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In 1865, the British surgeon Dr. Joseph Lister did something quite radical - he used phenol, then known as carbolic acid, to sterilise a young boy's compound fracture during surgery. The treatment was a success, and Lister managed to prevent infection throughout the entire healing process. He advocated for the use of carbolic acid-soaked dressings to treat wounds and encouraged all…
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Jetzt kostenlos anmeldenIn 1865, the British surgeon Dr. Joseph Lister did something quite radical - he used phenol, then known as carbolic acid, to sterilise a young boy's compound fracture during surgery. The treatment was a success, and Lister managed to prevent infection throughout the entire healing process. He advocated for the use of carbolic acid-soaked dressings to treat wounds and encouraged all of his fellow surgeons to wash their hands in a solution of 5% carbolic acid before and after surgery. Lister's antiseptic techniques managed to reduce the mortality rate from infectious diseases after operations to a third of their previous levels. As a result, he is widely regarded as the father of modern surgery. In this article, we're going to explore the molecule that led to Lister's fame: Phenol.
Phenol (systematically known as hydroxybenzene) is an aromatic organic compound made up of the phenyl group (C6H5-) joined to a hydroxyl group (-OH).
Phenol is an example of a benzene derivative. Benzene derivatives are formed when we take a benzene molecule and replace one or more of its hydrogen atoms with different substituents. Removing a hydrogen atom from benzene forms the phenyl group (C6H5-); the replacement substituent determines the identity of the benzene derivative. In phenol, we replace benzene's hydrogen atom with the hydroxyl group (-OH).
Like all benzene derivatives, phenol still contains benzene's characteristic ring of delocalised electrons. A benzene ring is also known as the arene functional group. This enables phenol to act similarly to benzene. However, it behaves in its own unique way thanks to its hydroxyl group. We'll explore this in more detail when we look at the properties and reactions of phenol
Unfamiliar with benzene? It is probably the simplest and most well-known example of an aromatic compound. We recommend that you learn about this molecule, its structure, and its typical reactions, as they set you up well for the rest of aromatic chemistry. Aromatic Compounds is a good starting point - check the article out for all that you need to know about benzene.
As we mentioned, phenol consists of a phenyl group (C6H5-) bonded to a hydroxyl group (-OH). Put the two together, and you end up with the formula for phenol: C6H5OH.
To show how phenol relates to benzene, we've shown their two structures below.
The term phenolic has two different meanings.
Now that we know what phenol is, we can look at its properties. Importantly, we can look at the interactions between its two functional groups (the phenyl group and the hydroxyl group) and how they set phenol apart from similar molecules.
You might know from articles such as Alcohols that molecules with the hydroxyl functional group (-OH) can form hydrogen bonds with each other. This is because of the large difference in electronegativity between oxygen and hydrogen. Hydrogen bonds are the strongest type of intermolecular force and require a lot of energy to overcome. As a result, molecules that experience hydrogen bonding have high melting and boiling points.
Head over to Intermolecular Forces to learn more about hydrogen bonding.
For example, compare methylbenzene (C6H5CH3) and phenol. Both have the same number of electrons but very different boiling points. This is because phenol experiences hydrogen bonding, thanks to its -OH group, whereas methylbenzene does not.
Species | Methylbenzene | Phenol |
Formula | C6H5CH3 | C6H5OH |
Number of electrons | 50 | 50 |
Boiling point (°C) | 111 | 182 |
Many short-chain molecules with the hydroxyl functional group are soluble in water. This is because they can form hydrogen bonds with H2O molecules. For example, ethanol (C2H5OH) and propanol (C3H7OH), both examples of alcohols, readily form aqueous solutions. But phenol, like other longer-chain alcohols, is only slightly soluble in water. This is because its non-polar phenyl group disrupts the hydrogen bonding between its hydroxyl group and water molecules, weakening the attraction between them.
Phenol is weakly acidic. It loses a proton from its hydroxyl group when in aqueous solutions, forming a phenoxide ion (C6H5O-).
Ethanol (C2H5OH) and water (H2O) both contain the hydroxyl group too, and like phenol, they're slightly acidic. But how do the acidities of the three molecules compare? It is all to do with the stability of the negative ion formed when they give up a proton in solution. Phenol ionises into phenoxide ions, water ionises into hydroxide ions, and ethanol ionises into ethoxide ions. The more stable the resulting ion, the more acidic the molecule.
