AQA Chemistry AS/A-level Year 1
The different structures can arise in any of three different ways: chain isomerism
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position isomerism
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functional group isomerism.
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Chain isomerism Chain isomers occur when there are two or more ways of arranging the carbon skeleton of a molecule. For example, C4H10 can be butane or 2-methylpropane, as shown in Fig 21. Fig 21 The isomers of C4H10
H
H
H
H
H
C
C
C
C
H
H
H
H
H
H
Essential Notes The number 2 is optional in 2-methylpropane, as was explained earlier (see this book, section 3.1.1.1).
Fig 22 The isomers of C5H12
H
H
H
C
C
C
H H
C
H H
H
H 2-methylpropane
butane
The three isomers of C5H12 are pentane, 2-methylbutane and 2,2-dimethylpropane, as shown in Fig 22.
H
H
H
H
H
H
H
C
C
C
C
C
H
H
H
H
H
H
H
H
H
H
H
C
C
C
C
H H
C
H H
H
H
H 2-methylbutane
pentane
H
H H
C
H H
C
C
C
H H
C
H H
H
H 2,2-dimethylpropane
These isomers have similar chemical properties, but slightly different physical properties. Branched isomers have smaller volumes, weaker van der Waals’ forces (see Collins Student Support Materials: A-Level year 1 – Inorganic and Relevant Physical Chemistry, section 3.1.3.7) and therefore lower boiling points. The number of structural isomers of alkanes rises steeply as the number of carbon atoms increases, as shown in Table 3.
Position isomerism Position isomers have the same carbon skeleton and the same functional group, but the functional group is joined at different places on the carbon skeleton. For example: CH3CH2CH2Br CH3CHBrCH3 1-bromopropane 2-bromopropane H2C==CHCH2CH3 CH3CH==CHCH3 but-1-ene but-2-ene
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