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Alcohols

Alcohols are organic compounds which contain a hydroxyl (—OH) group covalently bonded to a carbon atom.  Alcohols take part in a wide variety of chemical reactions, and are also frequently used as solvents.

The word “alcohol” comes from the Arabic term al kohl meaning “the fine powder.” Originally, this referred to an antimony sulfide compound (Sb2S3) used for eye shadow and as an antiseptic, which was ground up to form a fine powder, but then later came to refer to any finely divided powder. In the Middle Ages, this term came to mean the “essence” of anything.  When the Europeans took up alchemy in the Middle Ages, they referred to vapors from evaporating or boiling compounds as “spirits,” since they did not believe them to be material in the same sense that solids and liquids were. Alchemists began referring to “spirits of wine,” and since an alcohol when it boils away seems to powder away to nothing, they also began to refer to “alcohol of wine” and then simply “alcohol”.(1)

Simple alcohols are often referred to by common names derived by adding the word "alcohol" to the name of the appropriate alkyl group.  For instance, a chain consisting of one carbon (a methyl group, CH3) with an OH group attached to the carbon is called "methyl alcohol" while a chain of two carbons (an ethyl group, CH2CH3) with an OH group connected to the CH2 is called "ethyl alcohol."  For more complex alcohols, the IUPAC nomenclature must be used.  Straight-chain and branched alcohols are named by first selecting the longest continuous chain of carbon atoms containing the carbon to which the OH group is bonded.  The longest chain is named as an alkane, and the final -e is changed to an -ol.  A locator number is placed immediately in front of the prefix to indicate the location of the alcohol group.  Thus, the name "1-propanol" indicates a three carbon chain with an OH group on carbon number 1, and the name "2-propanol" indicates a three-carbon chain with an OH group on carbon number 2.  Cyclic alcohols, in which the OH group is attached to a carbon atom in a ring, are named in a similar fashion.  The ring is taken to be the longest chain, and the carbon bearing the OH group is numbered as carbon number 1.  Unless there is more than one OH group on the ring, the number "1" is usually omitted from the name, since it is understood that the OH group is on carbon 1.

The oxygen atoms in alcohols are sp3-hybridized, and have bent shapes, with bond angles of slightly less than 109.5 to each other.

Alcohols are polar, since they have oxygen-hydrogen bonds, which allow alcohol molecules to attract each other through hydrogen bonds.  Since oxygen atoms are much more electronegative than hydrogen atoms, the oxygen-hydrogen bond is especially polar.  The partially-negatively charged oxygen atom on one alcohol molecule is strongly attracted to the partially positively charged hydrogen atom on another alcohol molecule; this strong attraction results in much stronger intermolecular forces between alcohol molecules than there are between nonpolar alkanes of the same molar mass.  Alcohols are generally more soluble in water than alcohols of the same molecular mass; low-molecular weight alcohols such as methanol and ethanol are miscible with water, and solubility decreases as the number of carbons in the alcohol increases.  Alcohols also have much higher boiling points than alkanes of the same molecular weight:  for example, propane (molecular mass 42.08 g/mol) has a boiling point of -44.5C, while ethanol (MM 46.07 g/mol) has a boiling point of 78.3C.

Alcohols are classified as primary (1), secondary (2), or tertiary (3), depending on the number of carbons connected to the alcoholic carbon:

 

Methanol 3D

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Methanol, also known as methyl alcohol and wood alcohol, is the simplest of the alcohols.  The name is derived from the Greek words "methy" (wine) and "hule" (wood).  Methanol is is found in wood smoke, and contributes to the odor of wine.  It is metabolized in the body to produce formaldehyde and formic acid, and is toxic if more than 50 mL is consumed; smaller amounts can cause blindness.

Industrially, methanol is produced from synthesis gas, a mixture of carbon monoxide and hydrogen derived from coke (the coal, not the soda) or methane.  Methanol is used as the fuel in some racing cars, and is being investigated as an renewable alternative to the use of petroleum-based fuels.  The main use of methanol, however, is in the manufacture of other chemicals, such as formaldehyde, which is used in the manufacture of plastics, paints, plywood, etc.

 
   
Ethanol 3D

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Ethanol, also known as ethyl alcohol and grain alcohol, is the alcohol found in alcoholic beverages.  The fermentation of the sugars found in honey, grain, or fruit juices by yeasts to yield beers and wines was probably the first chemical reaction to be discovered.

Ethanol can be purified and concentrated by fractional distillation, but ethanol and water form an constant-boiling azeotrope at a concentration of 95% ethanol and 5% water which cannot be separated by distillation; absolute ethanol, which contains no water, is produced by fractional distillation of 95% ethanol with small amounts of benzene.  Ethanol that is intended for industrial use is "denatured" (rendered unfit for human consumption) by adding small amounts of methanol, denatonium benzoate, or other unpleasant or toxic substances, which exempts the alcohol from liquor taxes.

