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Substitution reaction

This is a reaction where one atom, or group of atoms, is replaced by another atom, or group of atoms. In the following generalized reaction you can see that the X has been replaced by the Y- ion:

RX + Y- RY + X-

The halogenoalkanes and arenes commonly undergo substitution reactions. In the reaction of an alkane with bromine in ultra-violet light a hydrogen atom is replaced by a bromine atom:

CH4 + Br2 CH3Br + HBr


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