Welcome to Hydroxy Compounds: Alcohols and Phenols!

Hello future Chemist! This chapter dives into the fascinating world of compounds containing the hydroxyl ($\text{-OH}$) functional group. These are known as Hydroxy compounds. You'll learn how these seemingly simple groups drastically change a molecule's properties and reactions.

This topic is fundamental to organic chemistry. Understanding alcohols (AS Level) and the special case of phenols (A Level) will unlock your ability to predict reactions and synthesize complex molecules. Let's get started on classifying and reacting these versatile molecules!

Section 1: Alcohols (AS Level Core Content)

1.1 Structure and Intermolecular Forces

Alcohols are organic compounds where an $\text{-OH}$ group is bonded to an aliphatic carbon atom (a $\text{C}$ atom that is not part of an aromatic ring like benzene). The general formula is $\text{R–OH}$, where $\text{R}$ is an alkyl group.

Hydrogen Bonding: The Sticky Handshake

The most important feature of the $\text{-OH}$ group is its ability to form hydrogen bonds.

  • Oxygen is highly electronegative, pulling electrons away from the hydrogen atom (making $\text{H}$ slightly positive, $\delta^+$).
  • This highly polarised $\text{O}-\text{H}$ bond allows the hydrogen atom of one molecule to form a strong intermolecular attraction with the lone pair of electrons on the oxygen atom of a neighbouring molecule.


Analogy: Think of hydrogen bonds as a "sticky handshake" between molecules. This handshake is much stronger than the weak van der Waals' forces found in alkanes.

Key Consequence: Because energy is needed to break these strong hydrogen bonds, alcohols have significantly higher melting and boiling points than alkanes of similar relative molecular mass. Smaller alcohols (like methanol and ethanol) are also soluble in water because they can form hydrogen bonds with water molecules.

1.2 Classification of Alcohols (Primary, Secondary, Tertiary)

Alcohols are classified based on the number of other carbon atoms attached to the carbon atom that carries the $\text{OH}$ group (the carbinol carbon).

Memory Aid: The Carbinol Carbon's Friends

  • Primary Alcohol (1°): The carbinol carbon ($\text{C}-\text{OH}$) is attached to one other alkyl group ($\text{R}$) or carbon atom.
    Example: Ethanol ($\text{CH}_3\text{CH}_2\text{OH}$).
  • Secondary Alcohol (2°): The carbinol carbon ($\text{C}-\text{OH}$) is attached to two other alkyl groups ($\text{R}$ and $\text{R}'$).
    Example: Propan-2-ol.
  • Tertiary Alcohol (3°): The carbinol carbon ($\text{C}-\text{OH}$) is attached to three other alkyl groups ($\text{R}, \text{R}', \text{R}''$).
    Example: 2-methylpropan-2-ol.

1.3 Preparation (Synthesis) of Alcohols (AS Syllabus 16.1.1)

Alcohols can be made in several ways, often starting from other functional groups:

Method 1: From Alkenes (Hydration)

This is an electrophilic addition reaction.

Reagents/Conditions: Steam ($\text{H}_2\text{O(g)}$) and a strong acid catalyst (e.g., concentrated $\text{H}_3\text{PO}_4$ or concentrated $\text{H}_2\text{SO}_4$) at high temperature and pressure.

\( \text{Alkene} + \text{H}_2\text{O(g)} \xrightarrow{\text{H}_3\text{PO}_4 \text{ catalyst, heat}} \text{Alcohol} \)

Example: Ethene + Steam $\rightarrow$ Ethanol.

Method 2: From Alkenes (Cold, Dilute $\text{KMnO}_4$)

Alkenes react with cold, dilute, acidified potassium manganate(VII) ($\text{KMnO}_4$) via oxidation to form a diol (a molecule with two $\text{OH}$ groups).

Reagents/Conditions: Cold, dilute, acidified $\text{KMnO}_4(\text{aq})$. The characteristic colour change is purple ($\text{MnO}_4^-$) to colourless/brown precipitate ($\text{MnO}_2$).

Method 3: From Halogenoalkanes

This is a nucleophilic substitution reaction. The strong nucleophile, $\text{OH}^-$, replaces the halogen, $\text{X}$.

