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dehydration of alcohol aluminium oxide mechanism
When alcohols are allowed to react with protic acids, they are prone to losing water molecules to form alkenes. This is known as a dehydration reaction and it is considered one of the basic examples of an elimination reaction.
Ethanol is a Lewis base, which means it can react with acids like phosphoric and sulphuric acid (H3PO4 and H2SO4) to eliminate water. Phosphoric and sulphuric acids are concentrated acid catalysts so they're much better for this type of dehydration reaction than hydrochloric and nitric acids, which are not so concentrated.
The first step in the dehydration process is protonation of ethanol by the acid. This is a reversible step because there is a single lone pair on the oxygen atom, making it a Lewis base.
This reaction is the rate-determining step because it involves the breaking of a C-O bond, which forms a carbocation. This is also the slowest and therefore the most difficult part of the dehydration process.
After the breakdown of the C-O bond, an alkene is formed with a double carbon-carbon (C=C) bond. This is called the dehydration reaction and it's categorized as an SN2 reaction in primary alcohols and an SN1 reaction in secondary and tertiary alcohols.
Among studied alcohols, 2-propanol and isobutanol have higher rates of olefin formation than ethanol. This is probably because the carbon chain length and substitution affect the stability of the carbocation-like transition state that allows olefin formation.
