HYDROXYL (OH): STRUCTURE, ION AND FUNCTIONAL GROUPS - CHEMISTRY - 2025

The hydroxyl group (OH) is one that has an oxygen atom and is similar to a water molecule. It can be found as a group, an ion or a radical (OH ^·). In the world of organic chemistry, it forms a bond essentially with the carbon atom, although it can also bond with sulfur or phosphorus.

On the other hand, in inorganic chemistry it participates as a hydroxyl ion (more specifically hydroxide or hydroxyl ion). That is, the type of bond between this and the metals is not covalent, but ionic or coordination. Because of this, it is a very important "character" that defines the properties and transformations of many compounds.

As can be seen in the image above, the OH group is linked to a radical denoted with the letter R (if it is alkyl) or with the letter Ar (if it is aromatic). In order not to distinguish between the two, it is sometimes represented linked to a “wave”. Thus, depending on what is behind that "wave", we speak of one organic compound or another.

What does the OH group contribute to the molecule to which it binds? The answer lies in their protons, which can be "snatched up" by strong bases to form salts; they can also interact with other surrounding groups through hydrogen bonds. Wherever it is, it represents a potential water-forming region.

Structure

What is the structure of the hydroxyl group? The water molecule is angular; that is, it looks like a boomerang. If they "cut" one of its ends -or what is the same, remove a proton- two situations can occur: the radical (OH ^·) or the hydroxyl ion (OH ^-) is produced. However, both have a molecular linear geometry (but not electronic).

Obviously this is due to the fact that the simple bonds orient two atoms to stay aligned, but the same does not happen with their hybrid orbitals (according to the valence bond theory).

On the other hand, being the water molecule HOH and knowing that it is angular, changing H for R or Ar originates ROH or Ar-OH. Here, the exact region involving the three atoms is of angular molecular geometry, but that of the two OH atoms is linear.

Hydrogen bonds

The OH group allows the molecules that possess it to interact with each other through hydrogen bonds. By themselves they are not strong, but as the number of OH in the structure of the compound increases, their effects multiply and are reflected in the physical properties of the compound.

Since these bridges require their atoms to face each other, then the oxygen atom of one OH group must form a straight line with the hydrogen of a second group.

This causes very specific spatial arrangements, such as those found within the structure of the DNA molecule (between nitrogenous bases).

Likewise, the number of OH groups in a structure is directly proportional to the affinity of water for the molecule or vice versa. What does it mean? For example, although sugar has a hydrophobic carbon structure, its large number of OH groups make it very soluble in water.

However, in some solids the intermolecular interactions are so strong that they "prefer" to stick together rather than dissolve in a certain solvent.

Hydroxyl ion

Although the ion and the hydroxyl group are very similar, their chemical properties are very different. The hydroxyl ion is an extremely strong base; that is, it accepts protons, even by force, to become water.

Why? Because it is an incomplete water molecule, negatively charged, and eager to complete with the addition of a proton.

A typical reaction to explain the basicity of this ion is the following:

R-OH + OH ^- => RO ^- + H ₂ O

This occurs when a basic solution is added to an alcohol. Here the alkoxide ion (RO ^-) associates immediately with a positive ion in the solution; that is, the Na ⁺ cation (RONa).

As the OH group does not need to be protonated, it is an extremely weak base, but as can be seen in the chemical equation, it can donate protons, although only with very strong bases.

Likewise, it is worth mentioning the nucleophilic nature of OH ^-. What does it mean? Since it is a very small negative ion, it can travel rapidly to attack positive nuclei (not atomic nuclei).

These positive nuclei are atoms of a molecule that suffer from an electronic deficiency due to their electronegative environment.

Dehydration reaction

The OH group accepts protons only in highly acidic media, leading to the following reaction:

R-OH + H ⁺ => RO ₂ H ⁺

In this expression H ⁺ is an acidic proton donated by a very acidic species (H ₂ SO ₄, HCl, HI, etc.). Here a water molecule is formed, but it is linked to the rest of the organic (or inorganic) structure.

The positive partial charge on the oxygen atom causes the weakening of the RO ₂ H ⁺ bond, resulting in the release of water. For this reason it is known as the dehydration reaction, since alcohols in acidic media release liquid water.

What comes next? The formation of what are known as alkenes (R ₂ C = CR ₂ or R ₂ C = CH ₂).

Functional groups

Alcohols

The hydroxyl group by itself is already a functional group: that of alcohols. Examples of this type of compound are ethyl alcohol (EtOH) and propanol (CH ₃ CH ₂ CH ₂ OH).

They are generally liquid miscible with water because they can form hydrogen bonds between their molecules.

Phenols

Another type of alcohols are aromatics (ArOH). Ar denotes an aryl radical, which is nothing more than a benzene ring with or without alkyl substituents.

The aromaticity of these alcohols makes them resistant to acid proton attacks; in other words, they cannot be dehydrated (as long as the OH group is directly attached to the ring).

This is the case of phenol (C ₆ H ₅ OH):

The phenolic ring can be part of a larger structure, as in the amino acid tyrosine.

Carboxylic acids

Finally, the hydroxyl group constitutes the acid character of the carboxyl group present in organic acids (-COOH). Here, unlike alcohols or phenols, OH itself is very acidic, its proton being donated to strong or slightly strong bases.

References

1. Helmenstine, Anne Marie, Ph.D. (February 7, 2017). Definition of Hydroxyl Group. Taken from: thoughtco.com
2. Wikipedia. (2018). Hydroxy group. Taken from: en.wikipedia.org
3. The Biology Project. (August 25, 2003). Hydroxyl Amino Acids. Department of Biochemistry and Molecular Biophysics University of Arizona. Taken from: biology.arizona.edu
4. Dr. JA Colapret. Alcohols. Taken from: colapret.cm.utexas.edu
5. Quimicas.net (2018). The Hydroxyl Group. Recovered from: quimicas.net
6. Dr. Ian Hunt. Dehydration of Alcohols. Department of Chemistry, University of Calgary. Taken from: chem.ucalgary.ca