Question

In which addition to alkene, syn addition takes place
A. $H_2\left(Pd\right)$
B. $Mn{O_4}^{-}/O{H}^{-}$
C. $Os{O}_4,NaHS{O}_3$
D. All

Answer 1

Hint: We know that in organic chemistry, syn and anti-addition are two ways in which we can add two substituents to a double bond or triple bond.

Complete step by step answer:
We know that syn and anti-addition denotes the face of the pi bond both groups would add. We consider syn addition when both groups/atoms are added to the same face whereas in anti-addition, the groups/atoms are added to the opposite face.
We can say that anti-addition is in direct contrast to syn addition. In anti-addition, we can add two substituents to opposite sides (or faces) of a double bond or triple bond. This would reduce the bond order and increase in the number of substituents. Bromination (any halogenation) of alkenes is an example of anti-addition.
We have to know that Syn addition is the addition of two substituents to the same side (or face) of a double bond or triple bond. This would lead to a decrease in bond order whereas there would be an increase in the number of substituents.
The substrate would be an alkene or alkyne. An example of syn addition is the oxidation of an alkene to a diol using a suitable oxidizing agent such as osmium tetroxide, potassium permanganate.
Syn addition takes place to alkene during the process of hydrogenation $H_2/Pd$, reaction of alkene with potassium permanganate $Mn{O_4}^{-}/O{H}^{-}$ and during the reaction of $Os{O}_4,NaHS{O}_3$.

So, the correct answer is Option D.

Note:
We must remember that Markovnikov's rule predicts the regioselectivity of electrophilic addition reactions of alkenes and alkynes. Addition of hydrogen bromide to an unsymmetrical alkene, the negative part of the reagent goes to the carbon in the pi bond having less hydrogen is called Markovnikov’s rule. Anti Markovnikov's rule is that in addition to an unsymmetrical alkene, the negative part of the reagent goes to the carbon constituting the pi bond which is poorer in hydrogen is called Markovnikov’s rule. However, in the presence of peroxide the addition occurs opposite to this rule. This is also called Kharash effect or peroxide effect.