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Alcohols, Aldehydes, and Ketones

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Alcohols, Aldehydes, and Ketones INTERCHAPTER P Alcohols, Aldehydes, and Ketones When an aldehyde is present, the Tollen’s test produces a silver mirror on the inside of a test tube or flask, as shown here. The oxidation-reduction reaction that takes place is described by the equation O O + – – R C H(aq)+ 2 [Ag(NH3)2] (aq)+ 3 OH (aq) R C O (aq)+ 2 Ag(s) + 4 NH3(aq) + 2 H2O(l ) aldehyde silver-ammonia carboxylic metallic complex acid silver In the reaction the aldehyde functional group, R-CHO, is oxidized to a carboxylic acid functional group, R-COOH (which is deprotonated in the basic solution), while the silver ions in the silver-ammonia complex are reduced to form metallic silver. The mirror forms because a thin film of metallic silver is deposited on the inside surface of the flask and adheres to the glass. This reaction was used in Victorian times to produce mirrors on glass. University Science Books, ©2011. All rights reserved. www.uscibooks.com P. ALCOHOLS, ALDehYDes, anD ketOnes P1 We shall continue our introduction to organic chem- CH3CHCH3 istry in this Interchapter. Up to now, we have learned CH OH CH H OH OH only about hydrocarbons. We studied saturated 3 3 2 hydrocarbons in Interchapter F, unsaturated hydro- methanol ethanol 2-propanol carbons in Interchapter G, and aromatic hydrocar- (methyl alcohol) (ethyl alcohol) (isopropyl alcohol or isopropanol) bons in Interchapter H. When we studied unsaturated hydrocarbons, we referred to the carbon–carbon These alcohols have both common names (given here double bond as a functional group. The chemical in parentheses) and IUPAC (or systematic) names. properties of unsaturated hydrocarbons are a result The common names are formed by naming the alkyl of how the double bond reacts with reagents. In this group to which the –OH is attached and adding the Interchapter we shall learn about alcohols, aldehydes, word alcohol. The IUPAC name is formed by replac- and ketones. Alcohols have –OH as their functional ing the -e from the end of the alkane name of the lon- H gest chain of carbon atoms containing the –OH group group, aldehydes have C O, and the functional with the suffix -ol. Thus, we have methanol, ethanol, and propanol. If the –OH group can be attached at group of ketones is C O. The simplest member of more than one position, then its position is denoted each class is by a number, as in H H H C 3 2 1 3 CH CHCH H C OH C O C O 3 3 3 2 1 H H3C CH CH CH OH H 3 2 2 OH methanol methanal propanone 1-propanol 2-propanol (methyl alcohol) (formaldehyde) Title (acetone)General Chemistry - 4th ed 2-propanol is also called isopropyl alcohol or iso- Author McQuarrie/Gallogy propanol. An aqueous solution of 70% by volume of Title General Chemistry - 4th ed Artist George Kelvin P-1. Alcohols Are Organic Compounds 2-propanol is sold as rubbing alcohol, the substance Author McQuarrie/Gallogy Figure # ethanol That Contain an –OH Group commonly used to cleanTitle theGeneral skin before Chemist administerry - 4th ed- Artist George Kelvin Date 07/30/10 ing an injection. FiguThere # classmethanol of organic compounds calledCheck alcohols if revision is charAppr- oved Author McQuarrie/Gallogy acterized by an –OH group attached to a hydrocarbon Date 07/30/10 Artist George Kelvin Checkchain. if re visionImportantAppr commonoved alcohols (Figure P.1) are Title General Chemistry - 4th edTitle General Chemistry - 4th ed Figure # 2-propanol Author McQuarrie/Gallogy Author McQuarrie/Gallogy Date 08/09/10 Check if revision Approved Artist George Kelvin Artist George Kelvin Figure # methanol Figure # ethanol Date 08/09/10 Date 08/09/10 Approved Check if revision Check if revision Approved methanol ethanol methanol ethanol 2-propanol CH OH CH CH OH 3 3 2 CH3CHCH3 OH Figure P.1 Space-filling and ball-and-stick models of common alcohols: methanol, ethanol, and 2-propanol. P2 GENERAL CHEMISTRY, FOURTH EDITION | McQuarrie, Rock, and Gallogly Methanol is sometimes called wood alcohol relatively small. The low-molecular-mass alcohols form because it can be produced by heating wood in the hydrogen bonds and mix completely (i.e., are miscible) absence of oxygen. Over three million metric tons of with water in all proportions. As the hydrocarbon por- methanol are produced annually in the United States tion becomes larger, however, the solubility of alcohols by the reaction described by in water decreases and eventually approaches the low catalyst solubility of the comparable hydrocarbon. In addition, CO(g) + 2 H (g)→ CH OH(l ) 2 high P, high T 3 the low-molecular-mass alcohols have much higher boiling points than their comparable hydrocarbons. Methanol is highly toxic and can cause blindness For