Alcohols, Carbonyls and Carboxylic Acids

This lesson covers the properties and reactions of alcohols, aldehydes, ketones, carboxylic acids, and their derivatives, including acyl chlorides, acid anhydrides, and esters. These functional groups are central to organic chemistry and form the basis of many synthetic routes. You will also learn about reducing agents (NaBH₄ vs LiAlH₄), the iodoform test, ester hydrolysis and biodiesel, and how IR spectroscopy links to oxidation products. This material aligns with the AQA specification for A-Level Chemistry.

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Alcohols

Alcohols contain the hydroxyl group (–OH) and have the general formula CₙH₂ₙ₊₁OH (for monohydric, saturated, aliphatic alcohols). They are classified as primary, secondary, or tertiary based on the number of carbon atoms bonded to the carbon bearing the –OH group.

Key Definition: A primary alcohol has the –OH group on a carbon bonded to one other carbon (or none, as in methanol). A secondary alcohol has the –OH group on a carbon bonded to two other carbons. A tertiary alcohol has the –OH group on a carbon bonded to three other carbons.

Physical Properties

• Alcohols have higher boiling points than alkanes of similar molecular mass due to hydrogen bonding between –OH groups. For example, ethanol (Mr = 46) has a boiling point of 78 °C, while propane (Mr = 44) boils at −42 °C.
• Short-chain alcohols (methanol, ethanol, propan-1-ol) are miscible with water because they can form hydrogen bonds with water molecules. As the hydrocarbon chain lengthens, the non-polar portion dominates and solubility decreases.
• Alcohols are polar solvents and can dissolve many ionic and covalent substances.

Combustion

Alcohols burn in excess oxygen to produce carbon dioxide and water. For example:

C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O

Ethanol is used as a biofuel, either on its own or blended with petrol (gasohol). It is produced industrially by fermentation of sugars (using yeast, at 25–37 °C, anaerobic conditions) or by hydration of ethene with an H₃PO₄ catalyst.

Oxidation of Alcohols

The oxidation of alcohols depends on their classification. The oxidising agent used is acidified potassium dichromate(VI), K₂Cr₂O₇/H₂SO₄, which changes colour from orange to green when reduction occurs (Cr₂O₇²⁻ is reduced to Cr³⁺).

graph LR
    A["Primary Alcohol<br/>RCH₂OH"] -->|"K₂Cr₂O₇/H₂SO₄<br/>Distil immediately"| B["Aldehyde<br/>RCHO"]
    B -->|"K₂Cr₂O₇/H₂SO₄<br/>Heat under reflux"| C["Carboxylic Acid<br/>RCOOH"]
    D["Secondary Alcohol<br/>R₂CHOH"] -->|"K₂Cr₂O₇/H₂SO₄<br/>Heat under reflux"| E["Ketone<br/>R₂CO"]
    E -->|"No further oxidation"| E
    F["Tertiary Alcohol<br/>R₃COH"] -->|"No reaction<br/>(resistant to oxidation)"| F

Type	Product of mild oxidation	Product of strong oxidation	Apparatus
Primary	Aldehyde	Carboxylic acid	Distil immediately for aldehyde; heat under reflux for acid
Secondary	Ketone	No further oxidation	Heat under reflux or distil
Tertiary	No reaction (stays orange)	No reaction	—

Exam Tip: To isolate an aldehyde, use distillation apparatus so the aldehyde distils off as it forms (before it can be further oxidised). To form the carboxylic acid, use reflux conditions with excess oxidising agent so the reaction goes to completion.

Confirming Oxidation Products by IR Spectroscopy

IR spectroscopy can confirm which product has formed after oxidation:

Functional group	Characteristic IR absorption
Alcohol (O–H)	Broad absorption at 3230–3550 cm⁻¹
Aldehyde (C=O)	Sharp absorption at 1720–1740 cm⁻¹
Carboxylic acid (O–H)	Very broad absorption at 2500–3300 cm⁻¹
Carboxylic acid (C=O)	Sharp absorption at 1700–1725 cm⁻¹

If you start with a primary alcohol showing a broad O–H peak and after oxidation you see a sharp C=O peak at ~1730 cm⁻¹ with no broad O–H, the product is an aldehyde. If the C=O peak appears alongside a very broad O–H absorption extending below 3000 cm⁻¹, the product is a carboxylic acid.

Dehydration

Alcohols can be dehydrated to form alkenes by heating with a concentrated acid catalyst (e.g. H₃PO₄ or H₂SO₄). This is an elimination reaction in which water is removed.

