Alcohols, phenols, and ethers are oxygen-containing organic compounds that form an important class of functional groups. Alcohols contain the hydroxyl (-OH) group attached to an aliphatic carbon, phenols have the hydroxyl group attached directly to an aromatic ring, and ethers have an oxygen atom connected to two alkyl or aryl groups. These compounds are used extensively as solvents, antiseptics, fuels, and intermediates in organic synthesis.
The chemistry of these compounds is dominated by the polarity of the oxygen-hydrogen and carbon-oxygen bonds. The hydroxyl group can act both as a hydrogen bond donor and acceptor, which explains the high boiling points of alcohols and their solubility in water. Phenols are more acidic than alcohols because the phenoxide ion is stabilised by resonance, while the acidity of alcohols depends on the stabilisation of the alkoxide ion by the inductive effect.
The chapter covers the preparation of alcohols from alkenes, carbonyl compounds, and Grignard reagents, and the preparation of phenols from chlorobenzene, diazonium salts, and cumene. Reactions such as the oxidation of alcohols, the Lucas test for distinguishing primary, secondary, and tertiary alcohols, and the electrophilic substitution reactions of phenols are studied in detail. Ethers are prepared by the Williamson synthesis and react by cleavage of the C-O bond.
2. Classification and Nomenclature
Alcohols
Alcohols are classified as primary, secondary, or tertiary based on the number of carbon groups attached to the carbon bearing the -OH group. Monohydric alcohols have one -OH group, while dihydric and trihydric alcohols have two and three respectively.
The IUPAC name of an alcohol is derived from the alkane by replacing the final -e with -ol; for example, CH3OH is methanol, C2H5OH is ethanol, and CH3CH(OH)CH3 is propan-2-ol.
Phenols
Phenols have the -OH group attached to an aromatic ring. The simplest member is C6H5OH, called phenol. Depending on the number of hydroxyl groups, phenols are classified as monohydric, dihydric, or trihydric phenols.
Ethers
Ethers have the general formula R-O-R'. They are named by identifying the two alkyl groups and adding the word ether, for example, CH3-O-CH3 is dimethyl ether. In the IUPAC system, the larger group is the parent and the smaller group is named as an alkoxy substituent; for example, CH3-O-C2H5 is methoxyethane.
3. Methods of Preparation
Preparation of Alcohols
From alkenes: Addition of water in the presence of acid follows Markovnikov's rule:
$$CH_3CH=CH_2 + H_2O \xrightarrow{H^+} CH_3CH(OH)CH_3$$
From carbonyl compounds: Reduction of aldehydes and ketones with H2/Ni, LiAlH4, or NaBH4 gives primary and secondary alcohols respectively.
From Grignard reagents: Grignard reagents react with formaldehyde to give primary alcohols, with other aldehydes to give secondary alcohols, and with ketones to give tertiary alcohols.
Hydration of alkenes: Direct hydration of alkenes gives the Markovnikov product.
Preparation of Phenols
From chlorobenzene: Chlorobenzene reacts with NaOH at high temperature and pressure to give sodium phenoxide, which is acidified to phenol.
From diazonium salts: Diazonium salts are hydrolysed by warming with water to give phenols.
From cumene: Cumene is oxidised in air to cumene hydroperoxide, which is decomposed with acid to give phenol and acetone.
From benzene: Benzene can be oxidised directly by air in the presence of a catalyst to give phenol.
Preparation of Ethers
Williamson synthesis: An alkyl halide reacts with an alkoxide ion to form an ether:
$$R-O^-Na^+ + R'-X \rightarrow R-O-R' + NaX$$
Dehydration of alcohols: Heating alcohol with concentrated sulphuric acid at 413 K gives diethyl ether.
$$2C_2H_5OH \xrightarrow{conc.\ H_2SO_4,\ 413\ K} C_2H_5OC_2H_5 + H_2O$$
4. Physical Properties
Alcohols and phenols form hydrogen bonds and therefore have higher boiling points than the corresponding alkanes and haloalkanes.
