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14 General, Organic, and Biochemistry, 7e Bettelheim, Brown, and March © 2003 Thomson Learning, Inc. All rights reserved 14-1 14 Chapter 14 Alcohols, Ethers, and Thiols © 2003 Thomson Learning, Inc. All rights reserved 14-2 14 Alcohols • Alcohol: a compound that contains an -OH (hydroxyl) group bonded to a tetrahedral carbon • methanol, CH3OH, is the simplest alcohol • Nomenclature 1.select the longest carbon chain that contains the -OH group as the parent alkane and number it from the end that gives the -OH the lower number 2.change the ending of the parent alkane from -e to -ol and use a number to show the location of the -OH group; for cyclic alcohols, the carbon bearing the -OH group is carbon-1 3.name and number substituents and list them in © 2003 Thomson Learning, Inc. alphabetical order All rights reserved 14-3 14 Nomenclature OH OH Ethan ol (Ethyl alcohol) OH 1-Propan ol (Prop yl alcohol) 2-Prop anol (Is op ropyl alcohol) OH OH 1-Butan ol (Bu tyl alcohol) OH 2-Bu tanol (sec-Bu tyl alcohol) OH 2-Methyl-2-p ropan ol (t ert -Butyl alcoh ol) © 2003 Thomson Learning, Inc. All rights reserved 2-Meth yl-1-propanol (Is ob utyl alcoh ol) OH Cycloh exanol (Cycloh exyl alcoh ol) 14-4 14 Nomenclature • Problem: write the I UPAC name for each alcohol OH (a) OH (b) OH (c) © 2003 Thomson Learning, Inc. All rights reserved (d) OH 14-5 14 Nomenclature • Solution: (a) 5 4 OH 1 OH (b ) 2 2 4-Methyl-2-pentan ol t rans-2-Meth ylcycloh exanol OH (c) 2 2-Heptanol © 2003 Thomson Learning, Inc. All rights reserved 7 (d) 3 2 1 OH 2,2-D imeth yl-1-p ropanol 14-6 14 Nomenclature • in the IUPAC system, a compound containing two hydroxyl groups is named as a diol, one containing three hydroxyl groups as a triol, and so on • IUPAC names for diols, triols, and so on retain the final "-e" in the name of the parent alkane • we commonly refer to compounds containing two hydroxyl groups on adjacent carbons as glycols CH2 CH2 OH OH CH3 CHCH2 HO OH 1,2-Eth anediol (Eth ylen e glycol) 1,2-Propan ediol (Propylene glycol) © 2003 Thomson Learning, Inc. All rights reserved CH2 CHCH2 OH OHOH 1,2,3-Propan etriol (Glycerol, Glycerin) 14-7 14 Physical Properties • Alcohols are polar molecules • the C-O and O-H bonds are both polar covalent © 2003 Thomson Learning, Inc. All rights reserved 14-8 14 Physical Properties • in the liquid state, alcohols associate by hydrogen bonding © 2003 Thomson Learning, Inc. All rights reserved 14-9 14 Physical Properties • bp increases as MW increases • solubility in water decreases as MW increases Molecular Weigh t bp (°C) Solubility in Water Structu ral Formula N ame CH3 OH CH3 CH3 methanol ethan e 32 30 65 -89 infinite ins olub le CH3 CH2 OH ethan ol propane 46 infinite 44 78 -42 1-propanol bu tane 60 58 97 0 1-bu tanol pen tane 74 117 72 36 CH3 CH2 CH3 CH3 CH2 CH2 OH CH3 CH2 CH2 CH3 CH3 CH2 CH2 CH2 OH CH3 CH2 CH2 CH2 CH3 © 2003 Thomson Learning, Inc. All rights reserved ins olub le infinite ins olub le 8 g/100 g ins olub le 14-10 14 Acidity of Alcohols • Alcohols have about the same pKa values as water; aqueous solutions of alcohols have the same pH as that of pure water • alcohols and phenols both contain an OH group • phenols are weak acids and react with NaOH and other strong bases to form water-soluble salts OH + NaOH Phenol H2 O O- Na+ + H2 O S od ium phenoxide (a w ater-soluble salt) • alcohols are weaker acids and do not react in this manner © 2003 Thomson Learning, Inc. All rights reserved 14-11 14 Dehydration • Dehydration: elimination of a molecule of water from adjacent carbon atoms gives an alkene • dehydration is most often brought about by heating an alcohol with either 85% H3PO4 or concentrated H2SO4 • 1° alcohols are the most difficult to dehydrate and require temperatures as high as 180°C • 2° alcohols undergo acid-catalyzed dehydration at