Alcohols react with H3PO4 by elimination of water to give an alkene
The double bond formed is between the carbon originally bearing the OH and one of its adjacent carbons.
A single alkene is produced from alcohols such as pentan-3-ol above where the hydrogens on carbons adjacent to the carbon bearing OH are in equivalent bonding environments.
A mixture of alkenes is formed from alcohols where the hydrogens on carbons adjacent to the carbon bearing OH are not in equivalent bonding environments (such as the alcohols below).
It is possible to predict which alkene will predominate in the reaction mixture.
Many experiments indicate that the relatively more stable alkene products are formed in larger amount.
Zaitsev's (Saytzeff's) rule states that: The most substituted alkene is formed in greatest amount.
The double bond formed is between the carbon originally bearing the OH and one of its adjacent carbons.
 | H3PO4 | CH3CH=CHCH2CH3 |
A single alkene is produced from alcohols such as pentan-3-ol above where the hydrogens on carbons adjacent to the carbon bearing OH are in equivalent bonding environments.
A mixture of alkenes is formed from alcohols where the hydrogens on carbons adjacent to the carbon bearing OH are not in equivalent bonding environments (such as the alcohols below).
 |  | |
| on C adjacent to C-OH two types of H (two alkenes) | on C adjacent to C-OH three types of H (three alkenes) |
It is possible to predict which alkene will predominate in the reaction mixture.
Many experiments indicate that the relatively more stable alkene products are formed in larger amount.
In general alkenes with the fewer hydrogens on the double bond are more stable.
Alkenes with fewer hydrogens on the double bond are referred to as more substituted.
Alkenes with fewer hydrogens on the double bond are referred to as more substituted.
Thus when butan-2-ol undergoes elimination, but-2-ene predominates in the product mixture.
 | H3PO4 |  minor | + |  major |
Zaitsev's (Saytzeff's) rule states that: The most substituted alkene is formed in greatest amount.