Alcohols and Phenols Properties - Chemistry Class 12 - NEET PDF Download

Alcohols and phenols are two important classes of organic compounds that contain the hydroxyl functional group (-OH). The presence of this group governs many of their characteristic physical and chemical properties. Alcohols are widely used as solvents, fuels, disinfectants and in the manufacture of many chemicals and consumer products. Phenols are important in the production of plastics, pharmaceuticals, antiseptics and certain personal-care products.

What are Alcohols?

Alcohols are organic compounds containing one or more hydroxyl (-OH) groups attached to a saturated carbon atom. The general formula is R-OH, where R is an alkyl or substituted alkyl group.

Alcohol as Stove Fuels

• The simplest alcohols are methanol (CH3OH), ethanol (C2H5OH), propanol (C3H7OH) and butanol (C4H9OH).
• Alcohols differ in properties according to chain length, branching, and the position (primary, secondary, tertiary) of the carbon bearing the -OH group.

What is Phenol?

Phenol (chemical formula C6H5OH) is an aromatic compound in which a hydroxyl group is directly bonded to a benzene ring. It is often classified separately from aliphatic alcohols because the aromatic ring strongly affects its chemistry and physical properties.

Phenol is a white crystalline solid and is corrosive; it can cause chemical burns. It is sparingly soluble in cold water (solubility increases with temperature) and dissolves readily in alkali to give phenoxide ions. Historically, phenol was first extracted from coal tar by Friedlieb Ferdinand Runge in 1834. Phenol was earlier employed in carbolic soaps and antiseptics; because of its toxicity and corrosiveness it must be handled with care.

Phenol Crystal

Natural and Industrial Sources of Phenols

Phenols occur in nature as decomposition products of organic materials and as constituents of coal tar. Forest fires and decomposition of biological wastes can increase phenol concentrations in the environment.

• Industrial sources include petroleum refineries, petrochemical plants, coal refining, pharmaceutical manufacturing, tannery operations and pulp and paper mills.

Phenol Structure

The simplest phenol has the formula C6H5OH. The -OH group is bonded to an sp2 carbon of the benzene ring. Interaction between the oxygen lone pair and the aromatic π system produces resonance stabilisation of the conjugate base (phenoxide ion), which strongly influences acidity and reactivity.

Phenol Structure

Types of Alcohols: Classification

Alcohols are classified by the nature of the carbon atom bearing the hydroxyl group, by the number of hydroxyl groups present, and by special structural features (allylic, benzylic, aromatic, etc.).

Classification by the type of carbon bearing the -OH group

Depending on how many carbon atoms are directly attached to the carbon bearing the -OH, alcohols are:

• Primary (1°) alcohols: The carbon bearing the -OH is bonded to only one other carbon. Example: ethanol.
• Secondary (2°) alcohols: The carbon bearing the -OH is bonded to two other carbons. Example: isopropanol.
• Tertiary (3°) alcohols: The carbon bearing the -OH is bonded to three other carbons. Example: tert-butanol.

Ethanol

Isopropanol

tert-Butanol

Classification Based on the Number of Carbon Atoms Attached 

Classification by number of hydroxyl groups

• Monohydric alcohols: contain one -OH group. Example: ethanol.
• Dihydric alcohols (glycols): contain two -OH groups. Example: ethane-1,2-diol (ethylene glycol).
• Trihydric alcohols: contain three -OH groups. Example: glycerol (1,2,3-propanetriol).

: They contain three -OH groups. Example, Propane-1,2,3-triol.

Aromatic alcohols and phenols

When the hydroxyl group is attached to an aromatic ring the compound is an aromatic alcohol or a phenol. Phenols are further classified as mono-, di- or trihydric depending on the number of -OH groups present.

• Monohydric phenols: one hydroxyl group (e.g., phenol).
• Dihydric phenols: two hydroxyl groups; these may be oriented as ortho, meta or para isomers (e.g., catechol, resorcinol, hydroquinone).
• Trihydric phenols: contain three hydroxyl groups (e.g., gallic acid derivatives and certain natural polyphenols).

They contain three -OH groups.

Other special types

• Allylic alcohols: the -OH is on a carbon adjacent to a C=C double bond. Example: allyl alcohol (CH2=CH-CH2OH).
• Benzyl alcohols: the -OH is attached to a benzylic (-CH2-) carbon. Example: benzyl alcohol (C6H5CH2OH).

