these two carbon atoms (previously double bonded) should be attached to at least one hydrogen atom each.

In $\text{2,3-dimethylbutane}$, the $\text{C2 and C3}$ carbon atoms are attached to one hydrogen atom each. So in the structure of the alkene, the double bond could be present between these two carbon atoms.

The $\text{C1 and C2}$ carbon atoms are also attached to at least one hydrogen atom each. So in the structure of the alkene, the double bond could be present between these two carbon atoms.

The alkane structure is symmetric, and the carbon atoms $\text{2 and 3}$ are attached to two methyl groups each. This means there cannot be any other alkene structure which would produce $\text{2,3-dimethylbutane}$ upon catalytic hydrogenation.

Hence, two alkenes can produce $\text{2,3-dimethylbutane}$ upon catalytic hydrogenation.

c) The structure of methylcyclobutane is shown below:

The hydrogen of alkenes adds one hydrogen atom on each carbon atom of the double bonded carbon atoms. So in the alkane, these two carbon atoms (previously double bonded) should be attached to one at least one hydrogen atom each.

In methylcyclobutane, the $\text{C1}$ carbon atom is attached to a hydrogen atom. It has an adjacent methyl group. So in the structure of the alkene, the double bond could be present between these two carbon atoms.

The $\text{C1 and C2}$ carbon atoms are also attached to at least one hydrogen atom each. So in the structure of the alkene, the double bond could be present between these two carbon atoms.

The $\text{C2 and C3}$ carbon atoms are attached to one hydrogen atom each. So in the structure of the alkene, the double bond could be present between these two carbon atoms.

The alkane structure is symmetric. This means, there cannot be any other distinct alkene structure which would produce methylcyclobutane upon catalytic hydrogenation.

Hence, three alkenes can produce methylcyclobutane upon catalytic hydrogenation.