Chemistry Question #3709

Shakira, a 17 year old female from Mississauga asks on November 22, 2006,

Why is it that alcohols do not follow a trend for their melting points, like their boiling points do? For example, their boiling points increase as the parent chain of the alcohol increases, so why is it that ethanol has a melting point of -114 degrees celsius, 1-propanol is -127 C and 1-butanol is -90 C?

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The answer

Reg Mitchell answered on November 23, 2006

Melting points depend on how well a substance fits into a crystal lattice, and so is much harder to predict than is boiling point which follows molecular weight (approx), shape (approx). In the case of alcohols Hydrogen-bonding is also very important in melting point. Remember that the hydrocarbon chains like to get close to each other, the -OH ends like to get close to each other, and the longer the chain, the more conformations there are, so in the end its a balancing act, propanol fits in a lattice with weaker overall molecular forces than the others and so it melts at the lower temperature.

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