For example: CH₃CH₂OH → CH₂=CH₂ + H₂O

With longer-chain alcohols, a mixture of alkene isomers may form (e.g. butan-2-ol can dehydrate to give but-1-ene and but-2-ene).

Esterification

Alcohols react with carboxylic acids in the presence of a concentrated sulfuric acid catalyst to form esters:

alcohol + carboxylic acid ⇌ ester + water

This reaction is reversible and reaches an equilibrium. Concentrated H₂SO₄ acts as a catalyst and also removes water, shifting the equilibrium to the right. Esters have pleasant, fruity smells and are used in flavourings, perfumes, and solvents.

For example: CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O (ethyl ethanoate)

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Aldehydes and Ketones (Carbonyl Compounds)

Both aldehydes and ketones contain the carbonyl group (C=O). In aldehydes, the carbonyl is at the end of the carbon chain (bonded to at least one H); in ketones, it is within the chain (bonded to two carbon groups).

Property	Aldehyde	Ketone
General formula	CₙH₂ₙO (C=O at position 1)	CₙH₂ₙO (C=O at position 2+)
Suffix	-al	-one
Example	Ethanal (CH₃CHO)	Propanone (CH₃COCH₃)
Oxidation	Easily oxidised to carboxylic acid	Resistant to oxidation

Distinguishing Aldehydes from Ketones

Three key tests can be used:

1. Tollens' reagent (silver nitrate in aqueous ammonia, [Ag(NH₃)₂]⁺): Warm gently. Aldehydes produce a silver mirror on the inside of the test tube (Ag⁺ is reduced to Ag). Ketones give no visible change.

2. Fehling's solution (an alkaline solution containing Cu²⁺ ions complexed with tartrate): Heat. Aldehydes produce a brick-red precipitate of copper(I) oxide, Cu₂O (Cu²⁺ is reduced to Cu⁺). Ketones give no visible change.

3. Acidified potassium dichromate(VI): Aldehydes turn the solution from orange to green (Cr₂O₇²⁻ reduced to Cr³⁺). Ketones produce no colour change.

All three tests work because aldehydes are readily oxidised (they have an H on the carbonyl carbon that can be replaced), whereas ketones have two carbon groups and resist oxidation.

flowchart TD
    A["Unknown carbonyl compound"] --> B{"Tollens’ reagent<br/>(warm gently)"}
    B -->|"Silver mirror forms"| C["ALDEHYDE"]
    B -->|"No change"| D["KETONE"]
    A --> E{"Fehling’s solution<br/>(heat)"}
    E -->|"Brick-red precipitate<br/>(Cu₂O)"| C
    E -->|"No change"| D
    A --> F{"K₂Cr₂O₇/H₂SO₄"}
    F -->|"Orange → Green"| C
    F -->|"Stays orange"| D

The Iodoform (Triiodomethane) Test

The iodoform test is used to identify compounds containing the CH₃CO– group (methyl carbonyl) or CH₃CH(OH)– group (which is oxidised in situ to CH₃CO–).

Reagents: Iodine solution (I₂) and sodium hydroxide (NaOH), warmed gently.

Positive result: A yellow precipitate of triiodomethane (CHI₃, iodoform) with an antiseptic smell.

The test works in two stages:

1. The three hydrogen atoms on the methyl group adjacent to the carbonyl are each replaced by iodine (I) in the presence of NaOH (halogenation).
2. The CI₃ group is then cleaved from the molecule by NaOH, forming CHI₃ (yellow precipitate) and a carboxylate salt.

Compounds that give a positive iodoform test:

• Ethanal (CH₃CHO) — the only aldehyde that gives a positive result
• Methyl ketones (CH₃COR), e.g. propanone, butanone
• Secondary alcohols of the form CH₃CH(OH)R, e.g. ethanol, propan-2-ol (these are oxidised to methyl carbonyls first)

Exam Tip: Only ethanal gives a positive result among aldehydes. If an unknown compound gives a positive iodoform test AND a positive Tollens' test, it must be ethanal.

Nucleophilic Addition of HCN to Carbonyls

The carbonyl group is polar (C is δ+ and O is δ−), so the carbon is susceptible to attack by nucleophiles. The key reaction is nucleophilic addition of hydrogen cyanide (HCN), using KCN as a source of CN⁻ with dilute H₂SO₄, to form a hydroxynitrile (cyanohydrin).

Key Definition: Nucleophilic addition is a reaction in which a nucleophile attacks a δ+ carbon, and the π bond of the C=O breaks so that both electrons move onto the oxygen. No leaving group departs — the nucleophile simply adds across the double bond.

Worked Mechanism: Nucleophilic Addition of HCN to Propanal