Re: aldehyde/ketone + HCN why product begin with 2-hydroxynitrile?

Both, aldehydes and ketones, contain a carbonyl group ( that is carbon 
bonded to an oxygen with a double bond: >C=O ). The reaction with hydrogen 
cyanide is a typical example for addition to this group. In the course of 
the reaction the hydrogen cyanide adds in such a manner that the cyano 
group attaches to the carbon, and the hydrogen attaches to the oxygen of 
the carbonyl group:


As you can see, in the end product the hydroxy and the cyano group are 
both attached to the same carbon atom.
For most of the nitriles  the carbon atom of the CN group is numbered as 
the first atom of the main chain. Therefore, the “2” refers to the number 
of the carbon atom to which the hydroxy group is attached. It is almost 
always “2”, except for some complex molecules. 

Here are some examples that might make the numbering clearer:


(Sometmes the 2-hydroxynitriles are also called “cyanohydrines” or alpha-
hydroxynitriles)

A very good site to acquaint yourself with the rules for naming of the 
organic compounds is: 
 http://www.sci.ouc.bc.ca/chem/nomenclature/index-2.htm

From the above link you can learn how to determine which is the main 
carbon chain in a molecule and how to number it.