Alcohols

In organic chemistry, an alcohol is a hydroxylic organic compound that contains a hydroxyl functional group (-OH), this being linked to a saturated carbon atom, in the sp3 hybridization state. The hydroxyl group can be attached to an alkane chain (saturated chain), to the saturated side of an alkene chain, or to the side chain of an aromatic hydrocarbon. The general formula for a simple, acyclic alcohol is CnH2n+1OH.

In common parlance, the term alcohol almost always refers to ethanol, also known as grain alcohol, and often to any beverage containing ethanol (see alcoholic beverage). This meaning is the basis of the term alcoholism (alcohol addiction). Refraining from drinking alcohol is called abstinence, and the 1900s US law against drinking alcohol was called prohibition. As a drug, ethanol is known to have a depressant effect, which decreases the sharpness of central nervous system reflexes. Other forms of alcohol are usually referred to with an adjective of rigor, such as isopropyl alcohol, or by the suffix -ol, as in isopropanol.

The word dates back to the 16th century, when it was used to name any substance that could be obtained by sublimation. It derives from the medieval Latin alcool ("powder of alum"), originally from the Arabic al-kuḥūl (كحول), which is also the source of the term kohl and related to the root k-ḥ-l, attested in the Arabic word for make-up.

Structure

The functional group of an alcohol is the hydroxyl group attached to an sp3 hybridized carbon atom. Thus alcohols can be said to be derivatives of water, with an alkyl group replacing a hydrogen. If an aryl group is present instead of an alkyl group, the compound is called a phenol. Also, if the hydroxyl group is bonded to an sp2 hybridized carbon of an alkenyl group, the compound is called an enol. The oxygen in an alcohol makes an angle of about 109° (cf. 104.5° in water) and two lone pairs of electrons. The O-H bond in methanol (CH3OH) is about 96 picometers long.

Primary, secondary and tertiary alcohols

There are three broad categories of alcohols - 'primary' (1°), 'secondary' (2°) and 'tertiary' (3°), based on the number of carbon atoms to which the C-OH carbon is attached (highlighted in red ). Methanol is the only "null" alcohol, the simplest primary alcohol being Ethanol. The simplest secondary alcohol is isopropanol (propan-2-ol) and the simplest tertiary alcohol is tert-butanol (2-methylpropan-2-ol).

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Saturated, unsaturated and aromatic alcohols

Depending on the nature of the radicals to which the hydroxyl functional group is attached, alcohols can be saturated (derived from alkanes), unsaturated (derived from alkenes or alkynes) or aromatic (derived from arenes).

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Monoalcohols and polyalcohols

Alcohols can also be classified according to the number of hydroxyl groups contained in the molecule. Thus, there are monoalcohols, with a single hydroxyl group (all of the above are monoalcohols), and polyalcohols, with more than one hydroxyl group.

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Methanol and ethanol

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

Methanol used to be obtained by distilling wood, so it was called "wood alcohol." Today, it is a readily available chemical produced by reacting carbon monoxide with hydrogen under pressure. Colloquially, the term "alcohol" often refers to ethanol or "grain alcohol". Methylated spirit, also called "medicinal spirit", is a form of ethanol made inedible by the addition of methanol and dyes, usually methyl blue. In addition to its primary use in alcoholic beverages, ethanol is used (albeit very well controlled) as an industrial solvent and raw material.

Applications

Alcohols are widely used in industry and science, as reactants, combustible solvents. Ethanol and methanol can burn creating less harmful substances than gasoline or diesel. Due to its low toxicity and ability to dissolve non-polar substances, ethanol is often used as a solvent in medicines, perfumes and plant essences such as vanilla. In organic synthesis, alcohols often appear as adaptable intermediates.

