Pharmacy Student Alcohols, Ethers and Epoxides Introduction—Structure and Bonding • Alcohols contain a hydroxy group (OH) bonded to an sp3 hybridized carbon. 2 Physical Properties of Alcohols Alcohols have considerably higher boiling points. Molecules of alcohols hydrogen bond to each other. •Alcohols can hydrogen bond to water and have solubility in water The alcohol groups form hydrogen bonding which makes the short chain molecules soluble in water. The solubility in water decreases as the chain length increases. Reactions of Alcohols 1- Reaction with alkali metal (Na- K) R OH + Na R ONa + ½ H2 CH3CH2OH + Na CH3CH2ONa + ½ H2 Reactions of Alcohols • 2- Action Of Hydrogen Halideds (HX): • Alcohol unlike alkyl halides in which the halogen atom serves as a good leaving group, the OH group in alcohols is a very poor leaving group. • For an alcohol to undergo nucleophilic substitution, OH must be converted into a better leaving group. By using acid, ¯OH can be converted into H2O, a good leaving group. 2- Action Of Hydrogen Halideds (HX): ROH + HX CH3CHCH3 + HBr OH R-X + H2O CH3CHCH3+ H2O Br 3- Formation of Haloalkane Ethanol and PCl5 C2H5OH(l) + PCl5(s) C2H5Cl(g) + POCl3(l) + HCl(g) Ethanol and SOCl2 C2H5OH(l) + SOCl2(l) C2H5Cl(g) + SO2(g) + HCl(g) 4-Dehydration by the action of heated alumina (Al2O3) or conc H2SO4 give Olefins: Primary alcohols can dehydrate to ethers This reaction occurs at lower temperature than the competing dehydration to an alkene. Step 1 CH3CH2-OH H+ CH3CH2-OH2 -H2O CH3CH2 Step2 CH3CH2 + HO-CH2CH3 Step 3 CH3CH2-O-CH2CH3 H CH3CH2-O-CH2CH3 H - H+ CH3CH2-O-CH2CH3 HSO4 - diethyl ether Williamson continuous etherification CH3-C (CH3)2-CH2OH CH3-C (CH3)2CH2 - H+ H+ CH3-C rearrangement -H O (CH3)2CH2OH2 2 CH3-C-CH2 CH3 CH3 CH3-C = CH CH3 2- methyl -2- butene CH3 R C Cl R'OH R OR' + HCl Acid chloride C O O Alkyl acetate Acid Anhydride R C O O C R'OH R R O C OR' + R C O Ester O Carboxylic acid R C O OH R'OH H + R C O OR' + H 2 O Ester OH Acid R-CO OH + H OR’ H2SO4 RCOOR’ + H2O OH OH .. H+ R-C-OH R-C-OH R’ O..H R-C-OH O O ‘R OH O H H + -H2O R-C R-C-OH2 -H OR’ OR’ R-C-OR’ O ESTER 2- Acid chloride: RCOCl +R’OH CH3COCl + C2H5OH Acetyl chloride 3- Acid Anhydride: (R CO)2O + R’OH (CH3CO)2O +C2H5OH CH3COOH acetic acid R-C-OOR’ + HCl CH3COOC2H5 + HCl ethyl acetate RCOOR’ + RCOOH CH3COOC2H5+ ethyl acetate Reactions of Alcohols: Dehydration • Dehydration, like dehydrohalogenation, is a elimination reaction in which the elements of OH and H are removed from the and carbon atoms respectively. • Dehydration is typically carried out using H2SO4 and other strong acids, or phosphorus oxychloride (POCl3) in the presence of an amine base. 20 • Typical acids used for alcohol dehydration are H2SO4 or ptoluenesulfonic acid (TsOH). • More substituted alcohols dehydrate more easily, giving rise to the following order of reactivity. 