Lower alcohols are soluble in water because of hydrogen bonding; solubility decreases as the carbon chain length increases.
Phenols are only slightly soluble in water.
Diethyl ether is a volatile, highly flammable liquid with a pleasant smell.
5. Chemical Reactions of Alcohols
Reactions Involving the O-H Bond
Acidity: Alcohols are weakly acidic and react with active metals like sodium to release hydrogen:
$$2C_2H_5OH + 2Na \rightarrow 2C_2H_5ONa + H_2$$
The acidity order is CH3OH > 1° alcohol > 2° alcohol > 3° alcohol because of the electron-releasing inductive effect of alkyl groups.
Esterification: Alcohols react with carboxylic acids in the presence of an acid catalyst to form esters:
$$CH_3COOH + C_2H_5OH \xrightarrow{H^+} CH_3COOC_2H_5 + H_2O$$
Reactions Involving the C-O Bond
Reaction with hydrogen halides: Alcohols react with HX to form haloalkanes. The reactivity order is 3° > 2° > 1°. The Lucas test distinguishes alcohols using ZnCl2 and HCl; tertiary alcohols give a cloudiness immediately, secondary after a few minutes, and primary only on warming.
Oxidation: Primary alcohols are oxidised to aldehydes and then carboxylic acids; secondary alcohols give ketones; tertiary alcohols resist oxidation.
$$RCH_2OH \xrightarrow{[O]} RCHO \xrightarrow{[O]} RCOOH$$
Dehydration: Alcohols lose water in the presence of acid to give alkenes; tertiary alcohols dehydrate most easily.
6. Chemical Reactions of Phenols
Acidity
Phenols are more acidic than alcohols because the phenoxide ion is stabilised by resonance, which distributes the negative charge over the ring. Electron-withdrawing groups at ortho and para positions increase acidity, while electron-donating groups decrease it.
Reactions
Electrophilic substitution: The -OH group is an activating and ortho/para directing group, so phenols undergo bromination (giving tribromophenol), nitration, and Friedel-Crafts reactions readily.
Reaction with FeCl3: Phenols give a characteristic violet or blue colouration with neutral ferric chloride.
Reaction with sodium: Phenols react with sodium to give sodium phenoxide and hydrogen.
Oxidation: Phenols are oxidised by chromic acid to quinones.
Kolbe's reaction: Sodium phenoxide reacts with carbon dioxide under pressure to give salicylic acid (hydroxybenzoic acid), the precursor of aspirin.
Reimer-Tiemann reaction: Phenol reacts with chloroform and NaOH to give salicylaldehyde (ortho-hydroxybenzaldehyde).
7. Chemical Reactions of Ethers
Ethers are relatively unreactive. Their most important reaction is cleavage by strong acids. Ethers react with concentrated hydrogen halides:
$$R-O-R' + HI \rightarrow ROH + RI$$
When unsymmetrical ethers are treated with HI, the smaller alkyl group forms the alkyl iodide because the smaller group is less hindered. Ethers also form oxonium salts with acids and can be oxidised to form peroxides on prolonged exposure to air.
