Organic Chemistry - Alcohols, Phenols and Ethers

SECTION 1 - Classification and Nomenclature

Q1. What are alcohols? How are they classified?

• Alcohols are organic compounds containing one or more hydroxyl (-OH) groups attached to a saturated carbon atom (sp³ hybridised).
• They are classified in two ways:
  • Based on the number of -OH groups:
    • Monohydric: one -OH (e.g., ethanol)
    • Dihydric: two -OH (e.g., ethylene glycol)
    • Trihydric: three -OH (e.g., glycerol)
    • Polyhydric: many -OH
  • Based on the type of carbon bearing the -OH group:
    • Primary alcohols: -OH on a carbon bonded to one other carbon (e.g., propan-1-ol)
    • Secondary alcohols: -OH on a carbon bonded to two other carbons (e.g., propan-2-ol)
    • Tertiary alcohols: -OH on a carbon bonded to three other carbons (e.g., 2-methylpropan-2-ol)

Q2. What are phenols? How do they differ from alcohols?

• Phenols are organic compounds in which the hydroxyl group (-OH) is directly attached to a benzene ring (aromatic ring).
• The parent compound is phenol (C6H5OH).
• Key difference from alcohols:
  • In phenols, the -OH is on an aromatic carbon (sp² hybridised).
  • In alcohols, it is on a saturated carbon (sp³ hybridised).
• This structural difference gives phenols distinctly different properties:
  • They are significantly more acidic than alcohols.
  • They undergo electrophilic aromatic substitution on the ring.

Q3. How are alcohols named by the IUPAC system?

• The longest carbon chain containing the -OH group is chosen as the parent chain.
• The terminal "e" of the corresponding alkane is replaced by "ol."
• The chain is numbered from the end nearest to the -OH group to give it the lowest possible locant.
• Examples:
  • CH3OH is methanol
  • CH3CH2OH is ethanol
  • CH3CH2CH2OH is propan-1-ol
  • CH3CH(OH)CH3 is propan-2-ol
• For diols, the suffix "diol" is used (e.g., ethane-1,2-diol (ethylene glycol)).

Q4. How are phenols named?

• Simple phenols are named as derivatives of the parent compound phenol.
• The position of substituents on the ring is indicated by numbers (or ortho, meta, para for disubstituted rings).
• Examples:
  • C6H5OH is phenol
  • 2-methylphenol is o-cresol
  • 4-nitrophenol is p-nitrophenol
  • 1,2-dihydroxybenzene is catechol
• In the IUPAC system, phenol is benzenol, but the common name phenol is universally accepted.

SECTION 2 - Preparation of Alcohols

Q5. How are alcohols prepared from alkenes?

Two important methods:

• Acid-catalysed hydration (Markovnikov addition):
  • An alkene reacts with water in the presence of dilute H2SO4 or H3PO4.
  • The -OH adds to the more substituted carbon (Markovnikov's rule).
  • For example, propene gives propan-2-ol as the major product.
• Hydroboration-oxidation (anti-Markovnikov addition):
  • The alkene reacts with diborane (BH3)2 to give a trialkylborane.
  • The trialkylborane is then oxidised with H2O2/NaOH to give an alcohol where -OH goes to the less substituted carbon.
  • For example, propene gives propan-1-ol.
  • This method is useful for making primary alcohols that Markovnikov addition would not yield.

Q6. How are alcohols prepared from carbonyl compounds?

• Aldehydes on reduction give primary alcohols.
• Ketones on reduction give secondary alcohols.
• Formaldehyde on reduction gives methanol (primary).
• The reducing agents used are:
  • LiAlH4 (lithium aluminium hydride): a powerful reducing agent that reduces all carbonyl compounds.
  • NaBH4 (sodium borohydride): a milder and safer reducing agent.
  • Catalytic hydrogenation (H2/Ni) also reduces carbonyl compounds to alcohols.

Q7. How are alcohols prepared using Grignard reagents?

• Grignard reagents (RMgX - alkyl magnesium halides) react with carbonyl compounds to give alcohols after hydrolysis.
• The Grignard reagent adds across the C=O bond.
• Formaldehyde (HCHO) gives primary alcohols.
• Aldehydes (RCHO) give secondary alcohols.
• Ketones (RCOR') give tertiary alcohols.
• CO2 gives carboxylic acids.
• This is one of the most important methods in organic synthesis for making alcohols of any desired structure.