Allyl Alcohol

Benzyl Alcohol

Nomenclature of Alcohols

Alcohols are named by selecting the longest chain containing the -OH, replacing the final -e of the corresponding alkane name by -ol, and numbering the chain so that the carbon bearing the hydroxyl group has the lowest possible locant. Substituents are named and located by numbers. The general substitutive IUPAC family name is alkanol.

1. Identify the longest continuous chain containing the -OH.
2. Number the chain so the carbon bearing -OH has the lowest possible number; use this number as a locant (e.g., butan-1-ol).
3. Name other substituents with appropriate locants.

IUPAC Nomenclature for Alcohols

• Select the longest continuous chain containing the hydroxyl group; change the alkane ending -ane to -ol.
• Number the chain to give the -OH the lowest possible number; indicate the position with that number.
• Indicate positions of other substituents using numbers as locants.

IUPAC Nomenclature for Alcohols

Types of nomenclature used historically

1. Common or trivial names (e.g., ethyl alcohol for ethanol).
2. Carbinol system (older).
3. IUPAC systematic names (preferred and standard).

Common Names of Alcohols

The common name "ethyl alcohol" and the trivial term "carbinol" were used historically. Hermann Kolbe contributed to early nomenclature ideas. The systematic IUPAC nomenclature was standardised and adopted in the mid-20th century.

Solved Example

Q1: Name the following compound.

Ans: This compound has a three-carbon chain, so the parent alkane is propane. Because the compound contains three hydroxyl groups the correct suffix is -triol. Hence the IUPAC name is 1,2,3-propanetriol. The common name is glycerol (also called glycerin), an important by-product in soap manufacture.

Nomenclature of Phenols

Phenols are organic compounds containing a benzene ring bonded to a hydroxyl group. They are also known as carbolic acids. Thus, a phenol molecule consists of two parts one aryl group part and the other hydroxyl group part. Based on the number of hydroxyl groups attached to the aryl group, it can be classified into mono-, di-, tri-, or polyhydric phenols.

Nomenclature of Phenols

(a) Earlier, most of the compounds with the same structural formula were known by different names depending on the regions where they were synthesized. This naming system was very trivial since it raised a lot of confusion. Finally, a common naming system enlisting standard rules was set up by IUPAC for the naming of compounds. 

(b) It is both a common name and an IUPAC name for the compounds containing a benzene ring attached to a hydroxyl group. Structurally phenols are the simplest hydroxy derivative of the benzene ring. IUPAC nomenclature of phenols follows a set of rules.

Rules underlying the Nomenclature of Phenols

1. Locate the position of a hydroxyl group attached to the benzene ring.

2. Benzene rings attached to more than one hydroxyl group are labeled with the Greek numerical prefixes such as di, tri, and tetra to denote the number of similar hydroxyl groups attached to the benzene ring. If two hydroxyl groups are attached to the adjacent carbon atoms of the benzene ring, it is named as benzene1,2-diol

3. In the case of substituted phenols, we start locating the positions of the other functional groups concerning the position where the hydroxyl group is attached. For example, if a methyl group is attached to the fourth carbon atom concerning the hydroxy group, the compound is named 4-Methyl phenol.

4. Depending on the position of the substituted functional group concerning the hydroxyl group, words like ortho (when the functional group is attached to the adjacent carbon atom), para (when the functional group is attached to the third carbon atom from the hydroxyl group), meta (when the functional group is attached to the second carbon atom from the hydroxyl group) are also used for the nomenclature of phenols.

Physical Properties of Alcohols and Phenols

Most physical properties of alcohols and phenols are governed by the presence of the hydroxyl group and by molecular structure (aromatic vs aliphatic, chain length, branching).

Boiling point

Alcohols and phenols generally have higher boiling points than hydrocarbons of similar molecular mass because of intermolecular hydrogen bonding between -OH groups.

• Intermolecular hydrogen bonding: hydrogen bonds between -OH groups increase the energy required to separate molecules, raising boiling points.
• Carbon chain length: boiling point increases with increasing chain length due to greater van der Waals forces.
• Branching: increased branching reduces surface contact and generally lowers boiling point (more branching ⇒ lower bp).
• Typical trend for isomeric alcohols: 1° often has higher boiling point than 2° and 3° (qualitatively 1° > 2° > 3°) because less branching allows stronger intermolecular interactions.

