Hydroxy compounds: alcohols and phenol (A Level only)

Q: What reagents and conditions are required for the dehydration (elimination) of an alcohol to an alkene?

Dehydration requires heated aluminium oxide (\(\text{Al}_2 ext{O}_3\)) catalyst or concentrated acid (such as \(\text{H}_2 ext{SO}_4\) or \(\text{H}_3 ext{PO}_4\)) at high temperatures (\(150^\circ\text{C}-180^\circ\text{C}\)).

Chemistry (9701) · Cambridge International A Level · 9 min read · Updated: 6 Sept 2026

Welcome to Hydroxy Compounds: Alcohols and Phenols!

Welcome to one of the most important chapters in A Level Organic Chemistry! Hydroxy compounds are molecules that contain the hydroxyl functional group, \(-\text{OH}\).
These notes cover both the fundamental chemistry of Alcohols (AS content review and A-Level reactions) and the special chemistry of Phenol (A-Level content). Understanding the differences between these two is critical for exam success, as the presence of an aromatic ring drastically changes reactivity.

Key Takeaway from the Introduction

The \(-\text{OH}\) group is responsible for almost all of the chemical properties of both alcohols and phenols.

Section 1: Alcohols (R–OH)

1.1 Classification of Alcohols (The 'Neighbour' Rule)

Alcohols are classified based on the number of alkyl groups (R groups) attached to the carbon atom (the carbinol carbon) that is directly bonded to the \(-\text{OH}\) group.

Primary Alcohols (1°): The carbinol carbon is attached to one alkyl group and two hydrogen atoms.
Example: Ethanol (\(\text{CH}_3\text{CH}_2\text{OH}\))
Secondary Alcohols (2°): The carbinol carbon is attached to two alkyl groups and one hydrogen atom.
Example: Propan-2-ol (\(\text{CH}_3\text{CH}(\text{OH})\text{CH}_3\))
Tertiary Alcohols (3°): The carbinol carbon is attached to three alkyl groups and zero hydrogen atoms.
Example: 2-methylpropan-2-ol

1.2 Preparation Methods of Alcohols

Alcohols can be made in several ways. You must know the reagents and conditions for all these routes.

From Alkenes (Electrophilic Addition of Steam)

Alkenes react with steam to form alcohols. This is the industrial method for ethanol production.
Reagents: \(\text{H}_2\text{O}(\text{g})\) (steam)
Conditions: High temperature (\(300^\circ\text{C}\)), High pressure (\(60-70\text{ atm}\)), \(\text{H}_3\text{PO}_4\) catalyst.
From Alkenes (Formation of Diols)

Alkenes react with cold, dilute, acidified potassium manganate(VII) (\(\text{KMnO}_4\)) to form a diol (a molecule with two \(-\text{OH}\) groups).
This is also a characteristic test for the \(\text{C}=\text{C}\) double bond; the purple \(\text{KMnO}_4\) is decolourised and a brown precipitate of \(\text{MnO}_2\) forms.
From Halogenoalkanes (Nucleophilic Substitution)

This is a classic nucleophilic substitution reaction.
Reagents: Aqueous \(\text{NaOH}(\text{aq})\) or \(\text{KOH}(\text{aq})\)
Conditions: Heat (Reflux)
From Carbonyl Compounds (Reduction of Aldehydes/Ketones)

Aldehydes and ketones can be reduced back into alcohols.
Aldehydes reduce to primary alcohols.
Ketones reduce to secondary alcohols.

Reagents: \(\text{NaBH}_4\) (Sodium borohydride) or \(\text{LiAlH}_4\) (Lithium aluminium hydride). \(\text{LiAlH}_4\) is a much stronger reducing agent and is used for carboxylic acids too.
From Carboxylic Acids (Reduction)

Carboxylic acids reduce to primary alcohols. This requires the powerful reducing agent \(\text{LiAlH}_4\).
Reagents: \(\text{LiAlH}_4\) in dry ether, followed by dilute acid.
From Esters (Hydrolysis)

Esters can be broken down (hydrolysed) to yield an alcohol and a carboxylic acid (or carboxylate salt).
Reagents: Dilute acid (\(\text{H}_2\text{SO}_4\)) or dilute alkali (\(\text{NaOH}\))
Conditions: Heat (Reflux)

Quick Review: Preparation Reagents

Alkene to Alcohol: Steam, \(\text{H}_3\text{PO}_4\), Heat/Pressure
Halogenoalkane to Alcohol: \(\text{NaOH}(\text{aq})\), Heat
Carboxylic Acid to Primary Alcohol: \(\text{LiAlH}_4\), dry ether

Section 2: Reactions of Alcohols

2.1 Oxidation Reactions (The Big Difference!)

The most characteristic set of reactions for alcohols is oxidation, as it depends entirely on the alcohol's classification (1°, 2°, or 3°).

