Organic chemistry · Lesson 3

What functional groups do

A handful of reaction types covers almost everything in this course: substitution, addition, oxidation and esterification. Learn the functional group and you can predict the reaction.

In 1902 the German chemist Wilhelm Normann patented a way to turn liquid vegetable oil into a soft, spreadable solid. He bubbled hydrogen through the hot oil with a finely divided nickel catalyst. Some of the C=C double bonds in the oil molecules added hydrogen and became single bonds, and the straighter molecules packed together into a solid fat.

The process made margarine cheap and widespread for a century, although partial hydrogenation also created unhealthy trans fats, and most countries now restrict it.

Normann’s process is an addition reaction, one of a small number of reaction types that account for almost all of the organic chemistry in this course. Learn to recognise the functional group, and you can predict what a molecule will do.

Alkanes: combustion and substitution

Alkanes are fairly unreactive. They burn: complete combustion gives CO₂ and water (C₃H₈ + 5O₂ → 3CO₂ + 4H₂O); with limited oxygen, carbon monoxide and soot form. In ultraviolet light they react slowly with halogens in a substitution reaction, in which a hydrogen atom is replaced:

CH₄(g) + Cl₂(g) → CH₃Cl(g) + HCl(g) (UV light)

Further substitution can give CH₂Cl₂, CHCl₃ and CCl₄, so a mixture of products forms. Bromine water is decolourised only slowly, and only in light.

Alkenes: addition

The C=C double bond makes alkenes much more reactive. In an addition reaction, one bond of the double bond breaks and a new atom or group adds to each carbon. There is only one product.

Hydrogen (hydrogenation, nickel catalyst, heat): CH₂=CH₂ + H₂ → CH₃CH₃
Halogens: CH₂=CH₂ + Br₂ → CH₂BrCH₂Br (1,2-dibromoethane). Orange bromine water is decolourised rapidly, even in the dark: the standard test for C=C.
Hydrogen halides: CH₂=CH₂ + HCl → CH₃CH₂Cl. With an unsymmetrical alkene such as propene, two products are possible (1-chloropropane and 2-chloropropane).
Water (hydration, steam with a phosphoric acid catalyst): CH₂=CH₂ + H₂O → CH₃CH₂OH. This is how industrial ethanol is made.

Alcohols: oxidation

Acidified potassium dichromate (orange to green) or acidified potassium permanganate (purple to colourless) oxidise alcohols, but what forms depends on the alcohol’s class:

Primary alcohols are oxidised first to an aldehyde, then to a carboxylic acid. Distilling the aldehyde off as it forms collects the aldehyde; heating under reflux with excess oxidant gives the acid. Ethanol → ethanal → ethanoic acid.
Secondary alcohols are oxidised to ketones, which are not oxidised further. Propan-2-ol → propanone.
Tertiary alcohols are not oxidised: the –OH carbon has no hydrogen atom to lose. The orange colour stays.

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Lab: identify six unknown liquids

Use bromine water, dichromate, sodium carbonate, water and a reflux test to identify each liquid from its functional group.

Carboxylic acids and alcohols: esterification

A carboxylic acid reacts with an alcohol, when heated under reflux with a few drops of concentrated sulfuric acid as a catalyst, to form an ester and water. The reaction is reversible:

CH₃COOH(l) + CH₃CH₂OH(l) ⇌ CH₃COOCH₂CH₃(l) + H₂O(l)

The ester is named from the alcohol (ethyl) and the acid (ethanoate). Esters have fruity smells and are used in flavours, perfumes and solvents. Carboxylic acids also react as acids: with carbonates they release CO₂, which is a useful test.

Telling compounds apart

Test	Positive result	Shows
Bromine water (no UV light)	orange colour disappears quickly	an alkene (C=C)
Acidified K₂Cr₂O₇, warm	orange to green	a primary or secondary alcohol, or an aldehyde
Acidified KMnO₄, warm	purple to colourless	as above (and alkenes)
Sodium carbonate solution	fizzing; the gas turns limewater milky	a carboxylic acid
Smell	fruity	an ester

Reaction pathways

Exam questions often chain reactions together. For example: ethene → (steam, H₃PO₄) ethanol → (acidified Cr₂O₇²⁻, reflux) ethanoic acid → (ethanol, conc. H₂SO₄) ethyl ethanoate. For each step, name the reaction type, the reagent and the conditions.

Common traps

Substitution (alkanes) gives two products, one of which is a hydrogen halide; addition (alkenes) gives one.
Tertiary alcohols are not oxidised by dichromate. Ketones are not oxidised further.
Concentrated sulfuric acid is a catalyst in esterification, not a reactant.

Now put it to workFive worked examples, twenty practice questions and ten harder problems on this lesson.

Go to the questions