Ethanol is also frequently used in beverages after fermentation to highlight flavors or to induce the euphoric intoxication called "drunkenness". The use of ethanol for this purpose is prohibited in some jurisdictions. In such cases of consumption, alcohol is a psychoactive drug, with an immediate potential for overdose, poisoning and physiological dependence (known as alcoholism). Alcoholism has become one of the most common causes of addiction (perhaps after caffeine) in the world. Physiological dependence caused by alcoholism means that the addicted person goes through withdrawal (in the form of a headache known as a "hangover," increased anxiety known as "chills," and fatigue or trouble sleeping) when use stops or decreases.

Surse

In the IUPAC system, the name of the alkane receives the suffix "ol", as, for example, "methanol" and "ethanol". When necessary, the position of the hydroxyl group is indicated by placing a number between the chain name and the "ol" suffix: propan-1-ol for CH3CH2CH2OH, propan-2-ol for CH3CH(OH)CH3. Sometimes the position number is written in front of the name: 1-propanol and 2-propanol. If a higher priority group (such as an aldehyde, ketone, or carboxylic acid) is present, then their position must also be specified. For example, 1-hydroxy-2-propanone for CH3COCH2OH.

Here are some examples of simple alcohols with naming patterns:

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The common names of alcohols usually appear by taking the name of the base alkyl radical and adding the word "alcohol", for example methyl alcohol, ethyl alcohol or tert-butyl alcohol. Propyl alcohol can be n-propyl alcohol or isopropyl alcohol, depending on the position at to which the hydroxyl is attached, respectively carbon 1 or carbon 2. Isopropyl alcohol is also occasionally called sec-propyl alcohol.

As mentioned above, alcohols are classified as primary (1°), secondary (2°) or tertiary (3°), and the common names show this by prefixing the alkyl radical. For example, (CH3)3COH is a tertiary alcohol and is commonly called tert-butyl alcohol. This would be called 2-methylpropan-2-ol under the IUCPA rules, indicating a propane chain that has methyl and hydroxyl groups attached to the middle (second) carbon.

An alcohol with two hydroxyl groups is generally called a "glycol", for example HO-CH2-CH2-OH is ethylene glycol. The UICPA name is ethane-1,2-diol, "diol" indicating the two hydroxyl groups, and 1,2 indicating the positions to which they are attached. Geminal glycols (having hydroxyls attached to the same carbon atom), such as ethane-1,1-diol, are usually unstable. For three or four groups, the suffixes "triol" and "tetraol" are used.

Etymology

The word "alcohol" almost certainly comes from the Arabic language (the prefix "al-" being the Arabic definite article); however, the precise origin is uncertain. It was introduced to Europe, along with the art of distillation and the substance itself, in about the twelfth 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 الكهل = ALKHL = al-kuhul, originally being the name of a very fine powder of stanium sulfide Sb2S3 used as an antiseptic and eye make-up. The powder is prepared by sublimating a natural mineral, stibbin, in a closed vessel. According to this theory, the meaning of the word alkuh would have been expanded first for distilled substances in general, and then narrowed only for ethanol. This etymology has been circulating in England since at least 1672 (DEO).

However, this derivation is not entirely accepted, since the current Arabic word for alcohol, alcohol = ALKHWL = al???, does not derive from al-kuhul. The Qur'an, in verses 37:47, uses the word الغول = ALGhWL = al-ghawl — meaning "spirit" ("spiritual being") or "demon" — with the meaning of "the thing that gives strength to wine." The word al-ghawl gave rise to the English word "ghoul" (imaginary creature that eats dead people; vampire), and the name of the star Algol.

According to a second theory, the popular etymology and spelling of the word "alcohol" would not have arisen due to the generalization of the word ALKHL, but the confusion of alchemists and Western authors of the words ALKHL and ALGhWL, which were indeed mistranslated in many ways.

Physical and chemical properties

The hydroxyl group generally makes alcohol a polar molecule. Those groups can form hydrogen bonds with each other and with other compounds. In alcohols there are two possibilities of dissolution: the tendency of the polar -OH group to make it soluble in water and that of the side chain to oppose it. Therefore, methanol, ethanol and propanol are soluble in water because the influence of the hydroxyl group is stronger than that of the chain. Butanol, with four carbons in the chain, is moderately soluble due to the balance between the two tendencies. Monohydric alcohols with five or more carbons (pentanol or higher) are insoluble in water due to the dominance of the side chain.

