Consider the following reactions,
Now consider the following reaction.
Here the sp2carbon of the aldehyde becomes a sp3 carbon and more importantly it becomes a stereo centre. So what?
Well a compound with one stereo centre can exist as a pair of enantiomers, with the configuration as either ( R ) or ( S ) and the compound can be either dextro or laevo rotatory. Now the question is, in the above reaction what is the configuration of the product? Is it (R) or (S)? Well experimentally it has been observed to be a mixture (racemic mixture). In organic synthesis, When a sp2carbon is converted to sp3, the product formed is always a racemic mixture. One more point let us look at the addition reaction once again, yes here a change in hybridsation state has occured, yet the product is not a racemic mixture, because it is not optically active, so the statement made has to be modified as follows. When a sp2 carbon is converted to sp3 the product formed is always a racemic mixture provided the carbon concerned in the product has become a stereo centre.
Why a racemic mixture? Why not only one of them?
The reason will be clear when we consider the mechanism of the reaction.