Getting Tripped By Markovnikov

Markovnikov’s rule is a decent tool to predict some simple organic reactions. It states that in simple electrophilic additions of alkenes, such as Hydrogen Chloride to n-Pentene, the electrophile will attach itself to the carbon of the double bond that has more Hydrogen atoms already attached; the less substituted carbon. It can trip a user up, however, if they do not understand the actual process taking place. Example:

1-phenyl-2-methylpropene

1-phenyl-2-methylpropene has two “sp2” hybridized carbons in its Propene backbone. In a reaction with Hydrogen Chloride, we will only consider these two points and will leave out attachments to the Benzene ring itself for simplicity.

When the PI bond reattaches itself to a passing Hydrogen (HCl), it creates an intermediate carbocation and a Chloride ion. Which of the two carbons the intermediate forms on is determined by stability considerations. Markovnikov’s rule would predict the hydrogen attaching to the Phenyl side, as there is only one substituent there. The reverse actually happens, creating a “non-Markovnikov” scenario where the Chloride ion attaches to the molecule close to the Phenyl substituent.

Instead of relying on Markovnikov’s rule for simple additions which create carbocations, use the idea of hyperconjugation. This boils down to the idea that carbocations (positively charged) can be stabilized by the sigma bonds of nearby substituents (usually carbon to hydrogen bonds) because the electrons involved in sigma bonding are negatively charged. The more substituents, the more stabilized a present carbocation becomes. The stability of carbons with positive charges becomes:

1^{\circ} < 2^{\circ} < 3^{\circ}

Primary carbocations are less stable than tertiary because tertiary carbocations have two more sets of hyperconjugation sigma bonds to stabilize the positive charge.

However, sigma hyperconjugation is nowhere near as stabilizing as the effects of lone pairs of nearby electrons such as those found in “sp2” hybridized carbons. The negative charge is less diluted in the lone pair. This makes Allyl and Benzene groups even more effective than tertiary carbons. The order stands at:

1^{\circ} < 2^{\circ} < 3^{\circ} < Allyl < Benzene

The point of this is that when a carbocation intermediate forms, it will form in the most probable place most of the time. The most probable is the most stable. Carbocations will form on carbons attached to Benzene before they attach to carbons with only two other carbons (methyl groups, for instance) attached to themselves.

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