Notes-Part-2-Class-12-Chemistry-Chapter-11-Alcohols, Phenols and Ethers-Maharashtra Board

Topics to be Learn : Part-1 Introduction Classification of alcohols Nomenclature Preparation of alcohols and phenols Topics to be Learn : Part-2 Physical and Chemical Properties of alcohols and phenols Ethers Uses of alcohols, phenols and ethers

Physical and Chemical Properties of alcohols and phenols :

Physical Properties of alcohols and phenols :

The properties of alcohols and phenols are mainly due to the hydroxyl group.

(i) Nature of intermolecular forces : Due to presence of —OH groups, alcohols and phenols are polar molecules, The polar —OH groups are held together by the strong intermolecular forces i.e. hydrogen bonding.

(ii) Physical State : Lower alcohols are colourless, toxic liquids having characteristic

alcoholic odour. Pure phenol is colourless, toxic, low melting solid having characteristic carbolic or phenolic odour.

(iii)  Boiling Points : The boiling points of alcohols and phenols increase with increase in their molecular mass.

(iv) Solubility :  Solubility of alcohols and phenols in water due to their ability to form intermolecular hydrogen bonding.

Phenols and lower alcohols (having upto three carbons) show appreciable solubility in water due to their ability to form intermolecular hydrogen bonding with water molecule.

Chemical properties of Alcohols and Phenols :

Laboratory test :

• Aqueous solution of alcohols and phenols can be tested with litmus paper.
• Aqueous solution of alcohols is neutral to litmus (neither blue nor red litmus change colour).
• Aqueous solutions of phenols turn blue litmus red. Thus, phenols have acidic character.
• Alcohol does not react with ag. NaHCO; or aq.NaOH
• Phenol does not react with ag. NaHCO, but reacts NaOH with NaOH

• Alcohol does not react with neutral ferric chloride.
• Phenol reacts with neutral ferric chloride solution to give deep (purple / violet / green) colouration of ferric phenoxide.

3Ar—OH +  FeCl₃  →     (Ar—O)₃Fe + 3HCl

      (neutral)         (deep colour)

• Alcohol is non-corrosive.
• Phenol is corrosive

. Distinguishing test for alcohols (Lucas test) :

• Lucas reagent is a mixture of concentrated HCI and anhydrous ZnCl,. It is used to distinguish between primary, secondary and tertiary alcohols.
• Alcohols are soluble in Lucas reagent. Alcohol with Lucas reagent forms an alkyl chloride, R—Cl which is insoluble and gives cloudiness and forms separate layer.

• The time required for cloudiness to appear is based on the type of the alcohol and its reactivity :

R - OH \(\frac{HCl}{ZnCl_2}\)> R - Cl

(i) Primary alcohol : It reacts with the Lucas reagent very slowly and on heating forms alkyl chloride. The cloudiness and separation of a layer takes place after a long time on heating.

(ii) Secondary alcohol : It reacts with the Lucas reagent much faster to form alkyl chloride. The cloudiness and separation of layer takes place slowly.

(iii) Tertiary alcohol : It reacts immediately with the Lucas reagent at room temperature to form alkyl chloride. The cloudiness and separation of layer takes place instantaneously.

Reactions due to breaking of O -H bond :

The reactivity of alcohols and phenols towards ionization of O-H bond in them is different. This difference lies in the extent of stabilization of their respective conjugate bases by electronic effects as shown below.

Ionization of alcohols :

The alkyl group's electron-donating inductive activity (+I effect) destabilises the alkoxide ion (the conjugate base of alcohol). As a result, alcohol does not ionise much in water and behaves in aqueous media like a neutral chemical.

Ionization of phenol :

Phenoxide ion, the conjugate base of phenol, is resonace stabilized by delocalization of the negative charge. Therefore phenol ionizes in aqueous medium to a moderate extent, and thereby shows a weak acidic character.