example, ethane (molecular mass 30) has a boil- and death if taken internally. During Prohibition in ing point of −88.6°C, while the boiling point of metha- the United States, many people died or became seri- nol (molecular mass 32) is 64.6°C. The relatively high ously ill from drinking methanol, either because they boiling points of the low-molecular-mass alcohols are were not aware of the difference between methanol due to extensive hydrogen bonding in the liquid phase. and ethanol or because the alcohol they purchased The –OH group on the relatively unreactive hydro- contained methanol as a major impurity. carbon chain accounts for the reactive properties of Ethanol is an ingredient in all fermented bev- alcohols. We say that the –OH group is a functional erages. Various sugars and the starch in potatoes, group, just as the double bond in alkenes serves as a grains, and similar substances can be used to produce functional group in that case. Alcohols undergo reac- ethanol by fermentation, a process in which yeast is tions analogous to those of water. For example, alco- used to convert sugar to ethanol. Although ethanol hols react with alkali metals to form alkoxides and may be best known as an ingredient in alcoholic bev- liberate hydrogen: erages, it is an important industrial chemical as well. Today most gasolines contain a certain percentage of 2 CH3OH(l ) + 2 Na(s) → 2 NaOCH3(alc) + H2(g) ethanol, and special multifuel vehicles exist that can sodium methoxide run entirely on ethanol (see Interchapter L). It is sometimes convenient to view alcohols as deriv- Alkoxides are strong Lewis bases, that is, electron- atives of water, in which one of the hydrogen atoms pair donors (see Section 20-1). is replaced by an alkyl group. This approach is most Table P.1 lists the boiling points and the solubili- useful when the hydrocarbon portion of the alcohol is ties in water of a number of alcohols. Note how the TabLE P.1 Boiling points and solubilities in water of several pairs of hydrocarbons and alcohols of similar molecular masses Molecular Boiling Solubility in water/g Name Formula mass point/°C per 100 g H2O at 25°C ethane CH3CH3 30.069 −88.6 insoluble methanol CH3OH 32.042 64.6 soluble in all proportions propane CH3CH2CH3 44.096 −42.1 insoluble ethanol CH3CH2OH 46.068 78.29 soluble in all proportions butane CH3CH2CH2CH3 58.122 −0.5 insoluble 1-propanol CH3CH2CH2OH 60.095 97.2 soluble in all proportions pentane CH3CH2CH2CH2CH3 72.149 36.06 insoluble 1-butanol CH3CH2CH2CH2OH 74.121 117.73 7.4 hexane CH3CH2CH2CH2CH2CH3 86.175 68.73 insoluble 1-pentanol CH3CH2CH2CH2CH2OH 88.148 137.98 2.20 heptane CH3CH2CH2CH2CH2CH2CH3 100.202 98.4 insoluble 1-hexanol CH3CH2CH2CH2CH2CH2OH 102.174 157.6 0.60 University Science Books, ©2011. All rights reserved. www.uscibooks.com P. ALCOHOLS, ALDehYDes, anD ketOnes P3 solubility decreases as the relative size of the alkyl portion of the molecule increases. Also, note that for pairs of hydrocarbons and alcohols of compara- ble molecular mass, the alcohols have considerably higher boiling points. Alcohols can be converted to alkenes by acid-cata- lyzed dehydration. For example, 1-propanol HH H H H2SO4 H CC OH(l ) C C(g) + H O (l ) 200°C 2 H H H H Many of the reactions that alcohols undergo depend on the number of carbon atoms that are bonded to the carbon atom bearing the –OH group. For this reason we define primary, secondary, and tertiary alcohols. A primary alcohol has one carbon atom bonded to the carbon atom bearing the –OH group; 2-propanol a secondary alcohol has two such carbon atoms; and a tertiary alcohol has three: H H3CCC H3 CH3 CH2 CH2 OH OH 1-propanol 2-propanol a primary alcohol a secondary alcohol CH3 H3CCC H3 OH 2-methyl-2-propanol 2-methyl-2-propanol a tertiary alcohol Table P.2 lists some other structural formulas of alco- hols, their IUPAC names, and respective classes, just for practice. The t- in the common name t-butyl alco- hol (Table P.2) stands for tertiary. It is possible to have more than one –OH group in an alcohol. Two examples are OH ethane-1,2-diol (ethylene glycol) HO CH2 CH2 OH HO CH2 CH CH2 OH ethane-1,2-diol propane-1,2,3-triol (ethylene glycol) (glycerol or glycerin) Ethylene glycol can be obtained by a controlled oxi- dation of ethylene, oxidation propane-1,2,3-triol H2C CH2 HO CH2 CH2 OH (glycerol or glycerine) P4 GENERAL CHEMISTRY, FOURTH EDITION | McQuarrie, Rock, and Gallogly TabLE P.2 The structural formulas, corresponding IUPAC names, and classes of some alcohols Structural formula IUPAC name Class CH3 tertiary H C C CH CH 2-methyl-2-butanol 3 2 3 alcohol OH CH CH CH CH 3 2 CH 2 3 3-pentanol secondary OH alcohol CH3 H C C OH 2-methyl-2-propanol tertiary 3 (t-butyl alcohol) alcohol CH3 and glycerol is obtained from the hydrolysis of fats.
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