ALCOHOLS, ETHERS and PHENOLS ALCOHOLS Molecules Containing –OH Group Are Termed As Alcohols

ALCOHOLS, ETHERS AND PHENOLS ALCOHOLS Molecules containing –OH group are termed as alcohols. Classification of alcohols they are classified as primary, secondary or tertiary alcohol according to the carbon that is bonded with –OH. Again when any molecule contains 1, 2 or 3 –OH groups then it is called mono, di or tri hydric alcohols respectively. (as in case of alkyl halides) OH OH OH OH OH CH32 – CH OH CH22 – CH CH22 – CH – CH Ethyl alcohol Ethylene glycol Glycerol (monohydric) (dihydric) (trihydric) Example 1: Classify the following into primary, secondary and tertiary alcohols: (a) CH3 (b) OH (c) HO OH H3C Solution: (a) Tertiary (b) Secondary (c) Tertiary GENERAL METHODS OF PREPARATION 1. From Alkenes : (i) By direct hydrolysis : OH H24 SO CH32 – CH = CH2 + H O CH33 – CH – CH (ii) Oxymercuration demercuration : OH OH NaBH4 Hg (OAc)2 CH – CH = CH + H O CH2 – CH – CH2 – CH33 – CH – CH 322 THF OH Hg(OAc) (iii) Hydroboration oxidation : OH BH26 H22 O /OH 6CH3 – CH = CH2 2(CH32 – CH – CH23 –) B 6CH3 – CH2 – CH 2 + 2H 3 BO 3 Overall result of above reaction is anti Markwonikoff addition of water and with no rearrangement. (iv) Oxo process followed by hydrogenation : O [Co(CO) ] 42 CH – CH – CH – C – H CH3 – CH = CH22 + CO + H high temperature 3 22 and high pressure H2 /Pd CH3 – CH 2 – CH 2 – CH 2 – OH Product has one more carbon. 2. From Alkyl Halides : When alkyl halides are treated with aq. KOH or aq. NaOH or moist Ag2O, alcohols are formed. – R X OHR OH X 3. Reduction of Carbonyl Compounds, Carboxylic Acids and their Derivatives : O red. agent R – C – H R – CH2 OH O OH red. agent R – C – R O red. agent R – C – OR R – CH2 OH + R – OH O red. agent R – C – X R – CH2 OH O O red. agent R – C – O – C – R 2R – CH OH 2 4. Using Grignard Reagent : (i) From aldehydes or ketones : O OMgX OH H O/H+ OH – C – + R – MgX – C – R 2 – C – R + Mg X In this reaction Formaldehyde gives 1°-alcohol Other aldehyde gives 2°-alcohol Ketones give 3°-alcohol (ii) From carboxylic acid and their derivatives : O O Z R – C – Z + R – MgX R – C – R + Mg X (iii) From epoxides : O OMgX OH + OH H2 O/H CH32 – CH — CH + R – MgX CH32 – CH – CH CH32 – CH – CH + Mg X R R 5. Hydrolysis of Ether : Ether undergo acid hydrolysis with dilute H2SO4 under pressure to give corresponding alcohols. dil. H2SO4 R O R H2O R OH R OH Example 2: (a) Find A, B, C, D, E. O Mg A H PhBrdry ether B C i CH3 MgBr (b) CH3 COOC 2 H 5 D E ii H2 O / H Solution: (a) O OH A = PhMgBr B =Ph C = Ph (b) CH3 H3C C OH C2H5OH CH3 Physical Properties (i) Physical state : At ordinary temperature, lower members are colourless liquids with burning taste and a pleasant smell. (ii) Boiling Point : The boiling point of alcohols rise regularly with the rise in the molecular mass. Amongst isomeric alcohols, the boiling point decrease in the order. 1° > 2° > 3° (iii) Solubility : The extent of solubility of any alcohol in water depends upon the capability of its molecules to form hydrogen bonds with water molecule. (iv) Alcohols are lighter than water however, the density increases with the increase in molecular mass. Chemical Properties 1. Reactions involving cleavage of O – H Bond Alcohols are acidic in nature but they are less acidic than water hence they do not give H+ in aqueous solution. They do not change the colour of litmus paper. Their acidic strength increases by increasing–I strength of the groups attached and decreases by increasing +I strength of the groups. (i) Alcohols do not react with aqueous alkali, as it does not give H+ in aqueous solution. (ii) Action of active metal : When alcohols are treated with active metal they form alkoxides with the liberation of H2 gas. 2ROH 2Na 2R ONa H2 (iii) Esterification : When carboxylic acid is treated with alcohols in the presence of acid as catalyst, esters are formed. O O H+ R – C – OH + H – O – R R – C – OR + HOH (iv) Reaction with Grignard Reagent : When Grignard reagents are treated with alcohol (or any proton donor) they form alkanes. R – MgX + H – OR R – H + R – OMgX Other proton donors can be carboxylic acids, phenols, alkynes, H2O, Amines, NH3 etc. 2. Reactions Involving Cleavage of C– O Bond (i) Reaction with HX : Most alcohols undergo SN1. Anhyd. ZnCl2 (a) R OH HCl(g) R Cl H2 O Note : HCl + anhyd. ZnCl2 is called Lucas reagent. H /H24 SO (b) R OH HBr conc. R Br H2 O H /H24 SO (c) R OH HI conc. R I H2 O Reactivity order of HX is HI > HBr > HCl (ii) Dehydration : Alkyl chlorides can also be prepared by following methods : R – OH + PCl5 R – Cl + POCl3 + HCl 3R – OH + PCl3 3R – Cl + H3PO3 R – OH + SOCl2 R – Cl + SO2 + HCl (Darzen's process) Darzen’s process is the best method as the other products are gases. 