Question : What are the electronic effects exerted by -OCH₃ and -Cl ? predict the acid strength of,

Esterification :

The reaction between alcohol or phenol with a carboxylic acid is carried out in the presence of concentrated sulphuric acid an ester is formed. This reaction is called esterification.

• The reaction is reversible and can be shifted in the forward direction by removing water as soon as it is formed.
• The formation of an ester is favoured using excess of alcohol in the presence of conc. H₂SO₄.

• Alcohols and phenols react with acid anhydrides in presence of acid catalyst to form ester.

• The reaction of alcohol and phenols with acid chloride is carried out in the presence of pyridine (base), which neutralizes HCl.

Acetyl derivatives : The CH₃-CO- group is called acetyl group. The acetate esters of alcohols or phenols are also called ‘acetyl derivatives’ of alcohols or phenols respectively.

When acetic anhydride is treated with salicyclic acid in presence of glacial acetic acid, acetyl salicyclic acid (aspirin) is obtained.

Reaction due to breaking of C-O bond in alcohols :

(i) Reaction with hydrogen halides : Tertiary alcohols react rapidly with hydrogen halides; secondary alcohols react somewhat slower; and primary alcohols, even more slowly. The order of reactivity of hydrogen halides is HI > HBr > HCl. HCl reacts only in the presence of anhydrous ZnCl₂. No catalyst is required in the case of HBr and HI.

When ethyl alcohol is heated with HBr, ethyl bromide is formed. (HBr is prepared in situ by adding NaBr to HCl or H₂SO₄)

(ii) Reaction with phosphorous halide : Alcohols react with phosphorous pentahalide (PX₅) and phosphorous trihalide (PX₃) to form alkyl halides.

Ethanol : When ethanol is treated with PCl₃, ethyl chloride is obtained.

3CH₃CH₂—OH + PCl₃ → 3CH₃—CH₂Cl + H₃PO₃

Ethanol : When ethanol is treated with PCl₅, ethyl chloride is obtained

CH₃CH₂—OH + PCl₅ → CH₃CH₂Cl + POCl₃ + HCl

(iii) Dehydration of alcohols to alkenes : Removal of water from an alcohol is called dehydration of alcohol.

• Alcohol when treated with concentrated sulphuric acid or phosphoric acid or alumina undergoes dehydration to form alkene and water.
• The reaction gives more substituted alkene as the major product, in accordance with Saytzeff rule.

The ease of dehydration of alcohols is in the following order :

tert-alcohol (3°) > secondary (2°) > primary (1°)

(a) Primary (1°) alcohol is dehydrated by heating it with 95% H₂SO₄ at 453 K.

(b) Secondary alcohol (2°) is dehydrated by heating with 60% H₂SO₄ at 373 K.

(c) A tertiary alcohol can be easily dehydrated by heating with 20% H₂SO₄ at 363K.

An alcohol can be dehydrated by passing vapours alcohols over heated alumina (AL₂O₃).

(iv) Oxidation of alcohols :

(a) Primary alcohol on oxidation with CrO₃ forms aldehyde. However, a better reagent to bring about this oxidation is PCC (pyridinium chlorochromate).

(b) Secondary alcohol on oxidation with chromic anhydride (CrO₃) forms ketone.

(c) Tertiary alcohols are very difficult to oxidise. When tertiary alcohol is oxidised with powerful oxidising agents at high temperatures, C-C bonds are broken, yielding a variety of carboxylic acids with fewer carbon atoms than the original 3° alcohol.

Heating with Cu : When vapours of primary, secondary and tertiary alcohols are passed over heated copper at 573 K, dehydrogenation (oxidation) of primary and secondary alcohol takes place while tertiary alcohols undergo dehydrogenation to give an alkene.

Reactions of phenols :

Phenol undergoes electrophilic substitution reactions more readily as compared to benzene. The -OH group in phenol is ring activating and an ortho-/paradirecting group.

(i) Halogentaion of phenol:

When phenol is treated with bromine water, a yellowish white precipitate of 2,4,6-tribromophenol is formed.

