Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Chapter 14: Alcohols, Ethers, and Thiols -OH 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 alphabetical order. Chapter 14: Alcohols, Ethers, and Thiols 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) 2-Meth yl-1-propanol (Is ob utyl alcoh ol) OH Cycloh exanol (Cycloh exyl alcoh ol) Chapter 14: Alcohols, Ethers, and Thiols Problem: Write the IUPAC name for each alcohol. OH (a) OH (b) OH (c) (d) OH Chapter 14: Alcohols, Ethers, and Thiols • 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 forth. • 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) CH2 CHCH2 OH OHOH 1,2,3-Propan etriol (Glycerol, Glycerin) Chapter 14: Alcohols, Ethers, and Thiols Alcohol Properties: • Alcohol molecules are polar • Alcohol molecules can create hydrogen bonds • Alcohols usually have a neutral pH, but phenols are weak acids Properties: Polarity and H-bonds Alcohols are polar molecules. • The C-O and O-H bonds are both polar covalent. Chapter 14: Alcohols, Ethers, and Thiols Alcohols have higher boiling points than hydrocarbons of comparable molecular weight. 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 ins olub le infinite ins olub le 8 g/100 g ins olub le Properties: Acidity 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 than phenols and do not react in this manner. Chapter 14: Alcohols, Ethers, and Thiols Alcohol Reactions: • Dehydration: Alcohol (-OH) Alkene (C=C) • Oxidation: Primary Alcohol Aldehide + Carboxylic acid 2. Secondary Alcohol Ketone 3. Tertiary Alcohol … Not possible 1. Chapter 14: Alcohols, Ethers, and Thiols 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. Chapter 14: Alcohols, Ethers, and Thiols 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) Chapter 14: Alcohols, Ethers, and Thiols 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) Chapter 14: Alcohols, Ethers, and Thiols Acid-catalyzed hydration of alkenes to give alcohols (Chapter 12) and acid-catalyzed dehydration of alcohols to give alkenes are competing reactions. • The following acid-catalyzed equilibrium exists. C C An alk ene + H2 O hydration dehydration 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. Chapter 14: Alcohols, Ethers, and Thiols 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 commonly carried out using potassium dichromate, K2Cr2O7, in aqueous sulfuric acid. CH3 ( CH2 ) 6 CH2 OH 1-Octanol K2 Cr2 O 7 H2 SO 4 O O K Cr O 2 2 7 CH3 ( CH2 ) 6 CH CH3 ( CH2 ) 6 COH H2 SO 4 Octanoic acid Octanal • It is sometimes 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 the carboxylic acid. Chapter 14: Alcohols, Ethers, and Thiols • Oxidation of a 2° alcohol gives a ketone. K2 Cr 2 O7 OH H2 SO 4 2-Is opropyl-5-methylcyclohexanol (Menthol) O 2-Is opropyl-5-methylcyclohexanone (Menthone) • Tertiary alcohols are resistant to oxidation. CH3 OH 1-Methylcyclopentanol K2 Cr2 O7 H2 SO4 (n o oxidation) 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. -OCH3 -O-CH3 CH3 CH2 -O-CH2 CH3 D imethyl ether Dieth yl eth er Chapter 14: Alcohols, Ethers, and Thiols Although ethers can be named according to the IUPAC system, chemists almost invariably use common names for low-molecularweight 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 OCH3 Cyclohexyl meth yl eth er (Methoxycycloh exane) Chapter 14: Alcohols, Ethers, and Thiols 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 O Tetrahydrofu ran (THF) Ethers: 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 CH3 OCH3 D imethyl ether b p -24°C Ethers: Reactions 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. 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 -SH Chapter 14: Alcohols, Ethers, and Thiols 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) CH3 CHCH2 SH 2-Methyl-1-propan ethiol (Isobu tyl mercaptan) Thiols 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 bp (°C) 6 35 98 Alcoh ol methanol ethan ol 1-bu tanol bp (°C) 65 78 117 Thiols Properties Thiols are weak acids (pKa 10) and are comparable in strength to phenols (pKa 10). • Thiols react with strong bases such as NaOH to form watersoluble thiolate salts. CH3 CH2 SH + NaOH Ethanethiol (pKa 10) H2 O CH3 CH2 S-Na+ + H2 O Sodium ethanethiolate Thiols reactions: oxidation 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 including H2 in the presence of a transition metal catalyst. 2HOCH2 CH2 SH A thiol oxidation reduction HOCH2 CH2 S-SCH2 CH2 OH A disulfide