Hazard, risk and organic nomenclatureEdexcel International A Level Chemistry: Revision notes

A hazard is a property of a substance or a procedure that has the potential to cause harm, for example a flammable liquid. Risk is the likelihood that the hazard will actually cause harm in a particular situation, together with how serious the harm would be.

Organic compounds present hazards such as being flammable (most hydrocarbons, alcohols and their vapours), toxic (methanol), corrosive (carboxylic acids), irritant, harmful to the environment or carcinogenic (benzene).

A risk assessment is carried out before practical work to identify the hazards, evaluate the risk and decide on precautions.

There are three ways to reduce risk:

• work on a smaller scale, using smaller volumes or masses, so less is present to cause harm
• take precautions specific to the hazard: eye protection and gloves for corrosive liquids, a fume cupboard for toxic or irritant vapours, no naked flames and an electric heater or water bath for flammable liquids
• use an alternative method involving less hazardous substances, for example replacing a toxic reagent with a safer one

The precaution must match the hazard: eye protection does not reduce the risk of a fire.

A functional group is the atom or group of atoms that is responsible for the characteristic reactions of a compound. A homologous series is a family of compounds with the same functional group and the same general formula, in which each member differs from the next by CH₂. Members have similar chemical properties and a gradual change in physical properties such as boiling temperature.

• alkanes, CₙH₂ₙ₊₂
• alkenes, CₙH₂ₙ, with C=C
• halogenoalkanes, CₙH₂ₙ₊₁X, with –Cl, –Br or –I
• alcohols, CₙH₂ₙ₊₁OH, with –OH
• aldehydes, with –CHO
• ketones, with C=O inside the carbon chain
• carboxylic acids, with –COOH

The stem shows the number of carbon atoms in the longest chain: meth (1), eth (2), prop (3), but (4), pent (5), hex (6), hept (7), oct (8), non (9), dec (10).

Rules:

1. Find the longest carbon chain that contains the functional group; this gives the stem.
2. The suffix shows the functional group: -ane (alkane), -ene (alkene), -ol (alcohol), -al (aldehyde), -one (ketone), -oic acid (carboxylic acid).
3. Prefixes show side chains and halogens: methyl, ethyl, chloro-, bromo-, iodo-. List them in alphabetical order.
4. Number the chain from the end that gives the lowest numbers to the functional group first, then the substituents.
5. Use di-, tri- for repeated groups, with a number for each.

Examples: CH₃CH(CH₃)CH₂CH₃ is 2-methylbutane; CH₃CH₂CHBrCH₃ is 2-bromobutane; CH₃CH=CHCH₂CH₃ is pent-2-ene; (CH₃)₂CHCH₂CH₂OH is 3-methylbutan-1-ol.

• Molecular formula shows the actual number of atoms of each element, e.g. C₄H₁₀.
• Structural formula shows the arrangement of atoms without showing every bond, e.g. CH₃CH₂CH₂CH₃ or (CH₃)₂CHCH₃.
• Displayed formula shows every atom and every bond, with every C–H bond drawn.
• Skeletal formula shows only the carbon skeleton as zigzag lines, with the functional groups drawn out; hydrogen atoms on carbon atoms are not shown and each corner and end is a carbon atom.

Check that every carbon atom has four bonds when you convert between formulae.