The hydroxyl group (OH) It is one that has an oxygen atom and resembles 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 do so 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. Due to 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 an 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.
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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-). However, both have a molecular linear geometry (but not electronic).
Obviously this is due to the fact that single 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, since the water molecule is H-O-H and knowing that it is angular, changing H for R or Ar originates R-O-H or Ar-O-H. Here, the exact region involving the three atoms is of angular molecular geometry, but that of the two O-H atoms is linear.
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 increases in the structure of the compound, 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 gives rise to 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 stay together rather than dissolve in a certain solvent..
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 forces, 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- => R-O- + HtwoOR
This occurs when a basic solution is added to an alcohol. Here the alkoxide ion (RO-) immediately associates with a positive ion in solution; that is, the Na cation+ (Scab).
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.
Also, 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..
The OH group accepts protons only in highly acidic media, leading to the following reaction:
R-OH + H+ => R-OtwoH+
In this expression H+ is an acidic proton donated by a highly acidic species (HtwoSW4, 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 R-O bondtwoH+, resulting in the release of water. For this reason it is known as a dehydration reaction, since alcohols in acidic media release liquid water..
What comes next? The formation of what are known as alkenes (RtwoC = CRtwo or RtwoC = CHtwo).
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 (CH3CHtwoCHtwoOH).
They are generally miscible liquids with water because they can form hydrogen bonds between their molecules.
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 (C6H5OH):
The phenolic ring can be part of a larger structure, as in the amino acid tyrosine.
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..
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