SECTION 3 - Preparation of Phenols

Q8. How is phenol prepared industrially from chlorobenzene (Dow process)?

• Chlorobenzene is fused with sodium hydroxide (NaOH) at 623 K and 320 atm pressure to give sodium phenoxide.
• Acidification of sodium phenoxide with dilute HCl or H2SO4 gives phenol.
• The reaction is: C6H5Cl + NaOH → C6H5ONa → C6H5OH.
• This reaction requires very high temperature and pressure because aryl halides are normally very unreactive in nucleophilic substitution.

Q9. How is phenol prepared from cumene (industrially important)?

• Cumene (isopropylbenzene, C6H5CH(CH3)2) is oxidised by atmospheric oxygen (air) to cumene hydroperoxide (C6H5C(CH3)2OOH).
• This hydroperoxide is then decomposed by dilute H2SO4 to give phenol and acetone as co-products.
• This is the most economical industrial method because it produces two valuable chemicals simultaneously: phenol and acetone.
• The reaction is: cumene → cumene hydroperoxide → phenol + acetone.

Q10. How is phenol prepared from benzene sulphonic acid and from diazonium salts?

• From benzene sulphonic acid:
  • Benzene sulphonic acid (C6H5SO3H) is fused with NaOH to give sodium phenoxide.
  • Acidification gives phenol.
  • This is called alkali fusion.
• From diazonium salts:
  • Benzenediazonium chloride (C6H5N2+Cl-) is hydrolysed by boiling with water to give phenol and nitrogen gas.
  • This is a useful laboratory method for making substituted phenols from the corresponding substituted anilines.

SECTION 4 - Physical Properties

Q11. Why do alcohols have much higher boiling points than ethers and hydrocarbons of similar molecular mass?

• Alcohols have strong intermolecular hydrogen bonding because of the highly polar O-H bond.
• The oxygen (very electronegative) forms hydrogen bonds with the hydrogen of another molecule's -OH group.
• These hydrogen bonds are much stronger than:
  • The van der Waals forces in hydrocarbons.
  • The weak dipole-dipole interactions in ethers (which have no O-H bond for hydrogen bonding).
• More energy is needed to break these hydrogen bonds, hence much higher boiling points.
• Example: ethanol (bp 78°C) boils much higher than dimethyl ether (bp -24°C) despite similar molecular masses.

Q12. Why are lower alcohols soluble in water while higher alcohols are not?

• Lower alcohols (up to 3 carbons) are completely miscible with water because the -OH group forms hydrogen bonds with water molecules.
• As the length of the alkyl chain increases, the hydrophobic (non-polar) part becomes dominant over the hydrophilic -OH group.
• Beyond four carbons, alcohols become increasingly insoluble in water.
• Tertiary alcohols with bulky groups are also slightly less soluble than primary alcohols of the same carbon number.

Q13. Why is o-nitrophenol steam volatile but p-nitrophenol is not?

• In o-nitrophenol, the -OH and -NO2 groups are adjacent on the ring, allowing intramolecular hydrogen bonding (hydrogen bonding within the same molecule).
  • This satisfies the hydrogen bonding capacity of the molecule internally, so there is very little intermolecular hydrogen bonding with water.
  • The compound therefore has a lower boiling point and can be carried by steam – it is steam volatile.
• In p-nitrophenol, the two groups are far apart on the ring and cannot form intramolecular hydrogen bonds.
  • They form extensive intermolecular hydrogen bonds with other molecules and with water, giving a much higher boiling point.
  • It is not steam volatile.
• This property is used to separate the two isomers by steam distillation.

SECTION 5 - Chemical Reactions of Alcohols

Q14. How do alcohols react with sodium metal? What does this show?

• Alcohols react with sodium metal to liberate hydrogen gas and form sodium alkoxide:
  • 2ROH + 2Na → 2RONa + H2↑.
• This reaction confirms the acidic nature of the O-H bond in alcohols.
• However, alcohols are much weaker acids than water – the reaction is slower and less vigorous than sodium with water.
• The order of reactivity is: methanol > ethanol > propanol > higher alcohols.
• This is because the electron-donating alkyl groups destabilise the alkoxide ion by pushing electron density onto the negatively charged oxygen.

