Revision: Organic Chemistry >> Alcohols, Phenols and Ethers Chemistry (Theory) ISC (Science) ISC Class 12 CISCE

Grignard reagents are organomagnesium halides (RMgX) used for the preparation of alcohols by reaction with aldehydes or ketones.

Alcohols in which the –OH group is attached to a primary carbon atom are called primary alcohols.

Alcohols in which the –OH group is attached to a secondary carbon atom are called secondary alcohols.

Alcohols in which the –OH group is attached to an sp³ carbon atom adjacent to a C=C double bond are called allylic alcohols.

Alcohols in which the –OH group is directly attached to a carbon atom of a C=C double bond are called vinylic alcohols.

Phenols containing only one –OH group attached to the aromatic ring are called monohydric phenols.

Chain isomerism occurs when alcohols have the same molecular formula but different carbon chain arrangements.

Functional isomerism occurs when alcohols have same molecular formula but different functional groups (e.g., alcohol and ether).

Fermentation is the conversion of carbohydrates into ethanol by enzymes in yeast under anaerobic conditions.

Alcohols containing two –OH groups attached to an aliphatic carbon chain are called dihydric alcohols or glycols.

Alcohols containing three –OH groups attached to an aliphatic carbon chain are called trihydric alcohols.

Hydroxy derivatives are organic compounds obtained when one or more hydrogen atoms of hydrocarbons are replaced by hydroxyl (–OH) groups.

Alcohols are hydroxy derivatives of aliphatic hydrocarbons in which the –OH group is attached to an sp³-hybridised carbon atom.

or

Organic compounds containing a hydroxyl (–OH) group attached to a saturated carbon atom are called alcohols.
General formula: R–OH

Alcohols in which the –OH group is present in the side chain attached to an aromatic ring are called benzylic alcohols.

Phenols containing two –OH groups attached to the aromatic ring are called dihydric phenols.

Phenols containing three –OH groups attached to the aromatic ring are called trihydric phenols.

A functional group is an atom or group of atoms that determines the chemical properties of an organic compound.

Optical isomerism occurs when alcohols contain a chiral (asymmetric) carbon atom and rotate plane-polarised light.

Hydration of alkenes is the addition of water to an alkene molecule to form an alcohol, usually in presence of an acid catalyst.

Alcohols containing more than three –OH groups are called polyhydric alcohols.

Position isomerism occurs when alcohols differ in the position of the –OH group on the same carbon chain.

Phenols are hydroxy derivatives of aromatic hydrocarbons in which the –OH group is directly attached to an sp²-hybridised carbon atom of the aromatic ring.

or

Aromatic compounds in which the hydroxyl (–OH) group is directly attached to a benzene ring are called phenols.

Alcohols in which the –OH group is attached to a saturated aliphatic carbon chain are called aliphatic alcohols.

Alcohols containing only one –OH group in the molecule are called monohydric alcohols.

Alcohols in which the –OH group is attached to a tertiary carbon atom are called tertiary alcohols.

1. Phenolic –OH group:
   In phenols, the –OH group is directly attached to the aromatic ring and is called a phenolic group, which behaves differently from alcoholic –OH.
2. Acidic character:
   Phenols are weak acids, stronger than alcohols but weaker than carboxylic acids, due to resonance stabilisation of phenoxide ion.
3. Reaction with alkalis:
   Phenols react with NaOH or alkali metals to form phenoxides, unlike alcohols.
   (They do not react with Na₂CO₃ or NaHCO₃.)
4. Acylation (ester formation):
   Phenols react with acid chlorides or acid anhydrides in presence of pyridine to form esters (acetylation / benzoylation).
5. Alkylation (Williamson type):
   Phenols react with alkyl halides in alkaline medium to form aryl ethers.
6. Electrophilic substitution:
   The –OH group is an activating, ortho-para directing group, so phenols undergo substitution reactions more readily than benzene.
7. Halogenation and nitration:
   Phenols undergo easy halogenation (e.g. 2,4,6-tribromophenol) and nitration even under mild conditions.
8. Special reactions:
   Phenols show characteristic reactions like Kolbe–Schmitt reaction, Reimer–Tiemann reaction, Liebermann’s nitroso test, and give colour with FeCl₃.

1. Physical state:
   Lower alcohols are colourless liquids with characteristic odour and burning taste, while higher alcohols are colourless, odourless waxy solids.
2. Polarity:
   Alcohols are polar compounds due to the presence of the –OH group, which makes the O–H bond polar.
3. Hydrogen bonding:
   Alcohols form intermolecular hydrogen bonds, causing association of molecules.
4. Solubility:
   Lower alcohols are completely miscible with water, but solubility decreases with increase in molecular mass.
5. Boiling point:
   Alcohols have higher boiling points than corresponding hydrocarbons due to strong hydrogen bonding; boiling point increases with molecular mass.
6. Effect of structure:
   For isomeric alcohols, boiling points follow the order:
   Primary (1°) > Secondary (2°) > Tertiary (3°) due to decreasing hydrogen bonding.

