Hydrocarbons

In short: Hydrocarbons are compounds of only carbon and hydrogen. Alkanes have only single bonds (saturated), while alkenes have a C=C double bond and alkynes a triple bond (unsaturated), and longer chains have higher boiling points, which is how crude oil is separated.

What is a hydrocarbon?

A hydrocarbon contains only carbon and hydrogen.

Series	Bonds	General formula	First member	Saturated?
Alkanes	C−C single only	$C_{n} H_{2n + 2}$	methane $C H_{4}$	Saturated
Alkenes	one C=C double	$C_{n} H_{2n}$	ethene $C_{2} H_{4}$	Unsaturated
Alkynes	one C≡C triple	$C_{n} H_{2n - 2}$	ethyne $C_{2} H_{2}$	Unsaturated

Saturated means every carbon has the most hydrogen atoms possible (no double or triple bonds). Unsaturated compounds have double or triple bonds and can add more atoms.

Cycloalkanes like cyclohexane, $C_{6} H_{12}$ , are rings of carbons with single bonds. They have the same formula as the alkenes but are saturated.

Shapes

The shape around each carbon depends on its bonds (VSEPR):

Bonds on the carbon	Shape	Angle	Example
Four single bonds	Tetrahedral	109.5°	every C in ethane
One double, two single	Trigonal planar	120°	each C in ethene (flat molecule)
One triple, one single	Linear	180°	each C in ethyne

Intermolecular forces and boiling points

Hydrocarbon molecules are non-polar. The only forces between them are dispersion forces (weak temporary attractions caused by moving electrons).

Longer chains have higher boiling points: more electrons and more surface contact, so stronger dispersion forces.
Branched chains have lower boiling points than straight chains of the same formula: they are more compact, with less surface contact.

Alkane	Carbons	Boiling point (°C)	State at 25 °C
Methane	1	−162	Gas
Butane	4	−1	Gas
Pentane	5	36	Liquid
Octane	8	126	Liquid
2,2-dimethylpropane	5	10	Gas

Alkenes and alkynes have similar boiling points to alkanes of the same size.

Solubility

Hydrocarbons are insoluble in water. Water molecules hold each other tightly with hydrogen bonds, and non-polar hydrocarbons cannot form hydrogen bonds to break into that network. Hydrocarbons do dissolve in other non-polar solvents ("like dissolves like"). Being less dense than water, they float on top, which is why oil spills spread across the sea.

Fractional distillation

Crude oil is separated by fractional distillation. It is heated so most of it boils, and the vapour rises up a tall column that is hot at the bottom and cool at the top. Each fraction condenses at a different height:

Small molecules (low boiling point) rise to the top: refinery gas, petrol.
Large molecules (high boiling point) condense near the bottom: diesel, lubricating oil, bitumen.

Safe handling

Hydrocarbons are flammable, and their vapours can catch fire well away from the liquid. Small ones are also volatile (evaporate easily). In the lab:

Keep them away from flames. Heat with a water bath or heating mantle, not a Bunsen burner.
Use a fume hood, because vapours can be toxic.
Store in sealed, labelled containers. Read the Safety Data Sheet (SDS).

Effects of using hydrocarbons

Area	Effects
Environmental	$C O_{2}$ from burning adds to the enhanced greenhouse effect and global warming. Extra $C O_{2}$ causes ocean acidification. Sulfur and nitrogen oxides cause acid rain. Incomplete combustion gives toxic CO and soot. Oil spills harm wildlife.
Economic	Fuels power transport and industry; oil sales earn money for countries; prices can rise and fall sharply. Cleaning up pollution costs money.
Social	Cheap energy improves living standards. Air pollution harms health. Supplies are non-renewable and will run out, so future generations will need other sources.

Note: the hole in the ozone layer was caused by CFCs (chlorofluorocarbons), not by burning hydrocarbons.

