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9.6: Alcohols, Phenols, and Ethers

  • Page ID
    177591
  • Learning Objectives

    • Describe the structure and properties of alcohols, ethers and phenols.
    • Know the name and uses of simple alcohols, phenols and ethers.

    An alcohol is an organic compound with a hydroxyl (OH) functional group on an aliphatic carbon atom. Because OH is the functional group of all alcohols, we often represent alcohols by the general formula ROH, where R is an alkyl group. Alcohols are common in nature. Most people are familiar with ethyl alcohol (ethanol), the active ingredient in alcoholic beverages, but this compound is only one of a family of organic compounds known as alcohols. The family also includes such familiar substances as cholesterol and the carbohydrates. Methanol (CH3OH) and ethanol (CH3CH2OH) are the first two members of the homologous series of alcohols.

    Nomenclature of Alcohols

    Alcohols with one to four carbon atoms are frequently called by common names, in which the name of the alkyl group is followed by the word alcohol:

    nomenclature.jpg

    According to the International Union of Pure and Applied Chemistry (IUPAC), the name of an alcohol comes from the hydrocarbon from which it was derived. The final -e in the name of the hydrocarbon is replaced by -ol, and the carbon atom to which the –OH group is bonded is indicated by a number placed before the name.

    Example \(\PageIndex{1}\): Naming Alcohols

    Consider the following example. How should it be named?

    alt

    Solution

    The carbon chain contains five carbon atoms. If the hydroxyl group was not present, we would have named this molecule pentane. To address the fact that the hydroxyl group is present, we change the ending of the name to -ol. In this case, since the –OH is attached to carbon 2 in the chain, we would name this molecule 2-pentanol.

    Exercise \(\PageIndex{1}\)

    Name the following molecule:

    The structure shown has a C H subscript 3 group bonded up and to the right to a C atom. The C atom is bonded down and to the right to a C H subscript 2 group. The C H subscript 2 group is bonded up and to the right to a C H subscript 2 group. The C H subscript 2 group is bonded down and to the right to a C H subscript 3 group. The second C atom (from left to right) is bonded to a C H subscript 3 group and an O H group.

    Answer

    2-methyl-2-pentanol

    Note

    The IUPAC adopted new nomenclature guidelines in 2013 that require this number to be placed as an “infix” rather than a prefix. For example, the new name for 2-propanol would be propan-2-ol. Widespread adoption of this new nomenclature will take some time, and students are encouraged to be familiar with both the old and new naming protocols.

    Example \(\PageIndex{2}\): Naming Alcohols

    Consider the following example. How should it be named according to the guidelines adopted in 2013?

    clipboard_e58f46c168a2790195072dea04595a769.png

    Solution

    The carbon chain contains four carbon atoms. If the hydroxyl group was not present, we would have named this molecule butane. To address the fact that the hydroxyl group is present, we change the ending of the name to -ol. In this case, since the –OH is attached to carbon 2 in the chain, we would name this molecule butan-2-ol.

    Note

    "Always place the OH on the lowest possible number for the chain."

    In the previous example, if the carbon atoms were numbered from left to right then the OH would have been placed on the third carbon and the molecule would have been named butan-3-ol, but butan-2-ol is the more preferred name.

    Exercise \(\PageIndex{2}\): Naming Alcohols

    Name the following molecules according to the guidelines adopted in 2013?

    a. clipboard_e39a5cb7603051a353d60546766b54051.png

    b. clipboard_e2bca8459eb9c3f16ee1e7dd17aa7ac13.png

    Answer

    a. propan-2-ol

    b. hexan-2-ol

    Table \(\PageIndex{1}\) names and classifies some of the simpler alcohols. Some of the common names reflect a compound’s classification as secondary (sec-) or tertiary (tert-). These designations are not used in the IUPAC nomenclature system for alcohols. Note that there are four butyl alcohols in the table, corresponding to the four butyl groups: the butyl group (CH3CH2CH2CH2) discussed before, and three others:

    sec, tert.jpg
    Table \(\PageIndex{1}\): Classification and Nomenclature of Some Alcohols
    Condensed Structural Formula Common Name IUPAC Name
    CH3OH wood alcohol (methyl alcohol) methanol
    CH3CH2OH grain alcohol (ethyl alcohol) ethanol
    CH3CH2CH2OH propyl alcohol 1-propanol
    (CH3)2CHOH rubbing alcohol (isopropyl alcohol) 2-propanol
    CH3CH2CH2CH2OH butyl alcohol 1-butanol
    CH3CH2CHOHCH3 sec-butyl alcohol 2-butanol
    (CH3)2CHCH2OH isobutyl alcohol 2-methyl-1-propanol
    (CH3)3COH tert-butyl alcohol 2-methyl-2-propanol
    Table 14.2.jpg cyclohexyl alcohol cyclohexanol

    Methanol (Methyl Alcohol)

    Methanol, also known as methyl alcohol among others, is a chemical with the formula CH3OH (a methyl group linked to a hydroxyl group, often abbreviated MeOH). Methanol acquired the name wood alcohol because it was once produced chiefly by the destructive distillation of wood. Today, methanol is mainly produced industrially by hydrogenation of carbon monoxide.

