Alkenes and Their Reactions

(Separate Chemistry only) The whole of 4.7.2 is assessed only in GCSE Chemistry (Triple), not in Combined Science.

Alkenes are unsaturated hydrocarbons: molecules of hydrogen and carbon only that contain a carbon–carbon double bond, written C=C. That double bond is what makes them different from the alkanes, which have single bonds only.

Because they contain a double bond, alkenes have two fewer hydrogen atoms than the alkane with the same number of carbons. This is what "unsaturated" means: the molecule could add more hydrogen if the double bond opened up.

An alkene contains a C=C double bond and follows the general formula $C_n{H}_{2n}$. Alkenes are unsaturated because the double bond means they hold fewer hydrogen atoms than the matching alkane.

Compare a two-carbon alkane and alkene: ethane is C₂H₆ (saturated), while ethene is C₂H₄ (unsaturated, two fewer hydrogens).

You need to know the names and formulae of the first four alkenes. Their names end in -ene, and every one fits the general formula $C_n{H}_{2n}$.

Name	Carbons ($n$)	Molecular formula ($C_n{H}_{2n}$)
Ethene	2	C₂H₄
Propene	3	C₃H₆
Butene	4	C₄H₈
Pentene	5	C₅H₁₀

There is no alkene with one carbon, because a C=C double bond needs at least two carbon atoms. That is why the series starts at ethene, not "methene".

The displayed formula of ethene shows the double bond clearly. Each carbon still forms four bonds in total: two of them are the double bond to the other carbon.

Ethene (C₂H₄)

    H       H
     \     /
      C = C
     /     \
    H       H

The C=C double bond is the alkene functional group: the reactive part that gives the whole family its characteristic reactions. Because they all share this group, all alkenes react in similar ways.

When alkenes burn in air, they tend to undergo incomplete combustion and burn with a smoky (sooty) flame. The high proportion of carbon in the molecule is not fully oxidised when oxygen is limited, so carbon (soot) and carbon monoxide are released alongside carbon dioxide and water.

Alkenes burn in air with smoky flames because of incomplete combustion. This sooty flame is a visible contrast with the cleaner flame of a small alkane.

This smoky flame is why alkenes are not used as fuels in the way alkanes are; their value lies in their addition reactions and in making polymers.

The characteristic reaction of an alkene is an addition reaction. The C=C double bond opens up and becomes a single bond, and new atoms add on to the two carbons that were double-bonded.

In an addition reaction the C=C double bond opens to a single C–C bond, and one atom or group joins each of the two carbon atoms. One product molecule is formed, with nothing left over.

Alkenes undergo addition with hydrogen, with water (steam) and with the halogens. In each case the same double bond opens and the reagent adds across it. The three reactions below all follow this single pattern.

Addition of hydrogen (hydrogenation). An alkene reacts with hydrogen at about 150 °C over a nickel catalyst. The double bond opens and a hydrogen atom joins each carbon, turning the alkene into the matching alkane.

${\text{C}}_2{\text{H}}_4+{\text{H}}_2\to {\text{C}}_2{\text{H}}_6$

Ethene becomes ethane. Drawn out, the double bond becomes a single bond and each carbon gains one hydrogen:

   H   H            H   H
    \ /             |   |
     C=C   + H₂ →   H—C—C—H
    / \             |   |
   H   H            H   H
   ethene           ethane

Addition of halogens (halogenation). An alkene reacts with a halogen, namely chlorine (Cl₂), bromine (Br₂) or iodine (I₂), at room temperature. One halogen atom joins each carbon, so ethene and bromine make 1,2-dibromoethane; with chlorine it makes 1,2-dichloroethane and with iodine 1,2-diiodoethane, following the same pattern:

${\text{C}}_2{\text{H}}_4+{\text{Br}}_2\to {\text{C}}_2{\text{H}}_4{\text{Br}}_2$

   H   H             H   H
    \ /              |   |
     C=C   + Br₂ →  Br—C—C—Br
    / \              |   |
   H   H             H   H
   ethene         1,2-dibromoethane

This is exactly the reaction behind the bromine water test: adding across the double bond removes the orange bromine, turning it colourless.

Alkenes react with steam (water) to form alcohols. The conditions are a temperature of about 300 °C, a pressure of around 60–70 atmospheres, and a phosphoric acid catalyst. The double bond opens; an –OH group joins one carbon and an –H joins the other.

Ethene plus steam makes ethanol:

${\text{C}}_2{\text{H}}_4+{\text{H}}_2\text{O}\to {\text{C}}_2{\text{H}}_5\text{OH}$

   H   H              H   H
    \ /               |   |
     C=C  + H₂O  →    H—C—C—O—H
    / \               |   |
   H   H              H   H
   ethene             ethanol

This is an important industrial route to ethanol. The same pattern holds for the larger alkenes: propene plus steam gives propanol, and so on, with the –OH and the –H adding across the C=C.

All three alkene additions share one idea: the C=C double bond opens to a single bond and the reagent (H₂, a halogen, or H and OH from water) adds across the two carbons.

1. Using the wrong general formula

Alkenes are $C_n{H}_{2n}$, not $C_n{H}_{2n+2}$ (that is the alkanes). Ethene is C₂H₄, not C₂H₆. Mixing up the two general formulae is one of the most common errors in this topic.

2. Forgetting there is no "methene"

The alkene series starts at ethene (2 carbons), because a C=C double bond needs at least two carbon atoms. There is no one-carbon alkene.

3. Leaving the double bond in the product of an addition reaction

After an addition reaction the C=C has opened to a single bond. A displayed formula of the product must show a single C–C bond with the added atoms, not a lingering double bond.

4. Drawing carbons with the wrong number of bonds

Every carbon must have four bonds in total. In ethene, two of a carbon's four bonds are the double bond to the other carbon; after addition, each carbon has four single bonds. Count the bonds on each carbon in your product.

5. Saying alkenes always burn cleanly

Alkenes burn with smoky, sooty flames due to incomplete combustion, unlike small alkanes. Do not describe an alkene flame as clean or non-smoky.