What are Alcohols?
The compounds obtained by replacing one hydrogen atom from aliphatic hydrocarbons by a hydroxyl group are alcohols whereas those obtained by replacing hydrogen atoms of aromatic hydrocarbons are phenols. Alcohols occur widely in nature and have various industrial and medicinal applications. In this article, we will talk about the classification of alcohols, alcohol physical and chemical properties and their uses.
Chemical and Physical Properties of Alcohol
What are The Physical Properties of Alcohol?
The important physical properties of alcohols are:
Physical State- The lower members are colourless liquids having a characteristic smell and burning taste. The higher members (having more than 12-13 carbon atoms) are colourless, odourless, wax-like solids.
Associated Nature- Alcohols exit as bonded molecules having intermolecular hydrogen bonds as shown below:
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This hydrogen bonding is due to the large difference in electronegativities of oxygen and hydrogen atoms. As a result, the OH bond is strongly polar and forms hydrogen bonds.
Boiling Points- The lower members have low boiling points but with the increase in molecular mass, the boiling points keep on increasing gradually. This is because of an increase in van der Waals forces. Isomers of alcohol have the same number of carbon atoms, the boiling points are in the order:
Primary > secondary > tertiary.
This is because, with branching, the surface area increases and therefore, van der Waals forces decrease. Consequently, the boiling point also decreases.
Solubility- The members with the low carbon of alcohols are highly soluble in water but the solubility in water decreases with the increase in molecular weight. The solubility of alcohols with less carbon in water is due to the formation of hydrogen bonds between alcohols and water molecules.
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However, as the number of carbon in the alcohol molecule increases, the alkyl group becomes larger and prevents the formation of hydrogen bonds with water molecules and hence the solubility goes on decreasing with increase in the length of the carbon chain.
Density- Generally, alcohols are lighter than water although the density increases with increase in molecular weight.
Chemical Properties of Alcohol
Alcohols can behave both as nucleophiles (electron-donating group) as well as electrophiles (electron-withdrawing group).
They behave as nucleophiles in the reaction where the bond between O-H is broken as shown below:
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They behave as electrophiles in which the bond between C-O is broken. These reactions are carried out in the presence of acids to form protonated alcohols.
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Protonated alcohols react as electrophiles.
Chemical Reactions of Alcohols
Reaction with active metals- Alcohols are weakly acidic in nature and react with active metals such as sodium, potassium, magnesium, aluminium, etc. to liberate hydrogen gas and form metal alkoxide. For example,
2CH3CH2OH + 2Na → 2CH3CH2ONa + H2
The above reaction shows that alcohols (R-OH) are acidic in nature (pH less than 7).
Reaction with metal hydrides- Alcohols react with metal hydrides and form sodium alkoxides and evolve hydrogen gas as a byproduct.
CH3OH + NaH →CH3O-Na+ + H2
Reaction with carboxylic acids- Alcohols react with the carboxylic acid, in the presence of concentrated sulphuric acid or dry hydrochloric gas as a catalyst, to form esters. The reaction is known as esterification. The function of concentrated sulphuric acid is to act as a protonating agent as well as a dehydrated agent.
CH3COOH + C2H5OH ⇌ CH3COOC2H5 + H2O
Reaction with grignard reagent
Alcohols react with Grignard reagents to form hydrocarbons. For example,
CH3OH + C2H5MgBr → C2H6 + CH3OMgBr
Reaction with acyl chloride or acid anhydride- When alcohols are treated with an acid chloride or acid anhydride in the presence of bases like pyridine, the hydrogen atom of -OH group is replaced by an acyl group.
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Reaction with hydrogen halide- Alcohols react with hydrogen halide and form alkane halide.
ROH + HX → RX + H2O
Reaction with phosphorus halide- Phosphorus halide when reacts with alcohols it forms haloalkanes.
ROH + PCl5 → R-Cl + POCl3 + HCl
Reaction with thionyl chloride- On treatment with thionyl chloride in the presence of pyridine, alcohols form chloroalkanes.
ROH + SOCl2 → R-Cl + SO2 ↑ + HCl↑
Did you know?
