Alkane

Alkanes are chemical compounds that consist only of the elements carbon (C) and hydrogen (H) (i.e., hydrocarbons), wherein these atoms are linked together exclusively by single bonds (i.e., they are saturated compounds).

In general, the number of carbon atoms is often used to define the size of the alkane.

An alkyl group, generally abbreviated with the symbol R, is a functional group or side-chain that, like an alkane, consists solely of single-bonded carbon and hydrogen atoms.

The simplest possible alkane (the parent molecule) is methane, CH₄.

Saturated hydrocarbons can be:

• linear (general formula C_nH_2n + 2) wherein the carbon atoms are joined in a snake-like structure
• branched (general formula C_nH_2n + 2, n > 3) wherein the carbon backbone splits off in one or more directions
• cyclic (general formula C_nH_2n, n > 2) wherein the carbon backbone is linked so as to form a loop.

Alkanes are not very reactive and have little biological activity. Alkanes can be viewed as a molecular tree upon which can be hung the interesting biologically active/reactive portions (functional groups) of the molecule.

Properties of Alkanes

Melting Point and Learning Points

The main intermolecular interaction in hydrocarbons is van der Waals forces: the attraction of the electrons of one molecule for the nuclei of another molecule. Since these intermolecular forces are weak, the molecules are readily separated.
There are two determinants for the strength of the London Forces:

• the number of electrons surrounding the molecule, which increases with the alkane's molecular weight
• the surface area of the molecule

Under standard conditions alkanes with:

• 1 to 4 carbons are gaseous
• 5 to 17 carbons are liquids
• alkanes with more than 18 C's are solids

As the boiling point of alkanes is primarily determined by weight, it should not be a surprise that the boiling point has almost a linear relationship with the size (molecular weight) of the molecule. As a rule of thumb, the boiling point rises 20 - 30 °C for each carbon added to the chain; this rule applies to other homologous series.

A straight-chain alkane will have a boiling point higher than a branched-chain alkane due to the greater surface area in contact, thus the greater van der Waals forces, between adjacent molecules.

On the other hand, cycloalkanes tend to have higher boiling points than their linear counterparts due to the locked conformations of the molecules, which give a plane of intermolecular contact.

Solubility
Hydrocarbons, being largely nonpolar, generally have very low solubility in polar solvents such as water, which is why gasoline remains separate from water.

Reactivity

Alkanes are not very reactive and have little biological activity. Alkanes can be viewed as a molecular tree upon which can be hung the interesting biologically active/reactive portions (functional groups) of the molecule.

Reactions of Alkanes

In general, alkanes show a relatively low reactivity, because their C bonds are relatively stable and cannot be easily broken. Unlike most other organic compounds, they possess no functional groups.

They react only very poorly with ionic or other polar substances.

However redox reactions of alkanes, in particular with oxygen and the halogens, are possible as the carbon atoms are in a strongly reduced condition

Combustion

All alkanes react with oxygen in a combustion reaction, although they become increasingly difficult to ignite as the number of carbon atoms increases. The general equation for complete combustion is:

C_nH_2n+2 + (1.5n +0.5) O₂ → (n +1) H₂O + n CO₂

For example methane:
CH₄ + 1.5 O₂ → CO + 2 H₂O

Reactions with Halogens

Alkanes react with halogens. The hydrogen atoms of the alkane are progressively replaced by halogen atoms.