hydrocarbon
Summary
A hydrocarbon is a compound made only of carbon and hydrogen, such as an alkane, alkene, alkyne or arene. It is nonpolar and held together by London dispersion forces, so boiling point rises with chain length and falls with branching.
Detail
Meaning: A hydrocarbon contains only carbon and hydrogen. Alkanes have only single bonds (saturated); alkenes, alkynes and arenes such as benzene are unsaturated. C–H bonds are nearly nonpolar, so hydrocarbons do not dissolve well in water and are held together only by London dispersion forces. Those forces grow with surface area, so boiling point rises with chain length and falls with branching, which makes a molecule more compact.
Equation: CnH2n+2 for an acyclic alkane, where n is the number of carbon atoms (a pure number with no unit). Each ring or pi bond removes two hydrogens.
On the MCAT: Expect rankings of boiling point or solubility for alkanes, alkenes and a similar-sized alcohol, plus degrees of unsaturation, combustion to CO₂ and H₂O, and free-radical halogenation of alkanes. In chromatography passages, nonpolar hydrocarbons usually elute first from a polar silica column.
Common confusion: Not every organic molecule is a hydrocarbon. Once O, N or a halogen is present, as in an alcohol or alkyl halide, the compound is a hydrocarbon derivative. Also, saturated does not mean acyclic: cycloalkanes are saturated hydrocarbons with the formula CnH2n.
Sources
- Organic Chemistry (Clayden, Greeves, Warren)
- Organic Chemistry (Wade)
- Organic Chemistry (Klein)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.