Ethene

Ethene is the simplest alkene and the one the syllabus builds all of alkene chemistry around. Its carbon-carbon double bond is a site of reactivity: it opens up in addition reactions, gives the positive bromine-water test, and lets thousands of molecules join into poly(ethene).

Key facts

Property	Detail
Formula	C2H4 (or CH2=CH2)
Relative molecular mass (Mr)	28 (C 12, H 1)
Homologous series	Alkenes, general formula CnH2n
Bonding / structure	Simple covalent molecule with a C=C double bond
Saturation	Unsaturated
Main exam roles	Bromine-water test, addition reactions, monomer for poly(ethene)

Where this compound appears in 0620

• Alkenes (Topic 12): unsaturation, the bromine-water test, and addition reactions.
• Fuels (Topic 12): ethene is a product of cracking long-chain alkanes from crude oil.
• Alcohols (Topic 12): the manufacture of ethanol by hydration of ethene.
• Polymers (Topic 12): ethene is the monomer for poly(ethene).

Source: cracking

C10H22 -> C8H18 + C2H4 (an example of thermal cracking)

Cracking breaks long, less useful alkanes into shorter alkanes and alkenes, using a high temperature (about 600-700 C) or a catalyst. Ethene can also be made by dehydrating ethanol: C2H5OH -> C2H4 + H2O.

Reactions you must know

Bromine-water test (addition):

C2H4 + Br2 -> C2H4Br2 (1,2-dibromoethane)

Orange-brown bromine water is decolourised. This is the standard way to tell an alkene (reacts) from an alkane (no reaction).

Hydrogenation (addition of hydrogen):

C2H4 + H2 -> C2H6 (nickel catalyst, about 150 C)

This is the reaction used to harden unsaturated vegetable oils into margarine.

Hydration (addition of steam) to make ethanol:

C2H4 + H2O -> C2H5OH (phosphoric acid catalyst, about 300 C and 60 atm)

Addition polymerisation:

n(CH2=CH2) -> -(CH2-CH2)-n

Complete combustion:

C2H4 + 3O2 -> 2CO2 + 2H2O

Ethene burns with a more luminous, smokier flame than an alkane of similar size because it has a higher proportion of carbon, so it tends towards incomplete combustion.

Ethene versus ethane

The two differ by a single feature — the double bond — and that feature drives all of ethene’s extra chemistry.

	Ethane	Ethene
Formula	C2H6	C2H4
Series	Alkane	Alkene
Bonding	All single bonds (saturated)	Has a C=C double bond (unsaturated)
Bromine water	No reaction	Decolourises it (addition)
Reactivity	Fairly unreactive	Reactive at the C=C

The double bond is a region of high electron density, which is why ethene readily takes part in addition reactions while the saturated ethane does not.

Worked exam question

Ethene is an unsaturated hydrocarbon obtained by cracking.

(a) State what is meant by ‘unsaturated’ and by ‘hydrocarbon’. (2 marks)

(b) Describe a test to show that a gas is an alkene and give the result. (2 marks)

(c) Ethene reacts with steam to form ethanol. Write the equation and state the conditions. (2 marks)

Mark scheme

• (a) Unsaturated = contains a carbon-carbon double bond (C=C) [1]; hydrocarbon = a compound of hydrogen and carbon only [1]
• (b) Add / shake with bromine water [1]; the orange-brown colour is decolourised (turns colourless) [1]
• (c) C2H4 + H2O -> C2H5OH [1]; phosphoric acid catalyst, about 300 C and 60 atm [1]

Examiner note: in (b) “goes clear” scores nothing — bromine water is already clear; the change is the loss of the orange-brown colour. In (a), specify a carbon-carbon double bond, not just “a double bond”.

Common exam mistakes

• Describing the bromine result as colourless-to-colourless. Bromine water is orange-brown; the alkene decolourises it (orange-brown to colourless).
• Confusing addition with substitution. Ethene undergoes addition — the double bond opens; alkanes undergo substitution, which needs UV light.
• Leaving the double bond in the product. After addition, C=C becomes a single C-C bond, with one bromine atom on each carbon in C2H4Br2.
• Swapping hydration and hydrogenation. Hydration adds steam to give ethanol; hydrogenation adds H2 (with a nickel catalyst) to give ethane — different reagents and products.