[Chemistry Class Notes] on Physical and Chemical Properties of Alcohols Pdf for Exam

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:

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.

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.

Chemical Properties of Alcohol

Alcohols can behave both as nucleophiles (electron-donating group) as well as electrophiles (electron-withdrawing group).

1. They behave as nucleophiles in the reaction where the bond between O-H is broken as shown below:

2. 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. 

Protonated alcohols react as electrophiles. 

Chemical Reactions of Alcohols

1. 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).

2. 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

3. 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

4. Reaction with grignard reagent

Alcohols react with Grignard reagents to form hydrocarbons. For example,

CH3OH + C2H5MgBr → C2H6 + CH3OMgBr

5. 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.

6. Reaction with hydrogen halide- Alcohols react with hydrogen halide and form alkane halide.

ROH + HX → RX + H2O

7. Reaction with phosphorus halide- Phosphorus halide when reacts with alcohols it forms haloalkanes.

ROH + PCl5 → R-Cl + POCl3 + HCl

8. 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.