Reagents/Conditions: Aqueous sodium hydroxide ($\text{NaOH(aq)}$) and heat (reflux).

\( \text{R}-\text{X} + \text{NaOH(aq)} \xrightarrow{\text{Heat}} \text{R}-\text{OH} + \text{NaX} \)

Method 4 & 5: Reduction of Carbonyl Compounds and Carboxylic Acids

Aldehydes, Ketones, and Carboxylic Acids can be reduced to form alcohols.

  • Aldehydes $\rightarrow$ Primary Alcohols
  • Ketones $\rightarrow$ Secondary Alcohols
  • Carboxylic Acids $\rightarrow$ Primary Alcohols

Reagent: Lithium aluminium hydride ($\text{LiAlH}_4$) dissolved in dry ether, followed by aqueous acid. Sodium borohydride ($\text{NaBH}_4$) can reduce aldehydes and ketones but is generally too weak for carboxylic acids.

\( \text{Carbonyl compound} \xrightarrow{\text{LiAlH}_4 \text{ or } \text{NaBH}_4} \text{Alcohol} \)

Method 6: Hydrolysis of Esters

Esters can be split back into a carboxylic acid (or carboxylate salt) and an alcohol.

Reagents/Conditions: Dilute acid (e.g., $\text{HCl}(\text{aq})$) or dilute alkali (e.g., $\text{NaOH}(\text{aq})$) and heat.


Quick Review: Alcohol Preparation

Alkenes become Alcohols (2 ways). Halogenoalkanes use $\text{NaOH(aq)}$. $\text{C=O}$ compounds (acids/aldehydes/ketones) use reducing agents like $\text{LiAlH}_4$.

Section 2: Reactions of Alcohols (AS Level)

2.1 Reactions involving the O–H bond (Acidity and Nucleophilic attacks)

a) Acidity of Alcohols and Reaction with Sodium (16.1.2c)

Alcohols are very weak acids. The $\text{O}-\text{H}$ bond is polar, but the hydrogen atom is much less acidic than in water or phenols.

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

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

Syllabus Check: Acidity Comparison (16.1.5)
In AS Chemistry, you need to understand that alcohols are slightly less acidic than water.

When ethanol loses a proton, it forms the ethoxide ion ($\text{CH}_3\text{CH}_2\text{O}^-$). The ethyl group ($\text{CH}_3\text{CH}_2–$) is an electron-donating group. It pushes electron density towards the $\text{O}$ atom, making the ethoxide ion ($\text{R–O}^-$) less stable than the hydroxide ion ($\text{HO}^-$).

A less stable conjugate base means the original alcohol is a weaker acid than water.

b) Esterification (16.1.2f & 16.1.2e, A-Level 32.1.1)

Alcohols react with carboxylic acids (or acyl chlorides, see A-Level content below) to form esters.

  • With Carboxylic Acids: This is a condensation reaction.
    Reagents/Conditions: Carboxylic acid + alcohol, using concentrated $\text{H}_2\text{SO}_4$ as a catalyst and heat.
  • With Acyl Chlorides (A-Level): This reaction is much faster than using a carboxylic acid.
    Reagents/Conditions: Alcohol + Acyl chloride (e.g., ethanoyl chloride) at room temperature.
    Example: $\text{CH}_3\text{CH}_2\text{OH} + \text{CH}_3\text{COCl} \rightarrow \text{CH}_3\text{COOCH}_2\text{CH}_3$ (ethyl ethanoate) + $\text{HCl}$

2.2 Reactions involving the C–O bond

a) Substitution to form Halogenoalkanes (16.1.2b)

Alcohols can be converted back into halogenoalkanes (alkyl halides). The $\text{OH}$ group is substituted by a halogen atom.

Reagents/Conditions:

  • Hydrogen halides ($\text{HX(g)}$) or $\text{KX}$ (e.g., $\text{KCl}$) with concentrated $\text{H}_2\text{SO}_4$ or $\text{H}_3\text{PO}_4$ (requires heating).
  • Phosphorus(III) chloride ($\text{PCl}_3$) and heat.
  • Phosphorus(V) chloride ($\text{PCl}_5$) at room temperature (produces steamy white fumes of $\text{HCl}$ - a good test for $\text{OH}$).
  • Thionyl chloride ($\text{SOCl}_2$).
b) Dehydration to an Alkene (Elimination) (16.1.2e)

Alcohols lose a molecule of water when heated with a catalyst, forming an alkene.

