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Transcript
ALCOHOLS SANTOSH CHEMISTRY DEPT Introduction • Alcohols constitute a class of compounds which are regarded as hydroxy derivatives of hydrocarbons. • Based on the no. of hydroxyl groups the alcohols are classified as mono-, di- and trihydric accordingly, if the molecules contain 1, 2 or 3 –OH groups respectively. The alcohols which contain four or more no. of –OH groups are called polyhydric alcohols. Classification CH3, 1o, 2o, 3o • Primary: carbon with –OH is bonded to one other carbon. • Secondary: carbon with –OH is bonded to two other carbons. • Tertiary: carbon with –OH is bonded to three other carbons. • Aromatic (phenol): -OH is bonded to a benzene ring. IUPAC Nomenclature • Find the longest carbon chain containing the carbon with the -OH group. • Drop the -e from the alkane name, add -ol. • Number the chain, starting from the end closest to the -OH group. • Number and name all substituents. Examples: CH3 CH3 CH CH2OH 2-methyl-1-propanol OH CH3 CH CH2CH3 2-butanol CH3 CH3 OH C OH CH3 2-methyl-2-propanol Br CH3 3-bromo-3-methylcyclohexanol Unsaturated Alcohols • Hydroxyl group takes precedence. Assign that carbon the lowest number. • Use alkene or alkyne name. OH CH2 CHCH2CHCH3 pent-4-ene-2-ol Hydroxy Substituent • When -OH is part of a higher priority class of compound, it is named as hydroxy. • Example: OH CH2CH2CH2COOH 4-hydroxybutanoic acid 7 Common Names • Alcohol can be named as alkyl alcohol. • Useful only for small alkyl groups. • Examples: CH3 CH3 CH CH2OH isobutyl alcohol OH CH3 CH CH2CH3 sec-butyl alcohol Naming Diols • Two numbers are needed to locate the two -OH groups. • Use -diol as suffix instead of -ol. OH HO 1,6-hexanediol Glycols • 1, 2 diols (vicinal diols) are called glycols. • Common names for glycols use the name of the alkene from which they were made. CH2CH2 CH2CH2CH3 OH OH OH OH 1,2-ethanediol 1,2-propanediol ethylene glycol propylene glycol Preparation Reactions Reduction of carbonyl compounds Hydration of Alkenes Grignard reactions Reduction of Carbonyl Compounds • Reduction of Aldehydes/ketones • Reduction of Carboxylic acids/Esters Reduction of Aldehydes/Ketones Hydrogenation H2 R C H Pt O RCH 2OH Primary ROH H2 R C R' Pt O H R C R' OH Secondary ROH Reduction of Aldehydes/Ketones Hydride Reductions LiAlH 4 RCH RCH 2OH or O NaBH 4 R C R' O LiAlH 4 or NaBH 4 H R C R' OH Primary ROH Secondary ROH Mechanism Reduction of Carboxylic Acids and Esters Lithium Aluminum Hydride Reduction R C OH LiAlH 4 RCH 2OH + OH - O R C OR' O LiAlH 4 RCH 2OH + R'OH Hydration of Alkenes Acid catalyzed Hydration Oxymercuration-Demercuration Hydroboration-Oxidation Acid-Catalyzed Hydration of Alkenes Markovnikov addition Formation of most stable carbocation Shifts/rearrangements possible H R C H C H H + R H 2O R' H R C C R'' H+ H 2O H H C C H OH H R R' H C C OH H R'' Hydration of Alkenes via Oxymercuration/Demercuration Y Markovnikov addition Y Typically no shifts/rearrangements Y Mercurinium ion involvement H H R C C H R' H R C C R'' Hg(OAc) 2 NaBH 4 H 2O Hg(OAc) H 2O R H H C C H OH H 2 NaBH 4 R R' H C C R'' OH H Hydroboration-Oxidation of Alkenes Anti-Markovnikov addition No shifts/rearrangements Syn addition H H R C C H R' H R C C R'' (BH 3)2 (BH 3)2 OH H 2O 2 R - OH H 2O 2 R H H C C