1. Chapter 12 Alkanes 12.2 IUPAC Naming System for Alkanes
12.3 Drawing Structural Formulas
CH CH3
| |
CH3—CH—CH2—CH—CH3
2,4-dimethylpentane

2. Branched-Chain Alkanes
In a branched-chain alkane, a side group called a branch or a substituent is attached to a carbon chain.
branch

3. Classification of Carbon Atoms
Carbon atoms are classified according to the number of attached carbon atoms.
Primary (1°) bonds to one carbon atom.
Secondary (2°) bonds to two carbon atoms.
Tertiary (3°) bonds to three carbon atoms.
CH3 |
CH3 —CH2—CH2—CH3 CH3—CH—CH3
secondary primary tertiary

4. Alkyl Groups An alkyl group:
Is composed of one or more carbon atoms attached to a carbon chain.
Is derived from the corresponding alkane by removing one hydrogen.
Is named by replacing the –ane ending of the corresponding alkane with –yl.
Derived from methane is methyl and from ethane is ethyl.

5. Alkyl Groups

6. Naming Branched-Chain Alkanes
The names of branched-chain alkanes indicate the groups bonded to the longest carbon chain.
CH methyl group on C |
CH3—CH2—CH2—CH—CH2—CH3 hexane
3-Methylhexane
A branched-chain alkane is named by indicating the attached groups on the longest carbon chain. In this example, the longest carbon chain has 6 carbon atoms.

7. Naming Summary In the IUPAC system:
The longest chain is named as the main chain.
Any carbon branches use their alkyl names.
Each branch is numbered by counting the main chain from the end nearest the first side group.
Branches are listed in alphabetical order.

8. What Is In A Name?
The structural formula is drawn from the IUPAC name.
2, 4-dimethylhexane
2, 4 dimethyl hexane
Location of Two CH3- groups carbon main chain
branches on attached with single C-C bonds
main chain

9. Drawing Structural Formulas
Draw the condensed formula of 2-methylpentane
1. Draw the main chain of 5 carbon atoms.
C—C—C—C—C
2. Add a CH3 branch to C-2 on the main chain.
CH3 |
3. Add hydrogen atoms to give 4 bonds to each C.
CH3—CH—CH2—CH2—CH3

10. Writing Isomers
The constitutional isomers for C4H10 can be written by first writing the continuous chain. Then remove one CH3 and attach it as a branch.
CH3 |
CH3—CH2—CH2—CH3 CH3—CH—CH3
Butane 2-Methylpropane