Alcohols contain anOOH group attached to a saturated carbon. The common names for alcohols are based on the name of the alkyl group.
The systematic nomenclature for alcohols adds the ending -ol to the name of the parent alkane and uses a number to identify the carbon that carries the OOH group. The sys-tematic name for isopropyl alcohol, for example, is 2-propanol.
Exercise O2.6
More than 50 organic compounds have been isolated from the oil that gives rise to the characteristic odor of a rose. One of the most abundant of the compounds is known by the common name citronellol. Use the systematic nomenclature to name this alcohol, which has the following structure.
Solution
The longest chain of carbon atoms in the compound contains eight atoms.
CH3
CH3
CH3
CH2
CH2
H2C H2C
CH
CH
OH
C B
G
G D
D G A
A A
O CH3OH
CH3CH2OH CH3CHOHCH3
Methanol Ethanol 2-Propanol CH3OH
CH3CH2OH CH3CHOHCH3
Methyl alcohol Ethyl alcohol Isopropyl alcohol CH4Cl2 CH3ClHCl
2
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The longest chain contains the OOH group, which means the compound is named as a de-rivative of octane. Because it is an alcohol, it would be tempting to name it as an octanol.
But it contains a CPC double bond, which means it must be an octenol. We now have to indicate that the OOH group is on one end of the chain and the CPC double bond oc-curs between the sixth and seventh carbon atoms of the chain, which can be done by con-sidering the compound to be a derivative of 6-octen-1-ol. Finally, we have to indicate the presence of a pair of CH3 groups on the third and seventh carbon atoms. The compound is therefore given the systematic name 3,7-dimethyl-6-octen-1-ol.
Methanol, or methyl alcohol, is also known as wood alcohol because it was originally made by heating wood until a liquid distilled. Methanol is highly toxic, and many people have become blind or have died from drinking it. Ethanol, or ethyl alcohol, is the alcohol associated with “alcoholic” beverages. It has been made for at least 6000 years by adding yeast to solutions that are rich in either sugars or starches. The yeast cells obtain energy from enzyme-catalyzed reactions that convert sugar or starch to ethanol and CO2.
C6H12O6(aq) 88n 2 CH3CH2OH(aq) 2 CO2(g)
When the alcohol reaches a concentration of 10% to 12% by volume, the yeast cells die.
Brandy, rum, gin, and the various whiskeys that have a higher concentration of alcohol are prepared by distilling the alcohol produced by the fermentation reaction. Ethanol isn’t as toxic as methanol, but it is still dangerous. Most people are intoxicated at blood-alcohol levels of about 0.1 gram per 100 mL. An increase in the level of alcohol in the blood to between 0.4 and 0.6 g/100 mL can lead to coma or death.
The method of choice for determining whether an individual is DUI—driving under the influence—or DWI—driving while intoxicated—is the Breathalyzer, for which a patent was issued to R. F. Borkenstein in 1958. The chemistry behind the Breathalyzer is based on the reaction between alcohol in the breath and the chromate or dichromate ion.
3 CH3CH2OH(g) 2 Cr2O72(aq) 16 H(aq) 88n
3 CH3CO2H(aq) 4 Cr3(aq) 11 H2O(l) The instrument contains two ampules that hold small samples of potassium dichromate dis-solved in sulfuric acid. One of the ampules is used as a reference. The other is opened and the breath sample to be analyzed is added. If alcohol is present in the breath, it reduces the yellow-orange Cr2O72 ion to the green Cr3 ion. The extent to which the color bal-ance between the two ampules is disturbed is a direct measure of the amount of alcohol in the breath sample. Measurements of the alcohol on the breath are then converted into
CH3
CH3
CH3
CH2
CH2
H2C H2C
CH
CH
OH
C B
G
G D
D G A
A A
5 O
6
7 8
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FUNCTIONAL GROUP CHEMISTRY 13
estimates of the concentration of the alcohol in the blood by assuming that 2100 mL of air exhaled from the lungs contains the same amount of alcohol as 1 mL of blood.
