Alcohol (chemistry)
In chemistry, an alcohol is a type of organic compound that carries at least one hydroxyl functional group bound to a saturated carbon atom. Alcohols range from the simple, like methanol and ethanol, to complex, like sugar alcohols and cholesterol. The presence of an OH group strongly modifies the properties of hydrocarbons, conferring hydrophilic properties. The OH group provides a site at which many reactions can occur.
History
The flammable nature of the exhalations of wine was already known to ancient natural philosophers such as Aristotle, Theophrastus, and Pliny the Elder. However, this did not immediately lead to the isolation of alcohol, even despite the development of more advanced distillation techniques in second- and third-century Roman Egypt. An important recognition, first found in one of the writings attributed to Jābir ibn Ḥayyān, was that by adding salt to boiling wine, which increases the wine's relative volatility, the flammability of the resulting vapors may be enhanced. The distillation of wine is attested in Arabic works attributed to al-Kindī and to al-Fārābī, and in the 28th book of al-Zahrāwī's Kitāb al-Taṣrīf. In the twelfth century, recipes for the production of aqua ardens by distilling wine with salt started to appear in a number of Latin works, and by the end of the thirteenth century, it had become a widely known substance among Western European chemists.The works of Taddeo Alderotti describe a method for concentrating alcohol involving repeated fractional distillation through a water-cooled still, by which an alcohol purity of 90% could be obtained. The medicinal properties of ethanol were studied by Arnald of Villanova and John of Rupescissa, the latter of whom regarded it as a life-preserving substance able to prevent all diseases.
Nomenclature
Etymology
The word "alcohol" derives from the Arabic kohl, a powder used as an eyeliner. The first part of the word is the Arabic definite article, equivalent to the in English. The second part of the word has several antecedents in Semitic languages, ultimately deriving from the Akkadian , meaning stibnite or antimony.Like its antecedents in Arabic and older languages, the term alcohol was originally used for the very fine powder produced by the sublimation of the natural mineral stibnite to form antimony trisulfide. It was considered to be the essence or "spirit" of this mineral. It was used as an antiseptic, eyeliner, and cosmetic. Later the meaning of alcohol was extended to distilled substances in general, and then narrowed again to ethanol, when "spirits" was a synonym for hard liquor.
Paracelsus and Libavius both used the term alcohol to denote a fine powder, the latter speaking of an alcohol derived from antimony. At the same time Paracelsus uses the word for a volatile liquid; alcool or alcool vini occurs often in his writings.
Bartholomew Traheron, in his 1543 translation of John of Vigo, introduces the word as a term used by "barbarous" authors for "fine powder." Vigo wrote: "the barbarous auctours use alcohol, or alcofoll, for moost fine poudre."
The 1657 Lexicon Chymicum by William Johnson glosses the word as "antimonium sive stibium." By extension, the word came to refer to any fluid obtained by distillation, including "alcohol of wine," the distilled essence of wine. Libavius in Alchymia refers to "". Johnson glosses alcohol vini as "." The word's meaning became restricted to "spirit of wine" in the 18th century and was extended to the class of substances so-called as "alcohols" in modern chemistry after 1850.
The term ethanol was invented in 1892, blending "ethane" with the "-ol" ending of "alcohol", which was generalized as a libfix.
The term alcohol originally referred to the primary alcohol ethanol, which is used as a drug and is the main alcohol present in alcoholic drinks.
The suffix -ol appears in the International Union of Pure and Applied Chemistry chemical name of all substances where the hydroxyl group is the functional group with the highest priority. When a higher priority group is present in the compound, the prefix hydroxy- is used in its IUPAC name. The suffix -ol in non-IUPAC names also typically indicates that the substance is an alcohol. However, some compounds that contain hydroxyl functional groups have trivial names that do not include the suffix -ol or the prefix hydroxy-, e.g. the sugars glucose and sucrose.