H H OH R' H C C R'' H OH Grignard Addition Reactions • Addition to Aldehydes/Ketones • Addition to Esters • Addition to Epoxides Grignard Additions to Aldehydes/Ketones Formation of primary, secondary, and tertiary alcohols H C H RMgX RCH 2OH Primary ROH O H R' C H RMgX O R' C R Secondary ROH OH R" RMgX R' C R" O R' C R OH Tertiary ROH Grignard Additions to Esters Formation of secondary and tertiary alcohols H C OR + 2R'MgX R' 2CHOH + ROH O Secondary ROH R' R" C OR + 2R'MgX O R" C OH + ROH R' Teriary ROH Grignard Addition to Epoxides O + RMgX RCH 2 CH 2 OH Primary ROH O + RMgX R' R' R' R OH C C H H R' Secondary ROH R' O R' + RMgX R' R' R' R OH C C R' R' Tertiary ROH R' Physical Properties • Unusually high boiling points due to hydrogen bonding between molecules. • Small alcohols are miscible in water, but solubility decreases as the size of the alkyl group increases. Boiling Points Solubility in Water Solubility decreases as the size of the alkyl group increases. CHEMICAL PROPERTIES The hydroxy gp present in alcohols is a very reactive gp and the characterstic rxns of alcohols are the rxns of –OH gp. In general, these are divided into 3 categories:I. Rxns involving the cleavage of O-H bond II. Rxns involving the cleavage of C-OH bond III. Rxns involving both alkyl and hydroxyl gps of the acohol molecules. Typical Alcohol Reactions Salt formation Dehydration Oxidation Alkyl halide formation Ester formation Ether synthesis Periodic acid cleavage of glycols Haloform reaction of methyl carbinols THP acetal formation Conversion of Alcohols to Salts Reaction with Active Metals ROH Na - + RO Na + H 2 Dehydration of Alcohols E-1 H R C C OH H+ E-2 H + R C C OH 2 H H rds H 2O + H H + R C C+ H H R C C OPOCl 2 H a dichlorophosphate intermediate H H H R C C+ POCl 3 H H R C C OH 2 H R C C OH H R C C OPOCl 2 R C C H H R C C H 1,2-shifts/rearrangements possible N Anti periplanar (coplanar) elimination No 1,2-shifts/rearrangements possible Oxidation of Alcohols PCC Primary RCH 2OH KMnO 4 or K 2Cr 2O 7 ² Secondary Tertiary RCHO R 2CHOH R 3COH PCC or KMnO 4 or K 2Cr 2O 7 ² PCC or KMnO ² RCOOH R C R O no reaction 4 Alcohol Conversion to Alkyl Halides Reaction with Hydrogen halides Reaction with Thionyl chloride Reaction with Phosphorus trihalides or pentahalides Hydrogen Halide Conversion of Alcohols to Alkyl Halides RCH 2OH HX RCH 2X SN 2 predominantly R 2CHOH HX R 2CHX SN 1 or SN 2 R 3COH HX R 3CX SN 1 predominantly where HX = HI, HBr, or HCl Mechanisms Ester Formation from Alcohols R C Cl R'OH C OR' + HCl R O R R'OH C O C R O R O C OH O R O R'OH H + R C OR' + R C OH O O C OR' + H 2O O Distinction b/w 1o, 2o & 3o alcohols • LUCAS TEST Lucas reagent : equimolar mixture of c.HCl and anhyd. ZnCl2 Appearance of cloudiness in the rxn mixture indicates the conversion of alcohol into alkyl halide. Observation30 alcohol:- reacts immediately & cloudiness appears immediately. 20 alcohol:- reacts within about 5 minutes when the cloudiness appears. 10 alcohol:- does not react appreciably at room temp. & therefore no cloudiness appears. • An older method known as Victor Meyer’s test is seldom used these days. Dihydric Alcohols Glycols Periodic Acid Cleavage of Glycols H H H C C H HIO OH OH R H H C C R HIO OH OH R R' R' C C R OH OH HIO 2H 4 3 O 2R 4 C H + HIO 3 O 2R 4 C H + HIO C R' + HIO O 3