Chapter 23, Problem 20PS

(a)

Interpretation Introduction

Interpretation: The structure of 2,2-dimethylhexane has to be drawn. The compound is whether chiral carbon containing molecule has to be determined

Concept introduction:

The structure of the alkanes can be drawn with the help of their systematic name.

(1) The parent name give the information about the longest chain of the carbon atoms for alkane.

(2) The prefix written in the systematic name tells about the side chains and their positions.

(3) Numbering should be given according to side chain, so that carbon atom having side chain get a least numbering.

(4) If molecule has more than one side chain which are different then name should be written alphabetically.

A chiral carbon is the carbon atom which is substituted to four different groups. Due to presence of chiral carbon a molecule become chiral and it gives optical isomers also that is $R\text{ and }S$ isomers. For example,

  

All the substituents are different hence is is a chiral carbon and so molecule.

Answer to Problem 20PS

The structure of the 2,2-dimethylhexane is drawn below.

It does not have any chiral carbon atom

Explanation of Solution

The given isomer is 2,2-dimethylhexane

From the name it is clear that:

• The parent name is hexane for the given alkane so the number of carbon atoms in longest chain are six.
• The alkane is disubstituted by methyl group at position $2$.

  

• Therefore there will be two methyl groups are present at position $2$ of the linear chain.

Therefore,

The structure is,

  

There is no such carbon in the molecule which is chiral. Therefore the isomer 2,2-dimethylhexane is achiral molecule.

(b)

Interpretation Introduction

Interpretation: The structure of 2,3-dimethylhexane has to be drawn. The compound is whether chiral carbon containing molecule has to be determined

Concept introduction:

The structure of the alkanes can be drawn with the help of their systematic name.

(1) The parent name give the information about the longest chain of the carbon atoms for alkane.

(2) The prefix written in the systematic name tells about the side chains and their positions.

(3) Numbering should be given according to side chain, so that carbon atom having side chain get a least numbering.

(4) If molecule has more than one side chain which are different then name should be written alphabetically.

A chiral carbon is the carbon atom which is substituted to four different groups. Due to presence of chiral carbon a molecule become chiral and it gives optical isomers also that is $R\text{ and }S$ isomers. For example,

  

All the substituents are different hence is is a chiral carbon and so molecule.

Answer to Problem 20PS

The structure of the 2,3-dimethylhexane is drawn below.

It has chiral carbon atom

Explanation of Solution

The given isomer is 2,3-dimethylhexane

From the name it is clear that:

• The parent name is hexane for the given alkane so the number of carbon atoms in longest chain are six.
• The alkane is disubstituted by methyl group at position $\text{2 and 3}$.

  

• There will be one methyl group at position $2$ and another will be at position $3$ in the linear chain.

Therefore,

The structure is,

  

The carbon $C-3$ has all four different groups so it is a chiral carbon.

  

It is denoted by asterisk (*). Therefore the isomer 2,3-dimethylhexane is chiral molecule.

(c)

Interpretation Introduction

Interpretation: The structure of 2,4-dimethylhexane has to be drawn. The compound is whether chiral carbon containing molecule has to be determined

Concept introduction:

The structure of the alkanes can be drawn with the help of their systematic name.

(1) The parent name give the information about the longest chain of the carbon atoms for alkane.

(2) The prefix written in the systematic name tells about the side chains and their positions.

(3) Numbering should be given according to side chain, so that carbon atom having side chain get a least numbering.

(4) If molecule has more than one side chain which are different then name should be written alphabetically.

A chiral carbon is the carbon atom which is substituted to four different groups. Due to presence of chiral carbon a molecule become chiral and it gives optical isomers also that is $R\text{ and }S$ isomers. For example,

  

All the substituents are different hence is is a chiral carbon and so molecule.

Answer to Problem 20PS

The structure of the 2,4-dimethylhexane is drawn below.

It has chiral carbon

Explanation of Solution

The given isomer is 2,4-dimethylhexane

From the name it is clear that:

• The parent name is hexane for the given alkane so the number of carbon atoms in longest chain are six.
• The alkane is disubstituted by methyl group at position $\text{2 and 4}$.

  

• Therefore there will be one methyl group at postion $2$ and one methyl at position $4$ of the linear chain.

Therefore,

The structure is,

  

The carbon $C-4$ has all four different groups so it is a chiral carbon.

  

It is denoted by asterisk (*). Therefore the isomer 2,4-dimethylhexane is chiral molecule.

(d)

Interpretation Introduction

Interpretation: The structure of 2,5-dimethylhexane has to be drawn. The compound is whether chiral carbon containing molecule has to be determined

Concept introduction:

The structure of the alkanes can be drawn with the help of their systematic name.

(1) The parent name give the information about the longest chain of the carbon atoms for alkane.

(2) The prefix written in the systematic name tells about the side chains and their positions.

(3) Numbering should be given according to side chain, so that carbon atom having side chain get a least numbering.

(4) If molecule has more than one side chain which are different then name should be written alphabetically.

A chiral carbon is the carbon atom which is substituted to four different groups. Due to presence of chiral carbon a molecule become chiral and it gives optical isomers also that is $R\text{ and }S$ isomers. For example,

  

All the substituents are different hence is is a chiral carbon and so molecule.

Answer to Problem 20PS

The structure of the 2,5-dimethylhexane is drawn below.

It does not have chiral carbon

Explanation of Solution

The given isomer is 2,5-dimethylhexane

From the name it is clear that:

• The parent name is hexane for the given alkane so the number of carbon atoms in longest chain are six.
• The alkane is disubstituted by two methyl groups at position $\text{2 and 5}$.

  

• There will be one methyl group at position $2$ and one methyl group at position $5$ of the linear chain.

Therefore,

The structure is,

  

There is no such carbon in the molecule which is chiral. Therefore the isomer 2,5-dimethylhexane is achiral molecule.