The standard oxidising agent is acidified potassium dichromate(VI) (\(\text{K}_2\text{Cr}_2\text{O}_7 / \text{H}^+\)). The colour change from orange (\(\text{Cr}_2\text{O}_7^{2-}\)) to green (\(\text{Cr}^{3+}\)) is used as a distinguishing test.

Primary Alcohols (1°)

Primary alcohols can be oxidised twice:

To Aldehydes: This requires gentle heating and distillation as soon as the aldehyde forms, preventing further oxidation.
\(\text{RCH}_2\text{OH} + [\text{O}] \rightarrow \text{RCHO} + \text{H}_2\text{O}\)
To Carboxylic Acids: This requires stronger heating under reflux (allowing the alcohol and aldehyde intermediate to be continuously oxidised).
\(\text{RCH}_2\text{OH} + 2[\text{O}] \rightarrow \text{RCOOH} + \text{H}_2\text{O}\)

Secondary Alcohols (2°)

Secondary alcohols are oxidised only once to form a ketone. Ketones are resistant to further oxidation under these conditions.
Conditions: Reflux with acidified \(\text{K}_2\text{Cr}_2\text{O}_7\) or \(\text{KMnO}_4\).
\(\text{R}_2\text{CHOH} + [\text{O}] \rightarrow \text{R}_2\text{CO} + \text{H}_2\text{O}\)

Tertiary Alcohols (3°)

Tertiary alcohols cannot be oxidised under these mild conditions because the carbinol carbon has no \(\text{H}\) atom attached, meaning no reaction occurs and the orange dichromate solution remains orange.

Common Mistake Alert! Students often confuse distillation and reflux. Remember:
Distillation = Delicate (stopping at aldehyde).
Reflux = Robust (forcing full oxidation to carboxylic acid).

2.2 Substitution Reactions (Forming Halogenoalkanes)

Alcohols can be substituted to replace the \(-\text{OH}\) group with a halogen (X) to form a halogenoalkane.

Using Hydrogen Halides (HX):
Reagents: \(\text{HX}(\text{g})\) or \(\text{KCl}\) (or \(\text{KBr}\)) with concentrated \(\text{H}_2\text{SO}_4\) or concentrated \(\text{H}_3\text{PO}_4\).
Using Phosphorus Halides:
\(\text{PCl}_5\) (Phosphorus(V) chloride) or \(\text{PCl}_3\) (Phosphorus(III) chloride, with heat).
Using Thionyl Chloride:
\(\text{SOCl}_2\) (Thionyl chloride) is highly efficient because the by-products (\(\text{SO}_2\) and \(\text{HCl}\)) are gases and bubble away, driving the reaction to completion.

2.3 Dehydration (Elimination)

Alcohols lose water (\(\text{H}_2\text{O}\)) to form an alkene (an elimination reaction).

Reagents: Heated \(\text{Al}_2\text{O}_3\) catalyst (aluminium oxide) OR Concentrated acid (like concentrated \(\text{H}_2\text{SO}_4\) or \(\text{H}_3\text{PO}_4\)).
Conditions: High temperature (typically \(150^\circ\text{C}\) to \(180^\circ\text{C}\) for concentrated \(\text{H}_2\text{SO}_4\)).

2.4 Reactions with Metals (Acidity)

Alcohols react with reactive metals like sodium, \(\text{Na}(\text{s})\), to produce a salt (an alkoxide) and hydrogen gas.

\(2\text{R-OH} + 2\text{Na} \rightarrow 2\text{R-O}^-\text{Na}^+ + \text{H}_2\)

This reaction demonstrates the very weak acidic nature of the \(\text{O-H}\) bond in alcohols.