Measurements taken with the Breathalyzer are reported in units of percent blood-alcohol concentration (BAC). In most states, a BAC of 0.10% is sufficient for a DUI or DWI conviction. (This corresponds to a blood-alcohol concentration of 0.10 gram of alco-hol per 100 mL of blood.)
Ethanol is oxidized to CO2 and H2O by the alcohol dehydrogenase enzymes in the body. This reaction gives off 30 kilojoules per gram, which makes ethanol a better source of energy than carbohydrates (17 kJ/g), and almost as good a source of energy as fat (38 kJ/g). An ounce of 80-proof liquor can provide as much as 3% of the average daily caloric intake, and drinking alcohol can contribute to obesity. Many alcoholics are mal-nourished, however, because of the absence of vitamins in the calories they obtain from alcoholic beverages.
As a general rule, polar or ionic substances dissolve in polar solvents; nonpolar sub-stances dissolve in nonpolar solvents. As a result, hydrocarbons don’t dissolve in water.
They are often said to be immiscible (literally, “not mixable”) in water. Alcohols, as might be expected, have properties between the extremes of hydrocarbons and water. When the hydrocarbon chain is short, the alcohol is soluble in water. There is no limit on the amount of methanol (CH3OH) and ethanol (CH3CH2OH), for example, that can dissolve in a given quantity of water. As the hydrocarbon chain becomes longer, the alcohol becomes less sol-uble in water, as shown in Table O2.4. One end of the alcohol molecules has so much non-polar character it is said to be hydrophobic (literally, “water-hating”). The other end con-tains an OOH group that can form hydrogen bonds to neighboring water molecules and is therefore said to be hydrophilic (literally, “water-loving”). As the hydrocarbon chain be-comes longer, the hydrophobic character of the molecule increases, and the solubility of the alcohol in water gradually decreases until it becomes essentially insoluble in water.
TABLE O2.4 Solubilities of Alcohols in Water
Formula Name Solubility in Water (g/100 g) CH3OH Methanol Infinitely soluble CH3CH2OH Ethanol Infinitely soluble CH3(CH2)2OH Propanol Infinitely soluble CH3(CH2)3OH Butanol 9
CH3(CH2)4OH Pentanol 2.7 CH3(CH2)5OH Hexanol 0.6 CH3(CH2)6OH Heptanol 0.18 CH3(CH2)7OH Octanol 0.054 CH3(CH2)9OH Decanol Insoluble in water
Alcohols are classified as either primary (1°), secondary (2°), or tertiary (3°) on the basis of their structures.
CH3CH2OH CH3CHCH3 CH3CCH3
OH
CH3
A OH
A A
3 2
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Ethanol is a primary alcohol because there is only one alkyl group attached to the carbon that carries the OOH substituent. The structure of a primary alcohol can be abbreviated as RCH2OH, where R stands for an alkyl group. The isopropyl alcohol found in rubbing alcohol is a secondary alcohol, which has two alkyl groups on the carbon atom with the OOH substituent (R2CHOH). An example of a tertiary alcohol (R3COH) is tert-butyl (or t-butyl) alcohol or 2-methyl-2-propanol.
Another class of alcohols are the phenols, in which an OOH group is attached to an aromatic ring, as shown in Figure O2.3. Phenols are potent disinfectants. When antiseptic techniques were first introduced in the 1860s by Joseph Lister, it was phenol (or carbolic acid, as it was then known) that was used. Phenol derivatives, such as o-phenylphenol, are still used in commercial disinfectants such as Lysol.
FIGURE O2.3 The structures of phenol and o-phenylphenol.
OH
Phenol o-Phenylphenol
OH
Water has an unusually high boiling point because of the hydrogen bonds between the H2O molecules. Alcohols can form similar hydrogen bonds, as shown in Figure O2.4.
FIGURE O2.4 A hydrogen bond between a pair of methanol molecules.