somewhat lower temperatures • 3° alcohols generally undergo acid-catalyzed dehydration at temperatures only slightly above room temperature © 2003 Thomson Learning, Inc. All rights reserved 14-12 14 Dehydration CH3 CH2 OH Ethan ol OH Cycloh exanol H2 SO4 180°C H2 SO4 140°C CH2 =CH2 + H2 O Ethylene + H2 O Cyclohexen e CH3 CH3 H2 SO4 CH3 CCH3 CH3 C=CH2 + H2 O 50°C OH 2-Meth yl-2-p ropanol 2-Methylpropene (Isobutylene) (t ert -Butyl alcoh ol) © 2003 Thomson Learning, Inc. All rights reserved 14-13 14 Dehydration • when isomeric alkenes are obtained, the alkene having the greater number of alkyl groups on the double bond generally predominates • examples: OH CH3 CH2 CHCH3 2-Butanol H3 PO4 -H2 O CH3 CH=CHCH3 + CH3 CH2 CH=CH2 2-Bu tene 1-Butene (80%) (20%) CH3 CH3 CH3 H2 SO4 CH3 CHCHCH3 CH3 C=CHCH3 + CH3 CHCH=CH2 -H2 O OH 3-Meth yl-2-b utanol 2-Methyl-2-b utene 3-Methyl-1-bu tene (major prod uct) © 2003 Thomson Learning, Inc. All rights reserved 14-14 14 Dehydration-Hydration • Acid-catalyzed hydration of alkenes to give alcohols and acid-catalyzed dehydration of alcohols to give alkenes are competing reactions • the following equilibrium exists C C + H2 O dehydration h yd ration An alk ene C C H OH An alcoh ol • in accordance with Le Chatelier's principle, large amounts of water favor alcohol formation, whereas removal of water from the equilibrium mixture favors alkene formation © 2003 Thomson Learning, Inc. All rights reserved 14-15 14 Oxidation • Oxidation of a 1° alcohol gives an aldehyde or a carboxylic acid, depending on the experimental conditions • oxidation of a 1° alcohol to a carboxylic acid is carried out using potassium dichromate, K2Cr2O7, in aqueous sulfuric acid CH3 ( CH2 ) 6 CH2 OH 1-Octanol K2 Cr2 O7 H2 SO4 O K Cr O 2 2 7 CH3 ( CH2 ) 6 CH H2 SO4 Octanal O CH3 (CH2 ) 6 COH Octanoic acid • it is often possible to stop the oxidation at the aldehyde stage by distilling the mixture; the aldehyde usually has a lower boiling point than either the 1° alcohol or © 2003 Thomson Learning, Inc. the carboxylic acid All rights reserved 14-16 14 Oxidation • oxidation of a 2° alcohol gives a ketone K2 Cr2 O7 OH H2 SO4 2-Is opropyl-5-methylcyclohexanol (Menthol) O 2-Isopropyl-5-methylcyclohexan on e (Menth on e) • tertiary alcohols are resistant to oxidation CH3 OH 1-Methylcyclopentanol © 2003 Thomson Learning, Inc. All rights reserved K2 Cr2 O7 H2 SO4 (no oxidation) 14-17 14 Ethers • The functional group of an ether is an oxygen atom bonded to two carbon atoms • the simplest ether is dimethyl ether • the most common ether is diethyl ether CH3 -O-CH3 CH3 CH2 -O-CH2 CH3 D imethyl ether Dieth yl eth er © 2003 Thomson Learning, Inc. All rights reserved 14-18 14 Nomenclature • Although ethers can be named according to the IUPAC system, chemists almost invariably use common names for low-molecular-weight ethers • common names are derived by listing the alkyl groups bonded to oxygen in alphabetical order and adding the word "ether” • alternatively, name one of the groups on oxygen as an alkoxy group CH3 CH2 OCH2 CH3 D iethyl eth er © 2003 Thomson Learning, Inc. All rights reserved OCH3 Cyclohexyl meth yl eth er (Methoxycycloh exane) 14-19 14 Nomenclature • Cyclic ether: an ether in which one of the atoms in a ring is oxygen • cyclic ethers are also known by their common names • ethylene oxide is an important building block for the organic chemical industry; it is also used as a fumigant in foodstuffs and textiles, and in hospitals to sterilize surgical instruments • tetrahydrofuran is a useful laboratory and industrial solvent O Eth ylene oxide © 2003 Thomson Learning, Inc. All rights reserved O Tetrahydrofu ran (THF) 14-20 14 Physical Properties • Ethers are polar compounds in which oxygen bears a partial negative charge and each carbon bonded to it bears a partial positive charge • however, only weak forces of