Question

Specify the reagents in which addition to alkene is syn or anti.

Answer 1

Hint: To determine the answer we should know what syn or anti addition is. If the reagents attach two groups from the same side of the alkene at each carbon by breaking the double bond of the alkene are known as syn addition. If the reagents attach two groups from the opposite side of the alkene at each carbon by breaking the double bond of the alkene are known as anti-addition.

Complete step by step answer:The reagent giving syn addition are as follows:
Osmium tetroxide$\text{Os}{\text{O}}_{\text{4}}$followed by hydrolysis: it adds two –Oh groups at each carbon of the double bond in syn manner.

Woodward reaction ${\text{I}}_{\text{2}}\text{/AgOCOC}{\text{H}}_{\text{3}}\text{(aq)}$: cause hydroxylation in syn manner.

Potassium permanganate$\text{KMn}{\text{O}}_{\text{4}}$: diol addition in syn manner.

Wilkinson catalyst${\left(\text{p}{\text{h}}_{\text{3}}\text{P}\right)}_{\text{3}}\text{RhC}l-{\text{H}}_{\text{2}}$, catalytic hydrogenation such as hydrogen in presence of nickel, platinum or palladium metal, $$\text{Pd}-{\text{H}}_{\text{2}}\text{/pt}-{\text{H}}_{\text{2}}/N\text{i}-{\text{H}}_{\text{2}}$$ , hydroboration followed by oxidation $\text{B}{\text{H}}_{\text{3}}-\text{THF/}{\text{H}}_2\text{O}$ all these gives syn addition.

The reagent giving anti addition are as follows:

Bromine$\text{B}{\text{r}}_2$: it adds two Br atoms at each carbon of the double bond in anti-manner.

Oxymercuration-demercuration$\text{HgC}{\text{l}}_{\text{2}}\text{/}{\text{H}}^{\text{ + }}\text{,}{\text{H}}_{\text{2}}\text{O}$: cause anti-addition of proton and hydroxide group.
Prerost reaction ${\text{I}}_{\text{2}}\text{/AgOCOC}{\text{H}}_{\text{3}}$: cause hydroxylation.
Epoxidation then nucleophilic ring opening gives anti-addition.
$\text{MCPBA}$gives anti-addition.

Note:Alkene has double bonds, so alkenes are electron rich, so alkenes work as nucleophiles. The alkene gives electrophilic addition reaction. Generally the reagent which forms a temporary three membered cyclic ring with double bond causes the anti-addition because the block the one side of the alkene. After the addition of one group from the opposite side the rings break and allow the attacks of another group. The stereo of the final product is decided on the basis of the stereo of alkene and reagent. The syn reagent with syn alkene and anti-reagent with anti-alkene gives the enantiomers or meso compounds.