H H
CH3
CH3
,
, O
O
As a result, alcohols have boiling points that are much higher than alkanes with similar molecular weights. The boiling point of ethanol, for example, is 78.5°C, whereas propane, with about the same molecular weight, boils at 42.1°C.
Alcohols are Brønsted acids in aqueous solution.
CH3CH2OH(aq) H2O(l ) 88nm88 H3O(aq) CH3CH2O(aq)
Alcohols therefore react with sodium metal to produce sodium salts of the corresponding conjugate base, as noted in Section O2.2.
2 Na(s) 2 CH3OH(l ) 88n 2 Na(alc) 2 CH3O(alc) H2(g) The conjugate base of an alcohol is known as an alkoxide.
[Na][CH3O] [Na][CH3CH2O]
Sodium methoxide Sodium ethoxide
As we saw in Section O1.8, alcohols can be prepared by adding water to an alkene in the presence of a strong acid such as concentrated sulfuric acid. The reaction involves adding an H2O molecule across a CPC double bond. Because the reactions follow Markovnikov’s rule, the product of the reaction is often a highly substituted 2° or 3°
alcohol.
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Less substituted 1° alcohols can be prepared by substitution reactions that occur when a primary alkyl halide is allowed to react with the OH ion.
CH3CH2CH2Br OH 88n CH3CH2CH2OH Br
Alcohols (ROH) can be thought of as derivatives of water in which one of the hydro-gen atoms has been replaced by an alkyl group. If both of the hydrohydro-gen atoms are replaced by alkyl groups, we get an ether (ROR). These compounds are named by adding the word ether to the names of the alkyl groups.
CH3CH2OCH2CH3 diethyl ether
Diethyl ether, often known by the generic name “ether,” was once used extensively as an anesthetic. Because mixtures of diethyl ether and air explode in the presence of a spark, ether has been replaced by safer anesthetics.
There are important differences between both the physical and chemical properties of alcohols and ethers. Consider diethyl ether and 1-butanol, for example, which are consti-tutional isomers with the formula C4H10O.
CH3CH2OCH2CH3 CH3CH2CH2CH2OH
BP 34.5°C BP 117.2°C
density 0.7138 g/mL density 0.8098 g/mL Insoluble in water Soluble in water
The shapes of the molecules are remarkably similar, as shown in Figure O2.5.
CH3CHPCHCH3 H2O CH3C OHA
HCH2CH3
H2SO4
FIGURE O2.5 The structures of diethyl ether and 1-butanol.
OH O
The fundamental difference between the compounds is the presence of OOH groups in the alcohol that are missing in the ether. Because hydrogen bonds can’t form between the molecules in the ether, the boiling point of the compound is more than 80°C lower than that of the corresponding alcohol. Because there are no hydrogen bonds to organize the structure of the liquid, the ether is significantly less dense than the corresponding alcohol.
Ethers can act as a hydrogen bond acceptor, as shown in Figure O2.6, but they can’t act as hydrogen bond donors. As a result, ethers are less soluble in water than alcohols with the same molecular weight.
FIGURE O2.6 Water acting as a hydrogen bond donor toward an ether.
CH3
CH3
H O
O H
The absence of an OOH group in an ether also has important consequences for its chemical properties. Unlike alcohols, ethers are essentially inert to chemical reactions. They
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don’t react with most oxidizing or reducing agents, and they are stable to most acids and bases, except at high temperatures. They are therefore frequently used as solvents for chem-ical reactions.
Compounds that are potential sources of an H ion, or proton, are often described as being protic. Ethanol, for example, is a protic solvent.
Substances that can’t act as a source of a proton are said to be aprotic. Because they don’t contain an OOH group, ethers are aprotic solvents.
Ethers can be synthesized by splitting out a molecule of water between two alcohols in the presence of heat and concentrated sulfuric acid.
They can also be formed by reacting a primary alkyl halide with an alkoxide ion.