Systematic names
is used in scientific publications, and in writings where precise identification of the substance is important. In naming simple alcohols, the name of the alkane chain loses the terminal e and adds the suffix -ol, e.g., as in "ethanol" from the alkane chain name "ethane". When necessary, the position of the hydroxyl group is indicated by a number between the alkane name and the -ol: propan-1-ol for, propan-2-ol for. If a higher priority group is present, then the prefix hydroxy-is used, e.g., as in 1-hydroxy-2-propanone. Compounds having more than one hydroxy group are called polyols. They are named using suffixes -diol, -triol, etc., following a list of the position numbers of the hydroxyl groups, as in propane-1,2-diol for CH3CHCH2OH.| Structural formula | Skeletal formula | Preferred IUPAC name | Other systematic names | Common names | Degree |
| propan-1-ol | 1-propanol; n-propyl alcohol | propanol | primary | ||
| propan-2-ol | 2-propanol | isopropyl alcohol; isopropanol | secondary | ||
| cyclohexanol | secondary | ||||
| 2-methylpropan-1-ol | 2-methyl-1-propanol | isobutyl alcohol; isobutanol | primary | ||
| tert-amyl alcohol | 2-methylbutan-2-ol; 2-methyl-2-butanol | TAA | tertiary |
In cases where the hydroxy group is bonded to an sp2 carbon on an aromatic ring, the molecule is classified separately as a phenol and is named using the IUPAC rules for naming phenols. Phenols have distinct properties and are not classified as alcohols.
Common names
In other less formal contexts, an alcohol is often called with the name of the corresponding alkyl group followed by the word "alcohol", e.g., methyl alcohol, ethyl alcohol. Propyl alcohol may be n-propyl alcohol or isopropyl alcohol, depending on whether the hydroxyl group is bonded to the end or middle carbon on the straight propane chain. As described under systematic naming, if another group on the molecule takes priority, the alcohol moiety is often indicated using the "hydroxy-" prefix.In archaic nomenclature, alcohols can be named as derivatives of methanol using "-carbinol" as the ending. For instance, can be named trimethylcarbinol.
Primary, secondary, and tertiary
Alcohols are then classified into primary, secondary, and tertiary, based upon the number of carbon atoms connected to the carbon atom that bears the hydroxyl functional group. The respective numeric shorthands 1°, 2°, and 3° are sometimes used in informal settings. The primary alcohols have general formulas. The simplest primary alcohol is methanol, for which R = H, and the next is ethanol, for which, the methyl group. Secondary alcohols are those of the form RR'CHOH, the simplest of which is 2-propanol. For the tertiary alcohols, the general form is RR'R"COH. The simplest example is tert-butanol, for which each of R, R', and R" is. In these shorthands, R, R', and R" represent substituents, alkyl or other attached, generally organic groups.Examples
Applications
Alcohols have a long history of myriad uses. For simple mono-alcohols, which are the focus of this article, the following are most important industrial alcohols:- methanol, mainly for the production of formaldehyde and as a fuel additive
- ethanol, mainly for alcoholic beverages, fuel additive, solvent, and to sterilize hospital instruments.
- 1-propanol, 1-butanol, and isobutyl alcohol for use as a solvent and precursor to solvents
- C6–C11 alcohols used for plasticizers, e.g. in polyvinylchloride
- fatty alcohol, precursors to detergents
Toxicity
With respect to acute toxicity, simple alcohols have low acute toxicities. Doses of several milliliters are tolerated. For pentanols, hexanols, octanols, and longer alcohols, range from 2–5 g/kg. Ethanol is less acutely toxic. All alcohols are mild skin irritants.Methanol and ethylene glycol are more toxic than other simple alcohols. Their metabolism 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.
Physical properties
In general, the hydroxyl group makes alcohols polar. Those groups can form hydrogen bonds to one another and to most other compounds. Owing to the presence of the polar OH alcohols are more water-soluble than simple hydrocarbons. Methanol, ethanol, and propanol are miscible in water. 1-Butanol, with a four-carbon chain, is moderately soluble.Because of hydrogen bonding, alcohols tend to have higher boiling points than comparable hydrocarbons and ethers. The boiling point of the alcohol ethanol is 78.29 °C, compared to 69 °C for the hydrocarbon hexane, and 34.6 °C for diethyl ether.