Producing Alcohols: Hydration, Substitution, Fermentation

This is part of the HSC Chemistry course under the topic Alcohols.

HSC Chemistry Syllabus

investigate the production of alcohols, including: hydration of alkenes, substitution reactions of halogenated organic compounds and fermentation

Reactions that Produce Alcohols

This video discusses various chemical reactions that produce alcohols including

hydration
substitution reactions of halogenated alkanes
fermentation of glucose

What are some Different Methods of Producing Alcohols?

Hydration of Alkene

Hydration is the addition reaction between an alkene and water. An alkene can react with water in the presence of an acid catalyst (H⁺/H₂SO₄ (aq)) to form an alcohol.

Alkene hydration obeys Markovnikov’s rule – the major alcohol product will be the one where the –OH group is connected to the carbon atom that has the most alkyl groups attached.

It is difficult to produce primary alcohols using acid-catalysed hydration of alkenes. This is because primary alcohol requires the formation of a primary carbocation which is the least stable. The primary alcohol is usually the minor product of hydration of an alkene.

Substitution of Halogenated Organic Compounds

Reaction between a halogenated alkane with hydroxide ions e.g. NaOH produces an alcohol as the halogen is substituted (replaced) by the OH^–

Substitution of halogenated alkane to produce alcohol can also be performed using water as the reagent. However, this is less effective as water is a poorer nucleophile compared to hydroxide ion.

Substitution of halogenated alkane does not produce more than one organic product.
The position of alcohol (–OH) depends on position of the halogen atom(s).

Fermentation of Carbohydrates

Fermentation of glucose (C₆H₁₂O₆) by yeast produces ethanol and carbon dioxide.

Fermentation is a renewable method of producing alcohol as carbohydrates, such as glucose, are derived from plants. For example, sugar canes are rich in glucose and therefore, are regarded as good raw materials for producing ethanol via fermentation.

Fermentation requires specific reaction conditions:

Conditions	Reasons
Yeast	Enzyme (biological catalyst) is required for fermentation.
Anaerobic environment (absence of oxygen)	Yeasts only catalyse fermentation when oxygen supply is insufficient. Ethanol can be oxidised to produce acetic acid in the presence of oxygen
Dilute solution of carbohydrate	If starting carbohydrate concentration is too high, large amount of alcohol is produced which damages and kills yeast cells. Ideally, the alcohol content is kept < 15%.
Temperature of 30-40ºC	This is an optimal temperature range for yeast’s enzymatic activity. If the temperature is below the range, the reaction rate will become too slow. If the temperature is above the range, yeast cells will die and the enzymes required for fermentation will be denatured.