Organic Compounds, Reactions and Polymers

Organic Chemistry becomes manageable when you follow the same order every time: count carbon atoms, identify the homologous series, then apply the reaction required at G2 depth.

1. Definition

A hydrocarbon contains carbon and hydrogen only. An alkane is a saturated hydrocarbon with only carbon–carbon single bonds. An alkene is an unsaturated hydrocarbon containing a carbon–carbon double bond.

A homologous series is a family with a general formula and similar chemical properties whose physical properties change gradually as molecular size and mass increase. For example, boiling point and viscosity generally increase along a series.

A monomer is a small molecule that can join to many similar molecules. A polymer is the long-chain molecule formed from many monomers; its repeat unit is the smallest section that repeats along the chain. Different monomers therefore produce polymers with different repeat units.

For the unbranched compounds required here:

• alkanes follow CₙH₂ₙ₊₂;
• alkenes follow CₙH₂ₙ.

2. Key Ideas

The first three unbranched members are:

Their prescribed displayed structures are shown below. Every line is a covalent bond.

Methane        Ethane          Propane
    H           H   H           H   H   H
    |           |   |           |   |   |
H - C - H   H - C - C - H   H - C - C - C - H
    |           |   |           |   |   |
    H           H   H           H   H   H

Ethene          Propene
H   H           H   H   H
|   |           |   |   |
C = C       H - C - C = C
|   |           |       |
H   H           H       H

3. Detailed Explanations

Fuels and crude oil

Natural gas, which is mainly methane, and crude oil are non-renewable fossil fuels. Crude oil is separated by fractional distillation because its hydrocarbons have different boiling ranges. Smaller molecules generally have lower boiling points and ignite more easily.

Crude-oil fractions have competing uses. Burning a fraction as a fuel releases energy immediately, while using it as a chemical feedstock preserves its molecules for making plastics and other useful chemicals. Refinery supply is often richer in less-demanded large molecules, while demand is higher for smaller fuel molecules and alkene feedstocks.

Bioethanol from sugarcane is renewable because more sugarcane can be grown. Compared with fossil fuel, some carbon dioxide released during burning is offset by carbon dioxide taken in while the crop grows. This prescribed comparison does not prove zero emissions.

Prescribed hydrocarbon reactions

• Complete combustion: hydrocarbon + oxygen → carbon dioxide + water.
• Incomplete combustion: limited oxygen can produce carbon monoxide and/or soot.
• Substitution: methane is generally unreactive, but it reacts with chlorine in UV light; one hydrogen atom is replaced to form chloromethane and hydrogen chloride: CH₄ + Cl₂ → CH₃Cl + HCl.
• Cracking: a larger alkane is heated with a catalyst to form smaller alkanes, alkenes and sometimes hydrogen. This converts less-demanded large refinery fractions into smaller fuels and chemical feedstocks for which demand is greater.
• Addition: atoms add across an alkene’s double bond.
• Hydrogenation: hydrogen adds across C=C with a nickel catalyst. Polyunsaturated vegetable oils contain molecules with more than one carbon–carbon double bond. Their hydrogenation makes a more saturated, solid product used to manufacture margarine.

In the bromine-water reaction, the two bromine atoms add across C=C. For ethene:

The product is saturated because the carbon–carbon double bond has become a single bond. The orange colour disappears because bromine is used up; “turns clear” is not a precise observation.

For example:

Addition polymers

Many alkene molecules join when their double bonds open. No small molecule is lost:

To deduce a polymer from an alkene monomer, open C=C to C–C, keep the substituents on the same carbon atoms, and put the repeat unit in brackets with continuation bonds. To work backwards, isolate the smallest repeat unit, remove its continuation bonds and restore C=C between the two backbone carbon atoms. For example, ethene gives [-CH₂-CH₂-]ₙ, and that repeat unit identifies ethene as its monomer.

Poly(ethene) is used for plastic bags and clingfilm. Its non-biodegradable waste can persist and accumulate.

A balanced discussion separates three kinds of issue. Social factors include convenient collection, clear sorting instructions, public participation in local recycling programmes and confidence that separated waste is handled properly. Economic factors include the cost of collection, sorting, cleaning and heating, compared with the value of the recycled material or cracking products. Environmental factors include reducing persistent waste, using energy and the emissions produced if recovered fuels are burned. A justified decision connects the waste type to a suitable route instead of claiming that one method is always best.

4. Common Mistakes

• Calling crude oil a pure substance.
• Writing that every fraction has one boiling point instead of a boiling range.
• Saying bromine water “turns clear”; state orange to colourless.
• Giving UV light as the condition for alkene addition instead of alkane substitution.
• Leaving C=C in an addition-polymer repeat unit.
• Reversing a repeat unit without restoring the monomer’s C=C bond.
• Calling every unsaturated oil polyunsaturated; polyunsaturated means that its molecules contain more than one C=C bond.
• Claiming recycling has no energy, sorting or contamination cost.
• Using butane, structural isomers, alcohols or condensation polymers in a G2 answer.

5. Exam Tips

6. Worked Examples

7. Mind Stretchers

8. Quiz