Why is the carbon-carbon double bond so reactive, and what does it react with?
Structure and bonding of alkenes (sigma and pi bonds, trigonal planar carbon), E/Z isomerism, electrophilic addition (hydrogen, halogens, hydrogen halides and steam), Markownikoff's rule, and addition polymerisation.
An OCR H432 module 4 answer on alkenes: pi bonding and trigonal planar shape, electrophilic addition with hydrogen, halogens, hydrogen halides and steam, Markownikoff's rule via the more stable carbocation, and addition polymerisation.
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What this topic is asking
OCR specification point 4.1.3 wants you to describe the bonding and shape of alkenes, explain E/Z isomerism, write equations and mechanisms for the electrophilic addition reactions of the double bond (with hydrogen, halogens, hydrogen halides and steam), apply Markownikoff's rule using carbocation stability, and describe addition polymerisation. The reactive double bond is the gateway to alcohols, haloalkanes and polymers.
Structure and bonding
Electrophilic addition
The four reactions and their conditions:
- Hydrogenation: with a nickel catalyst (about ), giving an alkane.
- Halogenation: or at room temperature, giving a dihaloalkane. Bromine water (orange to colourless) is the test for a double bond.
- Addition of hydrogen halides: , or , giving a haloalkane (Markownikoff product for an unsymmetrical alkene).
- Hydration: steam and a phosphoric acid catalyst at high temperature and pressure, giving an alcohol (the industrial route to ethanol from ethene).
Markownikoff's rule
Addition polymerisation
Examples in context
Example 1. Margarine from vegetable oils. Hydrogenation adds across the bonds of unsaturated vegetable oils over a nickel catalyst, raising the melting point and hardening the oil into a spread, a direct industrial use of alkene addition.
Example 2. The plastics problem. Poly(ethene) and poly(propene) are cheap and durable precisely because the addition polymer chains are unreactive, but the same inertness makes them persist in the environment, which is why recycling and alternative disposal routes matter.
Try this
Q1. State the reagent, conditions and observation for the test that distinguishes an alkene from an alkane. [2 marks]
- Cue. Bromine water at room temperature; the alkene decolourises the orange bromine, the alkane does not.
Q2. Name the product and the catalyst when ethene reacts with steam. [2 marks]
- Cue. Ethanol; phosphoric acid () catalyst.
Exam-style practice questions
Practice questions written in the style of OCR exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
OCR 20194 marksPropene reacts with hydrogen bromide. (a) Name the mechanism and outline it, showing the curly arrows. (b) State and explain which product forms in the greater amount.Show worked answer →
(a) Electrophilic addition (1). The electron-rich attacks the hydrogen of (curly arrow from the double bond to H and from the bond to Br), forming a carbocation and a bromide ion; the lone pair on then attacks the positive carbon (curly arrow from to that carbon) (1).
(b) The major product is 2-bromopropane (1). Markownikoff addition goes through the more stable secondary carbocation rather than a primary one, so bonds to the middle carbon (1).
Markers reward the mechanism name, both sets of curly arrows via the carbocation, the major product, and the more-stable-carbocation reason.
OCR 20213 marksEthene is converted into poly(ethene). (a) Name the type of polymerisation. (b) Draw the repeat unit. (c) State the atom economy of the reaction and explain your answer.Show worked answer →
(a) Addition polymerisation (1).
(b) The repeat unit is enclosed in brackets with outside (1).
(c) The atom economy is , because the alkene monomers add together to form the polymer with no other product (1).
Markers reward the polymerisation type, a correct repeat unit drawn in brackets with continuation bonds, and the atom economy with the no-by-product reason.
Related dot points
- Formulae (empirical, molecular, general, structural, displayed and skeletal), homologous series and functional groups, IUPAC nomenclature, and isomerism (structural and E/Z stereoisomerism with Cahn-Ingold-Prelog priority).
An OCR H432 module 4 answer on the basics of organic chemistry: the six types of formula, homologous series and functional groups, IUPAC nomenclature, and structural and E/Z stereoisomerism using Cahn-Ingold-Prelog priority rules.
- Structure and bonding of alkanes (sigma bonds, tetrahedral carbon), boiling-point trends, complete and incomplete combustion, pollutants, and free-radical substitution with halogens (initiation, propagation, termination).
An OCR H432 module 4 answer on alkanes: sigma bonding and tetrahedral shape, boiling-point trends from London forces, complete and incomplete combustion and pollutants, and the free-radical substitution mechanism with halogens.
- Classification and properties of alcohols (hydrogen bonding), combustion, oxidation by acidified dichromate (primary to aldehyde and carboxylic acid, secondary to ketone, tertiary not oxidised), dehydration to alkenes, and substitution to haloalkanes.
An OCR H432 module 4 answer on alcohols: classification and hydrogen bonding, combustion, oxidation by acidified potassium dichromate to aldehydes, ketones and carboxylic acids, dehydration to alkenes, and conversion to haloalkanes.
- Polarity of the carbon-halogen bond, nucleophilic substitution (with hydroxide, cyanide and ammonia), the trend in hydrolysis rate with carbon-halogen bond enthalpy, elimination to alkenes, and the role of CFCs in ozone depletion.
An OCR H432 module 4 answer on haloalkanes: the polar carbon-halogen bond, nucleophilic substitution with hydroxide, cyanide and ammonia, the hydrolysis-rate trend with bond enthalpy, elimination to alkenes, and CFCs and ozone depletion.
- Planning multi-step synthesis routes between functional groups using practical techniques (reflux, distillation, purification), and the analytical techniques of infrared spectroscopy (functional-group absorptions) and mass spectrometry (molecular ion and fragmentation).
An OCR H432 module 4 answer on organic synthesis and analysis: building multi-step reaction routes between functional groups, practical techniques, infrared spectroscopy for functional groups, and mass spectrometry for relative molecular mass and fragmentation.
Sources & how we know this
- OCR A-Level Chemistry A (H432) specification — OCR (2015)