AS Chemistry - Alkenes

AS AQA Chemistry unit - 2 cards on alkenes.

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  • Created by: Kaimlee
  • Created on: 24-02-11 10:04


Unsaturated hydrocarbons, made of carbon atoms and hydrogen atoms only with one or more C=C bond.

General formula - CnH2n

Naming - where the double bond is.

example: CH2=CH2 - ethene

               CH3CH=CH2 - prop-1-ene

               CH2=CH(CH3)CH2CH2 - 2-methylbut-1-ene

Structure: unlike C-C bond, the C=C can't rotate - because has a pi orbital (contains a single electron on each carbon, these 2 orbitals overlap to form 1 orbital with a cloud of e- density above and below the C-C bond (resistriction rotation)

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Alkenes can form chain isomers, positional isomers, geometrical isomers.

Positional Isomers:

Double bond in different positions (the longer the chain the more possible isomers that can form)

                  but-2-ene                           but -1-ene

Geometrical Isomer: - form of stereoisomerism

Same structural formula, different arrangement -occurs only round C=C

Z isomer (on zzzz same side)                   E isomer (on op side)

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High electron density around C=C bond - pretty reactive

this area is easily attacted by electrophiles

Electrophiles - electron pair acceptor that are attracted to negative (e- dense) areas, because it is short of electrons itself. i.e. H+ ions, NO2+ ions, polar molecules...etc

Test for alkenes:    alkene + halogen reaction example:

Shake unknown substance with orange/brown bromine water - if alkene present the solution turns colourless forming dibromoalkane, by electrophilic addition. Has no effect on an alkane (because no C=C, alkanes are saturated hydrocarbons)

2 steps - 1.) Electronphilic Addition - formation of carbocation, 2.) Rapid Reduction with ve-ion.

Draw mechanism:

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Reactions - Alkenes


   CH2=CH2 (g) + 3O(2) (g) ----------------> 2CO(2) (g) + 2H2O (l)

An alkene will burn in air (oxygen) to produce carbon dioxide and water.

However they are not used as fuels because there reactivity makes them useful for other purposes.

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Reactions - Alkenes

Electrophilic Addition

The 4 e-'s in the C=C bond, make a centre of high electron density, electronphilies are attracted to it and form a bond using 2 of the e-'s.

  • Electrophilie attracted to double bond
  • Electrophilies are positivily charged & accept the pair of e-'s from C=C bond
  • A positive ion is formed (a carbocation)
  • The negativily charged ion forms a bond with the carbocation.

Draw 3 mechanisms - using 1.) HCl 2.) HBr 3.) HI (hydrogen halides)

(step 1:)

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Electrophilic Addition

Of the hydrogen halide the halogen group is more electronegative than the hydrogen making a polar molecule:


The electrophilie is the hydrogen of the H-halide. The hydrogen is attracted to the C=C bond -(because of the high electron density)

(step 2:) Draw mechanism:

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Unsymmetrical Alkenes - Reactions

If a hydrogen halide (HBr) reacts with an unsymmetrical alkene - there are 2 possible products. The amount of each product formed depends on how stable the carbocation formed in the middle is.

Carbocations with more alkyl groups are more stable, because there is more of a electron releasing effect, - positive inductive effect, making the carboncation more stable.

Alkyl group - e.g. methyl, ethyl, (alkanes with a hydrogen removed)

Making a major product & a minor product. e.g. 2, bromopropane - major product - more positive inductive effect

1, bromopropane - minor product - less positive inductive effect - further from alkyl group.

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Alkene reaction - conc.sulphuric acid

Reaction Conditions - room temperature (is exothermic)

Electrophilie = H atom of H2SO4 with is delta +



the carbocation reacts with -ve hydrogensulphate ion



when water is added an alcohol is formed & sulphuric acid reforms (the H2SO4 acts as a catalyst) (way of making alcohols)

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Additional Polymers

Additional Polymerisation

the double bond of the alkene can open up and join = polymers. Polymers are made up of individual small alkenes = mononmers

example: poly(ethene) is made by additional polymerisation of ethene.

Poly(ethene) - is an alkane even through called an ene.

because the C=C is lost they are unreactive (like alkanes)

different structures of these polymers = different properties and uses.

(hard, soft, flexible plastics) High density - uses buckets - v.lit branching chains/packed well. Low density - uses bags, bottles - some branching.

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Uses for Polymers

Packaging - Polystyrine

Fibres for ropes - Poly(propene)

Electrical Insulation - Polychloro(ethene)

Clothing - Poly (propenenitrile)

Cooking - Polytetraflouro(ethene)

Problems with plastics:

Difficult to decompose - non-renewable - fill up land fill sites - but could reuse/recylce - melted & remoulded. When burnt gets off poisonous fumes.

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