The Amazing World of Alcohol 2
Definition
The term Alcohol, refers to any of a class of organic compounds characterized by one or more hydroxyl (―OH) groups attached to a carbon atom of an alkyl group (hydrocarbon chain).
Alcohols may be considered as organic derivatives of water (H2O) in which one of the hydrogen atoms has been replaced by an alkyl group, typically represented by R in organic structures. For example, in ethanol (or ethyl alcohol) the alkyl group is the ethyl group, ―CH2CH3.
Alcohols are among the most common organic compounds. They are used as sweeteners and in making perfumes, are valuable intermediates in the synthesis of other compounds, and are among the most abundantly produced organic chemicals in industry. Perhaps the two best-known alcohols are ethanol and methanol (or methyl alcohol). Ethanol is used in toiletries, pharmaceuticals, and fuels, and it is used to sterilize hospital instruments. It is, moreover, the alcohol in alcoholic beverages. The anesthetic ether is also made from ethanol. Methanol is used as a solvent, as a raw material for the manufacture of formaldehyde and special resins, in special fuels, in antifreeze, and for cleaning metals.
Classifications
Alcohols may be classified as primary, secondary, or tertiary, according to which carbon of the alkyl group is bonded to the hydroxyl group. Most alcohols are colorless liquids or solids at room temperature. Alcohols of low molecular weight are highly soluble in water; with increasing molecular weight, they become less soluble in water, and their boiling points, vapor pressures, densities, and viscosities increase.
Another way of classifying alcohols is based on which carbon atom is bonded to the hydroxyl group. If this carbon is primary (1°, bonded to only one other carbon atom), the compound is a primary alcohol. A secondary alcohol has the hydroxyl group on a secondary (2°) carbon atom, which is bonded to two other carbon atoms. Similarly, a tertiary alcohol has the hydroxyl group on a tertiary (3°) carbon atom, which is bonded to three other carbons. Alcohols are referred to as allylic or benzylic if the hydroxyl group is bonded to an allylic carbon atom (adjacent to a C=C double bond) or a benzylic carbon atom (next to a benzene ring), respectively.
Nomenclature
As with other types of organic compounds, alcohols are named by both formal and common systems. The most generally applicable system was adopted at a meeting of the International Union of Pure and Applied Chemistry (IUPAC) in Paris in 1957. Using the IUPAC system, the name for an alcohol uses the -ol suffix with the name of the parent alkane, together with a number to give the location of the hydroxyl group. The rules are summarized in a three-step procedure:
- Name the longest carbon chain that contains the carbon atom bearing the ―OH group. Drop the final -e from the alkane name, and add the suffix -ol.
- Number the longest carbon chain starting at the end nearest the ―OH group, and use the appropriate number, if necessary, to indicate the position of the ―OH group.
- Name the substituents, and give their numbers as for an alkane or alkene.
The example on the right has a longest chain of six carbon atoms, so the root name is hexanol. The ―OH group is on the third carbon atom, which is indicated by the name 3-hexanol. There is a methyl group on carbon 3 and a chlorine atom on carbon 2. The complete IUPAC name is 2-chloro-3-methyl-3-hexanol. The prefix cyclo- is used for alcohols with cyclic alkyl groups. The hydroxyl group is assumed to be on carbon 1, and the ring is numbered in the direction to give the lowest possible numbers to the other substituents, as in, for example, 2,2-dimethylcyclopentanol.
Common Names
The common name of an alcohol combines the name of the alkyl group with the word alcohol. If the alkyl group is complex, the common name becomes awkward and the IUPAC name should be used. Common names often incorporate obsolete terms in the naming of the alkyl group; for example, amyl is frequently used instead of pentyl for a five-carbon chain.
Physical Properties
Most of the common alcohols are colorless liquids at room temperature. Methyl alcohol, ethyl alcohol, and isopropyl alcohol are free-flowing liquids with fruity aromas. The higher alcohols—those containing 4 to 10 carbon atoms—are somewhat viscous, or oily, and they have heavier fruity odors. Some of the highly branched alcohols and many alcohols containing more than 12 carbon atoms are solids at room temperature.
Featured Alcohol: Pentanol
1-Pentanol, (or n-pentanol, pentan-1-ol), is an organic compound with the formula CH3CH2CH2CH2CH2OH (C5H11OH) and is classified as a primary alcohol. It is a colorless liquid with a distinctive aroma. It is one of 8 isomeric alcohols with the same formula. 1-Pentanol is commonly used as a solvent, a biological drying agent and in the synthesis of some fragrance compounds. It is also a common component of fusel alcohols (fusel oils or “tails”), which is the term given to a group of congeners/byproducts of alcoholic fermentation.
Pentan-1-ol is a short-chain primary fatty alcohol that is pentane in which a hydrogen of one of the methyl groups is substituted by a hydroxy group. It has been isolated from Melicope ptelefolia. It has a role as a plant metabolite and a human metabolite. It is a pentanol and a short-chain primary fatty alcohol.
1-Pentanol is a natural product found in Camellia sinensis, Angelica gigas, and other organisms. It is also a metabolite found in or produced by Saccharomyces cerevisiae, a yeast typically used for the fermentation of beverage and industrial grade ethanol.
Toxicity
Inhalation of amyl alcohol vapors by man caused vertigo, dyspnea and cough, double vision, deafness, delirium, and occasionally fatal poisoning (Patty, F. (ed.). Industrial Hygiene and Toxicology: Volume II)Repeated or prolonged contact with skin may cause dermatitis (IPCS, CEC; International Chemical Safety Card on 1-Pentanol, April 1997)
Aroma
Pentanol is often described as having a “Balsamic, Fruit, Green, Pungent, Yeast” aroma.Pentanal is often described as having a “fermented, bready, fruity, nutty, berry” aroma.Pentanoic Acid Pentyl Ester is often described as having a “fruity, slightly oily-fatty; apple & banana-like” aroma.