Hydrogen Bonding in Ethanol

Solubility in water

Solubility depends on the balance between the polar -OH (hydrophilic) and the hydrophobic hydrocarbon or aromatic part.

• Short-chain alcohols (methanol, ethanol, propanol) are miscible or highly soluble in water because hydrogen bonding with water dominates.
• Solubility decreases with increasing alkyl chain length because the hydrophobic part increases.
• Phenols are capable of hydrogen bonding with water, but the aromatic ring is hydrophobic; hence phenol is only moderately soluble in water. In alkaline solutions phenol is deprotonated to phenoxide ion and becomes highly soluble.

Intermolecular Hydrogen Bonding

Density

Most common alcohols have densities slightly lower than water and therefore tend to float on water. Density generally increases with molecular mass and chain length.

Alcohols Float on Water

• Phenol has density greater than water and will sink in water (phenol density ≈ 1.07 g cm-3 at room temperature).

Odour

• Many lower alcohols have characteristic odours: ethanol has a typical alcoholic smell.
• Phenol has a distinct medicinal/tarry odour and is readily recognisable.

Volatility

• Low-molecular-weight alcohols are more volatile (evaporate readily); volatility decreases with increasing molecular mass and hydrogen bonding strength.

Flammability

• Many alcohols are flammable. Lower alcohols (methanol, ethanol) burn easily and are used as fuels or fuel additives. Phenols are less volatile but can burn.

Alcohols are Flammable

Chemical Properties of Alcohols and Phenols

The chemical behaviour of alcohols and phenols is determined by the polarity of the O-H bond, the ability to form hydrogen bonds and - in phenols - resonance interaction between the oxygen lone pair and the aromatic ring.

Oxidation

• Primary alcohols can be oxidised to aldehydes and further to carboxylic acids under suitable oxidising conditions (common reagents: KMnO4, CrO3, PCC under controlled conditions to stop at aldehyde).
• Secondary alcohols oxidise to ketones.
• Tertiary alcohols resist oxidation under mild conditions because the carbon bearing the -OH lacks a hydrogen atom; strong/cleaving conditions are required to oxidise them.

Oxidation Reaction of Alcohols

Acid-base behaviour of alcohols

Alcohols are weak acids: they can donate the proton of the hydroxyl group to very strong bases to form alkoxide ions.

• Reaction with reactive metals or strong bases: 2 R-OH + 2 Na → 2 R-O-Na+ + H2↑. This is commonly used to prepare alkoxide salts for synthetic transformations.
• Alcohols are considerably weaker acids than water; typical pKa of simple aliphatic alcohols is around ~16, so they are deprotonated only by strong bases (e.g., NaH, alkali metals).
• Electron withdrawing substituents stabilise the conjugate base (alkoxide) and thereby increase acidity of the alcohol.

Alcohols with Metals

Relative acidity: Phenols vs alcohols

• Phenols > aliphatic alcohols in acidity because the negative charge on the phenoxide ion is stabilised by resonance with the aromatic ring; phenol pKa ≈ 10, so phenols are deprotonated by bases such as NaOH.
• Phenols are deprotonated by weaker bases (e.g., NaOH) to give phenoxide salts; aliphatic alcohols generally require stronger bases for deprotonation.

Phenol more Acidic due to Resonance

Acidity of Phenols - influence of substituents

• Phenols are acidic because the phenoxide ion formed on deprotonation is resonance-stabilised; the negative charge is delocalised over the aromatic ring.
• Electron withdrawing substituents (e.g., -NO2, -COOH) on the ring increase phenol acidity, especially at the ortho and para positions where resonance stabilisation is effective.
• Electron donating substituents (e.g., -CH3, -OCH3) decrease acidity by destabilising the conjugate base.

Phenol Generating Phenoxide and Hydrogen Ion

Dehydration of Alcohols

• Alcohols undergo dehydration (elimination of H2O) to give alkenes in the presence of strong acid and heat (e.g., concentrated H2SO4 or H3PO4). The reaction may follow an E1 mechanism (common for secondary and tertiary alcohols) or an E2 mechanism (possible for primary alcohols under certain conditions).
• Phenols do not undergo simple dehydration to form alkenes because the -OH is directly bonded to an sp2 carbon of the aromatic ring; removal of the hydroxyl would destroy aromaticity and is therefore unfavourable.

Dehydration of Alcohols

Esterification

Esterification is the reaction of an alcohol with a carboxylic acid (or its derivatives) to form an ester and water (Fischer esterification when acid catalysed).