Reagents/Conditions:

  • Passing alcohol vapour over a heated catalyst (e.g., aluminium oxide, $\text{Al}_2\text{O}_3$).
  • Heating with concentrated acid (e.g., concentrated $\text{H}_2\text{SO}_4$).

2.3 Oxidation of Alcohols (16.1.2d, 16.1.3b)

This is perhaps the most important set of reactions for distinguishing between primary, secondary, and tertiary alcohols.

Oxidising Agent: Acidified potassium dichromate(VI) ($\text{K}_2\text{Cr}_2\text{O}_7$) is commonly used.
Colour Change (Key Distinction): When oxidation occurs, the orange dichromate ion ($\text{Cr}_2\text{O}_7^{2-}$) is reduced to the green chromium(III) ion ($\text{Cr}^{3+}$).

1. Primary Alcohols (1°)

  • Gentle Heating and Distillation: Forms an aldehyde. Aldehydes are volatile and removed immediately by distillation, preventing further oxidation.
    \(\text{Primary Alcohol} \xrightarrow{[\text{O}], \text{Distillation}} \text{Aldehyde}\)
  • Heating under Reflux: Forms a carboxylic acid. The reflux setup ensures the aldehyde intermediate stays in the reaction vessel long enough to be fully oxidised.
    \(\text{Primary Alcohol} \xrightarrow{[\text{O}], \text{Reflux}} \text{Carboxylic Acid}\)

2. Secondary Alcohols (2°)

  • Heating under Reflux: Forms a ketone. Ketones are resistant to further oxidation under these mild conditions.
    \(\text{Secondary Alcohol} \xrightarrow{[\text{O}], \text{Reflux}} \text{Ketone}\)

3. Tertiary Alcohols (3°)

  • No Reaction: Tertiary alcohols cannot be easily oxidised by acidified dichromate(VI) because the carbinol carbon has no hydrogen atom directly attached to it that can be removed.


Did you know? In equations for organic redox reactions, the symbol $[\text{O}]$ is often used to represent one atom of oxygen supplied by the oxidising agent.

Section 3: Identification Tests

3.1 Distinguishing Alcohols (16.1.3)

The simplest way to distinguish 1°, 2°, and 3° alcohols is by their behaviour upon mild oxidation ($\text{K}_2\text{Cr}_2\text{O}_7$).

  • Primary & Secondary: Orange $\text{K}_2\text{Cr}_2\text{O}_7$ turns green.
  • Tertiary: Orange $\text{K}_2\text{Cr}_2\text{O}_7$ remains orange.

3.2 The Iodoform Test (Tri-iodomethane Test) (16.1.4)

This test is used to detect the presence of a specific group: the $\text{CH}_3\text{CH(OH)}–$ group in alcohols, or the $\text{CH}_3\text{CO}–$ group in carbonyl compounds (Topic 17.1.6).

  • Reagents: Alkaline iodine solution ($\text{I}_2(\text{aq})$ in $\text{NaOH}(\text{aq})$).
  • Result: If the $\text{CH}_3\text{CH(OH)}–\text{R}$ group is present, a pale yellow precipitate of tri-iodomethane ($\text{CHI}_3$) forms. The alcohol is oxidised and halogenated, forming the tri-iodomethane and an ion ($\text{RCO}_2^-$).
  • Example: Ethanol ($\text{CH}_3\text{CH}_2\text{OH}$) gives a positive test because it has the required $\text{CH}_3\text{CH(OH)}–$ group.

Key Takeaway for Alcohols: Classification (1°, 2°, 3°) dictates oxidation products and reactivity. Always remember the distinction between distillation (to get aldehydes) and reflux (to get acids/ketones).

Section 4: Phenol and its Chemistry (A Level Content)

4.1 Structure and Preparation of Phenol (32.2)

Phenol is the simplest hydroxy compound where the $\text{-OH}$ group is directly attached to a benzene ring ($\text{C}_6\text{H}_5\text{OH}$).