3. Reduction : Alcohols are reduced to alkanes when they are treated with Zn-dust or red P + HI. R OH Zn dust R H ZnO 4. Oxidation : O O [O] [O] [O] CH OH H – C – H H – C – OH CO 3 2 3°-alcohols can’t be oxidised. (i) Strong oxidising agent like KMnO4 or K2Cr2O7 cause maximum oxidation as above. (ii) If 1°-alcohol has to be converted into aldehyde PCC + CH2Cl2 or CrO3 should be used among which PCC + CH2Cl2 is the best. (iii) 2°-alcohol can converted to ketone best by PCC + CH2Cl2 or CrO3 or H2CrO4 in aq. acetone (Jones reagent). (iv) MnO2 selectively oxidises the –OH group of allylic and benzylic 1° and 2° alcohols to aldehydes and ketones respectively. 5. Action of Heated Copper : O Cu CH – CH – OH CH – C – H + H (Dehydrogenation) (i) 3 2 573K 32 OH O Cu (ii) CH3 – CH – CH3 573K CH3 – C – CH 3 + H 2 (Dehydrogenation) (iii) Tertiary alcohols undergo dehydration to give alkene under similar condition. CH3 CH2 Cu CH3 – C – OH 573 K CH32 – C + H O CH CH 3 3 Distinction Between 1°, 2° and 3° Alcohols 1. Lucas Test : Any alcohol is treated with Lucas reagent (HCl + an hyd. ZnCl2) at room temperature if (i) Solution becomes cloudy immediately, alcohol is 3°. (ii) Solution becomes cloudy after 5-min, alcohol is 2°. (iii) In solution cloud does not form at room temperature, alcohol is 1°. 2. Victor Meyer’s Method : P + I2 AgNO2 –H2 O R – CH2 OH R – CH2 I R – CH2 – NO 2 + HNO 2 R – C – NO2 (1°-alcohol) N OH (Nitrolic acid) NaOH blood red colour P + I2 AgNO2 –H2 O R2 CH – OH R2 – CH – I R2 CH – NO 2 + HNO 2 R2 C – NO2 (2°-alcohol) NO Pseudonitrol NaOH blue colour P + I2 AgNO2 HNO2 NaOH R3 – C – OH R3 C – I R32 C – NO No reaction Colourless (3°-alcohol) Note : Rectified Spirit : Azeotropic mixture of 95% C2H5OH and 5% H2O is called rectified spirit. Denatured Spirit : Azeotropic mixture of C2H5OH and CH3OH is called methylated spirit. Example 3 : Which of the following alcohols would react faster with Lucas reagent? CH3 CH CH CH CH OH , CH CH CHCH , CH CH CH OH , 3 2 2 2 3 2 3 3 2 H3C C CH3 OH OH CH3 Solution : CH COH, it being a tertiary alcohol. 3 3 Example 4 : List three methods with pertinet chemical equations for the preparation of alcohols from alkenes. Solution: Hydration, hydroboration and oxymercuration – demercuration of alkenes. Hydration: CH3 CH3 HO2 H3C C CH CH2 H H3C C CHCH3 CH CH OH 3 3 Hydroboration CH3 CH3 i BH3 H C C CH CH H C C CH CH OH 3 2 ii H22 O , OH 3 2 2 CH CH 3 3 Oxymercuration – demercuration CH3 CH3 i Hg OAC , THF, H O H C C CH CH 2 2 H C C CHCH 3 2 ii NaBH4 3 3 CH3 CH3 OH ETHERS Ethers are those organic compounds which contain two alkyl groups attached to an oxygen atom, i.e., R–O–R. They are regarded as dialkyl derivatives of water or anhydrides of alcohols. –2H H–O–H R–O–R HO R–OH HO–R Water 2R Ether 2 Alcohol (2 moles) Ethers may be of two types : (i) Symmetrical or simple ether are those in which both the alkyl groups are identical and (ii) unsymmetrical or mixed ethers are those in which the two alkyl groups are different. CH3–O–CH3; C6H5–O–C6H5 CH3–O–C2H5; CH3–O–C6H5 Symmetrical (simple) ethers Unsymmetrical (mixed)ethers Like water, ether has two unshared pair of electrons on oxygen atom, yet its angle is greater than normal tetrahedral (109°28´) and different from that in water (105°). This is because of the fact that in ethers the repulsion between lone pairs of electrons is overcome by the repulsion between the bulky alkyl groups. Preparation of Ethers : 1. By dehydrating excess of alcohols : Simple ethers can be prepared by heating an excess of primary alcohols with conc. H2SO4 at 413K. Alcohol should be taken in excess so as to avoid its dehydration to alkenes. Conc. H24 SO CH–OH2 5 HO–CH 2 5 CH–O–CH 2 5 2 5 HO 2 Ethanol (2 molecules)413K Diethyl ether Dehydration may also be done by passing alcohol vapours over heated catalyst like alumina under high pressure and temperature of 200 – 250°C.

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