Summarised information from topic 3.3.5 for AQA A-Level Chemistry, including key terms, mechanisms and diagrams to help you understand the subject in a concise and accurate way.
3.3.5 Alcohols
Classification
Alcohols contain the functional group -OH joined to a hydrocarbon chain.
The name of the -OH group is given the prefix hydroxy- or the prefix -ol, and the carbon joined to the -OH
group is carbon-1. If there are multiple -OH groups, di-, tri-, tetra- and so on are used to say how many -OH
groups are present.
Propane-1,2,3-triol is also known as glycerol, and can be obtained from the fats and oils in living
organisms.
The oxygen atom has two lone pairs of electrons, causing the C-O-H bond to shorten to 104.5°.
Alcohols are classified as primary (1°) secondary (2°) or tertiary (3°), depending on how many other groups
are bonded to the carbon that joins to the -OH group.
The -OH group causes hydrogen bonding, so alcohols have a higher melting point than corresponding
alkanes. Alcohols are soluble in water, as long as the chain is relatively short, as the polar end of the chain
dominates.
Ethanol Production
Ethanol (CH3CHOH) is the most common alcohol. It can be produced industrially, or by fermentation.
Cracking
Ethene is produced when crude oil fractions are cracked. Hydrating ethene by reaction with steam using a
catalyst of phosphoric acid produces ethanol.
• Faster
• High temperature (300°C)
• High pressure (60-70atm)
• No by-products made
• Requires a lot of energy.
• Continuous reaction.
Fermentation
Carbohydrates from plants are broken into sugars (glucose) then converted into ethanol by enzyme action
from yeast. At about 35°, a compromise between rate and denaturisation, yeast ferments glucose into
ethanol and carbon dioxide. When the reaction mixture contains 15% ethanol, the enzymes are unable to
function further and the product must be distilled.
• Slower
• Renewable
• Batch reaction.
• Impure solution produced.
• Low temperature (35°).
• Low energy costs.
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