2.5 Esterification

Esters are derivatives of carboxylic acids. They can be made from alcohols using two main methods:

With Carboxylic Acids (Traditional Esterification):
\(\text{R-OH} + \text{R'COOH} \rightleftharpoons \text{R'COOR} + \text{H}_2\text{O}\)
Reagents: Carboxylic acid (\(\text{R'COOH}\))
Conditions: Concentrated \(\text{H}_2\text{SO}_4\) catalyst, heat (reversible reaction).
With Acyl Chlorides (A-Level route - High Yield):
\(\text{R-OH} + \text{R'COCl} \rightarrow \text{R'COOR} + \text{HCl}\)
Reagents: Acyl chloride (\(\text{R'COCl}\))
Conditions: Room temperature (non-reversible, fast reaction). Example: Ethanol reacting with ethanoyl chloride to form ethyl ethanoate.

Key Takeaway: Alcohol Reactions

Alcohol reactions are dominated by oxidation, which provides the main way to distinguish between primary, secondary, and tertiary alcohols.

Section 3: Phenols (Aryl Hydroxy Compounds)

3.1 Structure and Preparation of Phenol

Phenol is a hydroxy compound where the \(-\text{OH}\) group is directly attached to a benzene ring. This direct attachment gives phenol chemical properties vastly different from simple aliphatic alcohols.

Preparation from Phenylamine (A-Level Synthesis)

Phenol can be synthesised from phenylamine via a two-step process involving the diazonium salt (diazotisation):

Phenylamine reacts with nitrous acid (\(\text{HNO}_2\), usually generated in situ from \(\text{NaNO}_2\) and dilute acid) below \(10^\circ\text{C}\) to form the benzenediazonium salt.
Warming the benzenediazonium salt solution with water (\(\text{H}_2\text{O}\)) causes hydrolysis, yielding phenol and \(\text{N}_2\) gas.

3.2 The Acidity of Phenol: Relative Acid Strength

Acidity Comparison: Carboxylic Acids, Phenol, Water, and Ethanol

The acidity of hydroxy compounds is measured by how easily they donate a proton (\(\text{H}^+\)). When the proton is released, a conjugate base is formed. The more stable this ion is, the stronger the acid.

Overall Order of Acidity: Carboxylic acid > Phenol > Water > Ethanol (Alcohol)

1. Ethanol (Weakest Acid):
\(\text{CH}_3\text{CH}_2\text{OH} \rightleftharpoons \text{CH}_3\text{CH}_2\text{O}^- + \text{H}^+\)
The negative charge on the ethoxide ion is concentrated on the oxygen atom. The electron-donating alkyl group (\(\text{CH}_3\text{CH}_2-\)) increases electron density on the oxygen, destabilising the ethoxide anion. Therefore, ethanol is a weaker acid than water.

2. Phenol:
\(\text{C}_6\text{H}_5\text{OH} \rightleftharpoons \text{C}_6\text{H}_5\text{O}^- + \text{H}^+\)
The negative charge on the phenoxide ion is delocalised into the benzene \(\pi\)-system. This delocalisation spreads the negative charge across the ring, making the phenoxide ion much more stable than the ethoxide ion. Consequently, phenol is a stronger acid than water and ethanol.

3. Carboxylic Acids (Strongest of the series):
Carboxylic acids are stronger acids than phenol because the negative charge in the carboxylate ion (\(\text{RCOO}^-\)) is delocalised over two strongly electronegative oxygen atoms.

Reactions with Bases and Carbonates:

Both alcohols and phenol react with \(\text{Na}(\text{s})\) to produce \(\text{H}_2\) gas and their respective salts.
Phenol is acidic enough to react with strong bases like aqueous sodium hydroxide, \(\text{NaOH}(\text{aq})\), forming soluble sodium phenoxide (\(\text{C}_6\text{H}_5\text{O}^-\text{Na}^+\)). Alcohols do not react with \(\text{NaOH}(\text{aq})\).
Crucially, phenol is not acidic enough to react with carbonates (\(\text{Na}_2\text{CO}_3\)) or hydrogencarbonates (\(\text{NaHCO}_3\)). There is no effervescence of \(\text{CO}_2\). This cleanly distinguishes phenols from carboxylic acids.

