Chapter 3, Problem 20P

Give the IUPAC names of each of the following:

(a) (b) (c)

Interpretation Introduction

Interpretation:

IUPAC names of the given structures are to be determined.

Concept introduction:

When writing the IUPAC name, the longest continuous carbon chain is first determined. The parent chain is numbered such that the substituents get the lowest numbers.

The location of each substituent group is designated by an appropriate number and name. Prefixes are used if more than one substituents of the same type are present. The substituents are written in alphabetical order.

A cyclic ring hydrocarbon is designated by the prefix cyclo- which appears in front of the base name.

Answer to Problem 20P

Solution:

$\text{2,4-dimethylpentane}$

$\text{2,2,4-trimethylhexane}$

$\text{trans -1- ethyl - 4 - isopropylcyclononane}$

Explanation of Solution

(a)

In the given Newman projection, the carbon atom at the front is a $-{\text{CH}}_3-$ group. The carbon atom at the back is attached to a $-{\text{CH}}_3-$ group, $\text{H}$, and the ${\left({\text{CH}}_3\right)}_2{\text{CHCH}}_2$ group. From this, the skeletal structure is drawn as shown below:

In this structure, the longest chain contains five carbon atoms, so the parent alkane is pentane. There are two methyl groups attached to this parent chain. The pentane chain is numbered such that these methyl groups get the lowest numbers.

The two methyl groups are attached to the carbon atoms $\text{C2 and C4}$. The prefix “di” is used to show the presence of two methyl groups. There are no chiral carbon atoms in this structure. Therefore, the IUPAC name of the compound is $\text{2,4-dimethylpentane}$.

(b)

The given sawhorse projection is converted to a skeletal structure as follows:

In this structure, the longest chain contains six carbon atoms, so the parent alkane is hexane. There are three methyl groups attached to the hexane chain. The chain is numbered such that the carbon atoms attached to these three methyl groups get the lowest numbers.

Two methyl groups are attached to $\text{C2}$ while one methyl group is attached to $\text{C4}$. The prefix “tri” is used to show the presence of three methyl groups. The IUPAC name of the compound is $\text{2,2,4-trimethylhexane}$.

(c)

In the given structure, there are nine carbon atoms in the ring. So the name of parent alkane is cyclononane. The ring is numbered such that the two substituents get the lowest numbers.

There is an ethyl group and an isopropyl group attached to the cyclononane ring. These two groups are trans to each other. Hence the ring is numbered such that the ethyl group gets the lower number since it comes first alphabetically. The IUPAC name of the compound is $\text{trans -1- ethyl - 4 - isopropylcyclononane}$.