Alkanes
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ALKANES
Alkanes are the simplest of hydrocarbons or rather the simplest family of hydrocarbons they are compounds containing carbon and hydrogen only. They only contain carbon-hydrogen bonds and carbon-carbon single bonds. The first six are:
| methane | CH4 |
| ethane | C2H6 |
| propane | C3H8 |
| butane | C4H10 |
| pentane | C5H12 |
| hexane | C6H14 |
Isomerism exibited by alkanes
All the alkanes with 4 or more carbon atoms in them show structural isomerism. This means that there are two or more different structural formulae that you can draw for each molecular formula.
For example, C4H10 could be either of these two different molecules:
These are called butane and 2-methylpropane respectively
What are Cycloalkanes
Cycloalkanes again only contain carbon-hydrogen bonds and carbon-carbon single bonds, but this time the carbon atoms are joined up in a ring. The smallest cycloalkane is cyclopropane.
the general formula for a cycloalkane is CnH2n.
Physical Properties Boiling PointsIt should be noted that the first four alkanes are gases at room temperature. Solids don’t start to appear until about C17H36.
and the fact that each isomer has a different melting and boiling point thus by the time you get 17 carbons into an alkane, there are unbelievable numbers of isomers!
Cycloalkanes have boiling points which are about 10 - 20 K higher than the corresponding straight chain alkane.
Explanations
There isn’t much electronegativity difference between carbon and hydrogen, so there is hardly any polarity. The molecules themselves also have very less polarity. A totally symmetrical molecule like methane is completely non-polar.
Thus the only attractions between one molecule and its neighbours are be Van der Waals dispersion forces. Although these are very small for a molecule like methane, but the force increases as the molecules get bigger. thus explaining why the boiling points of the alkanes increase with their molecular size.
In the case of isomers, the more branched the chain, the lower the boiling point tends to be. Van der Waals dispersion forces are less for smaller molecules, and only act over very short distances between the molecule and its neighbours. Thus it is more difficult for short fat molecules (with lots of branching) to lie as close together as long thin ones.
the boiling points of the three isomers of C5H12 are:
| boiling point (K) | |
|---|---|
| pentane | 309.2 |
| 2-methylbutane | 301.0 |
| 2,2-dimethylpropane | 282.6 |
The facts
Alkanes are virtually insoluble in water, but dissolve in organic solvents. The liquid alkanes are good solvents for many other covalent compounds.
Explanations
Solubility in water
In the mechanism of solution in water, the following happen
-
a break the intermolecular forces within the substance. In the case of the alkanes, these are Van der Waals dispersion forces.
-
a break the intermolecular forces in the water so that the substance can fit between the water molecules. In water the main intermolecular attractions are hydrogen bonds.
Breaking either of these attractions needs energy, although the amount of energy to break the Van der Waals dispersion forces in smaller alkanes like methane is pretty negligible. This fact howevee does not hold true for hydrogen bonds in water.
thus, a substance will dissolve if there is enough energy released when new bonds are formed between the substance and the water to make up for what is used in breaking the original attractions.
The only new attractions between the alkane and water molecules are Van der Waals. These don’t release anything like enough energy to compensate for what you need to break the hydrogen bonds in water . Thus alkanes doesn’t dissolve in water.
Solubility in organic solvents
In most of the organic solvents, the main forces of attraction between the solvent molecules are Van der Waals - either dispersion forces or dipole-dipole attractions.
That means that when an alkane dissolves in an organic solvent, thre is a breaking of Van der Waals forces and replacement by new Van der Waals forces. The two processes more or less cancel each other out energetically - so there isn’t any barrier to solubility.
Chemical Reactivity
Alkanes
Alkanes contain strong carbon-carbon single bonds and strong carbon-hydrogen bonds. The carbon-hydrogen bonds are very slightly polar and so there aren’t any bits of the molecules which carry any significant amount of positive or negative charge which other things might be attracted to.
Thus alkanes have a fairly restricted set of reactions.
- Burning - destroying the whole molecule;
- reaction with some halogens, breaking carbon-hydrogen bonds;
- Cracking resulting in breaking carbon-carbon bonds.