alcohols

Chemistry

2022

We explain what alcohols are, their classification, nomenclature and properties. Also, examples and importance in the industry.

Alcohols have one or more hydroxyl groups attached to a carbon atom.

What are alcohols?

Alcohols are true chemical compounds organic, which have in their structure one or more hydroxyl chemical groups (-OH) covalently linked still atom of saturated carbon (that is, with single bonds only to the adjacent atoms), forming a carbinol group (-C-OH).

Alcohols are compounds very common organic nature, who play important roles in organisms living organisms, especially in organic synthesis.

Its name comes from the Arabic al-kukhūl, which literally translates to "spirit" or "distilled liquid." This is because ancient Muslim alchemists called alcohols "spirit" and further perfected the methods of distillation in the 9th century. Later studies allowed to know the chemical nature of these compounds, especially the contributions of Lavoisier regarding the fermentation of the yeast of beer.

Alcohols can be toxic and even lethal to the human body if ingested in high doses. Furthermore, when consumed by the human being, can act as depressants of the Central Nervous System, cause the state of intoxication and provoke a more uninhibited behavior than normal.

On the other hand, alcohols have antibacterial and antiseptic properties that allow their use in chemical industry and in medicine.

Types of alcohols

Alcohols can be classified according to the number of hydroxyl groups that they present in their structure:

Monoalcohols or alcohols. These contain a single hydroxyl group. For example:

Polyalcohols or polyols. They contain more than one hydroxyl group. For example:

Another way to classify alcohols is according to the position of the carbon to which the hydroxyl group is attached, also taking into account how many carbon atoms this carbon is also attached to:

  • Primary alcohols. The hydroxyl group (-OH) is located on a carbon linked in turn to another single carbon atom. For example:

  • Secondary alcohols. The hydroxyl group (-OH) is located on a carbon linked in turn to two other different carbon atoms. For example:

  • Tertiary alcohols. The hydroxyl group (-OH) is located on a carbon linked in turn to three other different carbon atoms. For example:

Nomenclature of alcohols

Like other organic compounds, alcohols have different names, which we will explain below:

  • Traditional method (non-systemic). Attention is paid, first of all, to the carbon chain to which the hydroxyl (generally an alkane) adheres, to rescue the term with which it is named, prepend the word "alcohol" and then add the suffix -ilic instead of -ano. For example:
    • If it is a methane chain, it will be called methyl alcohol.
    • If it is an ethane chain, it will be called ethyl alcohol.
    • If it is a propane chain, it will be called propyl alcohol.
  • IUPAC method. Like the previous method, attention will be paid to hydrocarbon precursor, to rescue its name and simply add the ending -ol instead of -ano. For example:
    • If it is a methane chain, it will be called methanol.
    • If it is an ethane chain, it will be called ethanol.
    • If it is a propane chain, it will be called propanol.

Eventually, it will be necessary to indicate in some way the location of the hydroxyl group in the chain, for which a number is used at the beginning of the name. It is important to note that the longest hydrocarbon chain is always chosen as the main chain and the position of the hydroxyl group should be selected using the fewest numbering possible. For example: 2-butanol.

Physical properties of alcohols

Alcohols are generally liquids colorless that present a characteristic odor, although also, with less abundance, they can exist in solid state. They are soluble in water since the hydroxyl group (-OH) has a certain similarity to the water molecule (H2O), which allows them to form hydrogen bonds. In this sense, the most water-soluble alcohols are those with the lowest molecular mass, that is, those with smaller and simpler structures. As the number of carbon atoms and the complexity of the carbon chain increase, the less soluble the alcohols are in water.

The density of alcohols is greater according to the increase in the number of carbon atoms and the branches of their hydrocarbon chain. On the other hand, the formation of hydrogen bonds not only influences the solubility, but also their melting points Y boiling. The larger the hydrocarbon chain, the more hydroxyl groups it has and the more branches it has, the higher the values ​​of these two properties.

Chemical properties of alcohols

Alcohols have a dipole character, similar to that of Water, due to its hydroxyl group. This makes them polar substances (with a positive and a negative pole).

Because of this, alcohols can behave like acids or as bases depending on which reagent they react with. For example, if an alcohol is reacted with a strong base, the hydroxyl group deprotonates and the oxygen retains its negative charge, acting like an acid.

On the contrary, if an alcohol is faced with a very strong acid, the electronic pairs of the oxygen cause the hydroxyl group to protonate, acquire a positive charge and behave as a weak base.

On the other hand, alcohols can participate in the following chemical reactions:

  • Halogenation Alcohols react with hydrogen halides to give alkyl halides and water. Tertiary alcohols react more easily than primary and secondary alcohols. Some examples of these reactions are:

  • Oxidation.Alcohols are oxidized by reacting with certain oxidizing compounds, forming different products depending on the type of alcohol that is oxidized (primary, secondary or tertiary). For example:
    • Primary alcohols. They occur if, when oxidized, they lose a hydrogen atom that is attached to carbon, which in turn is attached to the hydroxyl group, they form aldehydes. On the other hand, if they lose the two hydrogen atoms from this carbon, they form carboxylic acids.
    • Secondary alcohols. As they oxidize, they lose the only carbon-bonded hydrogen atom that has the hydroxyl group and form ketones.
    • Tertiary alcohols. They are resistant to oxidationThat is, they do not oxidize, unless very specific conditions are imposed on them.
  • Dehydrogenation Alcohols (only primary and secondary) when subjected to high temperatures and in the presence of certain catalysts, they lose hydrogens to form aldehydes and ketones.
  • Dehydration It consists of adding a mineral acid to an alcohol to extract the hydroxyl group and obtain the corresponding alkene through elimination processes.

Importance of alcohols

Alcohol is used to make biofuels along with other organic substances.

Alcohols are substances of great chemical value. What raw material, are used to obtain other organic compounds in laboratories. Also as a component of everyday industrial products, such as disinfectants, cleaners, solvents, perfume base.

They are also used in the manufacture of fuels, especially in the food industry biofuels, alternative to those of fossil origin. It is common to see them in hospitals, first aid kits or the like.

On the other hand, certain alcohols are for human consumption (especially ethanol), part of many spirits in different degrees of refinement and intensity.

Examples of alcohols

Some examples of alcohols widely used on a daily basis are:

  • methanol or methyl alcohol (CH3OH)
  • ethanol or ethyl alcohol (C2H5OH)
  • 1-propanol, propanol, or propyl alcohol (C3H7OH)
  • isobutanol (C4H9OH)
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