If the reaction is carried out in a solvent of lower polarity than water, such as CHCl₃, CCl₄ or CS₂, a mixture of ortho- and parabromophenol is formed.

(ii) Nitration of phenol :

When phenol is treated with dilute nitric acid, a mixture of o-nitrophenol and p- nitrophenol is formed, In this reaction, p-nitrophenol is formed as the major product.

When phenol is warmed with a mixture of conc. nitric acid and conc. sulphuric acid (a nitrating mixture or the mixed acid), 2,4,6-trinitrophenol, commonly called picric acid, is formed.

(iii) Sulfonation of phenol : Phenol reacts with concerntrated sulfuric acid at room temperature to give o-phenolsulfonic acid and at 373K, p-phenol sulfonic acid.

(iv) Reimer-Tiemann Reaction : When phenol is treated with chloroform in aqueous sodium hydroxide solution followed by hydrolysis with acid, salicylaldehyde (2-hydroxy benzaldehyde) is formed. This reaction is known as Reimer-Tiemann reaction.

(v) Kolbe reaction : When phenol reacts with sodium hydroxide, sodium phenoxide is obtained. Phenoxide ion being more reactive than phenol towards electrophilic substitution, Phenoxide undergoes electrophilic substitution with carbon dioxide at 398 K under pressure of 6 atm (a weak electrophile) followed by acid hydrolysis, salicylic acid is formed as major product.

(vi) Oxidation of phenol : Phenol on oxidation with chromic anhydride or sodium dichromate in presence of H₂SO₄ gives p-benzoquinone. Phenol oxidizes slowly giving a dark coloured mixture in presence of air.

(vii) Catalytic hydrogenation of phenol: Phenol on catalytic hydrogenation gives cyclohexanol. In this reaction a mixture of vapours of phenol and hydrogen is passed over nickel catalyst at 433 K.

(viii) Reduction of phenol : Phenol is reduced to benezene on heating with zinc dust.

Ethers :

Preparation of ethers :

(i) By Dehydration of alcohols : When excess of ethyl alcohol is distilled with dehydrating agent like concentrated sulphuric acid (H₂SO₄) at 413 K, diethyl ether ethoxyethane is formed.

Mechanism of dehydration of alcohol to give ether :

Dehydration of alcohols to form ether is SN? reaction. The mechanism of dehydration of ethanol involves the following steps.

Step 1 (Protonation) : Initially ethyl alcohol gets protonated in the presence of acid to form ethyl oxonium ion.

Step 2 (SN² mechanism) : Protonated alcohol species undergoes a backside attack by second molecule of alcohol is a slow step.

Step 3 (Deprotonation) : Formation of diethyl ether by elimination of proton

(i) Williamson’s synthesis : When an alkyl halide(R—X) is heated with sodium alkoxide(R—O-Na), an ether is obtained, this reaction is known as Williamson’s synthesis.

• This method is used to prepare simple(or symmetrical) ethers and mixed (unsymmetrical) ethers.

Sodium alkoxide is obtained by a reaction of sodium with an alcohol.

(a) Simple(Symmetrical) ether : When an alkyl halide and sodium alkoxide having similar alkyl groups are heated, symmetrical ether is obtained.

Sodium ethoxide on heating with ethyl bromide gives diethyl ether.

(b) Mixed(Unsymmetrical) ether : When an alkyl halide and sodium alkoxide or sodium phenoxide having different alkyl groups are heated, unsymmetrical ether (dialkyl ethers or alkyl aryl ether) is obtained.

Sodium ethoxide on heating with methyl bromide gives ethyl methyl ether.

Sodium phenoxide on heating with ethyl bromide gives ethyl phenyl ether.

