The correct order of decreasing value of `K_("eq")` is :

Options

A. `a gt b gt c gt d `
B. `d gt a gt b gt c`
C. `d gt b gt a gt c `
D. `d gt a gt c gt d `

Answer

B

Explanation

(b) In simple nucleophilic additions where the rate-limiting step is attack by `Y^(theta)` , the positive character of the carbonyl carbon atom is reduced on going from the starting material (5) to the transition state(6) :

We should thus expect the rate of addition to be reduced by electron -donating R groups and enhanced by electron-withdrawing ones, this is borne out by the observed sequence :

In the above examples steric , as well as electronic , effect could be influencing relative rates of reaction , but the influence of electronic , effect could be influencing relative rates of reaction , but the influence of electronic effects alone may be seen in the series of compounds(11) :
So far as steric effects are concerned , the least energy-demanding direction of approach by the nucleophile to the carbonyl carbon atom will be from above , or below , the substantially planar carbonyl compound . it is also likely to be from slightly to the rear of the carbon atom , because of potential coulombic repulsion between the approaching nucleophile and the high electron density at the carbonyl oxygen atom :
Thus the Ks for cyanohydrin formation are found to reflect this operation of both steric and electronic factors :
`{:(, K), (CH_(3)CHO , "very large") , (C_(6)H_(5)CHO , 210), (CH_(3)COCH_(2)CH_(3) ,38), (C_(6)H_(5)COCH_(3) , 0.8):}`
Highly hindered ketones , such as `MeC COCMe_(3)` , may not react at all except possibly with very small , highly reactive nucleophilies .