Modified Markovnikov Rule

In this JC2 webinar we want to explain Markovnikov Rule and learn about Modified Markovnikov Rule.

1. Explaining Markovnikov Rule

Let's take a look at the electrophilic addition of HBr to propene.

modified markovnikov explain markovnikov rule

Markovnikov Rule states that the hydrogen will join to the carbon with more hydrogen to form major product.

You can watch my previous video for a more detailed explanation of Markovnikov Rule.

We can explain this using the stability of carbocations formed during electrophilic addition reaction.

There are 2 possible carbocations formed when electrophile hydrogen is added to the alkene.

When hydrogen is added to the carbon with more hydrogen, a carbocation that is attached to more electron donating alkyl groups is formed.

This secondary carbocation is less positive, more stable and more likely formed.

Hence it will eventually form major product 2-bromopropane.

When hydrogen is added to the carbon with less hydrogen, a carbocation that is attached to less electron donating alkyl groups is formed.

The primary carbocation is more positive, less stable, less likely formed hence forms the minor product 1-bromopropane.

2. Modified Markovnikov Rule

Markovnikov Rule works for addition of reagents that contain hydrogen, eg HBr, HCl, H2O.

But for reagents that do not contain hydrogen, such as aqueous bromine where Br and OH groups are added to alkene, Markovnikov Rule is not useful.

We can use a modified version of Markovnikov Rule to determine major product quickly.

Modified Markovnikov Rule: the electrophile will join to the carbon with more hydrogen to form major product.

modified markovnikov modified markovnikov rule

For aqueous bromine, Br is the electrophile hence it will be added to the carbon with more hydrogen to give us major product 1-bromopropan-2-ol.

The minor product will be 2-bromopropan-1-ol, where Br is added to carbon with less hydrogen.

Topic: Alkenes, Organic Chemistry, A Level Chemistry, Singapore

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