    CO(g) + 2H2(g) → CH3OH(l)

    Methanol is the simplest alcohol, consisting of a methyl group linked to a hydroxyl group. It is a light, volatile, colorless, flammable liquid with a distinctive odor similar to that of ethanol (drinking alcohol). At room temperature, it is a polar liquid. With more than 20 million tons produced annually, it is used as a fuel additive and as a precursor to other commodity chemicals, including formaldehyde, acetic acid, methyl tert-butyl ether, as well as a host of more specialized chemicals.[17]

    Ethanol (Ethyl Alcohol)

    Ethanol (also called ethyl alcohol, grain alcohol, drinking alcohol, or simply alcohol) is a chemical compound, a simple alcohol with the chemical formula C2H6O. Its formula can be also written as CH3−CH2−OH or C2H5OH (an ethyl group linked to a hydroxyl group), and is often abbreviated as EtOH. Ethanol is a volatile, flammable, colorless liquid with a slight characteristic odor. It is a psychoactive substance and is the principal type of alcohol found in alcoholic drinks.

    Ethanol is naturally produced by the fermentation of sugars by yeasts or via petrochemical processes, and is most commonly consumed as a popular recreational drug. Ethanol is the alcohol produced by some species of yeast that is found in wine, beer, and distilled drinks. It has long been prepared by humans harnessing the metabolic efforts of yeasts in fermenting various sugars:

    Large quantities of ethanol (for industrial use) are synthesized from the addition reaction of water with ethylene using an acid as a catalyst:

    This reaction shows two carbons connected by a double bond, each with two bonded H atoms plus H O H arrow labeled “H subscript 3 O superscript plus” followed by two carbon atoms connected with a single bond with 5 bonded H atoms and an O H group shown in red at the right end of the molecule. The O of this group is shown with 2 pairs of electron dots.

    Ethanol also has medical applications as an antiseptic and disinfectant. The compound is widely used as a chemical solvent, either for scientific chemical testing or in synthesis of other organic compounds, and is a vital substance used across many different kinds of manufacturing industries. Ethanol is also used as a clean-burning fuel source.

    Toxicity of Alcohols

    With respect to acute toxicity, simple alcohols have low acute toxicities. Doses of several milliliters are tolerated. For pentanols, hexanols, octanols and longer alcohols, LD50 (lethal dose, 50%) range from 2–5 g/kg (rats, oral). Methanol and ethanol are less acutely toxic. Methanol is however far more toxic than ethanol. All alcohols are mild skin irritants.

    The metabolism of methanol (and ethylene glycol) is affected by the presence of ethanol, which has a higher affinity for liver alcohol dehydrogenase. In this way methanol will be excreted intact in urine.

    The immediate effect of alcohol depends on the drinker's blood alcohol concentration (BAC). BAC can be different for each person depending on their age, sex, pre-existing health condition, even if they drink the same amount of alcohol.[7]

    Different BACs have different effects. Table \(\PageIndex{2}\) list the common effects of alcohol on the body depending on the BAC. However, tolerance varies considerably between individuals, as does individual response to a given dosage; the effects of alcohol differ widely between people. Hence in this context, BAC percentages are just estimates used for illustrative purposes.

    Table \(\PageIndex{2}\) Blood Alcohol Levels and Effects Source: Wikipedia.

    mg/dL mM

    Blood Alcohol Level

    % v/v

    Effects
    50 11 0.05% Euphoria, talkativeness, relaxation
    100 22 0.1% Central nervous system depression, nausea, possible vomiting, impaired motor and sensory function, impaired cognition
    >140 30 >0.14% Decreased blood flow to brain
    300 65 0.3% Stupefaction, possible unconsciousness
    400 87 0.4% Possible death
    500 109 >0.55% Death
    Rubbing alcohol refers to either isopropyl alcohol (propan-2-ol) or ethanol based liquids. Rubbing alcohol is undrinkable even if it is ethanol based, due to the bitterants added. Product labels for rubbing alcohol include a number of warnings about the chemical, including the flammability hazards and its intended use only as a topical antiseptic and not for internal wounds or consumption. It should be used in a well-ventilated area due to inhalation hazards. Poisoning can occur from ingestion, inhalation, absorption, or consumption of rubbing alcohol.

