Family Traits

Within the alkane, alkene and alkyne families certain trends appear. For example: at the low end methane, ethane, propane, ethene and ethyne are all gases at room temperature. The slightly larger molecules are liquids, then waxy solids. Because they are covalent molecules, they never get very strong, as one molecule is always rather easily parted from another. A candle is a typical mixed longer-chain alkane. Petrol is a typical mixed medium-chain alkane. The gas you burn in a Bunsen burner is a typical mixed short-chain alkane, although it is predominantly methane. We are still short of being able to build even the simplest living cell.

name	formula	melting point (°C)	boiling point (°C)
Methane	CH₄	-182	-162
Ethane	C₂H₆	-183	-89
Propane	C₃H₈	-188	-42
Butane	C₄H₁₀	-138	-0.5
Pentane	C₅H₁₂	-130	36
Hexane	C₆H₁₄	-95	69
Heptane	C₇H₁₆	-91	98
Octane	C₈H₁₈	-57	126
Nonane	C₉H₂₀	-53	151
Decane	C₁₀H₂₂	-30	174

Table 2. Melting and boiling point for first ten alkanes. (Alyward & Findlay, SI Chemical Data, Wiley & Sons.)

Thus you can draw a generalisation that longer chains have higher melting and boiling points than shorter ones. Similarly, straight chains lead to higher melting points than branched molecules with similar numbers of carbons. That is, you would expect hexane to have a higher melting point than dimethylbutane, even though both are of the formula C₆H₁₄, as indicated in Table 3 below. This last observation leads very handily into the next topic.

name	formula	melting pt (°C)	boiling pt (°C)
hexane	CH₃(CH₂)₄CH₃	-95	69
3-methylpentane	CH₃CH₂CH(CH₃)CH₂CH₃	?	63
2,2-dimethylbutane	CH₃C(CH₃)₂CH₂CH₃	-100	50
2,3-dimethylbutane	CH₃CH(CH₃)CH(CH₃)CH₃	-128	58

Table 3. Melting and boiling point for four alkanes with the formula C₆H₁₄. (Alyward & Findlay, SI Chemical Data, Wiley & Sons.)

Problem 6. From your recollection of bonding and intermolecular forces, suggest an explanation for this generalisation.

Next page: Isomers

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