Physical Properties of Alcohols vs Alkanes
Physical properties of alcohols differ significantly from alkanes, despite having the same number of carbon atoms. This is due to the presence of the polar O-H bond in alcohols, which leads to stronger intermolecular forces.
Highlight: Alcohols are less volatile, have higher melting points, and greater water solubility than corresponding alkanes.
The differences in properties between alcohols and alkanes can be explained by considering the polarity of their bonds:
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Alkanes:
Have non-polar bonds due to similar electronegativity of hydrogen and carbon
Molecules are non-polar
Intermolecular forces are weak London forces
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Alcohols:
Have a polar O-H bond due to electronegativity difference between oxygen and hydrogen
Molecules are polar
Intermolecular forces include weak London forces and stronger hydrogen bonds
Vocabulary: Volatility - The ability of a substance to change from solid/liquid to vapor, characterized by low vapor pressure and higher boiling point.
Classification of Alcohols
Alcohols are classified based on the number of hydrogen atoms and alkyl groups attached to the carbon atom containing the -OH functional group:
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Primary Alcohols:
-OH group attached to a carbon atom with two hydrogen atoms and one alkyl group
Examples: methanol, ethanol
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Secondary Alcohols:
-OH group attached to a carbon atom with one hydrogen atom and two alkyl groups
Examples: propan-2-ol, pentan-3-ol
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Tertiary Alcohols:
-OH group attached to a carbon atom with no hydrogen atoms and three alkyl groups
Examples: 2-methyl propan-2-ol, 2-methylbutan-2-ol
Example: Ethanol CH3CH2OH is a primary alcohol because the -OH group is attached to a carbon atom with two hydrogen atoms and one alkyl group.
Volatility and Boiling Point
Boiling point of alcohols is higher than that of alkanes with the same number of carbon atoms. This is due to the presence of hydrogen bonds in alcohols.
Definition: Volatility refers to a compound's ability to vaporize easily. Compounds with low boiling points are considered volatile.
Hydrogen bonding in alcohols affects their volatility and boiling point:
- In the liquid state, intermolecular hydrogen bonds hold alcohol molecules together
- These bonds must be broken to change the liquid alcohol into a gas
- Breaking hydrogen bonds requires more energy than overcoming the weaker London forces in alkanes
- As a result, alcohols have lower volatility and higher boiling points than alkanes with the same number of carbon atoms
Highlight: The effect of hydrogen bonding on alcohol properties becomes less pronounced as the carbon chain length increases.
Solubility in Water
Properties of primary, secondary and tertiary alcohols include their solubility in water. Alcohols are generally more soluble in water than alkanes due to their ability to form hydrogen bonds with water molecules.
Example: Methanol and ethanol are completely soluble in water due to hydrogen bonding between the polar -OH group of the alcohol and water molecules.
Reactions of Alcohols
Alcohols undergo various reactions, including:
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Combustion:
Alcohols burn completely in a plentiful supply of oxygen to produce carbon dioxide and water
The reaction is exothermic, releasing a large quantity of energy as heat
As the number of carbon atoms in the alcohol chain increases, the heat released per mole also increases
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Oxidation:
Primary alcohols can be oxidized to aldehydes or carboxylic acids
Secondary alcohols can be oxidized to ketones
The product depends on the reaction conditions
Example: The oxidation of butan-1-ol aprimaryalcohol to form butanal analdehyde using acidified potassium dichromate VI is represented as: CH₃CH₂CH₂CH₂OH O → CH₃CH₂CH₂CHO
In conclusion, the physical properties of alcohols are significantly influenced by their ability to form hydrogen bonds, which affects their boiling points, volatility, and solubility in water. Understanding these properties and the classification of alcohols is crucial for students studying organic chemistry in Class 12 and beyond.