    Multifunctional Alcohols

    A polyol is an organic compound containing multiple hydroxyl groups. Examples of polyols discussed in this section include ethylene glycol, propylene glycol, and glycerol.

    Alcohols containing two or more hydroxyl groups can be made. Examples include 1,2-ethanediol (ethylene glycol, used in antifreeze) and 1,2,3-propanetriol (glycerine, used as a solvent for cosmetics and medicines):

    Ethylene glycol (IUPAC name: ethane-1,2-diol) is an organic compound with the formula (CH2OH)2. It is mainly used for two purposes, as a raw material in the manufacture of polyester fibers and for antifreeze formulations. It is an odorless, colorless, sweet-tasting, viscous liquid. Ethylene glycol is toxic. Household pets are especially susceptible to ethylene glycol poisoning from vehicle antifreeze leaks.

    Wireframe model of ethylene glycol

    Ball and stick model of ethylene glycol

    Figure \(\PageIndex{1}\) Fluorescent green-dyed antifreeze is visible in the radiator header tank when car radiator cap is removed.

    Propylene glycol (IUPAC name: propane-1,2-diol) is a organic compound with the chemical formula CH3CH(OH)CH2OH. It is a viscous, colorless liquid which is nearly odorless but possesses a faintly sweet taste. Containing two alcohol groups, it is classed as a diol. It is misciblewith a broad range of solvents, including water, acetone, and chloroform. In general, glycols are non-irritating, have very low volatility and very low toxicity.

    ball-and-stick model

    Figure \(\PageIndex{2}\) A bottle of flavored "e-liquid" for "vaping" shows propylene glycol as the main ingredient. Source: Wikipedia

    Forty-five percent of propylene glycol produced is used as chemical feedstock for the production of unsaturated polyester resins. Propylene glycol is also used in various edible items such as coffee-based drinks, liquid sweeteners, ice cream, whipped dairy products and soda.[9][10]Vaporizers used for delivery of pharmaceuticals or personal-care products often include propylene glycol among the ingredients.[4] In alcohol-based hand sanitizers, it is used as a humectant to prevent the skin from drying.[11] Propylene glycol is used as a solvent in many pharmaceuticals, including oral, injectable and topical formulations, such as for diazepam and lorazepam which are insoluble in water.[12] Certain formulations of artificial tears, such as Systane, use proplyene glycol as an ingredient.[13]Propylene glycol is frequently used as a substitute for ethylene glycol in low toxicity, environmentally friendly automotive antifreeze. It is also used to winterize the plumbing systems in vacant structures.[

    Glycerol (/ˈɡlɪsərɒl/;[5] also called glycerine or glycerin; see spelling differences) is a simple polyol compound. It is a colorless, odorless, viscous liquid that is sweet-tasting and non-toxic. The glycerol backbone is found in many lipids which are known as glycerides. It is widely used in the food industry as a sweetener and humectant in pharmaceutical formulations. Glycerol has three hydroxyl groups that are responsible for its solubility in water and its hygroscopic nature.

    Ball-and-stick model of glycerol

    Glycerol

    In food and beverages, glycerol serves as a humectant, solvent, and sweetener, and may help preserve foods. It is also used as filler in commercially prepared low-fat foods (e.g., cookies), and as a thickening agent in liqueurs. Glycerol and water are used to preserve certain types of plant leaves.[14] As a sugar substitute, it has approximately 27 kilocalories per teaspoon (sugar has 20) and is 60% as sweet as sucrose. It does not feed the bacteria that form plaques and cause dental cavities.[citation needed] As a food additive, glycerol is labeled as E number E422. It is added to icing (frosting) to prevent it from setting too hard.

    Glycerol is used in medical, pharmaceutical and personal care preparations, often as a means of improving smoothness, providing lubrication, and as a humectant. Ichthyosis and xerosis have been relieved by the topical use glycerin.[16][17] It is found in allergen immunotherapies, cough syrups, elixirs and expectorants, toothpaste, mouthwashes, skin care products, shaving cream, hair care products, soaps, and water-based personal lubricants (Figure\(\PageIndex{3}\)). In solid dosage forms like tablets, glycerol is used as a tablet holding agent.

