Alcohol (chemistry)

In general usage, alcohol (from Arabic al-khwl الكحول, or al-ghawl الغول) refers almost always to ethanol, also known as grain alcohol, and often to any beverage that contains ethanol (see alcoholic beverage). This sense underlies the term alcoholism (addiction to alcohol). Other forms of alcohol are usually described with a clarifying adjective, as in isopropyl alcohol or by the suffix -ol, as in isopropanol.

In chemistry, alcohol is a more general term, applied to any organic compound in which a hydroxyl group (-O H) is bound to a carbon atom, which in turn is bound to other hydrogen and/or carbon atoms.

Structure

The functional group of an alcohol is a hydroxyl group bonded to an sp³ hybridized carbon.

General formula

The general formula for a mono-alcohol is C_nH_2n+1OH.

Primary, Secondary, and Tertiary Alcohols

There are three major subsets of alcohols - 'primary' (1°), 'secondary' (2°) and 'tertiary' (3°), based upon the number of carbons the C-OH carbon is bonded to. Ethanol and methanol, shown below, are both 'primary' alcohols. The simplest secondary alcohol is propan-2-ol, and a simple tertiary alcohol is 2-methylpropan-2-ol.

Methanol and ethanol

The simplest and most commonly used alcohols are methanol and ethanol (common names methyl alcohol and ethyl alcohol, respectively), which have the following structures:

     H           H H
     |           | |
   H-C-O-H     H-C-C-O-H
     |           | |
     H           H H

   methanol     ethanol

In common usage, "alcohol" often refers simply to ethanol or "grain alcohol".

Other common alcohols

isopropyl alcohol (sec-propyl alcohol, propan-2-ol, 2-propanol) H₃C-CH(OH)-CH₃, or "rubbing alcohol"
ethylene glycol (ethane-1,2-diol) HO-CH₂-CH₂-OH, which is the primary component in antifreeze
glycerin (or glycerol, propane-1,2,3-triol) HO-CH₂-CH(OH)-CH₂-OH bound in natural fats and oils, which are triglycerides (triacylglycerols)
phenol, an alcohol where the hydroxyl group is bound to a benzene ring

Fatty alcohols

These are derived from natural fats and oils. Those with common names include:

erucyl alcohol
ricinolyl alcohol
arachidyl alcohol
capryl alcohol
capric alcohol
behenyl alcohol
lauryl alcohol (1-dodecanol) -- 12 carbon atoms
myristyl alcohol (1-tetradecanol) -- 14 carbon atoms
cetyl (or palmityl) alcohol (1-hexadecanol) -- 16 carbon atoms
stearyl alcohol (1-octadecanol) -- 18 carbon atoms
isostearyl alcohol
oleyl alcohol (cis-9-octadecen-1-ol) -- 18 carbon atoms, unsaturated
palmitoleyl alcohol
linoleyl alcohol (9Z, 12Z-octadecadien-1-ol) -- 18 carbon atoms, polyunsaturated
elaidyl alcohol (9E-octadecen-1-ol)
elaidolinoleyl alcohol (9E, 12E-octadecadien-1-ol)
linolenyl alcohol (9Z, 12Z, 15Z-octadecatrien-1-ol)
elaidolinolenyl alcohol (9E, 12E, 15-E-octadecatrien-1-ol)

The smaller molecules are used in cosmetics and food, and as industrial solvents. Some of the larger molecules are important as biofuels.

Uses

Alcohols are in wide use in industry and science as reagents, solvents, and fuels. State-of-the-art engineering has achieved replacement of gasoline (and other hydrocarbons which produce toxic fumes) with forms of alcohol such as ethanol or methanol (which burn more cleanly). Because of its low toxicity and ability to dissolve non-polar substances, ethanol is often used as a solvent in medical drugs, perfumes, and vegetable essences such as vanilla.

Sources

Many alcohols can be created by fermentation of fruits or grains with yeast, but only ethanol is commercially produced this way, chiefly for fuel and drink. Other alcohols are generally produced by synthetic routes from natural gas, oil, or coal feedstocks.

Nomenclature

Systematic names

Common names for alcohols usually take the name of the corresponding alkyl group and add the word "alcohol", e.g. methyl alcohol, ethyl alcohol or tert-butyl alcohol. Propyl alcohol may be n-propyl alcohol or isopropyl alcohol depending on whether the hydroxyl group is bonded to the 1st or 2nd carbon on the propane chain. Isopropyl alcohol is also occasionally called sec-propyl alcohol.

