MCAT Organic Chemistry > Hydrocarbons

Background

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

Hydrocarbon Principles

The general chemical formula for alkanes is CnH2n+2.

# CarbonNameMolecular FormulaStructural Formula
1MethaneCH4CH4
2EthaneC2H6CH3CH3
3PropaneC3H8CH3CH2CH3
4ButaneC4H10CH3CH2CH2CH3
5PentaneC5H12CH3CH2CH2CH2CH3
6HexaneC6H14CH3(CH2)4CH3
7HeptaneC7H16H3(CH2)5CH3
8OctaneC8H18H3(CH2)6CH3
9NonaneC9H20H3(CH2)7CH3
10DecaneC10H22H3(CH2)8CH3

Important Hydrocarbons

MCAT Hydrocarbons - IsobuteneMCAT Hydrocarbons - Isobutene Model

Isobutene (C4H8)

MCAT Hydrocarbons - NeopentaneMCAT Hydrocarbons - Neopentane Model

Neopentane (C4H8)

Tert-Butyl ((CH3)3C-)

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.

Ring Strain

See Ring Strain

Hydrocarbon Reactions

Combustion

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

MCAT Hydrocarbons - Combustion Diagram

Halogenation

MCAT Hydrocarbons - Initiation, Propogation and Termnation

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

MCAT Organic Chemistry

Organic Chemistry Topics