    Figure \(\PageIndex{3}\)

    Personal lubricants commonly contain glycerol.

    Phenols

    Compounds in which an OH group is attached directly to an aromatic ring are designated ArOH and called phenols. Phenols differ from alcohols in that they are slightly acidic in water. They react with aqueous sodium hydroxide (NaOH) to form salts.

    \[\ce{ArOH (aq) + NaOH (aq) \rightarrow ArONa (aq) + H_2O}\]

    The parent compound, C6H5OH, is itself called phenol. (An old name, emphasizing its slight acidity, was carbolic acid.) Phenol is a white crystalline compound that has a distinctive (“hospital smell”) odor.

    Figure \(\PageIndex{4}\) (Right) Structure of phenol (left) Approximately two grams of phenol in glass vial. Image used with permisison from Wikipedia

    To Your Health: Phenols and Us

    Phenols are widely used as antiseptics (substances that kill microorganisms on living tissue) and as disinfectants (substances intended to kill microorganisms on inanimate objects such as furniture or floors). The first widely used antiseptic was phenol. Joseph Lister used it for antiseptic surgery in 1867. Phenol is toxic to humans, however, and can cause severe burns when applied to the skin. In the bloodstream, it is a systemic poison—that is, one that is carried to and affects all parts of the body. Its severe side effects led to searches for safer antiseptics, a number of which have been found.

    An operation in 1753, painted by Gaspare Traversi, of a surgery before antiseptics were used.

    One safer phenolic antiseptic is 4-hexylresorcinol (4-hexyl-1,3-dihydroxybenzene; resorcinol is the common name for 1,3-dihydroxybenzene, and 4-hexylresorcinol has a hexyl group on the fourth carbon atom of the resorcinol ring). It is much more powerful than phenol as a germicide and has fewer undesirable side effects. Indeed, it is safe enough to be used as the active ingredient in some mouthwashes and throat lozenges.

    The compound 4-hexylresorcinol is mild enough to be used as the active ingredient in antiseptic preparations for use on the skin.

    The compound 4-hexylresorcinol is mild enough to be used as the active ingredient in antiseptic preparations for use on the skin.

    In addition to acting as an antiseptic, phenol is also a useful precursor in many chemical syntheses to produce pharmaceuticals, food preservatives, polymers, resins and adhesives. Phenolics are also present in a number of biological systems and natural products such as neurotransmitters, flavouring agents, and vitamins to name a few.

    Ethers

    Ethers are compounds that contain the functional group –O–. Ethers do not have a designated suffix like the other types of molecules we have named so far. In the IUPAC system, the oxygen atom and the smaller carbon branch are named as an alkoxy substituent and the remainder of the molecule as the base chain, as in alkanes. As shown in the following compound, the red symbols represent the smaller alkyl group and the oxygen atom, which would be named “methoxy.” The larger carbon branch would be ethane, making the molecule methoxyethane. Many ethers as shown below, are referred to with common names instead of the IUPAC system names. For common names, the two branches connected to the oxygen atom are named separately and followed by “ether.”

    In the general formula for ethers, R—O—R, the hydrocarbon groups (R) may be the same or different.

    Ether molecules have no hydrogen atom on the oxygen atom (that is, no OH group). Therefore there is no intermolecular hydrogen bonding between ether molecules, and ethers therefore have quite low boiling points for a given molar mass. Indeed, ethers have boiling points about the same as those of alkanes of comparable molar mass and much lower than those of the corresponding alcohols (Table \(\PageIndex{3}\)).

    Table \(\PageIndex{3}\) Comparison of Boiling Points of Alkanes, Alcohols, and Ethers
    Condensed Structural Formula Name Molar Mass Boiling Point (°C) Intermolecular Hydrogen Bonding in Pure Liquid?
    CH3CH2CH3 propane 44 –42 no
    CH3OCH3 dimethyl ether 46 –25 no
    CH3CH2OH ethyl alcohol 46 78 yes
    CH3CH2CH2CH2CH3 pentane 72 36 no
    CH3CH2OCH2CH3 diethyl ether 74 35 no
    CH3CH2CH2CH2OH butyl alcohol 74 117 yes

    Ether molecules do have an oxygen atom, however, and engage in hydrogen bonding with water molecules. Consequently, an ether has about the same solubility in water as the alcohol that is isomeric with it. For example, dimethyl ether and ethanol (both having the molecular formula C2H6O) are completely soluble in water, whereas diethyl ether and 1-butanol (both C4H10O) are barely soluble in water (8 g/100 mL of water).

