What is Alcohol in Chemistry?

by Kate Onissiphorou

Alcohol is one of the prehistoric chemical discoveries of humankind, with the earliest archaeological evidence of winemaking dating back to about 8,000 years ago. An analysis of pottery, pollen and chemical residues excavated from a site in Georgia suggests a Stone Age farming community there was making wine as early as 6,000 BC.

Although prehistoric, Neolithic people didn’t understand the chemical composition and structure of alcohol. They knew how to make alcohol, specifically ethanol, from fermenting grapes. In chemistry, alcohol is defined as an organic compound that has a hydroxyl functional group attached to a saturated carbon.

Aside from ethanol, which has been used mainly for recreational and ceremonial purposes, there are other types of alcohol with a wide range of applications. Some types of alcohol are used as an antiseptic in medicines, for example, while others are used in various industrial processes. 

What are the Different Types of Alcohol?

There’s a wide variety of alcohols, especially ethanol or ethyl alcohol, which is the main active ingredient in alcoholic beverages such as wine, beer, vodka, rum, whisky, gin, and tequila. Although ethanol is the main ingredient, these alcoholic drinks differ in terms of fermentation source, concentration and other ingredients.

Wine, which includes ethanol, being poured

For example, beers are fermented from malted grains and hops. They usually contain relatively low levels of alcohol (typically around 5%). On the other hand, liquors or spirits like gin are fermented from various sources including grains, sugar canes, palm trees and fruits. They contain a higher concentration of ethanol after they’ve been distilled. So what is the strongest alcohol? The strongest commercially available brand of alcoholic drink is Spirytus Rektyfikowany, with a concentration of 96% ethanol by volume. Despite differences in fermentation sources and concentrations, the main active ingredients are the same all alcoholic beverages contain ethanol. 

In chemistry, there are hundreds of chemically distinct alcohols, including isomers of alcohols such as isopropyl alcohol (an isomer of propyl alcohol). However, these alcohols can be classified into three main categories based on the number of alkyl groups attached to the carbon with the hydroxyl group. These classifications apply to alcohols with saturated, unsaturated and cyclic alkyl groups.

Primary alcohols

These alcohols have only one alkyl group attached to the carbon with the hydroxyl group. Examples of primary alcohols include methanol and ethanol.A graphic showing primary alcohols

Secondary alcohols

Secondary alcohols have two alkyl groups attached to the carbon atom, as illustrated below. Some examples of secondary alcohols are propan-2-ol and butan-2-ol alcohols.

A graphic showing secondary alcohols

Tertiary alcohols

Tertiary alcohols have three alkyl groups attached to the carbon atom. Examples of tertiary alcohols include 2-methyl-propan-2-ol and 2-methyl-butan-2-ol.

A graphic showing tertiary alcohols

Alcohols are also classified based on how many hydroxyl groups are attached to the carbon atoms. Another option is to categorise them based on their bond type and shape of the R chains.

  • Monohydric alcohols these are alcohols with just one hydroxyl group. Examples include propan-2-ol (C3H7OH) and pentan-1-ol (C5H11OH).
  • Polyhydric alcohols polyhydric alcohols have two or more hydroxyl groups attached to the main chain. Examples include propane-1,2,3-triol, more commonly known as glycerol [C3H5(OH)3], and pentane-1,2,3,4,5-pentol [C5H7(OH)5].
  • Unsaturated aliphatic alcohols these are alcohols with double bonds in their R chains. Examples include geraniol and propargyl alcohol.
  • Alicyclic alcohols alicyclic alcohols contain one or more non-aromatic carbon rings. Menthol (C10H19OH) is the best example of this.

What is Rubbing Alcohol Used For?

Rubbing alcohol has numerous uses, some of which are outlined below.

Antiseptic

Ethyl alcohol and isopropyl alcohol are the most common alcohols that are commercially sold as rubbing alcohols. They are typically packaged as either 40% or 70% concentration and are mainly intended to be rubbed onto the skin as an antiseptic. Rubbing alcohol can be used as a hand sanitiser when you don’t have access to soap and water.

Surface disinfectant

Rubbing alcohol can kill bacteria, fungi and viruses. Medical instruments, such as forceps and scalpels, can be immersed in rubbing alcohol to disinfect them. You can also use it to clean surfaces like counters and desks. Nonetheless, rubbing alcohol is not as effective as other disinfectants on large surfaces such as floors and bathroom walls. In this instance, it may be better to use chlorinated water.

Surface disinfectant which includes rubbing alcohol

Nausea relief

According to one study, the smell of alcohol can mitigate the symptoms of nausea in postoperative patients. A cotton pad or cotton ball is soaked with alcohol and given to the patient, who then inhales the fumes to alleviate their nausea. 

Astringent

Rubbing alcohol can also be used as an astringent to clean and tighten skin pores. Although you may feel more refreshed after using it, alcohol can cause dry skin. You should therefore apply a moisturiser afterwards.

Muscle ache treatment

Rubbing alcohol on the skin may relieve muscle aches as it serves as a sort of liniment that  improves blood flow to the affected region. The cooling effect can also help to relax tense muscles. However, you should refrain from rubbing alcohol all over your body to avoid any possible neurological problems. This is because large amounts of alcohol can be absorbed by the skin. 

How is Alcohol Made?

One of the most common and oldest ways of making alcohol is through the fermentation of organic sources of sugar (carbohydrates). The alcohol produced by this process is then distilled to increase the concentration. However, this mainly produces ethanol. Other types of alcohol are produced in different ways, as summarised below:

  • Ziegler and oxidation processes Linear alcohols are the result of the reaction between ethylene and triethylaluminiumethanol. This is followed by oxidation and hydrolysis.
  • Hydration Water can be made to react with alkenes with the help of acidic catalysts. Alcohols with low molecular weights are produced in this manner. A less direct approach is to first convert alkenes to sulfate esters and then hydrolyse.
  • Substitution Primary alcohols can be produced this way. Alkyl halides can be induced to react with sodium hydroxide or potassium hydroxide in an aqueous solution. Meanwhile, substitution reactions can also occur with the reaction of Grignard reagents and carbonyl groups. This produces secondary and tertiary alcohols.

What are Alcohols in Organic Chemistry?

Just like other organic compounds, alcohols are defined by their functional group. In this case, the functional group is hydroxyl (-OH), which is attached to an alkyl group. An alkyl group consists of one or more hydroxyl groups attached to a hydrocarbon substituent. 

Alcohol naturally occurs as the biological byproduct of carbohydrate fermentation.

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