A Guidebook to Mechanism in Organic Chemistry

Hydration

This very closely resembles the electrophilic substitution of chloro­
benzene which, as we have seen (p. 123), is o/p-directive (preferential
stabilisation of the transition states for o- and p-substitution by
interaction of the unshared electron pairs on chlorine with the v
orbital system of the nucleus), yet slower than in benzene itself
(large inductive effect of chlorine resulting in overall deactivation of
the nucleus to electrophilic attack).
The rearrangements of structure that can take place during the
addition of acids to olefines, due to alkyl migrations in the carbonium
ion intermediates, have already been referred to (p. 89).
With hypochlorous acid, etc., the adduct polarises in the sense
t- »+
HO—Cl, thus yielding with propylene

«- .+

OH

HO—a © eoH I

Me—CH=CH, *• Me—CH—CH, »• Me—CH—CH,

| or H.O |

Cl Cl

this being in accordance with the Markownikov rule as OH is a

more negative group than CI. That such additions are also trans

(i.e. initial attack by X® to yield a cationic intermediate) is shown by

the conversion of cyclopentene to the trans chlorohydrin: _

H Cl

OTHER ADDITION REACTIONS

(i) Hydration

Hydration of a carbon-carbon double bond is, of course, the re­

versal of the acid-catalysed dehydration of alcohols to olefines (p. 192):

H

• ®OH OH

>

X H® V « / H,o \ | / -He \ I /

=c<^->>-c<-,>c-c<^>c-c<

H H H

(XV)