Synthesis of Alcohols by Hydration of Alkenes Chemistry Tutorial

Key Concepts

• Alkenes are unsaturated hydrocarbons.
• The double bond (C=C) is the active site on the molecule.
• Hydration of alkenes is an addition reaction.
• Water adds across the alkene's double bond in the hydration reaction.
• This addition follows Markovnikov's rule.

  (the H⁺ of H₂O goes to the C atom that has the greatest number of H atoms)

• Hydration of alkene reactions are used to produce industrial ethanol from ethene.
• General equation for the hydration of an alkene:

  word equation:	alkene	+	water	H+ →	alkanol
  chemical equation:	R- C = C -R'   | H   | H	+	H-O-H	H+ →	H |   O | -H   R- C - C -R'   | H   | H

  H⁺ represents a dilute, strong acid such as dilute sulfuric acid.

  R and R' represent hydrogen (H) or alkyl groups (alkane chains).

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Example of Hydration of Alkene: Hydration of ethene (ethylene) to ethanol

Ethene (ethylene) is an alkene with the condensed structural formula H₂C=CH₂.

The active site on the ethene molecule is the double bond (C=C).

In the presence of a dilute strong acid, water will add across the double bond in ethene (ethylene) to produce ethanol (ethyl alcohol).

The hydrogen atom of a water molecule will add to one of ethene's carbon atoms, and the OH from the water molecule will be added to the other carbon atom in ethene.

This synthesis of ethanol from ethene is shown in the chemical equations below:

Example of Hydration of Alkenes: Hydration of propene (propylene) to propanol

Propene (propylene) is an alkene with the condensed structural formula CH₃-H₂C=CH₂.

The active site on the propene molecule is the double bond (C=C).

In the presence of a dilute strong acid, water will add across the double bond in propene (propylene) to produce a mixture of alcohols: propan-1-ol (1-propanol) and propan-2-ol (2-propanol).
Propan-1-ol and propan-2-ol are structural isomers, they have the same molecular formula (C₃H₈O), but different structural formulae (CH₃-CH₂-CHOH and CH₃-CHOH-CH₃).
Propan-1-ol and propan-2-ol are position isomers, they are derived from the same parent alkane (propane), but, the OH (hydroxyl) functional group is located in a different position in the carbon chain (the first C atom for propan-1-ol, and, the second C atom for propan-2-ol).

These two isomers of propanol are not produced in equal quantities. We use Markovnikov's rule to decide which is the "major product" and which is the "minor product". Markovnikov's rule states that the hydrogen atom of the water molecule will add to the carbon with the greatest number of hydrogen atoms.
Propene: CH₃-C⁽²⁾H=C⁽¹⁾H₂
C⁽¹⁾ has 2 hydrogen atoms bonded to it.
C⁽²⁾ has only 1 hydrogen atom bonded to it.
Markovnikov's rule predicts that the hydrogen atom from water will mostly attach to C⁽¹⁾ and the OH of water will attached to C⁽²⁾ producing the major product propan-2-ol.
The other isomer, propan-1-ol, is less favoured so it will be present in a lower amount and is known as the minor product.

This synthesis of propan-1-ol and propan-2-ol from propene is shown in the chemical equations below:

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