21 • When an alcohol has two or three carbons, dehydration is regioselective and follows the Zaitsev rule. • The more substituted alkene is the major product when a mixture of constitutional isomers is possible. 22 A- 10 Alcohol gives aldehydes: Cu/ 350 oC CH3CH2OH CH3CHO + H2 B- Secondary Alcohol gives Ketones: OH O Cu/350oC CH3CHCH3 CH3CCH3 + H2 C- Tertiary alcohol gives Olefins: OH Cu/350oC CH3C- CH3 CH3-C=CH2 CH3 CH3 8-Oxidation of Alcohols The reactions of alcohols have a central role in organic chemistry because alcohols can be converted to many of the other functional groups. Oxidation is • a loss of electrons • a more positive oxidation number • a loss of hydrogen atoms • the addition of oxygen atoms. • more bonds to oxygen 1 Bond to O Alkane Alcohol (1°) CH3 2 Bonds to O Aldehyde [O] [O] CH3 Reduction is • a gain of electrons • a less positive oxidation number • a gain of hydrogen atoms • the loss of oxygen atoms • the loss of bonds to oxygen. OH CH3 CH2 C Carboxylic acid [O] O CH3 3 Bonds to O H [O] O CH3 C OH CO2 + H2O Primary alcohols Secondary alcohol aldehydes Ketones Don’t oxidise tertiary alcohol Carboxylic acid Subtraction: Remove -H from alcohol group Remove -H from alcohol carbon H H3C H2 C C H2 OH H2 H O C H3C C O H (O) + H H H3C H2 C O C H Subtraction: Remove -H from alcohol group Remove -H from alcohol carbon H H 3C H H OH CH O O CH3 H 3C C H + (O) CH3 O H 3C C CH3 CH3CH2OH NaOH I 2 oxid CH3CHO CHI3 + HCOONa 3I Cl3CHO iodal 2 substitution Victor Meyer Test: It is carried in four steps: Step 1. Add concentrated HI to the alcohol. The alcohol is converted to the alkyl halide. R-OH + HI RI + H2O Step 2. Add silver nitrite(AgNO2) solution. The alkyl halide is converted to nitroalkane. R-I + AgNO2 R-NO2 + AgI Step 3. Add nitrous acid, HONO (NaNO2 + H2SO4) Step 4. Add KOH solution. blood color represents a 1o alcohol blue color indicates a 2o alcohol no color indicates a 3o alcohol Dihydric alcohols Glycols They are saturated hydrocarbons in which 2 hydrogen atoms are replaced by 2(OH) groups. They are classified into α, β, γ according to the relative position of the OH groups in the molecule. 1- Hydrolysis of Vic-dihalides: Cl Cl OH OH CH2-CH2 + NaOH 2- Mild oxidation: CH2 =CH2 KMnO4/ (O) CH2-CH2 CH2-CH2 OH OH Chemical reactions: Chemical reactions: 3- Oxidation: CHO glyoxal CHO ( O) Preparation: 1- Hydrolysis of fats and oils under pressure at 220oC: CH2 OCOR CH OCOR + NaOH CH2 OCOR Triglyceride CH2 OH CH OH + 3 RCOONa CH2 OH Glycerol Soap Cl2/40oC ClCH2-CH=CH2 HOCL CH3-CH=CH2 CH2-CH-CH2 soda lime CH2-CH-CH2 Cl Cl Cl -HCl Cl O Hydrolysis CH2 CH CH2 OH OH OH 1- Action of H2SO4: CH2 OH H2SO4 CH OH conc CH2 OH CH2 CH CH2 OH OH CHO - H2O CH2 H2SO4 CH2OH CHO CH Acrolein CH2 : Oxidation of Alcohols oxidation [O] OH O H reduction [H] H OH C R1 R2 [O] R1 2° alcohols H H C R1 OH 1° alcohols O C R2 ketone [O] R1 O C aldehyde [O] H R1 O C OH carboxylic acids KMnO4 and chromic acid (Na2Cr2O7, H3O+) oxidize secondary alcohols to ketones, and primary alcohols to carboxylic acids.