Lesson 1: AlcoholThe hydroxyl (-OH) functional group with bond angle. In chemistry, an alcohol is an organic compound in which the hydroxyl functional group (-OH) is bound to a carbon atom. In particular, this carbon center should be saturated, having single bonds to three other atoms. An important class of alcohols are the simple acyclic alcohols, the general formula for which is CnH2n+1OH. Of these ethanol (C2H5OH) is the alcohol found in alcoholic beverages; in common speech the word alcohol refers to ethanol. Other alcohols are usually described with a clarifying adjective, as in isopropyl alcohol (propan-2-ol) or wood alcohol (methyl alcohol, or methanol). The suffix -ol appears in the IUPAC chemical nameof all substances where the hydroxyl group is the functional group with the highest priority; in substances where a higher priority group is present the prefix hydroxy- will appear in the IUPACname. The suffix -ol in non-systematic names (such as paracetamol or cholesterol) also typically indicates that the substance includes a hydroxyl functional group and, so, can be termed an alcohol. But many substances, particularly sugars (examples glucose and sucrose) contain hydroxyl functional groups without using the suffix. In everyday life "alcohol" without qualification usually refers to ethanol, or a beverage based on ethanol (as in the term "alcohol abuse"). Alkyl chain variations in alcohols
Short-chain alcohols have alkyl chains of 1-3 carbons. Medium-chain alcohols have alkyl chains of 4-7 carbons. Long-chain alcohols (also known as fatty alcohols) have alkyl chains of 8-21 carbons, and very long-chain alcohols have alkyl chains of 22 carbons or longer. Simple alcohols"Simple alcohols" appears to be a completely undefined term. However, 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. Simple alcohols, in particular ethanol and methanol, possess denaturing and inert rendering properties, leading to their use as anti-microbial agents in medicine, pharmacy, and industry. Higher alcoholsEncyclopædia Britannica states, "The higher alcohols - those containing 4 to 10 carbon atoms – are somewhat viscous, or oily, and they have heavier fruity odours. Some of the highly branched alcohols and many alcohols containing more than 12 carbon atoms are solids at room temperature." Like ethanol, butanol can be produced by fermentation processes. (However, the fermenting agent is a bacterium, Clostridium acetobutylicum, that feeds on cellulose, not sugars like the Saccharomyces yeast that produces ethanol.) Saccharomyces yeast are known to produce these higher alcohols at temperatures above 75 °F (24 °C). History and etymologyThe word alcohol appears in English as a term for a very fine powder in the 16th century. It was borrowed from French, which took it from medical Latin. Ultimately the word is from the Arabic كحل (al-kuḥl, "kohl, a powder used as an eyeliner"). Al- is the Arabic definitive article, equivalent tothe in English; alcohol was originally used for the very fine powder produced by the sublimation of the natural mineral stibnite to formantimony sulfide Sb2S3 (hence the essence or "spirit" of the substance), which was used as an antiseptic, eyeliner, and cosmetic (seekohl (cosmetics)). Bartholomew Traheron, in his 1543 translation of John of Vigo, introduces the word as a term used by "barbarous" (Moorish) authors for "fine powder." Vigo wrote: the barbarous auctours use alcohol, or (as I fynde it sometymes wryten) 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 (1594) refers to vini alcohol vel vinum alcalisatum. Johnson (1657) glosses alcohol vini as quando omnis superfluitas vini a vino separatur, ita ut accensum ardeat donec totum consumatur, nihilque fæcum aut phlegmatis in fundo remaneat. The word's meaning became restricted to "spirit of wine" (the chemical known today as ethanol) in the 18th century and was extended to the class of substances so-called as "alcohols" in modern chemistry after 1850. The first alcohol (today known as ethyl alcohol) was discovered by the tenth-century Persian alchemist al-Razi. The current Arabic name for alcohol (ethanol) is الغول al-ġawl – properly meaning "spirit" or "demon" – with the sense "the thing that gives the wine its headiness" (in the Qur'an sura 37 verse 47). The term ethanol was invented 1838, modeled on the German word äthyl (Liebig), which is in turn based on Greek aither ether and hyle "stuff. Lesson 2: Esters![]()
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Nomenclature
Systematic names 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-hydroxy-2-propanone (CH3COCH2OH). The IUPAC nomenclature is used in scientific publications and where precise identification of the substance is important. 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., methylalcohol, ethyl 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. Alcohols are classified into 0°, primary (1°), secondary (2°; also italic abbreviated sec- or just s-), and tertiary (3°; also italic abbreviatedtert- or just t-), based upon the number of carbon atoms connected to the carbon atom that bears the hydroxyl (OH) functional group. The primary alcohols have general formulas RCH2OH; secondary ones are RR'CHOH; and tertiary ones are RR'R"COH, where R, R', and R" stand for alkyl groups. Methanol (C H3O H or CH4O) is a 0° alcohol. Some sources include methanol as a primary alcohol,including the 1911 edition of the Encyclopædia Britannica,but this interpretation is less common in modern texts. Physical and chemical properties
Alcohols have an odor that is often described as “biting” and as “hanging” in the nasal passages. Ethanol has a slightly sweeter (or more fruit-like) odor than the other alcohols. In general, the hydroxyl group makes the alcohol molecule polar. Those groups can form hydrogen bonds to one another and to other compounds (except in certain large molecules where the hydroxyl is protected by steric hindrance of adjacent groups). This hydrogen bonding means that alcohols can be used as protic solvents. Two opposing solubility trends in alcohols are: the tendency of the polar OH to promote solubility in water, and the tendency of the carbon chain to resist it. Thus, methanol, ethanol, and propanol are miscible in water because the hydroxyl group wins out over the short carbon chain. Butanol, with a four-carbon chain, is moderately soluble because of a balance between the two trends. Alcohols of five or more carbons (pentanol and higher) are effectively insoluble in water because of the hydrocarbon chain's dominance. All simple alcohols are miscible in organic solvents. 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 (a common constituent of gasoline), and 34.6 °C fordiethyl ether. Alcohols, like water, can show either acidic or basic properties at the -OH group. With a pKa of around 16-19, they are, in general, slightly weaker acids than water, but they are still able to react with strong bases such as sodium hydride or reactive metals such assodium. The salts that result are called alkoxides, with the general formula RO- M+. Meanwhile, the oxygen atom has lone pairs of nonbonded electrons that render it weakly basic in the presence of strong acids such assulfuric acid. For example, with methanol: Alcohols can also undergo oxidation to give aldehydes, ketones, or carboxylic acids, or they can be dehydrated to alkenes. They can react to form ester compounds, and they can (if activated first) undergo nucleophilic substitution reactions. The lone pairs of electrons on the oxygen of the hydroxyl group also makes alcohols nucleophiles. For more details, see the reactions of alcohols section below.
As one moves from primary to secondary to tertiary alcohols with the same backbone, the hydrogen bond strength, the boiling point, and the acidity typically decrease. |