Organic reactions - how do they happen (AS)
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Why do the halogen molecules like Br2 behave as an electrophile in reaction with an alkene even though they are non-polar?
Corrie writes .....
Good question! One might expect the bromine molecule just to 'click' on to the >C=C< bond, forming a dibromo- deriviative of the alkene.
However, experimental evidence indicates a different mechanism. An intermediate carbonium ion (carbocation) appears to be involved, usually written at A-level as (using ethene as example):
CH2Br-CH2+
To produce this cation, we can imagine 'Br+' has become attached to one of the C-atoms on the C=C bond by accepting a pair of electrons from the C=C bond. This is suggested to occur as a result of polarisation of the bromine molecule as one end of it approaches the pi-electron cloud of the C=C bond. Repulsion between the two electron clouds results in the Br closest to the pi-bond becoming d+ and the other end d-. You would draw the Br2 molecule as a little dipole in an exam, if asked to show the mechanism:
Brd- -Brd+
The electron deficient Br atom then accepts a pair of electrons from the pi-bond, to form a bond to one of the C-atoms. Bromine is thus behaving as an electrophile (and the alkene as a nucleophile!) here.
At the same time the pair of electrons in the Br-Br bond moves to the other Br atom, breaking the Br-Br bond and forming a Br- ion.
In an exam you would be expected to show these shifts of pairs of electrons by means of 'curly arrows', which start at a pair of electrons on the move and end at where they finally finish up - as a lone pair or as a new bond.
The addition is complete when the Br - ion attaches itself to the C-atom carrying the + charge.
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updated: 31 May 2007
