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Double bonds and electrophiles

The double bond of an alkene is an area of high electron density, and therefore an area of high negative charge.

The negative charge of the double bond may be attacked by electron-deficient species, which will accept a lone pair of electrons.

Alkenes undergo addition reactions when attacked by electrophiles. This is called electrophilic addition.

These species have either a full positive charge or slight positive charge on one or more of their atoms. They are called electrophiles, meaning ‘electron loving’.

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Electrophilic addition mechanism: 1

In the first stage of electrophilic addition, the positive charge on the electrophile is attracted to the electron density in the double bond.

As the electrophile approaches the double bond, electrons in the A–B bond are repelled towards B.

The pi bond breaks, and A bonds to the carbon, forming a carbocation – an ion with a positively-charge carbon atom. The two electrons in the A–B bond move to B forming a B- ion.

δ+ δ-

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Electrophilic addition mechanism: 2

In the second stage of electrophilic addition, the B- ion acts as a nucleophile and attacks the carbocation.

The lone pair of electrons on the B- ion are attracted towards the positively-charged carbon in the carbocation, causing B to bond to it.

Because both electrons in the bond that joins B- to the carbocation ion come from B-, the bond is a co-ordinate bond.

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What is hydrogenation?

Hydrogen can be added to the carbon–carbon double bond using a nickel catalyst in a process called hydrogenation.

C2H4 + H2 C2H6

Vegetable oils are unsaturated and may be hydrogenated to make margarine, which has a higher melting point.

As well as a nickel catalyst, this requires a temperature of 200 °C and a pressure of 1000 kPa.

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Testing for alkenes

The presence of unsaturation (a carbon–carbon double bond) can be detected using bromine water, a red/orange coloured solution of bromine.

A few drops of bromine water are added to the test liquid and shaken. If a carbon–carbon double bond is present, the bromine adds across it and the solution turns colourless.

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More on the bromine water test

A simple equation for the bromine water test with ethene is:

However, because water is present in such a large amount, a water molecule (which has a lone pair) adds to one of the carbon atoms, followed by the loss of a H+ ion.

CH2=CH2 + Br2 + H2O CH2BrCH2Br + H2O

CH2=CH2 + Br2 + H2O CH2BrCH2OH + HBr

The major product of the test is not 1,2-dibromoethane (CH2BrCH2Br) but 2-bromoethan-1-ol (CH2BrCH2OH).

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Electrophilic addition reactions

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Addition to unsymmetrical alkenes

When an electrophile (e.g. HBr) attacks an alkene with three or more carbon atoms (e.g. propene), a mix of products is formed. This is because these alkenes are unsymmetrical.

Unequal amounts of each product are formed due to the relative stabilities of the carbocation intermediates.

minor product:1-bromopropane

major product:2-bromopropane

HBr

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A chain of carbon atoms can be represented by R when drawing organic structures. This is an alkyl group (general formula CnH2n+1).

Primary (1°) carbocations have one alkyl group attached to the positively-charged carbon.

Secondary (2°) carbocations have two alkyl groups attached to the positively-charged carbon.

Tertiary (3°) carbocations have three alkyl groups attached to the positively-charged carbon.

Structure of carbocations

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The stability of carbocations increases as the number of alkyl groups on the positively-charged carbon atom increases.

The stability increases because alkyl groups contain a greater electron density than hydrogen atoms. This density is attracted towards, and reduces, the positive charge on the carbon atom, which has a stabilizing effect.

Stability of carbocations

increasing stability

tertiaryprimary secondary

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Structure of carbocations

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Electrophiles: true or false?