Q15. How do alcohols react with hydrogen halides to form alkyl halides? What is the Lucas test?

• Alcohols react with HCl, HBr, or HI (or with PBr3, PCl5, SOCl2) to replace the -OH group with a halide.
• The rate of reaction with Lucas reagent (anhydrous ZnCl2 + conc. HCl) depends on the type of alcohol:
  • Tertiary alcohols react immediately at room temperature (turbidity appears at once).
  • Secondary alcohols react within 5 minutes (turbidity appears after time).
  • Primary alcohols do not react at room temperature (no turbidity).
• This is the Lucas test- used to distinguish primary, secondary, and tertiary alcohols.

Q16. What is the mechanism of dehydration of alcohols to alkenes?

• Dehydration of alcohols to alkenes involves heating with conc. H2SO4 or H3PO4.
• The mechanism has three steps:
  1. Protonation of the -OH group by the acid to give a good leaving group (water).
  2. Loss of water to form a carbocation (the rate-determining step).
  3. Loss of a proton from the adjacent carbon to form the alkene.
• For primary alcohols, the mechanism is E2 (one-step elimination).
• For secondary and tertiary alcohols, the mechanism is E1 (through a carbocation).
• Tertiary alcohols dehydrate most easily because they form the most stable (tertiary) carbocation.

Q17. What is Zaitsev's rule? How does it apply to dehydration of alcohols?

• Zaitsev's rule states that in elimination reactions, the more substituted alkene (having more alkyl groups on the double bond) is the major product because it is more stable.
• When an alcohol can give more than one alkene on dehydration, the major product is the one where more alkyl groups are attached to the double bond.
• For example: dehydration of butan-2-ol gives but-2-ene (major, more substituted) and but-1-ene (minor, less substituted).

Q18. How are alcohols oxidised? Compare oxidation of primary, secondary, and tertiary alcohols.

• Primary alcohols are oxidised first to aldehydes and then further to carboxylic acids.
  • Mild oxidising agents like PCC (pyridinium chlorochromate) stop oxidation at the aldehyde stage.
  • Strong oxidising agents like acidified KMnO4 or K2Cr2O7 oxidise primary alcohols all the way to carboxylic acids.
• Secondary alcohols are oxidised to ketones by K2Cr2O7/H2SO4, KMnO4, or CrO3.
  • Ketones are resistant to further oxidation under mild conditions.
• Tertiary alcohols have no hydrogen on the carbon bearing -OH, so they resist oxidation under mild conditions.
  • Under very vigorous conditions, they can be oxidised but the carbon chain breaks.

Q19. What is esterification of alcohols?

• Alcohols react with carboxylic acids in the presence of a mineral acid catalyst (H2SO4) to form esters and water.
• This is called Fischer esterification: RCOOH + R'OH ⇌ RCOOR' + H2O.
• It is a reversible reaction. To shift the equilibrium towards ester formation, water is removed or one reactant is taken in excess.
• Alcohols also react with acyl chlorides (RCOCl) and acid anhydrides to form esters – these reactions are faster and irreversible.
• Esters are used as flavouring agents and in the manufacture of perfumes and polymers.

Q20. What is the dehydration of alcohols to form ethers?

• When two molecules of a primary alcohol are heated with concentrated H2SO4 at around 140°C, an ether is formed with loss of water – this is intermolecular dehydration.
• For example: two molecules of ethanol give diethyl ether (C2H5OC2H5).
• At higher temperatures (above 170°C), the same reaction gives ethene (alkene) through intramolecular dehydration.
• Ether formation is favoured at lower temperatures and alkene formation at higher temperatures.
• This method is not suitable for secondary or tertiary alcohols because they tend to give only alkenes.

SECTION 6 - Chemical Reactions of Phenols

Q21. Why is phenol a stronger acid than ethanol?

