A carbonyl group is C?O. Of the two carbonyl functions in prostaglandin E1 one belongs to the ketone family, the other to the carboxylic acids.
O
O
OH
Ketone functional group
HO
Carboxylic acid functional group
OH
2.2
An unbranched alkane (n-alkane) of 28 carbons has 26 methylene (CH2) groups flanked by a methyl
(CH3) group at each end. The condensed formula is CH3(CH2)26CH3.
2.3
The alkane represented by the carbon skeleton formula has 11 carbons. The general formula for an alkane is CnH2nϩ2, and thus there are 24 hydrogens. The molecular formula is C11H24; the condensed structural formula is CH3(CH2)9CH3.
2.4
In addition to CH3(CH2)4CH3 and (CH3)2CHCH2CH2CH3, there are three more isomers. One has a five-carbon chain with a one-carbon (methyl) branch:
CH3
CH3CH2CHCH2CH3
or
The remaining two isomers have two methyl branches on a four-carbon chain.
CH3
CH3CHCHCH3
CH3 or CH3CH2CCH3
or
CH3
CH3
25
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ALKANES
2.5
(b)
(c)
2.6
Octacosane is not listed in Table 2.4, but its structure can be deduced from its systematic name. The suffix -cosane pertains to alkanes that contain 20–29 carbons in their longest continuous chain. The prefix octa- means “eight.” Octacosane is therefore the unbranched alkane having 28 carbon atoms. It is CH3(CH2)26CH3.
The alkane has an unbranched chain of 11 carbon atoms and is named undecane.
The ending -hexadecane reveals that the longest continuous carbon chain has 16 carbon atoms.
1
2
3
4
5
6
7
9
8
10
11
12
13
14
15
16
There are four methyl groups (represented by tetramethyl-), and they are located at carbons 2, 6, 10, and 14.
2,6,10,14-Tetramethylhexadecane
(phytane)
2.7
(b)
The systematic name of the unbranched C5H12 isomer is pentane (Table 2.4).
CH3CH2CH2CH2CH3
IUPAC name: pentane
Common name: n-pentane
A second isomer, (CH3)2CHCH2CH3, has four carbons in the longest continuous chain and so is named as a derivative of butane. Since it has a methyl group at C-2, it is 2-methylbutane.
CH3CHCH2CH3
CH3
IUPAC name: 2-methylbutane
Common name: isopentane methyl group at C-2
The remaining isomer, (CH3)4C, has three carbons in its longest continuous chain and so is named as a derivative of propane. There are two methyl groups at C-2, and so it is a
2,2-dimethyl derivative of propane.
CH3
CH3CCH3
CH3
IUPAC name: 2,2-dimethylpropane
Common name: neopentane
(c)
First write out the structure in more detail, and identify the longest continuous carbon chain.
H
CH3
CH3
C
CH3
CH2
C
CH3
CH3
There are five carbon atoms in the longest chain, and so the compound is named as a derivative of pentane. This five-carbon chain has three methyl substituents attached to it, making it
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ALKANES
a trimethyl derivative of pentane. Number the chain in the direction that gives the lowest numbers to the substituents at the first point of difference.
H
CH3
1
2
CH3
CH2
C
5
C
5
CH3
3
CH3C
not
2
CH2
1
C
CH3
CH3
CH3
2,2,4-Trimethylpentane (correct)
(d)
4
CH3
CH3
H
CH3
4
3
2,4,4-Trimethylpentane (incorrect)
The longest continuous chain in (CH3)3CC(CH3)3 contains four carbon atoms.
CH3 CH3
CH3
C
C
CH3
CH3 CH3
The compound is named as a tetramethyl derivative of butane; it is 2,2,3,3-tetramethylbutane.
2.8
There are three C5H11 alkyl groups with unbranched carbon chains. One is primary, and two are secondary. The IUPAC name of each group is given beneath the structure. Remember to number the alkyl groups from the point of attachment.
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