• Alcohols react with carboxylic acids in the presence of an acid catalyst to form esters: R-OH + R'COOH ⇌ R'COOR + H2O (Fischer esterification).
• Phenols are less nucleophilic than aliphatic alcohols and do not readily undergo Fischer esterification with carboxylic acids; they can form esters with acid chlorides or anhydrides (often using a base or catalyst such as pyridine or triethylamine).

Reaction with metals

• Alcohols react with reactive metals like sodium or potassium to give alkoxides and hydrogen gas: 2 R-OH + 2 Na → 2 R-O-Na+ + H2↑.
• Phenols react with aqueous sodium hydroxide to give sodium phenoxide (soluble); they do not react with weak bases such as sodium carbonate unless the phenol is activated (e.g., strongly electron withdrawing substituents present).

Alcohols with Metals

Phenol Reaction with Na

Phenol reacting with NaOH

Reaction with acid chlorides

• Alcohols and phenols react with acid chlorides (or acid anhydrides) to form esters; reaction with acid chlorides is often faster and proceeds under milder conditions than Fischer esterification (R-OH + R'COCl → R'COOR + HCl).

Nucleophilic substitution

Nucleophilic substitution at saturated carbon atoms bearing leaving groups is common in organic chemistry. Alcohols and phenols show different tendencies in such reactions because of bond characters and electronic effects.

• Alcohols can act as nucleophiles or can be converted into better leaving groups (e.g., tosylates) or protonated to undergo substitution (SN1 or SN2) with suitable nucleophiles/electrophiles.
• Phenols, when considered as nucleophiles for aromatic substitution, are governed by electrophilic aromatic substitution (EAS) patterns because the -OH group is an activating, ortho/para-directing substituent. Nucleophilic aromatic substitution on a simple phenol ring is uncommon and requires strongly deactivating substituents and special conditions.

Alcohols as Nucleophiles

Key Differences between Alcohols and Phenols

Although both contain the -OH group, alcohols and phenols differ in structure and reactivity because in phenols the -OH is bonded to an sp2 carbon of an aromatic ring. Key differences:

• Acidity: Phenols are significantly more acidic than aliphatic alcohols due to resonance stabilisation of the phenoxide ion (phenol pKa ≈ 10; aliphatic alcohols pKa ≈ 16).
• Reactivity: Alcohols readily undergo typical alcohol reactions (oxidation to aldehydes/ketones/acids, dehydration to alkenes). Phenols undergo electrophilic aromatic substitution and form phenoxide salts readily with bases.
• Solubility: Small alcohols (e.g., ethanol) are miscible with water; phenol is only moderately soluble in water but dissolves in alkali due to phenoxide formation.
• Boiling point: Both show elevated boiling points due to hydrogen bonding; detailed trends depend on molecular structure and intermolecular interactions.

Some Important Questions

Q1: Given below are two statements: One is labelled as Assertion A and the other is labelled as Reason R.
Assertion A: Butan-1-ol has higher boiling point than ethoxyethane.
Reason R: Extensive hydrogen bonding leads to stronger association of molecules.
In the light of the above statements, choose the correct answer from the options given below:
(a) A is false but R is true
(b) Both A and R are true but R is not the correct explanation of A
(c) Both A and R are true but R is the correct explanation of A
(d) A is true but R is false
Ans: (c)

The boiling point depends on intermolecular forces. Butan-1-ol has an -OH group and forms extensive hydrogen bonds, producing strong intermolecular attraction and a higher boiling point. Ethoxyethane (diethyl ether) lacks an -OH and cannot hydrogen-bond to the same extent; its forces are mainly van der Waals and dipole interactions. Thus both A and R are true, and R correctly explains A.

Q2: Find out the major product from the following reaction.

(a) 

(b) 

(c) 

(d) 

Ans: (d)

Q3: Which one of the following statements is not correct?
(a) Alcohols are weaker acids than water
(b) Acid strength of alcohols decreases in the following: RCH2OH > R2CHOH > R3COH
(c) Carbon-oxygen bond length in methanol, CH3OH is shorter than that of C-O bond length in phenol.
(d) The bond angle 

 in methanol is 108.9°.

Ans: (c)

The C-O bond in phenol is shorter than in aliphatic alcohols (e.g., methanol) because resonance (delocalisation of the oxygen lone pair into the aromatic ring) gives the C-O bond partial double-bond character. Thus statement (c) is not correct as written.