Quick Revision Tables
Table 1: Preparation Methods
Compound
Method
Reagents
Product
Primary alcohol
Aldehyde reduction
NaBH4/LiAlH4
RCH2OH
Secondary alcohol
Ketone reduction
NaBH4/LiAlH4
R2CHOH
Tertiary alcohol
Grignard + ketone
RMgX + R2CO
R3COH
Phenol
Chlorobenzene + NaOH
High T/P then H+
C6H5OH
Phenol
Diazonium salt + H2O
Warm water
C6H5OH
Ether
Williamson synthesis
RONa + R'X
R-O-R'
Table 2: Distinguishing Tests
Test
Primary
Secondary
Tertiary
Lucas test (ZnCl2/HCl)
No cloudiness
Cloudiness in 5 min
Immediate cloudiness
Oxidation
Aldehyde then acid
Ketone
No oxidation
Reaction with Na
Vigorous H2
Moderate H2
Slow H2
Acidic strength
Highest
Intermediate
Lowest
Mind Map
graph TD
A["Alcohols, Phenols, Ethers"] --> B["Alcohols"]
A --> C["Phenols"]
A --> D["Ethers"]
B --> B1["Preparation: alkenes, carbonyl, Grignard"]
B --> B2["Lucas test"]
B --> B3["Oxidation: 1° -> aldehyde -> acid"]
B --> B4["Dehydration to alkenes"]
C --> C1["More acidic than alcohols"]
C --> C2["Kolbe's and Reimer-Tiemann reactions"]
C --> C3["o/p directing electrophilic substitution"]
D --> D1["Williamson synthesis"]
D --> D2["Cleavage with HI"]
D --> D3["Peroxide formation"]
Important Diagrams (SVG)
Diagram 1: Acidity Comparison of Alcohols and Phenols
Diagram 2: Kolbe's and Reimer-Tiemann Reactions
Common Mistakes
Believing tertiary alcohols are most acidic; in fact, they are least acidic because alkyl groups release electrons.
Confusing the Lucas test outcome; immediate cloudiness indicates tertiary, several minutes indicates secondary, and primary alcohols show no cloudiness.
Stating that phenols are less acidic than alcohols; phenols are more acidic because of resonance stabilisation of the phenoxide ion.
Writing the wrong product in the Williamson synthesis; the alkoxide must be derived from the less hindered group for good yields.
Forgetting that the oxidation of primary alcohols gives carboxylic acids through the aldehyde intermediate, not directly the aldehyde.
Believing ethers are highly reactive; they are quite unreactive except for cleavage by strong acids.
In dehydration of alcohols, forgetting that tertiary alcohols dehydrate most readily.
Exam Tips
Memorise the acidity order: phenol > CH3OH > 1° > 2° > 3° alcohol, and be ready to justify it through resonance and inductive effects.
Learn the Lucas test and its results for all three types of alcohols.
Know the two name reactions: Kolbe's reaction (salicylic acid) and Reimer-Tiemann reaction (salicylaldehyde).
Remember that oxidation of a secondary alcohol gives a ketone, and a primary alcohol gives a carboxylic acid.
For the Williamson synthesis, write the reaction using sodium alkoxide and an alkyl halide, and explain why a primary alkyl halide gives the best yield.
Be careful with the preparation of phenol from cumene: oxidation to cumene hydroperoxide followed by acid decomposition gives phenol and acetone.
Conclusion
Alcohols, phenols, and ethers form a connected family of oxygen-containing compounds whose properties are governed by the polarity and hydrogen-bonding ability of the oxygen atom. The acidity trends, explained through resonance and inductive effects, illustrate how electronic structure controls chemical behaviour. Preparation methods ranging from hydration of alkenes to the Williamson synthesis provide practical routes to these important functional groups. Reactions such as the Lucas test, oxidation, and dehydration allow chemists to distinguish and transform these compounds systematically. Name reactions like Kolbe's and Reimer-Tiemann reactions highlight the ortho-directing power of the hydroxyl group in phenol. This chapter equips students with essential synthetic and analytical tools that form the foundation for studying carbonyl chemistry, carboxylic acids, and the biochemistry of sugars and lipids.
Test Your Understanding
Classify CH3OH, CH3CH(OH)CH3, and (CH3)3COH and write their IUPAC names.
Explain why phenols are more acidic than alcohols.
Write the equation for the Williamson synthesis of methoxyethane.
How would you distinguish primary, secondary, and tertiary alcohols using the Lucas test?
Give the products of the oxidation of propan-1-ol and propan-2-ol.