attraction exist between ether molecules in the pure liquid • consequently, boiling points of ethers are close to those of hydrocarbons of similar molecular weight • ethers have lower boiling points than alcohols of the same molecular formula CH3 CH2 OH Ethanol bp 78°C © 2003 Thomson Learning, Inc. All rights reserved CH3 OCH3 D imethyl ether b p -24°C 14-21 14 Reactions of Ethers • Ethers resemble hydrocarbons in their resistance to chemical reaction • they do not react with oxidizing agents such as potassium dichromate • they do not react with reducing agents such as H2 in the presence of a transition metal catalyst • they are not affected by most acids or bases at moderate temperatures • Because of their general inertness and good solvent properties, ethers such as diethyl ether and THF are excellent solvents in which to carry out organic reactions © 2003 Thomson Learning, Inc. All rights reserved 14-22 14 Thiols • Thiol: a compound containing an -SH (sulfhydryl) group • the most outstanding property of low-molecular-weight thiols is their stench • they are responsible for smells such as those from rotten eggs and sewage • the scent of skunks is due primarily to these two thiols CH3 CH3 CH=CHCH2 SH CH3 CHCH2 CH2 SH 2-Butene-1-thiol 3-Methyl-1-butanethiol © 2003 Thomson Learning, Inc. All rights reserved 14-23 14 Thiols - Nomenclature • IUPAC names are derived in the same manner as are the names of alcohols • to show that the compound is a thiol, the final -e of the parent alkane is retained and the suffix -thiol added • Common names for simple thiols are derived by naming the alkyl group bonded to -SH and adding the word "mercaptan" CH3 CH3 CH2 SH Ethaneth iol (Ethyl mercap tan) © 2003 Thomson Learning, Inc. All rights reserved CH3 CHCH2 SH 2-Methyl-1-propan ethiol (Isobu tyl mercaptan) 14-24 14 Physical Properties • Because of the small difference in electronegativity between sulfur and hydrogen (2.5 - 2.1 = 0.4), an S-H bond is nonpolar covalent • thiols show little association by hydrogen bonding • thiols have lower boiling points and are less soluble in water and other polar solvents than alcohols of similar molecular weight Thiol methanethiol ethan ethiol 1-bu tanethiol © 2003 Thomson Learning, Inc. All rights reserved bp (°C) 6 35 98 Alcoh ol methanol ethan ol 1-bu tanol bp (°C) 65 78 117 14-25 14 Reactions of Thiols • Thiols are weak acids (pKa 10), and are comparable in strength to phenols • thiols react with strong bases such as NaOH to form water-soluble thiolate salts CH3 CH2 SH + NaOH Ethanethiol (pKa 10) © 2003 Thomson Learning, Inc. All rights reserved H2 O CH3 CH2 S-Na+ + H2 O Sodium ethanethiolate 14-26 14 Reactions of Thiols • The most common reaction of thiols in biological systems is their oxidation to disulfides, the functional group of which is a disulfide (-S-S-) bond • thiols are readily oxidized to disulfides by O2 • they are so susceptible to oxidation that they must be protected from contact with air during storage • disulfides, in turn, are easily reduced to thiols by several reducing agents. 2HOCH2 CH2 SH A thiol © 2003 Thomson Learning, Inc. All rights reserved oxidation reduction HOCH2 CH2 S-SCH2 CH2 OH A disulfide 14-27 14 Important Alcohols Coal or meth ane 2 H2 CO Carbon monoxide H2 O, H2 SO4 CH2 =CH2 Ethylen e © 2003 Thomson Learning, Inc. All rights reserved O2 catalyst CO catalys t CH3 OH Meth anol CH3 CH2 OH Ethanol CH3 COOH Acetic acid O2 CH2 O oxidation Formaldehyde H2 SO4 180°C O H2 O, H2 SO4 H2 C CH2 Ethylene oxide CH3 CH2 OCH2 CH3 Dieth yl eth er HOCH2 CH2 OH Ethylene glycol 14-28 14 Important Alcohols H2 O, H2 SO4 CH3 CH=CH2 Propene © 2003 Thomson Learning, Inc. All rights reserved several s teps OH CH3 CHCH3 2-Propan ol OH HOCH2 CHCH2 OH Glycerin, glycerol 14-29 14 Alcohols, Ethers, and Thiols End Chapter 14 © 2003 Thomson Learning, Inc. All rights reserved 14-30