Because of hydrogen bonding, alcohols tend to have higher boiling points than hydrocarbons and ethers. All simple alcohols are soluble in organic solvents. Hydrogen bonds show that alcohols can be used as protic solvents.

The dielectronic orbital of the hydroxyl oxygen forms nucleophilic alcohols.

Alcohols, like water, can have either acidic or basic properties at the O-H group. With a pKa of around 16-19, they are slightly less acidic than water, but are capable of reacting with strong bases such as sodium hydride or reactive metals such as sodium. The resulting salts are called alkoxides, having the general formula RO- M+

Alcohols linked to benzene rings have lower acidity (a pKa around 10). Electron-withdrawing groups participate in increasing the acidity of alcohols. For example, para-nitro phenol has a pKa of 7.15.

Oxygen has a dielectron orbital on the last shell, which makes alcohol weakly basic in the presence of strong acids, such as sulfuric acid. For example, with methanol:

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Alcohols can also undergo oxidation to form aldehydes, ketones or organic acids, or they can be dehydrated to form alkenes. They can react to form de-esters and can undergo (if first activated) nucleophilic substitution reactions.

Toxicity

Ethanol has been consumed by humans since prehistoric times in the form of alcoholic beverages, for a variety of reasons: hygienic, food, medicinal, religious, recreational. Although the rare consumption of ethanol in small quantities does not have negative effects, on the contrary, larger doses lead to the state called "intoxication" or intoxication and, depending on the dose and the regularity of consumption, can cause acute respiratory problems or death, and chronic ingestion has serious medical repercussions.

Other alcohols are much more poisonous than ethanol, largely because they take longer to metabolize, and not infrequently their metabolism results in more toxic substances. Methanol, or wood alcohol, for example, is oxidized by enzymes in the liver and leads to the creation of formaldehyde, which can cause blindness or death.

An effective treatment to prevent formaldehyde toxicity after methanol ingestion is the administration of ethanol. This will prevent the conversion of methanol to formaldehyde, and the existing formaldehyde will be converted to formic acid and excreted before it can cause any harm.

Preparation of alcohols

Laboratory

There are several methods for preparing alcohols in the laboratory.

-Aliphatic halogenated compounds react with sodium hydroxide or potassium hydroxide, thus obtaining alcohols
-Aldehydes or ketones are reduced with sodium borohydride or lithium hydroborate to alcohols.
-Alkenes add water, in an acidic environment, using concentrated sulfuric acid as a catalyst.

Formation of a secondary alcohol by reduction and hydration:

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Industry

Industrially, alcohols are obtained in several ways.
by fermentation using glucose produced by sugar, obtaining starch hydrolysis, in the presence of yeast and a temperature below 37 °C to produce ethanol.
By direct hydration: using ethene or other alkenes, with a phosphoric acid catalyst, at high temperature and pressure.
Methanol is produced from water vapour: it is created from synthesis gas, where carbon monoxide and two equivalents of hydrogen are combined to produce methanol, using copper, zinc oxide and aluminum oxide as catalysts at 250 °C and a pressure of 50 -100 atm.

Reactions of alcohols

Deprotonation

Alcohols can behave as weak acids, undergoing deprotonation. The proton transfer reaction to produce an alkoxide salt is done with either a strong base, such as sodium or n-butyllithium hydride, or with sodium or potassium.

2 R-OH + 2 NaH → 2 R-O-Na+ + H2↑

2 R-OH + 2Na → 2R-O−Na+

Water is similar in pKa to many alcohols, so with sodium hydroxide an equilibrium usually shifted to the left occurs:

R-OH + NaOH⇔ R-O-Na+ + H2O (balance to the left)

However, it should be noted that the bases used in the deprotonation of alcohols are strong. The bases used and the alkoxides obtained are chemical reactants very sensitive to moisture.