Where alkanes come from and how they are made in the laboratory. How their boiling points follow the shape of the molecule, how ethane and butane twist about a C-C bond, and how alkanes burn, crack and react with halogens in a free-radical chain.
3 lessons, about 1 h 30 min3 simulations10 problems3 practice questionsPython in every lesson
Created by StemPy LabAlkanes are the saturated hydrocarbons, with the general formula . Natural gas and petroleum are their main sources. In the laboratory they are made by hydrogenating alkenes, reducing alkyl halides, the Wurtz reaction, heating a sodium salt with soda lime, and Kolbe electrolysis. Their boiling points rise with chain length and fall with branching.
A single bond lets the two groups at its ends turn. The different arrangements in space are called conformations. Ethane is lowest in energy when staggered and highest when eclipsed, a difference of about 12 kJ/mol. Butane has an anti form, a gauche form and two eclipsed forms. Newman projections show them, and a Boltzmann factor tells how many molecules are in each.
Alkanes resist acids, bases and most oxidising agents, but they burn, crack and react with halogens in light. Halogenation is a free-radical chain of initiation, propagation and termination. A halogen atom takes a tertiary hydrogen faster than a secondary one, and a secondary one faster than a primary one. Bromine is much more selective than chlorine. Cracking breaks a large alkane into a smaller alkane and an alkene.
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