3.3 Reactivity of the Benzene Ring in Phenol

The \(-\text{OH}\) group is an activating group. The lone pair of electrons on the oxygen atom overlaps with the delocalised \(\pi\)-system of the ring, increasing electron density. This makes the ring much more susceptible to electrophilic substitution than benzene itself.

The \(-\text{OH}\) group is a 2,4,6-directing group.

Key Reactions of Phenol's Ring

Bromination

Phenol reacts rapidly with aqueous bromine (\(\text{Br}_2(\text{aq})\)) at room temperature without requiring a catalyst.
Result: Rapid decolourisation of bromine water and formation of a white precipitate of 2,4,6-tribromophenol.
Nitration

Phenol reacts with dilute nitric acid (\(\text{HNO}_3(\text{aq})\)) at room temperature to produce a mixture of 2-nitrophenol and 4-nitrophenol.
Contrast this with benzene, which requires concentrated \(\text{HNO}_3\) and concentrated \(\text{H}_2\text{SO}_4\) catalyst at \(50^\circ\text{C}\).
Azo Coupling (Diazonium Salts)

Phenol reacts with a benzenediazonium salt (\(\text{C}_6\text{H}_5\text{N}_2^+\)) in alkaline solution (\(\text{NaOH}(\text{aq})\)) to form an azo compound (containing the \(-\text{N}=\text{N}-\) azo linkage). These compounds are intensely coloured and used as azo dyes.

3.4 Esterification with Acyl Chlorides (Phenol)

Phenols react with acyl chlorides at room temperature to form phenyl esters.

\(\text{C}_6\text{H}_5\text{OH} + \text{RCOCl} \rightarrow \text{RCOO}\text{C}_6\text{H}_5 + \text{HCl}\)

Reagents: Acyl chloride (\(\text{RCOCl}\))
Conditions: Room temperature.

Key Takeaway: Phenol

Phenol is much more reactive towards electrophiles than benzene (due to activation by \(-\text{OH}\)) and significantly more acidic than alcohols (due to delocalisation of negative charge in the phenoxide ion).

Section 4: Identifying Hydroxy Compounds (Distinguishing Tests)

Chemical tests are used to identify and distinguish functional groups within hydroxy compounds.

4.1 Oxidation Test (Dichromate)

This test distinguishes between primary, secondary, and tertiary alcohols.

Reagent: Acidified potassium dichromate(VI) (\(\text{K}_2\text{Cr}_2\text{O}_7 / \text{H}^+\)).
1° Alcohol: Colour change from orange to green (forms aldehyde or carboxylic acid).
2° Alcohol: Colour change from orange to green (forms ketone).
3° Alcohol: No reaction (solution remains orange).

4.2 Tri-iodomethane (Iodoform) Test

This test confirms the presence of the \(\text{CH}_3\text{CH}(\text{OH})-\) group in alcohols (or the \(\text{CH}_3\text{CO}-\) group in carbonyl compounds).

Reagents: Alkaline aqueous iodine (\(\text{I}_2(\text{aq})\) and \(\text{NaOH}(\text{aq})\)).
Condition: Gentle warming.
Positive Result: Formation of a bright yellow precipitate of tri-iodomethane (\(\text{CHI}_3\)).
Ethanol is the only primary alcohol that gives a positive iodoform test because it is oxidised to ethanal (\(\text{CH}_3\text{CHO}\)).

4.3 Distinguishing Phenol from Alcohols and Carboxylic Acids

A summary of the distinguishing tests:

Reaction with \(\text{NaOH}(\text{aq})\): Phenol dissolves to form sodium phenoxide; alcohols show no reaction.
Reaction with \(\text{Na}_2\text{CO}_3(\text{aq})\) / \(\text{NaHCO}_3(\text{aq})\): Carboxylic acids produce effervescence (\(\text{CO}_2\) gas); phenols and alcohols show no reaction.
Reaction with \(\text{Br}_2(\text{aq})\): Phenol rapidly decolourises bromine water and forms a white precipitate of 2,4,6-tribromophenol; simple alcohols show no reaction.

Key Takeaway: Distinguishing Features

Use oxidation to differentiate 1°, 2°, and 3° alcohols. Use \(\text{NaOH}(\text{aq})\) and \(\text{Br}_2(\text{aq})\) to identify phenol, and use \(\text{Na}_2\text{CO}_3\) to separate phenols from carboxylic acids.

