Question on A-Level Chemistry Mechanisms

Hi,
YK how for electrophilic addition, the more electronegative species will more likely bond with the more stable catiocarbon? In which reactions does this apply, for example, does this also work with hydration reactions? (BTW I've only completed the AS content)

Reply 1

UtterlyUseless69

12

Original post by mitostudent

Hi,
YK how for electrophilic addition, the more electronegative species will more likely bond with the more stable catiocarbon? In which reactions does this apply, for example, does this also work with hydration reactions? (BTW I've only completed the AS content)

For (uncatalysed) electrophilic addition reactions, when the molecule approaches the C=C bond, it is always the δ+ end of the dipole (whether permanent or induced) that approaches it first and it this always this atom which binds to the alkene to form a “carbocation”*, leaving the “more electronegative part” to react with the carbocation. This description applies for when electrophiles like the halogens and interhalogens (i.e ICl, BrI etc) react with alkenes.

Hydration is a little different as it takes place in the presence of an acid catalyst. For A level purposes, the acid protonates the alkene to a carbocation and then water acts as a nucleophile towards the carbocation, resulting in the formation of an alcohol with an extra hydrogen bound to the -OH group. The conjugate base of the acid catalyst then removes this extra hydrogen to form the alcohol and regenerate the acid catalyst.

*Strictly speaking there is no carbocation intermediate - you actually form something called a halonium ion, which behaves similarly to a carbocation. Carbocations are taught as the intermediates at A level purely for simplicity, but if you study chemistry at undergraduate level, you will have the evidence for the halonium intermediate explained to you.

(edited 1 month ago)

Reply 2

mitostudent

OP

11

Thanks for your reply! Does this mean for Hydration (at A-level at least) would have an equal chance of making either position isomer alcohol? And for addition/elimination, the conditions are just that an adjacent carbon must has a hydrogen?

Reply 3

UtterlyUseless69

12

Original post by mitostudent

Thanks for your reply! Does this mean for Hydration (at A-level at least) would have an equal chance of making either position isomer alcohol? And for addition/elimination, the conditions are just that an adjacent carbon must has a hydrogen?

For all addition reactions that proceed through a carbocation intermediate, there will be some formation of both possible carbocations (if the alkene is assymmetric), but it will heavily favour the formation of the more stable carbocation and so unequal amounts of each product form. Even with halonium intermediates, the tendency is for the leftover part to substitute onto the more substituted carbon despite steric effects due to something called “hyperconjugation” which is covered at undergrad level.

For elimination, there does need to be a hydrogen on the carbon adjacent to the carbon with the leaving group. However, I would also make the point that the reaction conditions also depend on what kind of group you are removing in the elimination reaction. For haloalkanes, you require a strong base in an ethanolic solution to remove this aforementioned hydrogen, triggering the remainder of the mechanism. For alcohols, you need a concentrated acid catalyst to protonate the -OH group to water, which ultimately leaves the molecule. Whether this reaction subsequently proceeds through a carbocation or the essentially the same way as with a haloalkane (where the conjugate base of the acid catalyst acts as the base) depends on how stable the carbocation intermediate would be.

(edited 1 month ago)

Reply 4

Nitrotoluene

6

Original post by mitostudent

Hi,
YK how for electrophilic addition, the more electronegative species will more likely bond with the more stable catiocarbon? In which reactions does this apply, for example, does this also work with hydration reactions? (BTW I've only completed the AS content)

In electrophilic addition reactions, a stable carbocation is preferred over others because of its affinity for an electric charge of opposite sign. This preference is influenced by a number of factors, including the nature of the reactants, the environment, and the conditions under which the reaction takes place. As a result, this phenomenon is common to a certain class of chemical reactions.

(edited 1 month ago)

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