IIT JEE , AIEEE Forum - Chemistry , Organic Chemistry

samarthrvittal

sir,

I read this column matching question recently which said -

Pentan-2-ol (A) undergoes the following reactions, predict the outcome :

1) A + PCl5 - ?

2) A + SOCl2- ?

3) A + TsCl/EtONa - ?

4) A + SOCl2/pyridine - ?

The four options to choose from were racemisation, inversion, retention, rearrangement.

Ans- 1) racemisation, rearrangement. 2)retention 3)inversion 4) inversion

I know the basics of Sn1 and Sn2 but how are we supposed to predict this !! pls help

joyfrancis

1) retention
2) retention
3) racemization
4) inversion.

is it right?

samarthrvittal

na iv typed the answers right there see .. u got 2 wrong ... and pls tell me how u do it

joyfrancis

To answer these you have to know the mechanism of all the reactions.
I'll tell you about the SOCl2 reaction since i got only that one right:(.
It basically is the formation of alkyl hallide from alcohol by Sni reaction. In Sni reactions , the configeration of the chiral carbon is retained.
If pyridine is also introduced, it would react with the acid (formed as a side product with R-Cl) and would enhance the rate of reaction (le-chattlier's principle). On reacting with the acid , a nucleophile is generatd which then atacks the substrate in a Sn2 manner. This is not Sni. And in Sn2 , inversion of conf does take place.

biren_iit

samarthrvittal

gee thanks , guys :) but how can u predict whether sumthings gonna invert or retain by just lookin at the reagents theyv used ?

kavindra

well SOCl2 (darzen's procedure) goes through SN2 mechanism and SOCl2/pyridine.. goes through SNi mechanism. TsCl that makes a tosylate, ALWAYS goes through SN2 because it is an excellent leaving group.

Maybe u know it but just for precaution, SNi is double SN2 from opposite sides thus leading to an overall retention.

A few interesting things about the reactions above...

Rxn with SOCl2 is SN2 but still goes with retention of configuration because of formation of ion-pair because of which, the Cl- attacks from the front side.
Rxn with SOCl2 pyridine is SNi but still goes with inversion.

kavindra

PCl5 too i think would give SN2

madhusudan.chavan

The reaction of alcohol with thionyl chloride follows substitution nucleophilic internal mechanism with almost complete retention of configuration. No inversion occurs in either stage. But in presence of pyridine, the chloride has now the inverted configuration. This has been explained on the basis that the SN2 mechanism is operating, but now involves a pyridine complex.
The mechanism has been well represented by one of our friends above.The tosyl chloride in presence of pyridine follows the same mechanism.
In case of phosphorous halides the primary alcohols give unrearranged products while secondary alcohols give rearranged products.

Ask & Discuss Questions with Community & Experts