Limitations of Williamson’s synthesis :

• In the preparation of unsymmetrical ethers by Williamson’s synthesis, the proper choice of the reactants namely alkyl halide and sodium alkoxide is necessary.
• The best yield of unsymmetrical ether is obtained when primary alkyl halide and tertiary alkoxide are heated, since primary alkyl halides are more susceptible to SN²
• Secondary or tertiary alkyl halide undergo α and β(halogen and hydrogen) elimination reaction giving an alkene instead of an ether since α carbon atom is sterically hindered by bulky alkyl groups.

Question : Explain Aryl halides do not give Williamson’s synthesis.

Physical properties :

(i) Physical states and boiling points :

• Dimethyl ether and ethyl methyl ether are gases. Other ethers are colourless liquids with pleasant odour.
• Lower ethers are highly volatile and highly inflammable substances.
• Boiling points of ethers show gradual increase with the increase in molecular mass.

Ether	B.P. / K
CH3-O-CH3	248
C2H5-O-CH3	284
C2H5-O- C2H5	308

• The solubility/miscibility of ethers in water is similar to that of alcohols of comparable molecular mass.

(ii) Polarity and solubility : Since bond angle is 110° and not 180°, the bond dipole moments of the two C-O bonds do not cancel each other; therefore ethers posses a small net dipole moment.

The solubility/miscibility of ethers in water is similar to that of alcohols of comparable molecular mass. This is because ethers can form hydrogen bonds with water through ethereal oxygen.

For example, diethyl ether and n-butyl alcohol have respective miscibilities of 7.5 and 9 g per 100 g of water.

Chemical properties of ethers :

(i) Laboratory test for ethers :

• Ethers are neutral compounds in aqueous medium.
• Ethers do not react with bases, cold dilute acids, reducing agents, oxidizing agents and active metals.
• However, ethers dissolve in cold concentrated H₂SO₄ due to formation of oxonium salts.

• This property distinguishes ethers from hydrocarbons.

(ii) Reaction involving alkyl group of ether :

Formation of peroxide : Ethers combine with atmospheric oxygen to form peroxide. Peroxides are hazardous because they decompose violently at high temperature.

(iii) Reaction involving C-O bond

Reaction with hot dilute sulphuric acid (Hydrolysis) : Ethers when heated with dilute sulfuric acid undergo hydrolysis to give alcohols/phenols.

Reaction with PCl₅ : Ethers react with PCl₅ to give alkyl chlorides

R-O-R' + PCl₅ \(\underrightarrow{Δ}\) R-Cl + R'-Cl + POCl₃

(iii) Reaction with hot concentrated acid :

Alkyl ethers react with hot and concentrated HI and HBr to give an alcohol and an alkyl halide.

The order of reactivity of HX is HI>HBr>HCl

Aryl alkyl ethers have stronger and shorter bond between oxygen and the aromatic ring. Hence an aryl alkyl ether undergoes cleavage of oxygen - alkyl bond and yields a phenol and an alkyl halide on reaction with HI.

(iv) Electrophilic substitution in aromatic ethers : The alkoxy group in aromatic

ether is a ring activating and ortho-, paradirecting group toward electrophilic aromatic substitution.

Resonance structures:

+R Effect of -OR group results in increased electron density at the para- and two orthopositions (see resonance structures II, III and IV).

(a) Halogenation : Anisole undergoes bromination with bromine in acetic acid even in the absence of ferric bromide catalyst. It is due to activation of benzene ring by the methoxy group.

When anisole is treated with bromine in acetic acid, p-bromoanisole (major product) is obtained.

(b) Friedel Crafts reaction : When anisole is treated with alkyl halide in the presence of anhydrous aluminium chloride (a Lewis acid) as catalyst, 4-Methoxy toluene is formed as major product. The alkyl groups are introduced at -ortho and -para positions in anisole, the reaction is known as Friedel-Crafts alkylation reaction.

(c) Nitration : Anisole reacts with concentrated nitric acid in presence of concentrated sulfuric acid (Nitrating mixture) to give a mixture of o-nitro anisole and p-nitro anisole.

Uses of alcohols, phenols and ethers :