Organic Chemistry: Naming, Reactions and the Marks You Lose

Organic chemistry is one of the four areas where 0620 students bleed the most marks, but not because it is conceptually deep. It is a new language bolted onto the back of the course, and the marks live in the precision of that language: exact names, fully drawn structures, the right reaction type, and polymer units drawn with every required detail. Students who treat organic as “vocabulary plus a handful of repeating reactions” find it the fastest of the four hard areas to fix. This is the exam technique; the full topic is taught under organic chemistry.

Start with the homologous series you must know

Almost every organic mark traces back to four families. Know their general formula, functional group, and one example, and most of the topic falls into place.

Series	General formula	Functional group	Example
Alkanes	CnH2n+2	none (saturated, single bonds)	methane CH4, ethane C2H6
Alkenes	CnH2n	C=C double bond	ethene C2H4, propene C3H6
Alcohols	CnH2n+1OH	-OH	ethanol C2H5OH
Carboxylic acids	CnH2n+1COOH	-COOH	ethanoic acid CH3COOH

A homologous series is a family with the same general formula and functional group, differing by CH2 each step, with a gradual trend in properties. Saying that sentence in those words earns the definition mark; paraphrasing it loosely usually does not.

Naming rules that score

0620 naming is built from a stem plus an ending:

• Stem = number of carbons: meth- (1), eth- (2), prop- (3), but- (4).
• Ending = family: -ane (alkane), -ene (alkene), -ol (alcohol), -oic acid (carboxylic acid).

So a three-carbon alcohol is propanol; a two-carbon acid is ethanoic acid; a four-carbon alkene is butene. Where a question gives a structure with the functional group not on the first carbon, a number shows its position. The marks are lost by mixing the stem and ending (writing “ethanol” for a three-carbon chain) or dropping the “-oic acid” form, so always count the carbons first, then attach the right ending.

Drawing displayed formulae without losing the mark

A displayed formula shows every atom and every bond drawn out, with no shorthand. This is pure accuracy marking, and two slips account for almost all the losses:

1. A missing hydrogen. Every carbon must have four bonds in total. A carbon drawn with three bonds is missing a hydrogen, and the structure is wrong.
2. A missing or wrong bond, especially forgetting the double bond in an alkene.

The reliable check before you commit: go carbon by carbon and count the bonds. If any carbon does not have exactly four, fix it. This one habit converts a frequently-dropped mark into a reliable one.

The reactions that repeat

The 0620 organic course tests a finite set of reactions. Learn this list with its conditions and you cover almost every reaction mark:

• Combustion. Complete combustion of alkanes and alcohols gives carbon dioxide and water; incomplete (limited oxygen) gives carbon monoxide or carbon (soot) and water.
• Substitution (alkanes). With a halogen in ultraviolet light: CH4 + Cl2 → CH3Cl + HCl. One atom swapped, a small molecule released.
• Addition (alkenes). The double bond opens and a molecule adds across it, with no by-product:
  • with bromine: C2H4 + Br2 → C2H4Br2 (this is also the test for unsaturation, below);
  • with hydrogen over a nickel catalyst: makes the alkane;
  • with steam (catalytic hydration): makes the alcohol, the industrial route to ethanol.
• Addition polymerisation. Many alkene monomers join: n ethene molecules form poly(ethene).
• Fermentation. Glucose to ethanol using yeast, warm and without air: C6H12O6 → 2C2H5OH + 2CO2.
• Oxidation of ethanol to ethanoic acid (by an oxidising agent or by microbes in air).
• Carboxylic acid reactions. They behave as acids: with reactive metals (giving hydrogen), with carbonates (giving carbon dioxide), and with bases (giving a salt and water).
• Esterification. An alcohol plus a carboxylic acid, with an acid catalyst, gives an ester plus water: ethanol + ethanoic acid → ethyl ethanoate + water.
• Cracking. A long-chain alkane is broken, with heat and a catalyst, into a shorter alkane plus an alkene (for example a useful short alkane and ethene for polymers).

Addition versus substitution: the distinction examiners test

This is a favourite trap. The deciding question is whether the molecule is saturated (only single bonds, an alkane) or unsaturated (a C=C double bond, an alkene). Alkanes undergo substitution (swap an atom, release a small molecule, needs UV light). Alkenes undergo addition (open the double bond, add across it, no by-product). Quoting the wrong type, or describing addition as “the bromine replaces a hydrogen”, loses the mark even when the rest of the answer is right.

The test for unsaturation

A guaranteed-to-appear mark: to tell an alkene from an alkane, add bromine water. With an alkene (unsaturated) it turns from orange to colourless (it is “decolourised”) as the bromine adds across the double bond. With an alkane (saturated) there is no change. The exact phrasing matters: “decolourises” or “orange to colourless” scores; “goes clear” does not, because clear and colourless are different things.

Drawing addition polymers

Polymer questions are worth easy marks that students throw away. To draw the repeat unit of an addition polymer:

1. Take the monomer and open the C=C double bond to a single bond.
2. Draw the two carbons with their side groups, inside brackets.
3. Put a continuation bond through each bracket and a small n outside, to show the unit repeats.

For poly(ethene) the repeat unit is two CH2 groups bonded, bracketed, with extension bonds and n. The marks are specifically for opening the double bond, the extension bonds, and the n. Leaving off the continuation bonds is the single most common reason this question scores zero.

The mistakes examiners flag every series

1. Naming slips: wrong stem for the carbon count, or the wrong ending for the family.
2. Displayed formulae with a missing hydrogen or bond: a carbon without four bonds.
3. Addition and substitution confused: the wrong reaction type for the molecule.
4. “Goes clear” instead of “decolourised” / “orange to colourless” in the bromine-water test.
5. Polymer repeat units without the extension bonds and n.

These are accuracy errors, not understanding errors, which is exactly why organic responds so well to deliberate practice; the same patterns appear in common exam mistakes, and the longer explain-style answers are handled in the 6-mark technique.

Worked exam question

Q (Paper 4 style): Ethene reacts with bromine. (a) State the type of reaction and explain why ethene reacts this way. (2) (b) Describe what you would see, and name the organic product. (2)

Model answer: (a) Addition (1), because ethene is unsaturated and has a carbon-carbon double bond that opens to let bromine add across it (1). (b) The orange bromine water is decolourised (1); the product is dibromoethane (1).

Mark-scheme logic: (a) the first mark is the reaction type; the second is specifically for linking it to the double bond, the reasoning a memorised answer omits. (b) “decolourised” (or “orange to colourless”) is the credited observation, not “goes clear”, and the named product secures the final mark. This is the whole organic pattern in miniature: the marks reward the precise word and the precise reason, not the general idea.

The Malaysia note

Organic chemistry usually lands late in Form 5, in the same crowded term as every other subject’s exam preparation, so it is the topic students most often meet tired and revise least. Because its marks are accuracy points rather than deep concepts, it is also the topic where focused practice pays back fastest: a short stretch spent drilling naming, displayed formulae, the repeating reactions, and polymer units typically lifts an organic score more than the same time spent anywhere else. If organic is one of the areas costing your child marks, a trial lesson can pinpoint which of these accuracy habits is leaking marks and fix it quickly.