MCAT Organic Chemistry > Hydrocarbons
A saturated hydrocarbon consists of only carbons and hydrogens and only contains single bonds in the structure. The structure must not be cyclic in nature, therefore, forming an open chain. These saturated hydrocarbons are most commonly referred to as alkanes
The general chemical formula for alkanes is CnH2n+2.
|# Carbon||Name||Molecular Formula||Structural Formula|
Overall hydrocarbons tend to have lower boiling points than other molecules with functional groups.
As molecular weight increases, so do the boiling point, melting point, and density. The heavier a molecule is, the harder it would be for it to break away from others and enter the high energy gas phase.
Branched molecules follow similar trends, but have slightly lower boiling points than their straight chained isomers. This is because branching reduces surface area available for interaction with other molecules as there are less van der Waals forces.
The more symmetrical the molecule, the higher the melting point.
All hydrocarbons are hydrophobic.
See Ring Strain
Combustion is the reaction of alkanes with molecular oxygen to form carbon dioxide, water, and heat. It is an exothermic reaction that often produces a flame (heat and light energy). Hydrocarbons are one of the most effective fuel sources for this reaction and can be in the gas, liquid or solid phase.
CH4 + 2O2 → CO2 + 2H2O
The general formula for combustion is
CxHy + zO2 + xCO2 + (y/2)H2O
Fuel + Oxygen → Carbon Dioxide + Water
z = x + 1/4y
Halogenation is a process in which one or more hydrogens are replaced with a halogen atom (Cl, Br, I) via the free-radical substitution mechanism. This reaction often requires the use of UV light or peroxides to initiate the reaction and provide the energy necessary to break the bonds. For example, the cholorination of methane gas proceeds as follows...
CH4 + Cl2 → CH3Cl + HCl
Alkane + Halogen → Alkyl Halide
Halogenation is a type of substitution that occurs via a free radical mechanism and has a preference of occurring at the most substituted carbon atom.
The free radical mechanism is dependent entirely on the presence of free radicals and therefore anything that inhibits free radicals will inhibit the reaction. Oxygen is a great inhibitor of halogenation reactions along with anti-oxidants which bond easily to free radicals.
Stability of Free Radicals
Free radicals can be sorted based on the following scheme.
Tertiary (3°) > Secondary (2°) > Primary (1°) > Methyl