Dehydration, Substitution, and Oxidation of Alcohols
This is part of the HSC Chemistry course under the topic Alcohols.
HSC Chemistry Syllabus
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write equations, state conditions and predict products to represent the reactions of alcohols, including but not limited to (ACSCH128, ACSCH136):
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investigate the products of the oxidation of primary and secondary alcohols
Reactions of Alcohols
This video discusses various common reactions involving alcohols including
- dehydration
- substitution with HX
- oxidation
Dehydration of Alcohol
- In the presence of concentrated acid e.g. concentrated H2SO4, an alcohol (hydroxyl) functional group can be eliminated to form an alkene and water.
Dehydration is considered as an elimination reaction, opposite to hydration of an alkene (addition reaction). The two reactions are reversible, with dehydration being endothermic and hydration exothermic.
Reagents and reaction condition:
- Concentrated H2SO4 (catalyst, increases reaction rate)
- Heat (Increases equilibrium yield of alkene as dehydration is endothermic)
Position isomers are produced during dehydration reaction of either a secondary (2º) or tertiary (3º) alcohol. The major product is always more substituted – the resultant C=C bond prefers to be formed between non-terminal carbon atoms. This is known as Zaitsev’s rule.
Zaitsev’s rule states that:
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In an elimination reaction, the major product will be one where the resultant double carbon-carbon bond is formed in a more substituted position. |
Dehydration of 2-butanol forms two isomers: 1-butene and 2-butene because the new C=C bond can formed in two positions adjacent to the carbon which was attached to the –OH group. In this example, 2-butene is the major product because the C=C bond is more substituted (towards the middle of the molecule).
Substitution of Alcohol with HX
When halogen halide is supplied in large concentration, the –OH group can be directly substituted by a halogen. This transforms an alcohol into a mono-substituted haloalkane.
Reagent:
- Concentrated hydrogen halide e.g. HCl, HBr
- ZnCl2 (catalyst)
This method of forming a haloalkane is slightly different to hydrohalogenation of an alkene.
- In the reaction between an alkene and a hydrogen halide, the halogen is selectively connected to the carbon with more alkyl substituents.
- In contrast, the position of the halogen in a haloalkane (produced from substitution with alcohol) will always depend on the position of its former alcohol.
For example,
Oxidation of Alcohol
Oxidation states of carbon atoms in organic molecules is determined by the following rules:- Every bond between C and another C does not alter the oxidation state
- Every bond between C and H will decrease the oxidation state by 1
- Every bond between C and a more electronegative element will increase the oxidation state by 1
- Oxidation of primary (1º) alcohol yields aldehyde as an intermediate and carboxylic acid as the final product.
- Oxidation of secondary (2º) alcohol produces ketone. Ketone cannot be oxidised further as this would break a C–C bond which requires too much energy.
- Tertiary (3º) alcohols cannot be oxidised because this would otherwise involve breaking a C–C bond (sigma bond) which requires too much energy.
Strong Oxidising Agents
Strong oxidising agents can oxidise primary alcohols to form carboxylic acids. An aldehyde is formed during the process, but is quickly oxidised further to form a carboxylic acid. Therefore, the final product of oxidation of a primary alcohol using strong oxidising agents is always a carboxylic acid.
Examples of strong oxidising agents:
- Acidified sodium dichromate (`H^+ & Cr_2O_7^{2-}`)
- Acidified potassium permanganate (`H^+ & MnO_4^-`)
Weak Oxidising Agents
Weak oxidising agents cannot oxidise an aldehyde. Therefore, when a primary alcohol is treated with a weak oxidising agent, the final product is an aldehyde (not carboxylic acid).
An example of a weak oxidising agent:
- Pyridinium chlorochromate (PCC)
Both strong and weak oxidising agents can transform a secondary alcohol into a ketone.