Welcome to the Alcohols Chapter!
Hello future chemists! This chapter is your guide to understanding alcohols—a super important class of organic molecules found everywhere, from hand sanitiser (ethanol) to antifreeze (glycol). Alcohols act as key intermediates in organic synthesis, meaning they are often the starting point or end product for many valuable reactions.
Don't worry if organic chemistry seems like a lot of names and structures. We will break down how to classify alcohols and look at their two main reaction types: oxidation (turning them into aldehydes, ketones, or acids) and elimination (turning them into alkenes).
3.3.5 Alcohols (International AS)
3.3.5.1 Classification of Alcohols
All alcohols belong to a homologous series because they share the same key feature: the hydroxyl functional group (\(–\text{OH}\)) attached to a carbon atom.
To classify an alcohol, we look at the number of other carbon atoms attached directly to the carbon atom holding the \(\text{OH}\) group.
1. Primary Alcohols (1°)
- The carbon atom bonded to the \(\text{OH}\) group is only attached to one other carbon atom (or no carbon atom, as in methanol).
- Example: Ethanol, \(\text{CH}_3\text{CH}_2\text{OH}\)
2. Secondary Alcohols (2°)
- The carbon atom bonded to the \(\text{OH}\) group is attached to two other carbon atoms.
- Example: Propan-2-ol, \(\text{CH}_3\text{CH(OH)}\text{CH}_3\)
3. Tertiary Alcohols (3°)
- The carbon atom bonded to the \(\text{OH}\) group is attached to three other carbon atoms.
- Example: 2-methylpropan-2-ol
Memory Tip: Count the number of C atoms "hugging" the C atom that has the \(\text{OH}\). That number is the classification degree (1, 2, or 3).
3.3.5.1 Oxidation of Alcohols: The Colour Change Reaction
Oxidation is one of the most important reactions for alcohols. It allows us to convert them into aldehydes, ketones, and carboxylic acids.
The Oxidising Agent
The syllabus specifies that acidified potassium dichromate(VI) is a suitable oxidising agent.
- Agent: Potassium dichromate(VI) (\(\text{K}_2\text{Cr}_2\text{O}_7\))
- Conditions: Acidified, usually with dilute sulfuric acid (\(\text{H}_2\text{SO}_4\)), and heated.
- Key Observation: During the reaction, the orange dichromate(VI) ions (\(\text{Cr}_2\text{O}_7^{2-}\)) are reduced to green chromium(III) ions (\(\text{Cr}^{3+}\)). This visible colour change (Orange \(\to\) Green) is a reliable test for the presence of a primary or secondary alcohol.
We represent the oxidising agent simply as \([\text{O}]\) in chemical equations.
Oxidation Pathways Based on Classification
A) Primary Alcohol Oxidation (1°)
Primary alcohols can be oxidised in two stages:
- \(\text{Primary Alcohol} \xrightarrow{\text{Oxidation}} \text{Aldehyde}\)
- \(\text{Aldehyde} \xrightarrow{\text{Further Oxidation}} \text{Carboxylic Acid}\)
\(\text{RCH}_2\text{OH} + [\text{O}] \rightarrow \text{RCHO} + \text{H}_2\text{O}\) (Aldehyde)
\(\text{RCHO} + [\text{O}] \rightarrow \text{RCOOH}\) (Carboxylic Acid)
Controlling the Product: Distillation vs. Reflux
You need to explain how the method used determines whether an aldehyde or a carboxylic acid is obtained. This is all about controlling the reaction time and temperature, and exploiting the different boiling points of the products.
1. To get an Aldehyde (Stop at the first stage): Use Distillation
- Aldehydes have a lower boiling point than the starting primary alcohol and the final carboxylic acid (because aldehydes cannot form hydrogen bonds with each other).
- The apparatus is set up for distillation so that as soon as the aldehyde forms, it vaporises and is immediately distilled off and collected in a separate container, preventing it from reacting further.
2. To get a Carboxylic Acid (Go all the way): Use Reflux
- For maximum oxidation, the mixture must be heated strongly for a long time.
- The apparatus is set up for reflux (heating in a flask with a condenser pointing straight up). This ensures all reactants and volatile products are kept in the flask by condensing them and letting them drip back down, ensuring complete reaction to the carboxylic acid.
B) Secondary Alcohol Oxidation (2°)
Secondary alcohols are oxidised to ketones in a single step. Ketones are resistant to further oxidation under these conditions.
\(\text{RCH(OH)R}' + [\text{O}] \rightarrow \text{RCOR}' + \text{H}_2\text{O}\) (Ketone)
C) Tertiary Alcohol Oxidation (3°)
Tertiary alcohols are not easily oxidised by acidified dichromate(VI) because the carbon atom bonded to the \(\text{OH}\) group does not have any hydrogen atoms attached that can be easily removed. A tertiary alcohol requires very harsh conditions (like strong concentrated acid and high heat) to react, and this usually leads to dehydration, not oxidation.
