11. Organic chemistry

Displayed formula of methane:
H
|
H—C—H
|
H

First place the atoms in the required arrangement, then add bonds until carbon has four bonds, hydrogen one bond, oxygen two bonds and halogens one bond. Finally count both atoms and bonds against the molecular formula.

A general formula represents every member of one homologous series. Substitute the stated value of n, then simplify the number of atoms without changing the functional group.

Scan the structure for the characteristic group before using the rest of the carbon chain. Molecules with the same functional group normally undergo similar types of chemical reaction.

Read from left to right and use brackets for branches when needed. The order of grouped atoms must preserve which atoms are bonded to which.

To test a proposed pair, count every atom to confirm the molecular formulae are identical, then compare connectivity to confirm the structural formulae differ.

The shared functional group controls the characteristic reactions. The remaining carbon chain can change in length without changing the family identity.

Inspect only the bonds between carbon atoms. If every C–C bond is a single bond, the molecule is saturated under this definition.

Look for a carbon–carbon double bond, C=C, or carbon–carbon triple bond, C≡C. Finding either makes the compound unsaturated.

When comparing adjacent members, add one carbon and two hydrogens. Predict a gradual physical-property change, but keep the functional group and characteristic chemistry unchanged.

Displayed formulae:
Methane Ethane Ethene
H H H H H
| | | | |
H—C—H H—C—C—H C=C
| | | | |
H H H H H

For a named reaction product from sections 11.4–11.7, first identify its homologous series and carbon count, then draw every atom and bond with valid valencies.

From a name, use the suffix. From a displayed or structural formula, locate the functional group or C=C bond. From a molecular formula, compare it with the relevant general formula and any stated structural information.

Choose the longest unbranched carbon chain, number it from the end that gives C=C or –OH the lowest position, then add the correct suffix. Draw the structural formula first and expand it to a displayed formula when every atom and bond is required.

Name the alcohol-derived alkyl group first, then the acid-derived alkanoate group. To draw the ester, place the acid carbonyl next to –O– and attach the alcohol-derived carbon chain after that oxygen.

Treat the three names as a complete set: coal, natural gas, petroleum.

'Main constituent' means methane is the component present in the greatest proportion; natural gas can still contain smaller amounts of other gases.

Check both parts of the definition: the substance must be a compound containing carbon and hydrogen, and it must contain no other element.

It contains many different hydrocarbon molecules, including molecules with different chain lengths and boiling points. Because it is a mixture, it can be separated physically into fractions.

A hydrocarbon condenses when the column temperature falls below its boiling point. High-boiling molecules condense low down; low-boiling molecules rise further before condensing.

Shorter-chain molecules have weaker intermolecular attractions, so less energy is needed to separate them. They therefore have lower boiling points, evaporate more easily and flow more readily.

Learn each fraction and use as one pair. For vehicle fuels, distinguish gasoline for cars, kerosene for aircraft and diesel oil for diesel engines.

Methane, CH₄, ethane, CH₃CH₃, and propane, CH₃CH₂CH₃, are alkanes. None contains a C=C or C≡C bond.

Complete combustion of methane: CH₄ + 2O₂ → CO₂ + 2H₂O.

Identify the part that leaves and the part that takes its place. The carbon skeleton remains present while one bond is changed.

General pattern: R–H + Cl₂ → R–Cl + HCl. For methane: CH₄ + Cl₂ → CH₃Cl + HCl, with ultraviolet light above the arrow.

For a structural product, keep the alkane carbon skeleton, replace exactly one H with Cl, then include HCl as the second product and check atom balance.

Ethene, CH₂=CH₂, and propene, CH₃CH=CH₂, are alkenes. The C=C bond is the reactive part of each molecule.

Example: C₁₀H₂₂ → C₂H₄ + C₈H₁₈. Another possible cracking pattern is an alkane → alkene + hydrogen, provided the equation is balanced.

Cracking converts a surplus of less useful large molecules into products whose properties and chemical reactivity better match industrial demand.

Add aqueous bromine (bromine water) to the sample and observe the initial and final colour. State both the reagent and the observation.

For an alkene, the C=C double bond becomes a C–C single bond and new atoms attach to the two carbon atoms.

Locate the two carbon atoms of C=C, change the double bond to a single bond, and attach one part of the reagent to each carbon. Keep the original carbon skeleton and check every carbon has four bonds.

Yeast provides enzymes for fermentation. The glucose must be in aqueous solution, the temperature must be warm enough for enzyme activity, and oxygen is excluded so fermentation rather than aerobic respiration occurs.

Complete combustion: C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O.

If a question asks for the syllabus uses, state both: ethanol is used as a solvent and as a fuel.

Make each comparison relative: fermentation uses renewable feedstock and gentler conditions, whereas hydration is faster, continuous and gives purer ethanol.

The salt name ends in ethanoate. Its formula combines CH₃COO⁻ ions with the metal ion in the ratio needed for electrical neutrality.

The oxidation change can be represented as: C₂H₅OH + 2[O] → CH₃COOH + H₂O.

carboxylic acid + alcohol ⇌ ester + water

The alcohol supplies the first, -yl part of the ester name; the acid supplies the second, -oate part.

Polymerisation joins many monomer molecules into a long chain. The repeating structural pattern in the chain is called the repeat unit.

n CH₂=CH₂ → [–CH₂–CH₂–]ₙ

Draw the repeat unit with two backbone carbon atoms, single bonds, continuation bonds through brackets and n outside the brackets.

A plastic material contains long polymer molecules and may also contain additives that adjust colour, flexibility, strength or durability.

A disposal explanation must connect a property to a consequence. For example: chemical resistance makes a container durable in use, but also slows degradation in landfill.

Name the disposal route or location, then state a specific consequence rather than only saying 'pollution'.

A repeat unit must reproduce the chain when copied end-to-end. Include the exact atoms between equivalent continuation points, not an arbitrary visual segment.

Propene CH₂=CHCH₃ gives [–CH₂–CH(CH₃)–]ₙ. Chloroethene CH₂=CHCl gives [–CH₂–CH(Cl)–]ₙ.

Connect one functional group at each end of a monomer to a functional group on the next monomer. Remove OH from –COOH and H from –NH₂ or –OH to form water, then draw the new linkage.

Look first for C=C versus paired functional groups, then check whether a small molecule is produced.

Nylon contains repeating amide links, –CONH–, between diamine and dicarboxylic-acid residues.

Show continuation bonds through a complete repeat unit and include every atom and bond in each amide or ester linkage.

PET polymer → chemical breakdown to monomers → purification of monomers → condensation polymerisation → PET

General amino acid structure: H₂N–CH(R)–COOH. Every amino acid has an amino group, a carboxylic acid group and an R side-chain attached to the central carbon.

The –NH₂ group of one amino acid reacts with the –COOH group of another, forming an amide (peptide) link and water.

The amide/peptide linkage is –CO–NH–. It joins the carbonyl carbon of one amino acid residue to the nitrogen of the next.