What Are the Physical and Chemical Properties of Alcohols with Examples and Reactions
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 in Organic Chemistry
1. What are the physical properties of alcohols?
The physical properties of alcohols include high boiling points, hydrogen bonding, solubility in water (for lower alcohols), and characteristic odor.
- Boiling point: Higher than corresponding alkanes due to intermolecular hydrogen bonding.
- Solubility: Lower alcohols like CH3OH and C2H5OH are completely miscible with water.
- State: Lower alcohols are liquids; higher alcohols become waxy solids.
- Polarity: Alcohols are polar due to the –OH (hydroxyl) group.
2. Why do alcohols have higher boiling points than alkanes?
Alcohols have higher boiling points than alkanes because they form strong intermolecular hydrogen bonds.
- The –OH group in alcohols allows hydrogen bonding between molecules.
- Alkanes only have weak van der Waals forces.
- More energy is required to break hydrogen bonds, increasing the boiling point.
3. How does the solubility of alcohols in water change with chain length?
The solubility of alcohols in water decreases as the carbon chain length increases.
- Lower alcohols (methanol, ethanol, propanol) are highly soluble due to hydrogen bonding with water.
- As the non-polar hydrocarbon chain grows, it reduces overall polarity.
- Higher alcohols become less soluble or nearly insoluble in water.
4. What are the chemical properties of alcohols?
The chemical properties of alcohols include combustion, oxidation, dehydration, and reaction with active metals.
- Combustion: C2H5OH(l) + 3O2(g) → 2CO2(g) + 3H2O(l)
- Reaction with sodium: 2C2H5OH(l) + 2Na(s) → 2C2H5ONa(aq) + H2(g)
- Oxidation: Primary alcohols oxidize to aldehydes and acids.
- Dehydration: Alcohols form alkenes on heating with acid.
5. How do alcohols react with active metals like sodium?
Alcohols react with active metals like sodium to form alkoxides and release hydrogen gas.
- General reaction: 2ROH + 2Na → 2RONa + H2
- Example: 2C2H5OH(l) + 2Na(s) → 2C2H5ONa(aq) + H2(g)
6. What happens when alcohols undergo combustion?
When alcohols undergo complete combustion, they produce carbon dioxide and water along with heat energy.
- Example (ethanol): C2H5OH(l) + 3O2(g) → 2CO2(g) + 3H2O(l)
- The reaction is highly exothermic.
- Incomplete combustion may produce CO or soot.
7. How are primary, secondary, and tertiary alcohols different in chemical properties?
Primary, secondary, and tertiary alcohols differ mainly in their oxidation behavior.
- Primary (1°) alcohols: Oxidize to aldehydes, then to carboxylic acids.
- Secondary (2°) alcohols: Oxidize to ketones.
- Tertiary (3°) alcohols: Resist oxidation under mild conditions.
8. What is dehydration of alcohols?
Dehydration of alcohols is the removal of a water molecule to form an alkene in the presence of a strong acid.
- Usually heated with concentrated H2SO4 or H3PO4.
- Example: C2H5OH(l) → C2H4(g) + H2O(l)
- Follows Saytzeff’s rule in unsymmetrical alcohols.
9. Why are alcohols considered weak acids?
Alcohols are considered weak acids because they can donate a proton from the –OH group but only to a very small extent.
- They react with active metals like sodium to release H2.
- They do not completely ionize in water.
- Their acidity is much lower than that of carboxylic acids.
10. How does hydrogen bonding affect the properties of alcohols?
Hydrogen bonding significantly increases the boiling point, viscosity, and water solubility of alcohols.
- The hydrogen of the –OH group forms hydrogen bonds with oxygen of neighboring molecules.
- This creates strong intermolecular attraction.
- It explains why alcohols have higher boiling points than ethers of similar molar mass.
