Chemistry - A Molecular Science

Chapter 13 Organic Chemistry

Example 13.3

Draw the skeletal structure of 3-methyl-4-ethyloctane. 1. Draw the eight-carbon continuous

chain of an octane. See margin.

2. Attach a methyl group to the 3 position.
   3. Add an ethyl group to the 4 position.

13.3

ISOMERS Isomers

are different molecules that have differe

nt structures and properties but the same

molecular formula. There are two types:

constitutional isomers and stereoisomers

.

CONSTITUTIONAL ISOMERS Constitutional isomers

have the same molecular formula but differ in the connectivity of

the atoms; that is, they differ in how the atom

s are connected to one another. For example,

the carbon atoms of the two isomers of butane shown in Figure 13.3 are connected to one another in different ways, so they are constitu

tional isomers. Also, consider the two Lewis

structures for the molecules having the molecular formula C

H 2

O shown in Figure 13.9. 6

The structure in Figure 13.9a contains C-O-C linkage, which makes it is an

ether

, while

that in Figure 13.9b contains a hydroxyl group (-OH), which makes it an

alcohol

. Alcohols

and ethers are very different. Alcohols are able to hydrogen bond to one another while ethers cannot. The hydrogen bond can dramatically

affect the solubility properties and the

physical properties of alcohols. As an example,

Figure 13.9a is dimethyl ether, which boils

at -23

°C, while Figure 13.9b is ethanol, a hydrogen-bonding molecule that boils at 78

°C.

Now consider the six structures fo

r molecules having the formula C

H 4

O (Figure 10

13.10). Figures 13.10a and 13.10b contain a C-O-C linkage, so they are ethers, while the remaining structures all contain hydroxyl groups, which make them alcohols. Note that, as the number of carbon atoms in the molecular formula increases, the number of constitutional isomers also increases. For C

H 12

O, there are well over one hundred 26

isomers! The existence of so many constitu

tional isomers gives rise to a wealth of

structural diversity in organic chemistry. While there is no simple mathematical relationship between the molecular formula a

nd the number of constitutional isomers, you

can be certain that molecules with a large number of carbon atoms will have a large number of constitutional isomers.

HCOC

H H

H

H

H

HC

H H

H C H

OH

CH

OCH 3

3

CH

CH 3

OH 2

O

OH

(a)

(b)

Figure 13.9 Constitutional isomers of C

H 2

O 6

HCOC^3

H^2

CH^2

CH

3

HCC^3

H^2

OC

H^2

CH

3

HCC^3

H^2

CH

CH

3

OH

O

O

OH

(a)

(b)

(c)

HCC^3

H^2

CH^2

CH^2

OH

OH

HCC^3

CH H

3 CH

2

OH

HCC^3

CH

3

OH

CH

3

OH

OH

(d)

(e)

(f)

Figure 13.10 Six constitutional isomers of C

H 4

O 10

1

2

3

1

2

3

4

Step 1

Step 2

Step 3

Example 13.3

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