Why are alkenes so much more reactive than alkanes?
Alkenes as unsaturated hydrocarbons containing a C=C double bond. The bonding in a double bond as a pi bond. Electrophilic addition of alkenes with hydrogen halides, sulfuric acid and bromine. Markownikoff addition and carbocation stability. Addition polymerisation.
A focused answer to the AQA A-Level Chemistry 3.3.4 specification points on alkenes. Covers the C=C double bond and pi bonding, electrophilic addition with hydrogen halides, bromine and sulfuric acid, carbocation stability and Markownikoff addition, and addition polymerisation.
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What this dot point is asking
AQA wants you to describe the bonding in a double bond, give electrophilic addition mechanisms with hydrogen halides, bromine and sulfuric acid, use carbocation stability to predict the major product (Markownikoff addition), and explain addition polymerisation.
Bonding in the double bond
Electrophilic addition
The pi bond is an exposed region of high electron density, so it attacks an electrophile (an electron-pair acceptor or a centre). Addition means the double bond opens and two new groups add across it, so the product is saturated. Because there is only one product, addition reactions have a high atom economy.
- Bromine: . Decolourising bromine water from orange to colourless is the test for a double bond.
- Concentrated sulfuric acid: adds to give an alkyl hydrogensulfate, which is hydrolysed by water to an alcohol (an overall route from alkene to alcohol via two steps).
- Hydrogen halides: add to give a halogenoalkane, following Markownikoff's rule (the major product comes from the more stable carbocation).
The pi bond also explains the restricted rotation about the double bond, which is what gives rise to E-Z stereoisomerism in suitable alkenes. Because the pi electrons sit outside the line between the nuclei and are not held as tightly as the sigma electrons, they are the reactive site, and the typical reaction is addition (electrophile plus nucleophile add across the bond) rather than the substitution seen in alkanes and benzene.
Addition polymerisation
Many alkene monomers join, with the double bond opening, to form a long saturated chain with no small molecule lost:
The repeat unit is drawn in brackets with a trailing bond at each end and the subscript . Addition polymers such as poly(ethene), poly(propene) and poly(chloroethene) (PVC) are chemically inert because the chain has only strong, non-polar and bonds, which makes them durable but also non-biodegradable, raising the waste and disposal issues that AQA expects you to discuss (recycling, incineration for energy recovery, and development of feedstock recycling back to monomers).
Try this
Q1. State the colour change when an alkene is shaken with bromine water. [1 mark]
- Cue. Orange to colourless.
Q2. Explain why 2-bromobutane is the major product when but-1-ene reacts with HBr. [3 marks]
- Cue. Forms a secondary carbocation, which is more stable than primary because alkyl groups donate electron density.
Exam-style practice questions
Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AQA 20195 marksPropene reacts with hydrogen bromide to form a major and a minor product. Outline the electrophilic addition mechanism, explain which product forms in the greater amount, and name both products.Show worked answer →
The mechanism: the pi bond attacks the hydrogen of the polar molecule, so a curly arrow goes from the pi bond to the H and a second arrow from the bond to the bromine. This forms a carbocation and a bromide ion. A curly arrow then goes from a lone pair on to the positive carbon.
Two carbocations are possible: a secondary one, , and a primary one, . The secondary carbocation is more stable because the two alkyl groups donate electron density and spread the positive charge.
The major product forms via the more stable secondary carbocation: 2-bromopropane. The minor product, via the primary carbocation, is 1-bromopropane.
Markers reward correct curly arrows (from the pi bond, not the H), the two possible carbocations, the stability comparison, and naming 2-bromopropane as major.
AQA 20213 marksDescribe a chemical test to distinguish hexane from hex-1-ene, stating the reagent, the observation with each, and the type of reaction.Show worked answer →
Reagent: bromine water (orange).
Observation: with hex-1-ene the bromine water is decolourised (turns from orange to colourless); with hexane there is no change (it stays orange).
Type of reaction: electrophilic addition of bromine across the double bond of the alkene; the saturated alkane has no double bond, so it does not react.
Markers reward the named reagent, the correct colour change for the alkene, no change for the alkane, and electrophilic addition.
Related dot points
- Alkanes as saturated hydrocarbons from crude oil, fractional distillation, cracking. Combustion of alkanes and the formation of pollutants. Free-radical substitution of alkanes by halogens, including initiation, propagation and termination.
A focused answer to the AQA A-Level Chemistry 3.3.2 specification points on alkanes. Covers fractional distillation and cracking of crude oil, complete and incomplete combustion, pollutants and catalytic converters, and the free-radical substitution mechanism.
- Nucleophilic substitution of halogenoalkanes by hydroxide, cyanide and ammonia. Elimination of halogenoalkanes to form alkenes. The effect of bond enthalpy on rate of hydrolysis. CFCs and the depletion of the ozone layer.
A focused answer to the AQA A-Level Chemistry 3.3.3 specification points on halogenoalkanes. Covers nucleophilic substitution with hydroxide, cyanide and ammonia, elimination to alkenes, how bond enthalpy controls hydrolysis rate, and the role of CFCs in ozone depletion.
- Alcohols as products of fermentation and hydration of alkenes. Classification as primary, secondary and tertiary. Oxidation of alcohols with acidified potassium dichromate(VI) to aldehydes, carboxylic acids and ketones. Elimination to form alkenes. The biofuel debate.
A focused answer to the AQA A-Level Chemistry 3.3.5 specification points on alcohols. Covers fermentation and hydration routes, primary, secondary and tertiary classification, oxidation with acidified dichromate, dehydration to alkenes, and the ethanol-as-biofuel debate.
- Addition polymers from alkenes. Condensation polymers, including polyesters and polyamides, from two monomers or one monomer with two functional groups. Identifying the repeating unit and the monomers. Hydrolysis of condensation polymers. Biodegradability and disposal of polymers.
A focused answer to the AQA A-Level Chemistry 3.3.13 specification points on polymers. Covers addition polymerisation of alkenes, condensation polyesters and polyamides, identifying repeat units and monomers, hydrolysis of condensation polymers, and the disposal and biodegradability of plastics.
- Synthetic routes for preparing one organic compound from another in several steps. Reagents and conditions for the interconversion of functional groups in aliphatic and aromatic chemistry. Practical techniques for organic preparation, including purification and the determination of percentage yield.
A focused answer to the AQA A-Level Chemistry 3.3.15 specification points on organic synthesis. Covers planning multi-step routes, the key reagents and conditions for functional-group interconversions, and practical preparation, purification and percentage-yield techniques.
Sources & how we know this
- AQA A-level Chemistry (7405) specification — AQA (2015)