There are a number of fairly commonly encountered electrophilic additions that have their own very particular mechanisms, some of which have been collected here. Some sections of the mechanisms may be unfamiliar; if they are, simply focus upon the electrophilic addition step that is important for this chapter – the other aspects of the mechanism should become more clear as later chapters are covered.


The addition of ozone to an alkene. The final reduction step has been included to show a likely synthetic use of this reaction – to cleave an alkene to two ketones.


The addition of two OH groups to an alkene. The reagent of choice for this is osmium tetra-oxide. However, because this reagent is very expensive and highly toxic, N-morpholene-oxide is used as a ‘co-oxidant’ to regenerate the OsO4, so the OsO4 can be used in catalytic quantities;

Note that the two OH groups are added cis to each other because of the concerted nature of the reaction.


The addition of H2 across an alkene, with a transition metal catalyst – the energy barrier for straight reaction between an alkene and H2 is too great.

The mechanism probably involves adsorption of both the alkene and the dihydrogen onto the metal surface, and thus activation of each. Note as with reaction 2, the addition is cis.