Phenol, like many benzene derivatives, takes part in electrophilic substitution reactions. These reactions swap one of the hydrogen atoms in the phenyl group's aromatic benzene ring for a different atom or group. However, substitution reactions aren't random, and the hydroxyl group plays a part in determining which hydrogen atom is replaced. The hydroxyl group encourages electrophiles to attack certain carbon atoms in the benzene ring; this is known as a directing effect.
In particular, hydroxyl groups are electron-donating groups. If we count phenol's C-OH bonded carbon as number 1 in the aromatic carbon ring, the hydroxyl group encourages electrophiles to attack carbons 2, 4, and 6. This means that substituents tend to bond to these carbons, replacing a hydrogen atom.
Before we learn about the reactions of phenol, it is helpful to understand how we produce it.
For your exams, you need to know about producing phenol from aromatic amines. This is a three-step procedure, although the first two steps happen in situ. Briefly put, we react phenylamine (C6H5NH2) with nitrous acid (HNO2) to produce an unstable diazonium salt. The salt then decomposes into phenol. Here's the reaction.
Our second reactant, besides phenylamine, is nitrous acid. Nitrous acid (HNO2; also known as nitric(III) acid) is a very unstable acid and so needs making in situ by mixing sodium nitrite (NaNO2) with an acid (typically hydrochloric acid, HCl). We carry out this step in an ice bath, keeping the temperature below 10 oC. The reaction also forms a salt, depending on the acid used, but here we have simply represented the acid using H+:
NaNO2 + H+ → HNO2 + Na+
In next step of the reaction, the nitrous acid reacts with phenylamine and an acid. The reaction is once again cooled to below 10 oC using an ice bath. Overall, we produce a diazonium salt and water.
$$C_{6}H_{5}NH_{2} + HNO_{3} + H^{+} \rightarrow C_{6}H_{5}N^{+}\equiv N + 2H_{2}O$$
Diazonium salts aren't very stable. Heating the solution we just formed causes the diazonium salt to react with water. It decomposes into phenol (C6H5OH) and nitrogen (N2), and regenerates the acid (H+) used in step 2:
$$C_{6}H_{5}N^{+}\equiv N + H_{2}O \leftarrow C_{6}H_{5}OH + N_{2} + H^{+}$$
Recognise this process? It's the same reaction used at the start of azo compound production. Azo compounds are used as dyes, and you can explore their synthesis in Uses of Amines.
Making phenols via phenylamine and diazonium salts isn't very economically viable, and so alternate methods tend to be used in industry. These include:
Let's conclude by introducing you to the reactions of phenol. We can roughly break them down into two categories:
Phenol's hydroxyl group reacts in much the same way as the hydroxyl group in water or alcohols. That's handy for you as a student, because it means you have fewer reactions to learn!
This isn't an exhaustive list of the reactions involving phenol's hydroxyl group. Other reactions include esterification, in which phenol reacts with an acid derivative to form an ester. Check your exam specification to find out which reactions you need to know about.
Phenol also takes part in some of the typical reactions of benzene, thanks to its aromatic phenyl group. These include electrophilic substitution reactions such as bromination.
You might notice that the conditions for the nitration and bromination of phenol are a lot milder than the conditions for the respective reactions with benzene. You can find out why when you explore all of the phenol's above reactions in more depth in the article Reactions of Phenol. The production of azo compounds is also discussed in more detail in Uses of Amines, as we mentioned earlier on.
Here's a mind-map summarising the reactions of phenol.
Phenol (systematically known as hydroxybenzene) is an aromatic organic compound made up of the phenyl group (C6H5-) joined to a hydroxyl group (-OH).
One way of making phenol is by reacting phenylamine with nitrous acid (HNO2) at temperatures under 10 oC. The nitrous acid must be made in situ using sodium nitrite (NaNO2) and an acid. We then heat the solution formed to create phenol. However, a more common method of phenol production is the Cumene process, which is used to make 95% of benzene in industry.
Phenol is toxic. Prolonged exposure can cause burns as well as damage to the lungs, eyes, kidneys, and liver. Phenol's toxicity is due to its ability to denature proteins, which also makes it a good antiseptic. However, there is no evidence that phenol is a carcinogen.
Phenol is used in plastics, detergents, a wide range of pharmaceuticals (such as aspirin), and as an antiseptic.
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