Mada za sehemu hiiHydroxyl CompoundsMada 4
- Structure and Nomenclature
- Preparation of Hydroxyl Compounds
- Properties of Hydroxyl Compounds
- Uses and Hazards of Hydroxyl Compounds
Properties of hydroxyl compounds
- Boiling points:
- The boiling point of alcohols increases with an increase in the number of carbon atoms due to higher molecular weight. Example: The boiling point of C3H7OH is higher than C2H5OH.
- Less branched alcohols have higher boiling points compared to highly branched alcohols due to:
- Poor packing of branched molecules.
- Reduced surface area, making heating easier.
- Comparison with alkenes: Alcohols have higher boiling points than alkenes of similar molecular weight due to hydrogen bonding in alcohols.
- Polyhydric alcohols: Alcohols with multiple hydroxyl groups have higher boiling points due to more hydrogen bonding sites. Example: Ethylene glycol (C2H6O2) has a higher boiling point (197°C) compared to propanol (97°C) and butane (-0.5°C) because it has two hydroxyl groups.
- Solubility in water:
- Decreases with an increase in hydrophobic groups (more carbon atoms).
- Polyhydric alcohols are more soluble due to increased hydrogen bonding sites. Example: Glycerol is more soluble than ethanol.
- Color: Alcohols are usually colorless liquids.
- Phenols are more acidic than alcohols because:
- Easier hydrogen release: Lone pair electrons on oxygen in phenols are delocalized through resonance, stabilizing the phenoxide ion.
- Phenoxide ion stability: The negative charge in the phenoxide ion is delocalized, enhancing stability.
Alcohols do not exhibit such resonance stabilization.
Reactions highlighting acidity
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Reaction with alkaline solutions: Phenols react with NaOH to form sodium phenoxide, while alcohols do not.
C6H5OH+NaOH→C6H5ONa+H2O
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Reaction with sodium carbonate: Phenols release CO2, while alcohols do not.
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Reaction with carboxylic acids: Alcohols form esters, but phenols do not react in this manner.
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Replacement of hydrogen in the –OH group:
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React with alkali metals to form alkoxides and release hydrogen gas.
ROH+Na→RONa+H2
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Replacement of the –OH group:
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Halogenation: Alcohols react with halogenating agents to form alkyl halides.
C2H5OH+PCl5→C2H5Cl+POCl3+HCl
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Oxidation:
- Primary alcohols: Form aldehydes with mild oxidizing agents and carboxylic acids with strong oxidizing agents.
- Secondary alcohols: Oxidize to ketones.
- Tertiary alcohols: Resistant to oxidation due to the absence of a hydrogen atom on the carbon bonded to –OH.
Lucas test
- Primary alcohols: No turbidity.
- Secondary alcohols: Turbidity within 5 minutes.
- Tertiary alcohols: Immediate turbidity.
Phenol tests
- Turns blue litmus paper red.
- Forms violet color with FeCl3.
- Reacts with bromine water to form white precipitate.
Similarities
- Evolve hydrogen gas with alkaline metals.
- Form ethers with haloalkanes.
- React with hydrogen halides.
Differences
| Alcohol | Phenol | |
|---|---|---|
| 1. | Undergo oxidation easily (for primary and secondary alcohol) | Do not undergo oxidation easily |
| 2 | Do not react with bromine water | Form white ppt of 2, 4, 6 – tribomophenol with bromine water |
| 3 | Do not react with FeCl3 | Form purple coloration with ion(III) chloride |
| 4 | Form ester (has fruity smell) | Do not react with carboxylic acid |
| 5 | Do not react with sodium hydroxide solution | Form white ppt of sodium phenoxide with NaOH(aq) |
| 6 | Do not react with sodium carbonate | Form white ppt of sodium phenoxide and gas which turns lime water milky (i.e CO2 is evolved) |
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