Preparation of Phenol (from Diazonium Salts) (32.2.1)

Phenol can be synthesized starting from phenylamine ($\text{C}_6\text{H}_5\text{NH}_2$):

  1. Phenylamine reacts with nitrous acid ($\text{HNO}_2$) (formed in situ from $\text{NaNO}_2$ and dilute acid) at a temperature below $10^\circ\text{C}$ to form a diazonium salt ($\text{C}_6\text{H}_5\text{N}_2^+\text{Cl}^-$).
  2. Warming the diazonium salt solution with water ($\text{H}_2\text{O}$) causes the $\text{N}_2$ group to be substituted by the $\text{OH}$ group, forming phenol.

4.2 Acidity of Phenol (32.2.3, 32.2.4)

Phenol is much more acidic than aliphatic alcohols (like ethanol) but less acidic than carboxylic acids.

Relative Acidities: Ethanol < Water < Phenol

Why is phenol more acidic than water and ethanol?

When phenol loses a proton ($\text{H}^+$), it forms the phenoxide ion ($\text{C}_6\text{H}_5\text{O}^-$).

  • The negative charge on the oxygen atom in the phenoxide ion can be delocalised into the benzene ring via resonance.
  • This delocalisation spreads the charge over the ring, making the phenoxide ion much more stable than the alkoxide ion (or the hydroxide ion).
  • A more stable conjugate base means that phenol is a stronger acid.

Contrast with Ethanol: Ethanol forms an ethoxide ion ($\text{CH}_3\text{CH}_2\text{O}^-$). The alkyl group destabilises the negative charge (electron-donating effect), making ethanol a very weak acid.

Reaction with Bases (32.2.2a): Phenol is acidic enough to react with strong bases like aqueous sodium hydroxide ($\text{NaOH(aq)}$) to form a salt (sodium phenoxide).
\( \text{C}_6\text{H}_5\text{OH} + \text{NaOH(aq)} \rightarrow \text{C}_6\text{H}_5\text{O}^-\text{Na}^+ + \text{H}_2\text{O} \)

Reaction with Sodium (32.2.2b): Similar to alcohols, phenol reacts with sodium metal to produce hydrogen gas and sodium phenoxide.
\( 2\text{C}_6\text{H}_5\text{OH} + 2\text{Na(s)} \rightarrow 2\text{C}_6\text{H}_5\text{O}^-\text{Na}^+ + \text{H}_2\text{(g)} \)

4.3 Reactions of the Benzene Ring in Phenol (32.2.5, 32.2.6)

The $\text{OH}$ group in phenol activates the benzene ring towards electrophilic substitution. It does this by donating a lone pair of electrons into the ring, increasing the electron density.

This activation makes the ring so reactive that substitution occurs under much milder conditions than for benzene itself.

a) Bromination (32.2.2e)

Phenol reacts instantly with aqueous bromine ($\text{Br}_2(\text{aq})$) at room temperature.


Contrast with Benzene: Benzene requires a catalyst ($\text{AlBr}_3$) and heat for bromination.

Result: A white precipitate of 2,4,6-tribromophenol is formed, and the bromine water is decolourised. The $\text{-OH}$ group directs the incoming electrophile ($\text{Br}^+$) to the 2, 4, and 6 positions (ortho and para).

b) Nitration (32.2.2d)

Phenol reacts with dilute nitric acid ($\text{HNO}_3(\text{aq})$) at room temperature.


Contrast with Benzene: Benzene requires concentrated $\text{HNO}_3$ and concentrated $\text{H}_2\text{SO}_4$ (catalyst) heated to $50-60^\circ\text{C}$.

Result: A mixture of 2-nitrophenol and 4-nitrophenol is produced.

c) Azo Coupling (32.2.2c)

Phenol reacts with a cold solution of a diazonium salt (e.g., benzenediazonium chloride) in aqueous sodium hydroxide ($\text{NaOH(aq)}$) to form an azo compound (a dye) containing the characteristic azo group ($\text{-N}=\text{N}-$).

A Common Mistake to Avoid: Don't confuse the conditions for nitrating benzene (harsh, conc. acids) with nitrating phenol (mild, dilute $\text{HNO}_3$). Phenol is much more reactive!

Key Takeaway for Phenol: Phenol's acidity and enhanced reactivity are due to the delocalisation of electrons between the oxygen atom and the benzene ring, which stabilises the phenoxide ion and activates the ring.