Quick Review: Oxidation Products
- 1° Alcohol \(\to\) Aldehyde (Distill) or Carboxylic Acid (Reflux)
- 2° Alcohol \(\to\) Ketone
- 3° Alcohol \(\to\) No reaction (or elimination)
Chemical Tests to Distinguish Aldehydes and Ketones
After oxidation, you might need to prove whether you made an aldehyde or a ketone. Since aldehydes are easily oxidised (to carboxylic acids) but ketones are not, we use mild oxidising agents for testing.
1. Tollens’ Reagent (Silver Mirror Test)
- Reagent: A solution of aqueous silver nitrate dissolved in aqueous ammonia, containing \([\text{Ag}(\text{NH}_3)_2]^+\) ions.
- Aldehydes: The aldehyde reduces the silver ions, forming a shiny layer of metallic silver on the inside of the test tube (the "silver mirror").
\(\text{RCHO} + 2[\text{Ag}(\text{NH}_3)_2]^+ + 3\text{OH}^- \rightarrow \text{RCOO}^- + 2\text{Ag}(s) + 4\text{NH}_3 + 2\text{H}_2\text{O}\)
- Ketones: No reaction. The solution remains colourless.
2. Fehling’s Solution
- Reagent: An alkaline solution containing copper(II) ions (\(\text{Cu}^{2+}\)), which are deep blue.
- Aldehydes: The aldehyde reduces the blue copper(II) ions to copper(I) oxide (\(\text{Cu}_2\text{O}\)), which is seen as a brick-red precipitate.
\(\text{RCHO} + 2\text{Cu}^{2+} + 5\text{OH}^- \rightarrow \text{RCOO}^- + \text{Cu}_2\text{O}(s) + 3\text{H}_2\text{O}\)
- Ketones: No reaction. The solution remains blue.
Common Mistake Alert! Always remember that Fehling's and Tollens' reagents are used to distinguish between the products of alcohol oxidation (aldehydes vs. ketones), not the alcohols themselves.
3.3.5.2 Elimination Reactions of Alcohols
Alcohols can undergo an elimination reaction, where a small molecule is removed from the starting compound. In this case, water is eliminated (hence the reaction is also called dehydration), resulting in the formation of an alkene.
Reaction Summary
- Reaction Type: Acid-catalysed elimination (dehydration).
- Reactant: An alcohol.
- Product: An alkene and water.
- Conditions: Concentrated acid catalyst (e.g., \(\text{H}_2\text{SO}_4\) or \(\text{H}_3\text{PO}_4\)) and heat.
Example: Ethanol is dehydrated to ethene.
\(\text{CH}_3\text{CH}_2\text{OH} \xrightarrow{\text{Conc. H}_2\text{SO}_4, \text{Heat}} \text{CH}_2=\text{CH}_2 + \text{H}_2\text{O}\)
Did you know? Alkenes formed by the elimination of water from alcohols can be used to produce addition polymers (like polyethene) without using monomers derived from crude oil. This is important for sustainable chemistry!
Outline of the Elimination Mechanism
The acid acts as a catalyst by protonating the alcohol group. The mechanism involves three key steps:
Step 1: Protonation of the Alcohol
The \(\text{OH}\) group is a very poor "leaving group" (it doesn't like to detach). The acid catalyst donates a proton (\(\text{H}^+\)) to the oxygen atom, turning the alcohol into an oxonium ion. The \(\text{OH}\) group is now converted into a much better leaving group: a water molecule (\(\text{H}_2\text{O}\)).
Step 2: Loss of the Leaving Group
The weak \(\text{C}-\text{O}\) bond breaks, and the water molecule leaves. This forms a positively charged carbon atom called a carbocation.
Step 3: Elimination of a Proton
A proton (\(\text{H}^+\)) is removed from an adjacent carbon atom, resulting in the formation of a carbon-carbon double bond, which creates the alkene product. The \(\text{H}^+\) is released back into the solution, meaning the acid is regenerated (it is a catalyst).
Key Takeaway on Elimination: The acid catalyst is essential because it transforms the poor \(\text{OH}\) leaving group into the excellent \(\text{H}_2\text{O}\) leaving group, making the reaction possible.
Key Takeaway Summary: Alcohols (3.3.5)
- Alcohols are classified (1°, 2°, 3°) based on how many carbons are attached to the \(\text{C}-\text{OH}\) carbon.
- Oxidation uses acidified \(\text{K}_2\text{Cr}_2\text{O}_7\) (Orange \(\to\) Green).
- The 1° alcohol product is controlled: Distillation for volatile Aldehyde; Reflux for Carboxylic Acid.
- 2° alcohols form Ketones. 3° alcohols do not oxidise easily.
- Aldehydes and ketones are distinguished using Tollens' (Silver Mirror) or Fehling's (Red ppt).
- Elimination (Dehydration) requires concentrated acid and heat to form an alkene by removing \(\text{H}_2\text{O}\).