Isomerism
Alkanes with more than three carbon atoms can be arranged in various different ways, forming structural isomers. The simplest isomer of an alkane is the one in which the carbon atoms are arranged in a single chain with no branches. This isomer is sometimes called the n-isomer (n for "normal", although it is not necessarily the most common). However the chain of carbon atoms may also be branched at one or more points. The number of possible isomers increases rapidly with the number of carbon atoms (sequence A000602 in OEIS). For example:
- C1: no isomers: methane
- C2: no isomers: ethane
- C3: no isomers: propane
- C4: 2 isomers: n-butane & isobutane
- C5: 3 isomers: pentane, isopentane, neopentane
- C6: 5 isomers: hexane, 2-Methylpentane, 3-Methylpentane, 2,3-Dimethylbutane & 2,2-Dimethylbutane
- C12: 355 isomers
- C32: 27,711,253,769 isomers
- C60: 22,158,734,535,770,411,074,184 isomers, many of which are not stable.
Branched alkanes can be chiral. For example 3-methylhexane and its higher homologues are chiral due to their stereogenic center at carbon atom number 3. In addition to these isomers, the chain of carbon atoms may form one or more loops. Such compounds are called cycloalkanes.
Read more about this topic: Alkane