6 Characteristics of Alcohols

Alcohols are organic compounds that have the hydroxyl group (OH) attached to one or more saturated carbon atoms.

If it were just one OH group attached to one carbon, we have a monoalcohol, but if it is two or more OH groups attached to carbon atoms, then we have polyalcohols.

Due to this type of structure, alcohols present some very important physical properties for their use in some areas, among them, the use of ethanol as a gasoline additive, helping to reduce polluting emissions released from the burning of this fossil fuel.

Main Properties or Characteristics of Alcohols

Intermolecular force

Alcohol molecules attract each other through hydrogen bonds : the most intense type of intermolecular force that exists.

Hydrogen bonds occur when a hydrogen atom joins an atom of fluorine, oxygen, or nitrogen, which are strongly electronegative elements. In the case of alcohols, hydrogen bonds with oxygen.

This force of molecular interaction of alcohols explains other of their properties, such as solubility, polarity, and melting and boiling points.

Melting and boiling points

They are high, since the hydrogen bonds that the alcohol molecules make with each other are very intense electrostatic forces. Therefore , it takes a lot of energy to break those bonds.

Monoalcohols have lower boiling points than polyalcohols, since the more OH groups there are, the more hydrogen bonds there will be.

An interesting aspect is that when 95% ethanol is mixed with 5% water, an azeotropic mixture is formed, which means that it behaves like a pure substance at the time of boiling, since the boiling temperature is maintained. constant at 78.15 ºC, at sea level, until the entire mixture passes into the gaseous state.

The boiling points of water and ethanol separated are, respectively, 100ºC and 78.3ºC at sea level.

It is not possible to separate this mixture by means of simple distillation; For this, a chemical process is necessary, in which virgin lime (CaO) is added, which reacts with water, forming extinct lime, which is insoluble in ethanol. Then it’s a matter of making a filtration.

Polarity

Alcohols have a polar part of the molecule (the part that has the OH group) and another non-polar part (the carbon chain).

Molecules that have few carbon atoms in the chain tend to be polar. But as the carbon chain increases, it tends to be non-polar. In addition to that, polyalcohols are more polar than monoalcohols.

Solubility

Short chain alcohols, which have a greater polar tendency, are quite soluble in water, because their molecules make hydrogen bonds with water molecules.

As the size of the carbon chain increases and the non-polar tendency increases, alcohols become insoluble. Monoalcohols with 4 or 5 carbons in the chain are practically insoluble in water.

However, polyalcohols have more hydroxyls that form hydrogen bonds with water molecules. Thus, although they have a larger carbon chain, the more hydroxyls the polyalcohol has, the more soluble it is in water.

Since the ethanol shown in the previous item has a polar and a non-polar part, it dissolves both in water, which is polar, and in gasoline, which is non-polar. That is why ethanol can be used as an additive in gasoline.

In addition to that, fuel ethanol has a part of water in its composition. The 70% ethyl alcohol used as an antiseptic and disinfectant is 70% ethanol and 30% water. Ethanol is very soluble in water due to hydrogen bonds.

Physical state

Monoalcohols of 12 carbons or less are liquids; Above that number they are solid. Polyalcohols with 5 carbons or less are liquids and with 6 carbons or more are solids.

The viscosity of alcohols increases if the number of hydroxyls increases.

Density

Most monoalcohols are less dense than liquid water. To cite an example, the density of alcohol is 0.79 g/cm 3 while that of water is greater (1.0 g/cm 3 ).

For comparison, the density of ice is 0.92 g/cm 3 , denser than alcohol, however less dense than water. That’s why an ice cube floats on the surface of water, but sinks in some alcoholic drink.

Classification of Alcohols

The classification of alcohols depends on the position of the hydroxyl

Primary alcohols

They present their hydroxyl attached to a carbon at the end of the chain, possessing the characteristic group – CH 2 OH.

Secondary alcohols

They present their hydroxyl attached to a secondary carbon of the chain, having the characteristic group – CHOH.

Tertiary alcohols

They present their hydroxyl attached to a tertiary carbon, possessing the –COH group.

Normal chain primary and saturated alcohols with up to eleven carbons are colorless liquids; the others are solid.

Alcohols with up to three carbons have a pleasant aroma and as the carbon chain increases, these liquids become viscous, so that above eleven carbons, they become odorless solids similar to paraffin.

Applications

Methanol (methyl alcohol)

Formula H 3 C ─ OH, produced on an industrial scale from carbon and water, they are used as solvents in many reactions and as raw materials in polymers.

Glycerol

Viscous, colorless and sweet liquid, it is obtained through saponification (reaction that produces soap) of the esters that constitute oils and fats. Used in the manufacture of inks, cosmetics and in the preparation of nitroglycerin (explosive).

Ethanol (ethyl alcohol)

It is used as a solvent in the production of alcoholic beverages, in the preparation of acetic acid, ether , inks, perfumes and as automobile fuel.

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