CHEM 2 - Alkenes

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  • Created by: Franklin
  • Created on: 27-05-14 17:42


  • Alkenes are unsaturated hydrocarbons
  • General formula CnH2n
  • They are the starting material for many different products mainly plastics
  • There is no rotation about the double bond
  • pi bond made from 2 p orbitals overlap: half above half below. This causes restricted rotation


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Isomers of alkenes

Alkenes have structural isomers and geometrical isomers.



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Isomers (cont.)


If the two groups with the higher priorities are on the same side of the double bond, that is described as the (Z)- isomer. If not it is the (E)- isomer.

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Properties of alkenes

  • The double bond doesn't really affect melting point and boiling point
  • However the double bond does affect reactivity
  • The double bond is an electron-rich area so it attracts electrophiles
  • Alkenes undergo electrophilic addition
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Reactions of alkenes

Electrophilic addition

  • The electrophile is attracted to the C-C double bond 
  • Electrophiles are positively charged and accept a pair of electrons
  • A carbocation is formed


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Reactions of alkenes (cont.)

When bromine reacts with propene bromopropane is produced. However this can be 1-bromopropane or 2-bromopropane

  • Due to the inductive effect, it is almost all 2-bromopropane
  • Alkyl groups have a tendency to release electrons which stabilizes the intermediate carbocation
  • Therefore the more alkyl groups, the more stable
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Addition of sulfuric acid

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Addition of sulfuric acid

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Nucleophillic Substitution - ammonia

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Free radical substitution

When a mixture of a halogen and alkane are exposed to sunlight, a haloalkane forms

Free-radical substitution

  • Initiation - Breaking of the diatomic halogen molecule into two free radicals
  • Propagation - The free-radical takes a hydrogen from the alkane to form a hydrogen halide and another free radical
  • Termination - Two free radicals join to form a stable molecule


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