• When phenol loses a proton, it forms a phenoxide ion (C6H5O-), which is highly stabilised by resonance.
  • The negative charge on oxygen is delocalised into the benzene ring through five resonance structures.
• This makes the phenoxide ion much more stable than the ethoxide ion (C2H5O-) from ethanol, which has no resonance stabilisation.
• A more stable conjugate base means the acid is stronger.
• Phenol (pKa ≈ 10) is therefore a much stronger acid than ethanol (pKa ≈ 16).
• However, phenol is still a weaker acid than carbonic acid – it reacts with NaOH but not with NaHCO3 (unlike carboxylic acids).

Q22. How does the presence of substituents affect the acidity of phenols?

• Electron-withdrawing groups (like -NO2, -CHO, –Cl) at ortho or para positions increase the acidity of phenol.
  • They stabilise the phenoxide ion further by withdrawing electron density from the ring, making it more capable of bearing the negative charge.
  • Nitrophenols are stronger acids than phenol – p-nitrophenol and o-nitrophenol are both more acidic than phenol.
• Electron-donating groups (like -CH3, -OCH3) at ortho or para positions decrease acidity by increasing electron density on oxygen, destabilising the phenoxide ion.

Q23. What is the reaction of phenol with bromine water? Why does it not need a catalyst?

• When bromine water is added to phenol, an immediate white precipitate of 2,4,6-tribromophenol is formed.
• No catalyst is needed because the -OH group on the ring is a very powerful activating and ortho/para-directing group.
  • It donates electron density into the ring through resonance, making the ring so electron-rich that it reacts with Br2 even without a Lewis acid catalyst.
• This is in contrast to bromination of benzene, which requires AlBr3 as catalyst.
• The formation of a white precipitate with bromine water is a characteristic test for phenol.

Q24. What is Kolbe's reaction?

• In Kolbe's reaction, sodium phenoxide (C6H5ONa) is treated with carbon dioxide (CO2) at 400 K under pressure of 4–7 atmospheres, followed by acidification.
• The product is 2-hydroxybenzoic acid (salicylic acid) as the major product, along with a small amount of 4-hydroxybenzoic acid.
• The phenoxide ion, which is more reactive than phenol itself towards electrophiles, attacks CO2 as an electrophile.
• Salicylic acid is the starting material for making aspirin (acetylsalicylic acid).

Q25. What is the Reimer-Tiemann reaction?

• In the Reimer-Tiemann reaction, phenol is treated with chloroform (CHCl3) in the presence of aqueous NaOH at 340 K.
• A -CHO (formyl) group is introduced at the ortho position of the benzene ring, giving 2-hydroxybenzaldehyde (salicylaldehyde) as the major product.
• The intermediate is a dichlorocarbene (:CCl2), which acts as the electrophile.
• On heating with alkali and then acidification, the product is the ortho aldehyde.
• This is an electrophilic aromatic substitution reaction specific to phenol.

Q26. What is nitration of phenol? Why is dilute HNO3 used instead of the nitrating mixture?

• Phenol reacts with dilute nitric acid (HNO3) at room temperature to give a mixture of o-nitrophenol (major) and p-nitrophenol.
• The concentrated nitrating mixture (HNO3/H2SO4) is not used because it would oxidise phenol and give polynitration products.
• Dilute HNO3 is sufficient because the ring is already highly activated by the -OH group.
• At higher temperatures or with concentrated HNO3, 2,4,6-trinitrophenol (picric acid) is formed.

Q27. What is sulphonation of phenol?

• Phenol reacts with concentrated sulphuric acid (H2SO4) to give sulphonated products.
• At lower temperatures (around 15–20°C), the major product is ortho-hydroxybenzenesulphonic acid (o-phenolsulphonic acid).
• At higher temperatures (around 100°C), the major product is para-hydroxybenzenesulphonic acid (p-phenolsulphonic acid).
• This temperature dependence reflects kinetic versus thermodynamic control – the ortho product is kinetically favoured and the para product is thermodynamically more stable.

SECTION 7 - Ethers

Q28. What are ethers? How are they classified?

• Ethers are organic compounds in which an oxygen atom is bonded to two alkyl or aryl groups – general formula R-O-R'.
• Classification:
  • If both groups are the same, it is a symmetrical (simple) ether.
  • If the two groups are different, it is an unsymmetrical (mixed) ether.
• Examples:
  • Diethyl ether (C2H5OC2H5) is symmetrical.
  • Methyl ethyl ether (CH3OC2H5) is unsymmetrical.
• Diethyl ether (common ether) was historically used as an anaesthetic.