In the IUPAC system, the name of the alkane chain loses the terminal "e" and adds "ol", e.g. "methanol" and "ethanol". When necessary, the position of the hydroxyl group is indicated by a number between the alkane name and the "ol": propan-1-ol for CH₃CH₂CH₂OH, propan-2-ol for CH₃CH(OH)CH₃. Sometimes, the position number is written before the IUPAC name: 1-propanol and 2-propanol. If a higher priority group is present (such as an aldehyde, ketone or carboxylic acid), then it is necessary to use the prefix "hydroxy", for example: 1-hydroxypropan-2-one (CH₃COCH₂OH).

Some examples of simple alcohols:

As mentioned above alcohols are classified as primary (1°), secondary (2°) or tertiary (3°), and common names often indicate this in the alkyl group prefix. For example (CH₃)₃COH is a tertiary alcohol is commonly known as tert-butyl alcohol. This would be named 2-methylpropan-2-ol under IUPAC rules, indicating a propane chain with methyl and hydroxyl groups both attached to the middle (2) carbon.

An alcohol with two hydroxyl groups is commonly called a "glycol", e.g. HOCH₂CH₂OH is ethylene glycol. The IUPAC name is ethane-1,2-diol, "diol" indicating two hydroxyl groups, and 1,2 indicating their bonding positions. Geminal glycols (with the two hydroxyls on the same carbon atom), such as ethane-1,1-diol, are generally unstable. For three or four groups, "triol" and "tetraol" are used.

Etymology

The word "alcohol" almost certainly comes from the Arabic language (the "al-" prefix being the Arabic definite article); however, the precise origin is unclear. It was introduced into Europe, together with the art of distillation and the substance itself, around the 12th century by various European authors who translated and popularized the discoveries of Islamic alchemists.

A popular theory, found in many dictionaries, is that it comes from الكحل = ALKHL = al-kuhul, originally the name of very finely powdered antimony sulfide Sb₂S₃ used as an antiseptic and eyeliner. The powder is prepared by sublimation of the natural mineral stibnite in a closed vessel. According to this theory, the meaning of alkuhul would have been first extended to distilled substances in general, and then narrowed to ethanol. This conjectured etymology has been circulating in England since 1672 at least (OED).

However, this derivation is suspicious since the current Arabic name for alcohol, الكحول = ALKHWL = al???, does not derive from al-kuhul. The Qur'an in verse 37:47 uses the word الغول = ALGhWL = al-ghawl — properly meaning "spirit" ("spiritual being") or "demon" — with the sense "the thing that gives the wine its headiness". The word al-ghawl also originated the English word "ghoul", and the name of the star Algol. This derivation would, of course, be consistent with the use of "spirit" or "spirit of wine" as synonymous of "alcohol" in most Western languages. (Incidentally, the etymology "alcohol" = "the devil" was used in the 1930s by the U.S. Temperance Movement for propaganda purposes.)

According to the second theory, the popular etymology and the spelling "alcohol" would not be due to generalization of the meaning of ALKHL, but rather to Western alchemists and authors confusing the two words ALKHL and ALGhWL, which have indeed been transliterated in many different and overlapping ways.

Physical and chemical properties

The hydroxyl group generally makes the alcohol molecule polar. Those groups can form hydrogen bonds to one another and to other compounds. Two opposing solubility trends in alcohols are: the tendency of the polar OH to promote solubility in water, and of the carbon chain to resist it. Thus, methanol, ethanol, and propanol are miscible in water because the hydroxyl group predominates. Butanol is moderately soluble because of a balance between the two trends. Pentanol and branched butanols are effectively insoluble because of the hydrocarbon chain's dominance. Because of hydrogen bonding, alcohols tend to have higher boiling points than comparable hydrocarbons and ethers. All simple alcohols are miscible in organic solvents.

Alcohols are protic solvents and are weaker acids than water (except for methanol), but still stronger than ammonia or acetylene.

One important class of reactions undergone by alcohols is nucleophilic substitution, where one nucleophilic group attached to a carbon atom is replaced by another. So, for instance, tertiary alcohols react with hydrochloric acid to produce tertiary alkyl halides, where the hydroxyl group is replaced by a chlorine atom. If primary or secondary alcohols are to be reacted with hydrochloric acid, an activator such as zinc chloride is needed. Alternatively the conversion may be performed directly using thionyl chloride.^[1] Alcohols may likewise be converted to alkyl bromides using bromic (I) acid or phosphorus (III) tribromide. Some typical examples of alkyl chloride preparations are:

Alcohols are themselves nucleophilic, so can react with one another to produce ethers and water. They also react with carboxylic acids (in the presence of acid) or acid chlorides (in the presence of base) to produce compounds called esters, of which the esters of carboxylic acids are the most important. At high temperatures, alcohols can undergo an elimination reaction to produce alkenes. The reverse of this, the addition of water to an alkene to produce an alcohol, is catalyzed by acids but is of limited use for synthesis because it generally results in mixtures. Some other techniques exist to convert alkenes to alcohols more reliably.

