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Alkanes: Structure and Constitution

Alkanes: Branched Alkanes

Branching of the chain in alkanes dramatically increases the diversity of conceivable structures. Structures with identical molecular formulas but with different connectivities of atoms are called constitutional isomers. Constitutional isomers exhibit different physical properties, for example, melting and boiling points.

butane (n-butane)
2-methylpropane (isobutane)
pentane (n-pentane)
2-methylbutane (isopentane)
2,2-dimethylpropane (isobutane)
Fig.1
Number of isomeric short-chain alkanes in relation to the number of C atoms

Table with the number of isomers of various alkanes

Despite an equal number of C atoms, isomeric branched alkanes exhibit different melting points. As a rule of thumb, the more asymmetric the molecule the lower the melting point. Responsible for this behavior are the decreasing van der Waals forces between molecules with increasing branching. If, however, branching results in a symmetrical structure, e.g. 2,2,3,3-teramethylbutane, the melting point increases. In this case, van der Waals forces increase because these molecules are well packed.

Melting points of octane isomers are used as examples. The correlation of melting point and molecular symmetry is very well illustrated. Initially, melting points decrease with increasing branching but then become larger again as the symmetry increases. Clicking on the structures will open up a ball-and-stick model of the molecule.

Tab.1
Octane isomers
StructureMelting point [°C]
Fig.
Octane
-57°C
Fig.
2-Methylheptane
-109°C
Fig.
2,2-Dimethylhexane
-121°C
Fig.
2,2,3-Trimethylpentane
>-112°C
Fig.
2,2,3,3-Tetramethylbutane
+ 101°C
Fig.2
Melting points of octane isomers
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