Cycloalkanes

Cycloalkanes

• Compounds that contain rings of carbon atoms

• Have the general formula CnH2n

• Represented by polygons in skeletal drawings

cyclopropane cyclobutane cyclopentane

Naming cycloalkanes

1. Count the number of carbon atoms in the ring and the number in the largest substituent.

– If the number of carbon atoms in the ring is equal to or greater than the number in the largest substituent, the compound is named as alkyl-substituted cycloalkane.

Substituted cycloalkane

Methylcyclopentane

Cycloalkyl-substituted alkane

3 carbons 4 carbons

1-cyclopropylbutane

2 1

3 6

4 5

6 1

5 2

4 3

2. For alkyl-substituted cycloalkanes, start at a point of attachment and number the substituents on the ring so as to arrive at the lowest sum.

1,3-dimethylcyclohexane NOT 1,5-dimethylcyclohexane

3. When two or more different alkyl groups or halogens are present, number them by alphabetical priority.

CH3

CH2CH3

1-ethyl-2-methylcyclopentane

NOT

2-ethyl-1-methylcyclopentane

Cis-trans isomers

• Because there is not free rotation in cycloalkanes, geometric isomers called stereoisomers are formed.

• Cis (L. “on the same side”)

• Trans (L. “across from”)

• Geometric isomers do not convert back and forth.

H H

H H H Cl

H H

Cl H

H H

H H

Cl H H Cl

H H

H H

H H

trans-1,4-dichlorocyclohexane

cis-1,4-dichlorocyclohexane

Chair Conformation

Homework

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