Ethanol is metabolized in the body, primarily by the enzyme alcohol dehydrogenase, to produce acetaldehyde; the buildup of acetaldehyde in the blood is one of the factors which contributes to the symptoms of a hangover.  Physiologically, ethanol acts as a depressant, but since it frees parts of the cortex from inhibitory controls, to its consumer, it seems to be a stimulant.

Ethanol is a primary (1) alcohol, and is easily oxidized by mild oxidizing agents, such as chromic acid (H2CrO4) or potassium dichromate (K2Cr2O7), in which the chromium is in the +6 oxidation state.  These substances undergo a distinct color change on reaction with ethanol:  in the +6 oxidation state chromium compounds are typically a dark reddish-orange color; after the alcohol is oxidized, the chromium is reduced to the +3 oxidation state, which is green.  The color change from red-orange to green forms the basis of some of the simple breathalyzer tests that are used to test motorists who are suspected of driving while drunk.

Ethanol can also be oxidized in air, forming acetic acid, the active ingredient in vinegar.  Alcoholic beverages that are not stored properly can end up tasting like vinegar because of the formation of acetic acid (see entry for acetic acid).

 
   
1-Propanol (Propyl alcohol) 3D

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1-Propanol, or propyl alcohol, is a three-carbon alcohol with the OH group on an end carbon.  Its structural isomer, 2-propanol, is described below.

 
   
2-Propanol (Isopropyl alcohol) 3D

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2-Propanol, or isopropyl alcohol, is a three-carbon alcohol with the OH group on the middle carbon.  Isopropyl alcohol is a secondary alcohol,   Rubbing alcohol is a solution of 70% isopropyl alcohol and 30% water, which is commonly used in sterilizing swabs and disinfectants.

Isopropyl alcohol is a secondary (2) alcohol, and is easily oxidized by mild oxidizing agents.

 
   
1-Butanol (Butyl alcohol) 3D

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1-Butanol, or butyl alcohol, is a four-carbon chain, with the OH group on an end carbon.  It is used as a solvent and a paint thinner, and has some potential use as a biofuel.

Butyl alcohol is a primary (1) alcohol, and is easily oxidized.

There are three other structural isomers of 1-butanol:  2-butanol (sec-butyl alcohol), 2-methyl-1-propanol (isobutyl alcohol), and 2-methyl-2-propanol (tert-butyl alcohol).

 
   
2-Butanol (sec-Butyl alcohol) 3D

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2-Butanol, or sec-butanol, or sec-butyl alcohol, or s-butyl alcohol, is a four-carbon chain, with the OH group on the second carbon.  (Since the alcoholic carbon is connected to two other carbons, it is secondary, hence the prefix "sec".)  It is used as a solvent and an intermediate in the manufacture of other compounds.

sec-Butyl alcohol is a secondary (2) alcohol, and is easily oxidized.

2-Butanol is a chiral compound, and exists in two enantiomeric forms:  (R)-2-butanol and (S)-2-butanol:

The 3D structure shown above is the R stereoisomer.
   
2-Methyl-1-propanol (Isobutyl alcohol) 3D

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2-Methyl-1-propanol, or isobutanol, or isobutyl alcohol, is a three-carbon chain, with the OH group on and end carbon and a methyl group on the middle carbon.  It is used as a solvent, in paints and inks, and in the manufacture of some coatings and varnishes.

Isobutyl alcohol is a primary (1) alcohol, and is easily oxidized.

 
   
2-Methyl-2-propanol (tert-Butyl alcohol) 3D

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2-Methyl-2-propanol, or tert-butanol, or tert-butyl alcohol, or t-butyl alcohol, is a three-carbon chain, with the OH group and a methyl group on the middle carbon.  (Since the alcoholic carbon is connected to three other carbons, it is tertiary, hence the prefix "tert".)  It is used as a solvent, a denaturant for ethanol, as an octane booster in gasoline, and in some pain thinners.

tert-Butyl alcohol is a tertiary (3) alcohol, and does not react with oxidizing agents.  It is useful in organic synthesis, in the form of the t-butoxide anion, which is generated by the reaction of tert-butanol with sodium or a strong base such as sodium hydride, which removes the slightly acidic hydrogen from the OH group, leaving behind a negative charge on the oxygen.  The t-butoxide anion is a strong base, but its steric bulk makes it slow to participate in nucleophilic substitution reactions, making it more likely to participate in elimination reactions.

 
   
Ethylene glycol (1,2-Ethanediol) 3D

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1,2-Ethanediol, better known as ethylene glycol, is an alcohol which contains two OH groups.  Molecules which contain two OH groups on adjacent carbons are often known as glycols.  It is a viscous, colorless, odorles liquid, with a sweet taste.  It is miscible with water, since there are two locations on the molecule which can form hydrogen bonds with water.  It has an extremely high boiling point for such a small molecule, 197C; this is because these molecules can form more than one set of hydrogen bonds to each other.

Ethylene glycol is produced industrially from ethylene:  the ethylene is oxidized to form ethylene oxide, which reacts with water to produce ethylene glycol.  It is used in the manufacture of they polyester polyethylene terephthalane (PET), and also to remove water vapor from natural gas.