1. Nature of bonds:
   Alcohols contain polar C–O and O–H bonds, making them reactive towards polar and ionic reagents.
2. Acidic character:
   Alcohols behave as weak acids and ionise slightly to give alkoxide ions and H⁺; they are less acidic than water.
3. Order of acidity:
   The acidic strength of alcohols decreases in the order:
   Primary (1°) > Secondary (2°) > Tertiary (3°).
4. Reaction with active metals:
   Alcohols react with Na, K, or Al to form metal alkoxides with the evolution of hydrogen gas.
5. Esterification:
   Alcohols react with carboxylic acids in the presence of conc. H₂SO₄ to form esters and water.
6. Acylation:
   Alcohols react with acid chlorides or acid anhydrides (in presence of pyridine) to form esters.
7. Substitution (C–OH bond cleavage):
   Alcohols react with HX, PCl₅, PBr₃, SOCl₂ to form haloalkanes; reactivity order is
   3° > 2° > 1° alcohol.
8. Oxidation behaviour:

• Primary alcohols → aldehydes → acids
• Secondary alcohols → ketones
• Tertiary alcohols → resistant to oxidation

1. Physical state:
   Simple phenols are colourless liquids or crystalline solids with low melting points; they darken on exposure to air and light and have a characteristic carbolic odour.
2. Solubility:
   Phenols are only slightly soluble in water; however, phenol and some di- and trihydric phenols show moderate solubility.
3. Reason for low solubility:
   Although phenols form hydrogen bonds with water, the hydrophobic aromatic ring reduces solubility compared to alcohols.
4. Boiling point:
   Phenols have higher boiling points than corresponding aromatic hydrocarbons and haloarenes due to intermolecular hydrogen bonding.

1. Physical nature:
   Ethyl alcohol is a colourless liquid with a characteristic smell and burning taste; it boils at 351 K.
2. Solubility:
   It is miscible with water in all proportions, and mixing with water is exothermic (heat is evolved).
3. Physiological effect:
   It acts as a central nervous system stimulant followed by depressant; prolonged consumption is harmful to health.
4. Solvent property:
   Ethanol is a good solvent for fats, oils, paints, varnishes, resins, perfumes, dyes, and many organic substances.
5. Azeotrope formation:
   It forms a constant boiling (azeotropic) mixture with water containing 95.6% alcohol and 4.4% water.
6. Uses:
   Ethanol is used as a fuel, industrial solvent, raw material for chemicals (ether, acetic acid, chloroform), in alcoholic beverages, as an antifreeze, and as a preservative.

1. Physical properties:
   Phenol is a colourless, crystalline, hygroscopic solid with a characteristic carbolic odour; it melts at 315 K and boils at 455 K.
2. Solubility & corrosive nature:
   Phenol is partially soluble in water and becomes completely miscible above 339 K; it is strongly corrosive and poisonous, causing blisters on skin.
3. Chemical behaviour:
   Phenol shows all characteristic reactions of phenols due to the presence of the phenolic –OH group.
4. Uses:
   Phenol is used in the manufacture of drugs (aspirin, salol), dyes, explosives, bakelite, picric acid, phenolphthalein, as a disinfectant, preservative, and as a starting material for nylon and artificial tannins.

1. Physical state, colour and odour:
   Lower ethers are colourless liquids or gases with a pleasant ethereal odour; higher ethers are colourless liquids at room temperature.
2. Density:
   The density of ethers increases with molecular mass, but all ethers are lighter than water.
3. Polarity and structure:
   Ethers contain polar C–O bonds and have a bent (angular) structure, so they behave as weakly polar molecules.
4. Hydrogen bonding:
   Ethers do not form intermolecular hydrogen bonds due to the absence of –OH group, though they can form weak hydrogen bonds with water.
5. Boiling point and solubility:
   Ethers have much lower boiling points than corresponding alcohols and are sparingly soluble in water but readily soluble in organic solvents.

1. General reactivity:
   Ethers are less reactive because the –O– group is relatively inert towards dilute acids, alkalis, and reducing agents.
2. Halogenation (alkyl group):
   In the dark, ethers undergo substitution at the α-carbon when treated with chlorine or bromine.
3. Combustion:
   Ethers are highly inflammable and burn in air to give CO₂ and H₂O.
4. Peroxide formation:
   On standing in air and light, ethers form explosive peroxides, especially dangerous in old samples.
5. Lewis base behaviour:
   The ether oxygen acts as a Lewis base and forms coordination compounds (etherates) with BF₃, AlCl₃, and Grignard reagents.
6. Cleavage by halogen acids:
   Ethers are cleaved by HI or HBr to form alkyl halides and alcohols; reactivity order is HI > HBr > HCl.
7. Hydrolysis and dehydration:
   On boiling with water or dilute acids, ethers give alcohols; on heating over Al₂O₃ at 633 K, they form alkenes.
8. Aromatic ethers:
   In aryl ethers, the –OR group is activating and o-, p-directing, so they undergo electrophilic substitution (bromination, nitration, sulphonation).

1. Physical nature:
   Diethyl ether is a colourless, highly volatile liquid, lighter than water and boils at 308 K.
2. Odour & safety:
   It has a pleasant odour, but its vapours can cause unconsciousness and it is highly inflammable, forming explosive mixtures with air.
3. Solubility:
   It is slightly soluble in water but readily miscible with alcohol, benzene, etc.
4. Chemical nature:
   It is a typical ether, showing general reactions of ethers and acts as a Lewis base (forms etherates).
5. Uses:
   Used as a solvent, in ether extraction, as a general anaesthetic, refrigerant, and as a reaction medium for Grignard reagents.