References

Cambridge International AS & A Level Chemistry (9701) Syllabus for examination in 2025, 2026 and 2027 — Cambridge Assessment International Education

Quick check

Can you answer these now?

Open each question to check the key ideas from this chapter.

How are primary (1°), secondary (2°), and tertiary (3°) alcohols classified based on the carbinol carbon?

Alcohols are classified by the number of alkyl (R) groups attached to the carbinol carbon: 1° has one alkyl group, 2° has two, and 3° has three.

What reagents and conditions are required to reduce aldehydes, ketones, and carboxylic acids to alcohols?

Aldehydes and ketones are reduced using \(\text{NaBH}_4\) or \(\text{LiAlH}_4\), while carboxylic acids require the stronger reducing agent \(\text{LiAlH}_4\) in dry ether followed by dilute acid.

How do primary, secondary, and tertiary alcohols differ in their reactions with acidified potassium dichromate(VI)?

Primary alcohols oxidise to aldehydes (under distillation) or carboxylic acids (under reflux), secondary alcohols oxidise to ketones, and tertiary alcohols do not oxidise.

Which reagents are commonly used to substitute the \(-\text{OH}\) group of an alcohol to form a halogenoalkane?

Halogenoalkanes can be formed by reacting alcohols with hydrogen halides (\(\text{HX}\)), phosphorus halides (\(\text{PCl}_5\) or \(\text{PCl}_3\)), or thionyl chloride (\(\text{SOCl}_2\)).

What reagents and conditions are required for the dehydration (elimination) of an alcohol to an alkene?

Dehydration requires heated aluminium oxide (\(\text{Al}_2\text{O}_3\)) catalyst or concentrated acid (such as \(\text{H}_2\text{SO}_4\) or \(\text{H}_3\text{PO}_4\)) at high temperatures (\(150^\circ\text{C}-180^\circ\text{C}\)).

How does the esterification of an alcohol with an acyl chloride differ from esterification with a carboxylic acid?

Reaction with an acyl chloride occurs rapidly at room temperature and is non-reversible (forming \(\text{HCl}\)), whereas reaction with a carboxylic acid is slow, reversible, and requires heat and a concentrated \(\text{H}_2\text{SO}_4\) catalyst.

How is phenol synthesised from phenylamine in the laboratory?

Phenylamine is diazotised with \(\text{HNO}_2\) below \(10^\circ\text{C}\) to form a benzenediazonium salt, which is then warmed with water to undergo hydrolysis, producing phenol and \(\text{N}_2\) gas.

Why is phenol significantly more acidic than aliphatic alcohols like ethanol?

The lone pair on the oxygen in the phenoxide ion delocalises into the benzene \(\pi\)-system, stabilising the conjugate base, whereas alkyl groups in alkoxide ions are electron-donating and destabilise the negative charge.

Why does the benzene ring in phenol undergo electrophilic substitution much more readily than benzene?

The lone pair of electrons on the oxygen atom delocalises into the ring's \(\pi\)-system, activating the ring by increasing its electron density and directing incoming electrophiles to the 2, 4, and 6 positions.

What observation occurs when phenol reacts with aqueous bromine (\(\text{Br}_2(\text{aq})\)) at room temperature?

Bromine water is rapidly decolourised, and a white precipitate of 2,4,6-tribromophenol is formed without requiring a catalyst.

What structural feature is required for an alcohol to give a positive tri-iodomethane (iodoform) test, and what is the positive observation?

The alcohol must contain the \(\text{CH}_3\text{CH}(\text{OH})-\) group, which forms a bright yellow precipitate of tri-iodomethane (\(\text{CHI}_3\)) when warmed with alkaline aqueous iodine.

How can chemical tests distinguish between an alcohol, phenol, and a carboxylic acid using \(\text{NaOH}(\text{aq})\) and \(\text{Na}_2\text{CO}_3(\text{aq})\)?

Alcohols react with neither; phenol dissolves in \(\text{NaOH}(\text{aq})\) but does not react with \(\text{Na}_2\text{CO}_3\); carboxylic acids react with both, producing \(\text{CO}_2\) effervescence with \(\text{Na}_2\text{CO}_3\).

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