Core Concepts
Markovnikov’s rule helps predict the product of nucleophilic addition to alkenes. The nucleophile adds to the carbon with the highest degree (primary, secondary, etc.).
Topics Covered in Other Articles
- Electronegativity
- Organic Molecules
- Ions
- Nucleophiles and Electrophiles
- Cycloaddition Reaction
- Alkenes
- Cannizzaro reaction
Electrophilic Addition
Electrophilic addition is the process by which a nucleophile adds to an alkene by first forming a carbocation intermediate (e.g. the addition of halogens to alkenes). These additions have a common mechanism; for example:
1.Hydrogen Chloride heterolytically disassociates to form a proton and a chloride nucleophile.
2. Later, the alkene pi bond transfers the thy hydrogen to form a carbocation. Then, a hydride shift moves this carbocation to the more substituted carbon (Markovnikov’s Rule).
3. The extra electron pair on the chloride nucleophile shares these electrons with the carbocation to form a covalent bond.
Positive Inductive Effect
All elements in a covalent molecule have electronegativity. This means that they pull the electrons away from them. When removed from a carbocation, electrons make the structure more stable. Because of this, the most stable carbocation will be the one that feels the most electron pull. Thus, carbons connected to more carbons (i.e. secondary tertiary or quaternary carbons) feel more electron pull, and become more stable. This is the positive inductive effect.
When on a straight chain, the most stable carbocation will be in the middle, as the pull is slightly more balanced than anywhere in the chain. This effect diminishes with increasing length of the chain, however. If two sites are identical, the carbocation should be equally stable in both spots.
Can you predict where the most stable carbocation will form in the three molecules below?
Markovnikov’s Rule Mechanism and Hydride Shift
Markovnikov’s rule explains that the nucleophile adds in an electrophilic addition reaction to the more substituted carbon because the carbocation (electrophile) is more stable there. However, the carbocation does not choose where it forms, but rather moves to the correct spot once formed. This is a process called a hydride shift.
This hydride shift is responsible for the regioselectivity (how likely a reaction is to occur at one atom but not another) of the electrophilic addition, and can lead to yields of up to 999:1 desired product to undesired product under optimal conditions for simple molecules.
Exceptions to the Markovnikov’s Rule
Markovnikov’s rule applies to electrophilic addition, but there are some other reactions where Markovnikov’s rule is broken. This is usually due to a competing mechanism that forces the carbocation to the less stable position before the nucleophile adds. There are three main families of reactions where this is the case:
Free Radical Addition
Radicals are even more stable than the carbocation, and so during the reaction, the radical is preferentially formed there, forcing the nucleophile to add the other, less-substituted carbon.
Hydroboration Oxidation
In Hydroboration-Oxidation, an induced dipole forces the addition of the borane to the less substituted carbon before the hydride shift can occur. When this borated molecule is oxidized with hydrogen peroxide and some catalytic base, this borane is converted to an alcohol on the less substituted carbon.
Peroxides
Organic peroxides cause the addition of the nucleophile to the less substituted carbon. Organic peroxides are fragile, and break the O-O bond to form two alkoxide radicals. These radicals then react with a hydro-halogen to from the halogen radical which reacts according to the free radical addition mechanism shown below.
Markovnikov’s Rule Practice Problems
