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Physical Properties of Alcohols and Phenols

By Chemca Editorial Team • Last Updated: January 2026 • 9 min read

Alcohols ($R-OH$) and Phenols ($Ar-OH$) exhibit unique physical properties primarily due to the presence of the polar hydroxyl group ($-OH$). This group allows for intermolecular hydrogen bonding, significantly influencing boiling points and solubility compared to hydrocarbons and ethers.

1. Boiling Points

The boiling points of alcohols and phenols are much higher than those of other classes of compounds (hydrocarbons, ethers, haloalkanes) with comparable molecular masses.

Reason: Intermolecular Hydrogen Bonding

The $-OH$ group is highly polar. Molecules associate through strong intermolecular hydrogen bonds, requiring more energy to separate them during boiling.

A. Trend with Molecular Mass

Boiling points increase with an increase in the number of carbon atoms (increase in Van der Waals forces).

Example: Methanol < Ethanol < Propanol

B. Effect of Branching (Isomeric Alcohols)

For isomeric alcohols, boiling point decreases with an increase in branching.

Reason: Branching makes the molecule more spherical, reducing the surface area for Van der Waals interactions.

$$ n\text{-Butyl alcohol} > \text{iso-Butyl alcohol} > \text{sec-Butyl alcohol} > \text{tert-Butyl alcohol} $$

2. Solubility

Solubility in water depends on the ability to form Hydrogen bonds with water molecules.

A. Alcohols

• Lower Alcohols: (Methanol, Ethanol, Propanol) are miscible with water in all proportions due to extensive H-bonding.
• Higher Alcohols: Solubility decreases rapidly as the size of the hydrophobic alkyl group ($R-$) increases. The large non-polar chain hinders the formation of H-bonds.

B. Phenols

Phenols are sparingly soluble in water due to the large hydrophobic benzene ring, although they can form H-bonds. They are readily soluble in organic solvents (alcohol, ether).

3. Acidity (Comparison)

Alcohols and phenols are weak acids. They can donate a proton ($H^+$).

$$ \text{Acidity Order:} $$ $$ \text{Carboxylic Acids} > \text{Carbonic Acid} > \text{Phenol} > \text{Water} > \text{Alcohols} > \text{Alkynes} $$

Why are Phenols more acidic than Alcohols?

1. Resonance Stabilization: The Phenoxide ion formed after losing a proton is stabilized by resonance (delocalization of negative charge into the benzene ring). The Alkoxide ion ($RO^-$) has no such stabilization.
2. Hybridization: The $sp^2$ hybridized carbon in phenol is more electronegative than the $sp^3$ carbon in alcohols, making the O-H bond more polar.

Effect of Substituents on Phenol Acidity

• Electron Withdrawing Groups (EWG): (e.g., $-NO_2, -X, -CHO$) stabilize the phenoxide ion and increase acidity. (o- and p- positions are most effective).
• Electron Donating Groups (EDG): (e.g., $-CH_3, -OCH_3$) destabilize the phenoxide ion and decrease acidity.

4. Physical State and Color

• Alcohols: Lower members are colorless liquids with a characteristic sweet/alcoholic smell and burning taste. Higher members are waxy solids. Toxic (Methanol causes blindness).
• Phenols: Colorless crystalline solids or liquids. They often turn pink or reddish-brown upon exposure to air due to slow oxidation. They have a characteristic "phenolic" (antiseptic) odor.

5. Summary Table

Property	Alcohols	Phenols
Boiling Point	High (H-bonding)	High (Stronger H-bonding)
Solubility in Water	Lower members soluble	Sparingly soluble
Acidity	Weaker than Water	Stronger than Water
Litmus Test	Neutral to Litmus	Turns Blue Litmus Red

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