Markovnikoff's rule
Addition to an alkene can give two products if neither the alkene nor the reagent are symmetrical.
In most addition reactions where two products form, one product forms faster than the other and predominates in the reaction mixture. The major product by far in the reaction above is 2-chloropropane.
Halides (and alcohols) are classed as primary, secondary or tertiary depending on how many carbons are bonded to the carbon bearing the halogen (or OH).
Markovnikoff's Rule is a general guideline for predicting the major product.
Markovnikoff's Rule restated in a more general form:
Electronegativity is the ability of a bonded atom to attract the electrons to itself.
A table of electronegativities is given.
Elements with lower electronegativities are the more electropositive atom in a bond.
As can be seen from the table of electronegativities H is more electropositive than most other nonmetals. Thus in the major product, the H from HCl in the reaction above becomes bonded to the carbon of the original double bond that had more hydrogens.
Asymmetric reagents have two different halves (for example H-Cl).
For asymmetric alkenes each doubly-bonded carbon has a different pair of groups.
For example, both 2-chloropropane and 1-chloropropane are produced from the asymmetric alkene propene.
For asymmetric alkenes each doubly-bonded carbon has a different pair of groups.
For example, both 2-chloropropane and 1-chloropropane are produced from the asymmetric alkene propene.
CH3CH=CH2 + HCl 
In most addition reactions where two products form, one product forms faster than the other and predominates in the reaction mixture. The major product by far in the reaction above is 2-chloropropane.
Halides (and alcohols) are classed as primary, secondary or tertiary depending on how many carbons are bonded to the carbon bearing the halogen (or OH).
The major product above is secondary because there are two carbons bonded to the carbon bearing chlorine.
The minor product is primary because there is one carbon bonded to the carbon bearing chlorine.
Tertiary halides or alcohols have three carbons bonded to the carbon bearing the halogen (or OH).
The minor product is primary because there is one carbon bonded to the carbon bearing chlorine.
Tertiary halides or alcohols have three carbons bonded to the carbon bearing the halogen (or OH).
Markovnikoff's Rule is a general guideline for predicting the major product.
In addition of HX to an alkene, the H becomes bonded to the carbon already bearing the larger number of hydrogens.
Markovnikoff's Rule restated in a more general form:
The more electropositive portion of the reagent becomes bonded to the carbon having the larger number of hydrogen atoms.
| arrows show trends |  | H 2.2 |  | He | ||||
| Li 1.0 | Be 1.6 | B 2.0 | C 2.5 | N 3.0 | O 3.5 | F 4.0 | Ne | |
| Na 0.93 | Mg 1.3 | Al 1.6 | Si 1.9 | P 2.2 | S 2.6 | Cl 3.2 | Ar | |
| K 0.82 | Ca 1.0 | Ga 1.6 | Ge 1.8 | As 2.2 | Se 2.6 | Br 3.0 | Kr | |
| Rb 0.82 | Sr 0.95 | In 1.8 | Sn 2.0 | Sb 2.1 | Te 2.1 | I 2.7 | Xe | |
| Cs 0.79 | Ba 0.89 | Tl 1.6 | Pb 2.3 | Bi 2.0 | Po 2.0 | At | ||
A table of electronegativities is given.
Elements with lower electronegativities are the more electropositive atom in a bond.
As can be seen from the table of electronegativities H is more electropositive than most other nonmetals. Thus in the major product, the H from HCl in the reaction above becomes bonded to the carbon of the original double bond that had more hydrogens.