Primary alcohols may be oxidised to aldehydes, and secondary alcohols oxidised to ketones, using oxidising agents such as PCC or via a Swern oxidation. Tertiary alcohols have no H attached to the alcohol carbon, and so they resist oxidation.

Toxicity

Alcohols often have an odor described as 'biting' that 'hangs' in the nasal passages. Ethanol in the form of alcoholic beverages has been consumed by humans since pre-historic times, for a variety of hygienic, dietary, medicinal, religious, and recreational reasons. While infrequent consumption of ethanol in small quantities may be harmless or even beneficial, larger doses result in a state known as drunkenness or intoxication and, depending on the dose and regularity of use, can cause acute respiratory failure or death and with chronic use can cause severe health problems, such as liver and brain damage.

Other alcohols are substantially more poisonous than ethanol, partly because they take much longer to be metabolized, and often their metabolism produces even more toxic substances. Methanol, or wood alcohol, for instance, is oxidized by alcohol dehydrogenase enzymes in the liver to the poisonous formaldehyde, which can cause blindness or death. Interestingly, an effective treatment to prevent formaldehyde toxicity after methanol ingestion is to administer ethanol. This will bind to alcohol dehydrogenase, preventing methanol from binding and thus its acting as a substrate.

Preparation and Reaction Alcohols

Preparation

a)Laboratory

From halogenoalkanes: reflux with aqueous NaOH or KOH
From aldehydes : reduction with sodium tetrahydridoborate (III)
From alkenes : an acid catalysed reaction using concentrated sulfuric (VI) acid

b)Industrial

Fermentation: using glucose from the hydrolysis of starch, in the presence of yeast and temperature of <37°C
Direct Hydration: using ethene from cracking of fractions of distilled crude oil. Uses a catalyst of phosphoric (V) acid under high temperature and pressure

c)Of methanol

A synthesis gas is first produced by the reaction of methane with water with a Ni catalyst at 10-20 atm and a temperature of 850°C. CO and 3 hydrogen molecules are produced.
The synthesis gas is reacted after removing one mole of the hydrogen. This is done using Cu, ZnO and Al₂O₃ catalyst at 250°C and a pressure of 50-100 atm. This produces 1 mole of methanol.

Reactions

Alcohols react using a nucleophilic substitution, where a negatively charged nucleophile is attracted to the positive part of the molecule (the C-OH bond), substituting the hydroxyl group.

a)With metallic Na

        2R-OH + 2Na → 2R-O^-Na⁺

     eg.2CH₃CH₂OH + 2Na → 2CH₃CH₂-O^-Na⁺

b)With H-X (hydrogen halides)

         R-OH + H-X + ZnCl₂ → R-X + H₂O + ZnCl₂

     eg. CH₃OH + HCl + ZnCl₂ → CH₃Cl + H₂O + ZnCl₂

c)With PX₅/PX₃

PX₅

         R-OH + PCl₅ → POCl₃ + HCl + R-Cl

PX₃

         3R-OH + PCl₃ → 3RCl + H₃PO₃

d)With SOCl₂

         R-OH + SOCl₂ → R-Cl + SO₂ + HCl

e)Esterification

The alcohol and carboxylic acid must undergo reflux with a catalyst of conc. sulfuric (VI) acid.

         R-OH + R^ICOOH →  R^ICOOR

f)Wtih H₂SO₄

This reaciton must be performed at a temperature of 0°C.

         CH₃CH₂OH + H₂SO₄ → CH₃CH₂OSO₃H + H₂O

g)Oxidation

Acidified dichromate (VI) or acidified manganate (VII). Acidified manganate (VII)is the stronger oxidising agent.

  i)Of 1° alcohols

The alcohol should be in excess, and the product distilled, to obtain the aldehyde, and the weaker oxidising agent should be used. To obtain the carboxylic acid, the manganate (VII) is used, and the solution refluxed.

      -  OHCH₂CH₂OH → OHCH₂CHO

      -  CH₃CHCH₂OH → CH₃CHCOOH
            |            |
            OH           CH₃

  ii)Of 2° alcohols

External links

Alcohol and the Distillation of Wine in Arabic Sources by Ahmad Y. al-Hassan.