Q29. How are ethers named by the IUPAC system?

• In the IUPAC system, ethers are named as alkoxyalkanes.
• The larger alkyl group is chosen as the parent chain and the smaller alkyl group along with the oxygen is named as an alkoxy substituent.
• For example:
  • CH3OC2H5 is methoxyethane.
  • CH3OCH3 is methoxymethane.
  • C2H5OC2H5 is ethoxyethane.
• Aryl ethers are named as alkoxybenzenes – for example, C6H5OCH3 is methoxybenzene (common name: anisole).

Q30. What is Williamson ether synthesis? What are its limitations?

• Williamson ether synthesis is a laboratory method to prepare symmetrical and unsymmetrical ethers.
• An alkyl halide reacts with a sodium alkoxide (RO-Na+) through an SN2 mechanism to give an ether: R'X + RONa → ROR' + NaX.
• Limitations:
  • For best results, the alkyl halide must be a primary alkyl halide.
  • Secondary and tertiary alkyl halides predominantly give alkenes (elimination) instead of ethers because the alkoxide is a strong base and bulky secondary or tertiary systems favour E2 elimination.
  • Aryl halides and vinyl halides cannot be used as substrates because they are unreactive in SN2.
  • However, aryl sodium alkoxide can be used as the nucleophile with a primary alkyl halide to give aryl alkyl ethers.

Q31. What are the physical properties of ethers? Why do they have lower boiling points than alcohols?

• Ethers are polar molecules because of the C-O-C dipole, but they cannot form hydrogen bonds with each other (no O-H bond).
• Therefore their intermolecular forces are weaker than alcohols and their boiling points are much lower than the corresponding alcohols.
• For example: diethyl ether (bp 34.6°C) versus pentan-1-ol (bp 138°C) despite similar molecular masses.
• Lower ethers are slightly soluble in water because the oxygen can accept hydrogen bonds from water molecules.
• Ethers are good solvents for many organic reactions because they dissolve a wide range of compounds and are relatively inert.

Q32. What happens when ethers react with HI?

• Ethers react with HI (or HBr) under heating to cleave the C-O bond.
• For dialkyl ethers:
  • Cleavage gives an alkyl iodide and an alcohol: R-O-R' + HI → RI + R'OH.
  • With excess HI, the alcohol is further converted to alkyl iodide.
• For aryl alkyl ethers (like anisole (C6H5OCH3)):
  • The cleavage always gives phenol and methyl iodide.
  • The C-O bond to the aryl group is very strong (due to partial double bond character from resonance), so the alkyl-oxygen bond breaks preferentially.
  • Aryl iodides are not formed in this reaction.

Q33. Why are ethers used as solvents in organic reactions?

Ethers are used as solvents for several reasons:

• They are chemically inert – they do not react with most reagents like Grignard reagents, LiAlH4, and many oxidising or reducing agents.
• They are polar enough to dissolve ionic reagents and non-polar enough to dissolve organic substrates.
• They have low boiling points, making them easy to remove after a reaction.
• Diethyl ether and tetrahydrofuran (THF) are the two most widely used ethereal solvents in organic synthesis.

Q34. What are some commercially important alcohols? Give their uses.

• Methanol (CH3OH):
  • Also called wood spirit.
  • Used as fuel, solvent, antifreeze, and in the manufacture of formaldehyde and acetic acid.
  • Highly toxic – ingestion causes blindness and death.
• Ethanol (C2H5OH):
  • Used in alcoholic beverages, as fuel (flex-fuel), as industrial solvent, in hand sanitisers, and in the manufacture of many chemicals.
  • Industrial ethanol is made by fermentation of sugars.
• Ethylene glycol (HOCH2CH2OH):
  • Used as antifreeze in radiators and in the manufacture of polyester (terylene).
• Glycerol (HOCH2CH(OH)CH2OH):
  • Used in soaps, cosmetics, pharmaceuticals, and as a food additive.
  • It is a by-product of soap making.

NCERT topics including classification, nomenclature, preparation methods, physical properties, and all chemical reactions of alcohols, phenols, and ethers including named reactions are suitably covered.