    Diethyl ether, the most widely used compound of this class, is a colorless, volatile liquid that is highly flammable. It was first used in 1846 as an anesthetic, but better anesthetics have now largely taken its place. Diethyl ether and other ethers are presently used primarily as solvents for gums, fats, waxes, and resins. Tertiary-butyl methyl ether, C4H9OCH3 (abbreviated MTBE—italicized portions of names are not counted when ranking the groups alphabetically—so butyl comes before methyl in the common name), is used as an additive for gasoline. MTBE belongs to a group of chemicals known as oxygenates due to their capacity to increase the oxygen content of gasoline.

    To Your Health: Ethers as General Anesthetics

    A general anesthetic acts on the brain to produce unconsciousness and a general insensitivity to feeling or pain. Diethyl ether (CH3CH2OCH2CH3) was the first general anesthetic to be used.

    William Morton, a Boston dentist, introduced diethyl ether into surgical practice in 1846. This painting shows an operation in Boston in 1846 in which diethyl ether was used as an anesthetic. Inhalation of ether vapor produces unconsciousness by depressing the activity of the central nervous system. Source: Painting of William Morton by Ernest Board.

    Diethyl ether is relatively safe because there is a fairly wide gap between the dose that produces an effective level of anesthesia and the lethal dose. However, because it is highly flammable and has the added disadvantage of causing nausea, it has been replaced by newer inhalant anesthetics, including the fluorine-containing compounds halothane, enflurane, and isoflurane. Unfortunately, the safety of these compounds for operating room personnel has been questioned. For example, female operating room workers exposed to halothane suffer a higher rate of miscarriages than women in the general population.

    halothane.jpg

    These three modern, inhalant, halogen-containing, anesthetic compounds are less flammable than diethyl ether.

    A list of important ethers and their uses are given in Table \(\PageIndex{4}\)

    Table \(\PageIndex{4}\) Important Ethers and Their Uses
    Chemical structure of ethylene oxide Ethylene oxide Also the simplest epoxide. Most ethylene oxide is used for synthesis of ethylene glycols (antifreeze), including diethylene glycol and triethylene glycol, that accounts for up to 75% of global consumption.
    Chemical structure of dimethyl ether Dimethyl ether An aerosol spray propellant. A potential renewable alternative fuel for diesel engines with a cetane rating as high as 56–57.
    Chemical structure of diethyl ether Diethyl ether A common low boiling solvent (b.p. 34.6 °C) and an early anaesthetic. Used as starting fluid for diesel engines. Also used as a refrigerant and in the manufacture of smokeless gunpowder, along with use in perfumery.
    Chemical structure of dimethoxyethane Dimethoxyethane(DME) A water miscible solvent often found in lithium batteries (b.p. 85 °C):
    Chemical structure of dioxane Dioxane A cyclic ether and high-boiling solvent (b.p. 101.1 °C).

    Example \(\PageIndex{2}\): classifying functional groups

    Identify the following compounds as alcohol, phenol or ether.

    a. b. 3b.jpg

    Solution

    a. The compound has the general formula R—O—R, so it is an ether. b. The compound has two hydroxyl (–OH) groups, so it is an alcophol.

    Exercise \(\PageIndex{2}\)

    Identify the following compounds as alcohol, phenol or ether.

    a. The structure shown has a C H subscript 3 group bonded up and to the right to a C atom. The C atom is bonded down and to the right to a C H subscript 2 group. The C H subscript 2 group is bonded up and to the right to a C H subscript 2 group. The C H subscript 2 group is bonded down and to the right to a C H subscript 3 group. The second C atom (from left to right) is bonded to a C H subscript 3 group and an O H group.b. A molecular structure shows a C H subscript 3 group bonded up and to the right to an O atom. The O atom is bonded down and to the right to a C H group. The C H group is bonded up and to the right to a C H subscript 3 group. The C H group is also bonded down and to the right to another C H subscript 3 group.

    Answer

    a. alcohol b. ether

    Summary

    • The –OH group is the functional group of an alcohol. Various alcohols have a wide range of applications in the medical field as well as in the transportation, food, cosmetic industries.
    • The name of an alcohol comes from the hydrocarbon from which it was derived.
    • The –R–O–R– group is the functional group of an ether. Diethyl ether and other ethers are presently used primarily as solvents for gums, fats, waxes, and resins. Ethylene oxide in particular is used in the manufacture of ethylene glycol (antifreeze).
    • Phenols are compounds in which an OH group is attached directly to an aromatic ring. Many phenols are used as antiseptics.

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