Organic Chemistry, 7th Edition
L. G. Wade, Jr.

Chapter 3
Structure and Stereochemistry of Alkanes

2010, Prentice Hall
Chapter 3

1

Hydrocarbons
Hydrocarbons are molecules that are made of carbon and hydrogen ONLY.

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2

Alkanes
• General formula: CnH2n+2
• Found in everything from natural gas to petroleum. • The smaller alkanes have very low boiling points (b.p.) therefore they are gases.

b.p.

CH4
-160oC

C 2H 6
-89oC
Chapter 3

C 3H 8
-42oC
3

Alkane Examples

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4

Small Alkanes (CnH2n+2)
• Methane

CH4

• Ethane

CH3 CH3

• Propane

CH3 CH2 CH3

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Butane: C4H10
CH3CH2CH2CH3

n-butane

CH3

CH3CHCH3

iso -butane

Constitutional isomers are compounds with the same molecular formula but the carbons are connected differently.
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Pentanes: C5H12
CH3CH2CH2CH2CH3

n-pentane

CH3 iso-pentane CH3CHCH2CH3
CH3
CH3CCH3

neo-pentane

CH3
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IUPAC
• International Union of Pure and Applied
Chemistry
• Common names kept: methane, ethane, propane, butane.
• Alkanes: suffix “-ane” will be used after the number of carbons.
– Example: An alkane with 5 carbons is “penta” for five and the suffix “-ane”: pentane Chapter 3

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IUPAC Rules
• Rule 1: Find the longest continuous chain of carbon atoms, and use the name of this chain as the base name of the compound.
• Rule 2: Number the longest chain, beginning with the end of the chain nearest a substituent. • Rule 3: Name the groups attached to the longest chain as alkyl groups. Give the location of each alkyl group by the number of the main chain carbon atom to which it is attached. • Write the alkyl groups in alphabetical order regardless of their position on the chain.
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Rule 1: Find the Longest Chain of
Consecutive Carbons.

The longest chain is six carbons: hexane Chapter 3

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Main Chain

When there are two longest chains of equal length, use the chain with the greatest number of substituents.
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Common Alkyl Groups

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Rule 2: Number the Longest Chain.
Methyl is closest to this end of the main chain.

Number the longest chain, beginning with the end of the chain nearest a substituent.
3-methylehexane
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Rule 3: Alkyl Substituents
4-ethyl

2-methyl

CH3

CH2CH3

CH3CHCH2CHCH2CH3
1

2

3

4

5

6

• Name the groups attached to the longest chain as alkyl groups.
• Give the location of each alkyl group by the number of the main chain carbon atom to which it is attached.
• Write the alkyl groups in alphabetical order regardless of their position on the chain.
4-ethyl-2-methylhexane
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Organizing Multiple Groups
• When two or more of the same substituents are present, use the prefixes di-, tri-, tetra-, etc. to avoid having to name the alkyl group twice.

CH3

CH3

CH3CH2CH2CHCH2CHCH2CH2CHCH3
10

9

8

7

6

5

4
CH3

3

2

1

Three methyl groups at positions 2, 5, and 7
2,5,7-trimethyldecane
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Solved Problem 3-1
Give the structures of 4-isopropyloctane and
5-t-butyldecane.
Solution: 4-Isopropyloctane has a chain of eight carbons, with an isopropyl group on the fourth carbon. 5-t-Butyldecane has a chain of ten carbons, with a t-butyl group on the fifth.

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Solved Problem 3-2
Give a systematic (IUPAC) name for the following compound. Chapter 3

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Solved Problem 3-2: Solution
The longest carbon chain contains eight carbon atoms, so this compound is named as an octane. Numbering from left to right gives the first branch on C2; numbering from right to left gives the first branch on C3, so we number from left to right.

4-isopropyl-2,2,3,6-tetramethyloctane
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Boiling Points of Alkanes
As the number of carbons in an alkane