Hydrocarbons

Types

Alkanes: CnH2n+2, saturated
Alkenes: CnH2n, C=C
Alkynes: CnH2n−2, C≡C

Shapes

Tetrahedral 109.5°
Trigonal planar 120°
Linear 180°

Forces

Non-polar
Dispersion only
Longer chain, higher bp
Branched, lower bp

Properties

Insoluble in water
Flammable, volatile

Crude oil

Fractional distillation

Effects

Global warming
Acid rain
Non-renewable

Year 11Intermolecular forces compared

The strength of intermolecular forces, from weakest to strongest:

dispersion forces < dipole-dipole forces < hydrogen bonds

Dispersion forces act between all molecules. They are the only forces in non-polar molecules like hydrocarbons.
Dipole-dipole forces act between polar molecules, such as chloromethane or propanone.
Hydrogen bonds form when H is bonded to N, O or F and is attracted to an N, O or F on another molecule.

So for molecules of similar size, boiling point increases: alkane < haloalkane or ketone < alcohol. For example, propane (−42 °C) < propanone (56 °C) < propan-1-ol (97 °C).

Carboxylic acids boil highest. The −COOH group has both a C=O and an O−H, so acid molecules hydrogen bond more strongly than alcohols (pairs of acid molecules even hold together in twos). For similar molar masses: pentane < butan-1-ol < propanoic acid.

Year 12Why carbon makes so many compounds

Carbon has four valence electrons, so it forms four strong covalent bonds. C−C bonds are strong enough to make long stable chains (this is called catenation), and carbon can form single, double and triple bonds and rings. Silicon, below carbon in group 14, makes much weaker Si−Si bonds, so it cannot form long stable chains in the same way.

Combustion energy and efficiency. All hydrocarbons burn in excess oxygen to make carbon dioxide and water:

$C_{8} H_{18} (l) + \frac{25}{2} O_{2} (g) \to 8C O_{2} (g) + 9 H_{2} O (l)$

In limited oxygen, incomplete combustion produces carbon monoxide or soot (C) instead of $C O_{2}$ , and releases less energy.

Compare the equations: $2 C_{8} H_{18} + 17 O_{2} \to 16CO + 18 H_{2} O$ needs 17 mol of oxygen, while complete combustion of the same octane needs 25. Less oxygen is used, less energy is released, and the CO is a poisonous gas.

Telling isomers apart. Cyclohexane and hex-1-ene are both $C_{6} H_{12}$ , but cyclohexane has only single bonds. Hex-1-ene turns bromine water colourless at once; cyclohexane does not.

Quick summary

Hydrocarbons contain only C and H. Alkanes are saturated; alkenes and alkynes are unsaturated.
Shapes: tetrahedral (single bonds), trigonal planar (C=C), linear (C≡C).
Only dispersion forces: longer chains have higher boiling points, branching lowers them.
Hydrocarbons are insoluble in water, flammable and volatile.
Fractional distillation separates crude oil by boiling point.
Burning hydrocarbons causes global warming, ocean acidification and acid rain, and the supply is non-renewable.

Quiz

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A hydrocarbon contains:
1. A Only carbon and hydrogen
2. B Carbon, hydrogen and oxygen
3. C Only carbon
4. D Carbon and any other element
Show answer
A: Only carbon and hydrogen
Hydro (hydrogen) + carbon.
Alkanes are described as saturated because they have:
1. A Only single C−C bonds
2. B A double bond
3. C A triple bond
4. D Oxygen atoms
Show answer
A: Only single C−C bonds
Every carbon holds as many hydrogens as possible.
The general formula of the alkanes is:
1. A $C_{n} H_{2n}$
2. B $C_{n} H_{2n + 2}$
3. C $C_{n} H_{2n - 2}$
4. D $C_{n} H_{n}$
Show answer
B: $C_{n} H_{2n + 2}$
For example, propane is $C_{3} H_{8}$ .

What is a hydrocarbon?

Shapes

Intermolecular forces and boiling points

Solubility

Fractional distillation

Safe handling

Effects of using hydrocarbons

Quick summary

Quiz

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