What is regioselective reactions explain with example?

The chemical reaction which give two or more different product and these different products are structural or constitutional isomers  to each other but of the isomer is major product , such type of reactions are also called "regioselective".

What is the stereospecific reaction explain with example ?

The chemical reaction in which a stereoselective reactant gives another stereoselective product called Stereospecific reaction for example trans addition of HBr with with but-2-ene give 92% meso-product but cis but-2ene give almost l00%(dl)/ racemic mixture. The sterespecificity of the addition can be explained in terms of a bridged structure in analogy to the bridged bromonium ion in the ionic addition of HBr to alkenes.

What are the groups that NaBH4 can and cannot reduce?

(1) NaBH4 is a milder and selective reducing agent than LiAIH4. It can reduce aldehydes,  ketones and acid chlorides without attacking other reducible groups such as esters epoxides, catboxylic acids, esters, amides olefIns etc.

(2) NaBH4 is also used in the second step of the Oxymercuration Demurcuration addition reaction to replace mercury (Hg)(Demurcuration) with H

NaBH4  can reduces:  

(1) Aldehydes into primary alcohols.     

(2) Ketones into secondary alcohols.  

(3) Acid chlorides into primaryalcohols.

 NaBH4 can't reduce:

Acids, Amides, Esters, anhydrides, Nitriles, Isonitriles, alkenes, alkynes etc.