What is a hydroxyl group?

In chemistry, a hydroxyl group is a group of atoms formed by an oxygen atom with two unshared pairs of electrons; on the one hand it is linked to a hydrogen atom by means of a single covalent bond, and on the other it can be linked to a carbon chain in an organic compound, to some other non-metal (for example, sulfur, nitrogen, etc.) , or it may not be bonded to any other atom but have an unpaired electron.

The word hydroxyl literally means a radical made up of hydrogen and oxygen. Here, the word radical can be used, in the context of organic chemistry, to refer to an atom or, in this case, a group of atoms that replaces a hydrogen in a hydrocarbon. On the other hand, it can also refer to a free radical with an electron deficient oxygen that has one unpaired electron.

In some cases, the hydroxyl group is confused with the hydroxide anion. This is a very common mistake, but there are very important differences between one and the other. The most notorious of all is that, while the hydroxyl group is an electrically neutral group, the hydroxide is an anion (that is, it has a negative charge). On the other hand, the hydroxyl free radical is a very reactive and unstable chemical species while the hydroxide anion is reactive, but not so much.

The following figure shows the different forms in which you can find spices made up of hydrogen and oxygen.

To avoid confusion, from now on we will refer to the hydroxyl group as the central structure of the previous figure, that is, as part of a molecule in which oxygen is directly linked to a carbon chain or to another non-metal.

Properties of the hydroxyl group

is a polar group

Because oxygen is more electronegative than hydrogen, the covalent bond between these two atoms is polarized, with the partial negative charge on the oxygen atom. This makes most organic compounds that have a hydroxyl group, such as alcohols, polar compounds.

Can form hydrogen bonds

The polarity of the OH bond means that the hydroxyl group can act as a hydrogen donor in a hydrogen bond. In addition, the oxygen of the hydroxyl group has two lone pairs of electrons, so it can also receive two hydrogen bonds as an acceptor. In other words, the hydroxyl group can form a total of three simultaneous hydrogen bonds.

It is a Brønsted-Lowry acid.

Again because of the polarity of the OH bond, and also because oxygen has a good ability to bear a formal negative charge by losing the hydrogen, the hydroxyl groups can give up the proton by acting like a Brønsted-Lowry acid.

The particular pKa value or acidity of the hydroxyl group will depend on the structure of the rest of the molecule to which it is attached. If –OH is directly attached to a carbonyl group (as in carboxylic acids), then it will be highly acidic, with pKa values ​​in the order of 3 to 5. If it is attached to an aromatic group, as in the case of phenols, their pKa will be in the order of 7 to 10; if it is linked to an aliphatic chain, its pKa will be 15 or more.

Can act as a Lewis base

The fact that the OH group has two unpaired pairs of electrons means that it can also act as a base, donating an electron pair to a proton or some other electron-deficient species (Lewis acid). Simply put, it can be protonated by a strong enough acid.

Functional groups that have a hydroxyl group

The hydroxyl group by itself is not a functional group, as it depends on what it is attached to. In the case of organic compounds, the most common functional groups that have hydroxyl groups are:

alcohols

Alcohols are the simplest functional group that possesses a hydroxyl group. In this case, the oxygen is directly bonded to a saturated aliphatic carbon. Alcohols are generally represented as follows:

where R represents an alkyl group.

These are polar compounds, most are soluble in water, and are liquid at room temperature.

enols

The main difference between an alcohol and an enol is that in the second case the hydroxyl group is attached to an unsaturated carbon atom with sp ² hybridization and that it is attached to another carbon by a double covalent bond, as shown in the following figure. .

This double bond stabilizes the conjugate base by resonance, so enols are usually more acidic than alcohols.

phenols

Phenols are very similar to alcohols except that, in this case, the hydroxyl group is attached to a carbon that is part of an aromatic ring.

An example of this type of compound is phenol, which has the following structure:

As in the case of enols, the aromatic ring is able to stabilize a negative charge on oxygen by means of resonance, so phenols are always considerably more acidic than alcohols.

Carboxylic acids

Carboxylic acids or organic acids have a hydroxyl group linked to a carbonyl.

The presence of the carbonyl double bond stabilizes the conjugate base by resonance after losing the proton. But, in addition to this, it distributes this negative charge between the two oxygens, which is much more favorable than distributing it on carbons, as happens in the two previous cases. This gives these hydroxyl groups greater acidity than in the other cases; for this reason these compounds are called acids.

sulfonic acid

This is an example of a functional group that has a hydroxyl group, but in which it is not bonded to a carbon but to another heteroatom, in this case, sulfur.

The multiple resonance structures mean that compounds that possess this functional group are also acidic in character.

References

Carey, F., & Giuliano, R. (2014). Organic Chemistry (9th ^ed .). Madrid, Spain: McGraw-Hill Interamericana de España SL

Functional Groups and Organic Nomenclature . (2020, October 29). Retrieved from https://espanol.libretexts.org/@go/page/2313

Alcohols and Ethers . (nd). (2021, January 9). Retrieved from https://espanol.libretexts.org/@go/page/1973

Smith, MB, & March, J. (2001). March’s Advanced Organic Chemistry: Reactions, Mechanisms, and Structure, 5th Edition (5th ed.). Hoboken, NJ: Wiley-Interscience.