In simple terms
A friendly intro before the formal notes — no formulas yet.
Alcohols: The Versatile Functional Group
Alcohols are organic molecules containing the hydroxyl (-OH) group, which dictates their chemical behaviour. Their classification as primary, secondary, or tertiary determines how they react, particularly in oxidation.
Think of alcohols like different types of employees in a workshop. A primary alcohol is a junior apprentice who can be promoted twice (to an aldehyde, then a carboxylic acid). A secondary alcohol is a skilled worker who can be promoted once (to a ketone). A tertiary alcohol is the workshop owner, already at the top and resistant to further promotion (oxidation).
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Primary: –CH₂OH; secondary: –CHOH–; tertiary: –COH–. | Sim hint: Lucas test distinguishes 1°, 2°, 3°.
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Oxidation: 1° → aldehyde → acid; 2° → ketone; 3° resistant. | Sim hint: Acidified dichromate or KMnO₄.
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Dehydration with conc H₂SO₄ gives alkene. | Sim hint: Say which H is eliminated for 2° alcohol.
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Esterification with carboxylic acid + acid catalyst. | Sim hint: Reversible — remove water.
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Key formulas
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Full topic notes
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Classification of Alcohols
Alcohols are classified as primary (1°), secondary (2°), or tertiary (3°) depending on the number of alkyl groups attached to the carbon atom that is bonded to the hydroxyl group. This classification is crucial as it dictates the products formed during oxidation reactions.
Primary (1°): The carbon atom bonded to the –OH group is attached to one other alkyl group (or zero in methanol). Example: Ethanol, .
Secondary (2°): The carbon atom bonded to the –OH group is attached to two other alkyl groups. Example: Propan-2-ol, .
Tertiary (3°): The carbon atom bonded to the –OH group is attached to three other alkyl groups. Example: 2-methylpropan-2-ol, .
Oxidation of Alcohols
Alcohols can be oxidised by strong oxidising agents, most commonly acidified potassium dichromate(VI), , or acidified potassium manganate(VII), . The outcome of the reaction depends on the class of the alcohol and the reaction conditions. The oxidising agent is often represented by [O] in simplified equations.
Primary alcohols can be oxidised in two stages. Partial oxidation, achieved by using a limited amount of oxidising agent and immediately distilling off the product, yields an aldehyde. Aldehydes have lower boiling points than alcohols as they cannot form hydrogen bonds. Complete oxidation, achieved by heating under reflux with an excess of the oxidising agent, yields a carboxylic acid.
Partial Oxidation: Complete Oxidation:
Secondary alcohols are oxidised to ketones when heated under reflux with an oxidising agent. Further oxidation is not possible without breaking the carbon chain, which requires much harsher conditions.
Tertiary alcohols are resistant to oxidation. This is because the carbon atom attached to the -OH group does not have any hydrogen atoms that can be easily removed. Therefore, there is no reaction under typical oxidation conditions, and the oxidising agent (e.g., orange dichromate(VI)) remains unchanged in colour.
Dehydration and Esterification
Dehydration is an elimination reaction where a molecule of water is removed from an alcohol to form an alkene. This is typically achieved by heating the alcohol with a concentrated acid catalyst, such as sulfuric acid or phosphoric(V) acid, or by passing the alcohol vapour over a heated solid catalyst like aluminium oxide.
Esterification is the condensation reaction between an alcohol and a carboxylic acid to form an ester and water. The reaction is reversible and is catalysed by a strong acid, typically a few drops of concentrated sulfuric acid. To improve the yield of the ester, the water produced can be removed as it forms, shifting the equilibrium to the right according to Le Chatelier's principle.
Worked examples
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An alcohol, A, with the molecular formula is heated under reflux with acidified potassium dichromate(VI). The product, B, is a neutral compound that does not react with Fehling's solution. Identify the structures of A and B.
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The molecular formula corresponds to butanol isomers (butan-1-ol, butan-2-ol, 2-methylpropan-1-ol, 2-methylpropan-2-ol).
Propan-1-ol is warmed with ethanoic acid in the presence of a few drops of concentrated sulfuric acid. Name and draw the displayed formula of the organic product formed.
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Identify Reactants: The reaction is between an alcohol (propan-1-ol, ) and a carboxylic acid (ethanoic acid, ). This is an esterification reaction.
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Glossary
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What is a primary (1°) alcohol?
An alcohol where the carbon atom bonded to the -OH group is attached to only one other alkyl group (or none, for methanol). General structure: RCH₂OH.
Key takeaways
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Primary (1°): The carbon atom bonded to the –OH group is attached to one other alkyl group (or zero in methanol). Example: Ethanol, .
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Secondary (2°): The carbon atom bonded to the –OH group is attached to two other alkyl groups. Example: Propan-2-ol, .
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Tertiary (3°): The carbon atom bonded to the –OH group is attached to three other alkyl groups. Example: 2-methylpropan-2-ol, .
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Test Your Knowledge on Alcohols
Test Your Knowledge on Alcohols
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