Question

Consider the following reactions. Which of the following are regioselective reactions?

A.\[(I),(II),(V)\]
B.\[(III),(IV),(VI)\]
C.\[(I),(II),(IV)\]
D.\[(III),(IV),(V)\]

Answer 1

Hint: In the case of regioselectivity, the major product will be formed as a more substituted product. And it always esteem, at a particular atom, the bond formation will take place over other possible atoms. Here always, one product will predominate. For example, if an asymmetric reagent is added to an asymmetric alkene, there is a formation of two different products and this reaction is regioselective.

Complete answer:
Among these reactions, I, II and V are the regioselective reactions.
In the case of first reaction, cis-but-2-ene reacted with aqueous potassium permanganate. And the cis-addition will be takes place in the reaction. And there is a formation of meso tartaric acid. The reaction can be written as,
  

In the case of second reaction, cis-but-2-ene reacted with osmium tetroxide in the presence of sodium bi carbonate and there is a formation of meso tartaric acid by cis addition and the reaction can be written as,

In fifth reaction, the cis-but-2-ene reacted with osmium tetroxide in the presence of hydrogen peroxide, there is a formation of meso product by cis addition.
In these three reactions, only one product will form as the major product. Therefore, reaction I, II and III are regioselective. Hence, option (A) is correct.
In the reactions, III, IV and VI, there is a formation of more than one product as major. Hence, the option (B) is incorrect.
 In the fourth reaction, cis-but-2-ene reacted with magnesium monoperoxy phthalate in the presence of methanol and water, there is a formation of trans product. Thus the product will be (+) and (-) tartaric acid. Hence, option (C) is incorrect.
In third and fourth reactions, there is a formation of both (+) and (-) tartaric acid. Hence, the option (D) is incorrect.

Note:
In the case of regioselective reaction, there is a formation of only one product as major one. And here, reactions I, II and V will gives only one product as major which is cis tartaric acid. Therefore, these reactions are regioselective. And the other three reactions give, both (+) and (-) tartaric acid as products. Hence, these are not regioselective.