Paul Walden, the brilliant son of a farmer succeeded in finding a concept that is now widely known as umbrella effect in `S_N2` reactions. In an `S_N2` reaction, the configuration at the reaction centre is inverted as a umbrella turns inside out in a strong wind. An ideal `S_N1` reaction, on the other hand, yield nearly racemised product as it proceeds through planar carbocation. `S_N2` reactions proceed smoothly on primary alkyl halide whereas tertiary halides are fit for `S_N1` mechanism. However, evidences indicate that all the molecules of particular substances do not necessarily react by a single mechanism.After all, the intention of the molecule is very clear, they want to follow the low energy path.If both paths have nearly the same energy barrier, then follow a mixed mechanism.For example, secondary alkyl halides follow a mixed mechanism, say 60% molecule react by `S_N2` and rest 40% by `S_N1` Stereochemical features, as mentioned above, are noticeable only when the reaction is carried out on a particular enantiomer.Even the reaction centres, which are paths of suitably substituted rings, also exemplify the said features.
A reaction gives the following results.

What should be the % age of back side attack assuming complete reactions ?

Options

A. `65%`
B. `30%`
C. `35%`
D. `85%`

Answer

A

Explanation

Percentage of backside attack=Percentage of inversion + 1/2 (Percentage of racemisation) 70% racemisation and rest 30% inversion gives 65% backside attack.