Ethanol and methanol are alcohols that act as fuel.
Alcohol can lower blood sugar level.
Alcohol can turn blue litmus to red.
Ethanol is used as an intoxicating agent.
FAQs on Physical and Chemical Properties of Alcohols
1. What are the key physical properties of alcohols as per the CBSE syllabus?
Based on the NCERT curriculum for the 2025-26 session, the main physical properties of alcohols are:
- Physical State: Lower alcohols are colourless liquids with a distinct smell, while higher members (with more than 12 carbons) are waxy solids.
- Boiling Point: Alcohols have significantly higher boiling points than alkanes of similar mass due to strong intermolecular hydrogen bonding. Boiling points increase with molecular mass.
- Solubility: Lower alcohols are highly soluble in water due to their ability to form hydrogen bonds with water molecules. Solubility decreases as the hydrocarbon part (alkyl group) gets larger.
- Density: Most alcohols are less dense than water, though density increases with molecular mass.
2. Why do alcohols have higher boiling points than hydrocarbons of comparable molecular mass?
Alcohols have higher boiling points because their molecules can form intermolecular hydrogen bonds with each other. The hydroxyl (-OH) group in an alcohol is highly polar. This strong attraction between molecules requires more energy to break compared to the much weaker van der Waals forces present between nonpolar hydrocarbon molecules. Therefore, more heat is needed to vaporise alcohols, resulting in higher boiling points.
3. Are alcohols acidic or basic? Explain with a chemical reaction.
Alcohols are weakly acidic in nature. This is because the polar O-H bond can break to release a proton (H⁺). They demonstrate this acidity by reacting with active metals like sodium (Na) to form a metal alkoxide and liberate hydrogen gas. This reaction is a key test for the presence of the hydroxyl group.
Reaction: 2R-OH + 2Na → 2R-O⁻Na⁺ (Sodium alkoxide) + H₂(g)
4. Why does the solubility of alcohols in water decrease as the molecular mass increases?
The solubility of an alcohol in water depends on the balance between its two parts: the polar, hydrophilic hydroxyl (-OH) group and the nonpolar, hydrophobic alkyl (-R) group. The -OH group forms hydrogen bonds with water, promoting solubility. However, as the alkyl chain gets longer, the nonpolar part dominates, disrupting the hydrogen bonding between water molecules. This makes it harder for the alcohol to dissolve, hence the solubility decreases.
5. How can an alcohol molecule act as both a nucleophile and an electrophile?
An alcohol's dual nature is central to its chemistry:
- As a Nucleophile: In reactions where the O-H bond breaks, the oxygen atom has lone pairs of electrons that it can donate. For example, in esterification, the alcohol acts as a nucleophile attacking the carbonyl carbon of a carboxylic acid.
- As an Electrophile: In reactions where the C-O bond breaks, the alcohol first gets protonated by an acid. This protonated alcohol, R-OH₂⁺, has a positively charged oxygen, which makes the attached carbon atom electron-deficient (electrophilic) and susceptible to attack by a nucleophile.
6. What is the esterification reaction for alcohols, and what is the role of the acid catalyst?
Esterification is a reversible reaction where an alcohol reacts with a carboxylic acid to form an ester and water. A strong acid catalyst, such as concentrated sulphuric acid (H₂SO₄), is used for two primary reasons:
- It protonates the carbonyl oxygen of the carboxylic acid, making the carbonyl carbon a much stronger electrophile and speeding up the nucleophilic attack by the alcohol.
- It acts as a dehydrating agent, removing the water produced during the reaction. This shifts the equilibrium to the right, favouring the formation of the ester product.
7. How does branching in isomeric alcohols affect their boiling points?
Among isomeric alcohols (alcohols with the same chemical formula but different structures), branching decreases the boiling point. This is because branching leads to a more compact, spherical shape, which reduces the surface area available for intermolecular contact. A smaller surface area results in weaker van der Waals forces between the molecules. Since less energy is needed to overcome these weaker forces, the boiling point is lower. The order of boiling points for isomeric alcohols is: Primary > Secondary > Tertiary.