Ethylene glycol is used in antifreeze — pure ethylene glycol freezes at -12.9C (8.8F), but a 50:50 mixture of ethylene glycol and water freezes at around -40C (-40F).  It is also used as an airplane de-icer, a humectant (an anti-drying agent that keeps other substances moist), used in ball point pen inks.

Ethylene glycol is toxic; in the body it is metabolized into glycolic acid (if one alcohol group is oxidized to a carboxylic acid) and oxalic acid (if both alcohol groups are oxidized), which can cause irregular heartbeat and respiration, and kidney failure.  The antidote for ethylene glycol poisoning is ethanol:  since ethanol competes for the same alcohol dehydrogenase enzyme that metabolizes ethylene glycol, flooding the body with alcohol can help to flush the ethylene glycol out of the system.

 
   
Propylene glycol (1,2-Propanediol) 3D

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1,2-Propanediol, or propylene glycol, is a tasteless, odorless, clear liquid.  Like ethylene glycol, it is miscible with water, because of hydrogen bonding.  It is used in antifreeze, as a moisturizer in lotions, foods, and some medicines and cosmetics, and as a solvent for food dyes.  It is also used to make artificial smoke.

 
   
Glycerol / Glycerin (1,2,3-Propanetriol) 3D
 

1,2,3-Propanetriol, better known as glycerol or glycerin, is a viscous, colorless, odorless, sweet-tasting liquid.  It is used as an emollient (softening agent) in cosmetics; as a humectant (anti-drying agent) in toothpaste, candies, medicines, tobacco (where it keeps leaves from drying and crumbling), and glues (prevent glue from drying in the bottle). It is also used in some plastics, especially cellophane, as a plasticizer to keep the plastic soft and pliable. Glycerol also contributes to the sweet, smooth taste of some wines.  Because it can form three sets of hydrogen bonds, it is extremely soluble in water, and has a very high boiling point (290C).

Glycerol combines with fatty acids (long-chain carboxylic acids) to make an series of biologically important molecules called the triglycerides (fats and oils).


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Benzyl alcohol 3D

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Benzyl alcohol, or phenylmethanol, is used in perfumes, flavors, soaps, cosmetics, ointments, and ball point pen inks; it is also used in some anti-itching medications.

 
   
Cinnamic alcohol 3D

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Cinnamic alcohol, or cinnamyl alcohol, is found in cinnamon leaves (hence the name), usually in the form of an ester; it is also found in Tolu balsam, the resin of the Myroxylon toluifera tree.  It has an odor similar to that of hyacinth, and is used in perfumes (particularly in lilac and other floral scents), deodorants, flavoring agent, soaps, and cosmetics.

 
   
Diethylene glycol 3D

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Diethylene glycol (DEG) is a colorless, odorless, toxic liquid used as an industrial solvent, and in the synthesis of other organic molecules.  It is also used as a humectant for tobacco, and some inks and glues.

In 1937, the S.E. Massengill Co. marketed sulfanilamide (a sulfa drug) dissolved in diethylene glycol as "Elixir Sulfanilamide"; over a hundred people died of poisoning by the DEG, and this incident catalyzed the passage of the 1938 Federal Food, Drug, and Cosmetic Act, which greatly expanded the power of the Food and Drug Administration (FDA) to regulate the safety of foods, medicines, and cosmetics.  Several other cases of DEG poisoning have resulted from DEG either being added to wines in an attempt to sweeten them, or because of its presence in improperly purified medications and cosmetics.  In May of 2007, several cheap brands of toothpaste originating in China were removed from the market in several countries because they were found to be contaminated with DEG (labeled on the containers as "glycerine").

 
   
Grandisol 3D

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Grandisol, or (+)-(1R,2S)-1-(2'-hydroxyethyl)-1-methyl-2-isopropenylcyclobutane, is a pheromone which acts as the sex attractant of the cotton boll weevil (Anthonomus grandis) and some related insects.  It is used in a insecticide called grandlure, which attracts and traps or kills these damaging insects.  
   
Cyclohexanol 3D

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Cyclohexanol is a cyclic, secondary alcohol.  It is used in some organic synthesis reactions, in the manufacture of celluloid, textiles, and some insecticides.

 
   

 

 

References

(1) Isaac Asimov, "You, Too, Can Speak Gaelic" in Asimov on Chemistry.  Garden City:  Anchor Books, 1975, p. 127.

P. W. Atkins, Molecules, 2nd ed.  Cambridge: Cambridge University Press, 2003, p. 48-52, 58-59.

Richard J. Lewis, Sr., Hawley's Condensed Chemical Dictionary, 13th ed.  New York:  Van Nostrand Reinhold, 1997.

D. W. A. Sharp, The Penguin Dictionary of Chemistry, 2nd ed.  London:  Penguin Books, 1990.

Graham Solomons and Craig Fryhle, Organic Chemistry, 7th ed.  New York:  John Wiley & Sons, Inc., 2000, p. 477-479, 482-483.

Martha Windholz (ed.), The Merck Index, 10th ed. Rahway: Merck & Co., Inc., 1983.