StudySmarter: Study help & AI tools
4.5 • +22k Ratings
More than 22 Million Downloads
Free
|
|
Phenol

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 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.

Mockup Schule Mockup Schule

Explore our app and discover over 50 million learning materials for free.

Illustration

Lerne mit deinen Freunden und bleibe auf dem richtigen Kurs mit deinen persönlichen Lernstatistiken

Jetzt kostenlos anmelden

Nie wieder prokastinieren mit unseren Lernerinnerungen.

Jetzt kostenlos anmelden
Illustration

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 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.

  • This article is an introduction to phenol in organic chemistry.
  • We'll start by looking at what exactly phenol is, including its structure and formula.
  • We'll then discuss its properties, such as its melting and boiling points, acidity, and directing effects.
  • After that, we'll learn about the production of phenol.
  • To finish, we'll provide you with an overview of the reactions of phenol. These include bromination, chlorination, and nitration.

What is 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.

Phenol structure and formula

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.

Phenol benzene phenol structure functional groups StudySmarterFig. 1: The structures of benzene (left) and phenol (right), with their functional groups highlighted.StudySmarter Originals

Phenolic meaning

The term phenolic has two different meanings.

  1. It more simply refers to any aromatic compound, like phenol itself, where one or more of the aromatic ring's hydrogen atoms has been replaced by the hydroxyl group. To make life confusing, all of these compounds can also just be called phenols! For example, an aromatic benzene ring with hydroxyl groups instead of hydrogen atoms attached to carbons 1 and 3 is a phenolic compound known systematically as 1,3-dihydroxybenzene.
  2. Phenolic can also be used more specifically to refer to synthetic thermosetting resins made by reacting phenol with an aldehyde. These resins are used as adhesives and coatings.

Phenol phenolic compound example 1,3-dihydroxybenzene StudySmarterFig. 2: 1,3-dihydroxybenzene: An example of a phenolic compound, confusingly also known as a type of phenol.StudySmarter Originals

Properties of phenol

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.

Melting and boiling point

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.

SpeciesMethylbenzenePhenol
FormulaC6H5CH3C6H5OH
Number of electrons5050
Boiling point (°C)111182

Solubility

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.

Acidity

Phenol is weakly acidic. It loses a proton from its hydroxyl group when in aqueous solutions, forming a phenoxide ion (C6H5O-).

Phenol acidity ionisation delocalisation StudySmarterFig. 3: Phenol is weakly acidic and so ionises in aqueous solution.StudySmarter Originals

Acidity of phenol, ethanol, and water

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 is more acidic than both ethanol and water. When it ionises, it manages to spread the oxygen atom's negative charge over the whole of the aromatic ring of delocalised electrons, stabilising the phenoxide ion.
  • Water is less acidic than phenol, but more acidic than ethanol. The hydroxide ion formed when water ionises doesn't have any factors influencing its charge distribution or stability.
  • Ethanol is less acidic than both phenol and water. This is because its oxygen atom is bonded to an alkyl group, which are electron-donating. When ethanol ionises, the alcohol group pushes the oxygen atom's negative charge away from itself and increases the oxygen's charge density, making the ethoxide ion less stable.

Phenol relative acidity phenol water ethanol StudySmarterFig. 4: The relative acidities of phenol, water, and ethanol. Phenol is the most acidic because it forms the most stable ion when it gives up a proton in solution.StudySmarter Originals

Directing effects

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.

Phenol directing effects hydroxyl group StudySmarterFig. 5: The directing effects of phenol's hydroxyl group favour substitution of the hydrogen atoms bonded to carbons 2, 4, and 6.StudySmarter Originals

Production of phenol

Before we learn about the reactions of phenol, it is helpful to understand how we produce it.

Production of phenol using phenylamine

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.

Forming nitrous acid

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+

Forming a diazonium salt

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.

$$C_{6}H_{5}NH_{2} + HNO_{3} + H^{+} \rightarrow C_{6}H_{5}N^{+}\equiv N + 2H_{2}O$$

Forming phenol

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.

Other methods of phenol production

Making phenols via phenylamine and diazonium salts isn't very economically viable, and so alternate methods tend to be used in industry. These include:

  • The Cumene process, which reacts cumene (C6H5CH(CH3)2; also known as 1-(methylethyl)benzene or propan-2-ylbenzene) with oxygen. This method accounts for about 95% of phenol production worldwide.
  • The direct oxidation of benzene. Although theoretical, this method isn't yet industrially profitable using oxygen gas by itself. However, nitrogen oxide (NO2) shows potential as a cheap and more efficient oxidising agent.

Reactions of phenol

Let's conclude by introducing you to the reactions of phenol. We can roughly break them down into two categories:

  • Reactions involving the hydroxyl group (-OH).
  • Reactions involving the phenyl group (C6H5-).

Reactions involving the hydroxyl group

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!

  • Phenol's hydroxyl group is weakly acidic and so reacts with strong bases to produce a phenoxide salt and water. For example, reacting phenol with sodium hydroxide (NaOH) produces sodium phenoxide (C6H5O-Na+) and water. Phenoxide salts dissolve readily in aqueous solution.
  • Phenol's hydroxyl group also reacts with sodium itself, or in fact any other reactive metal. This forms a type of phenoxide salt once again, but this time the additional product is hydrogen gas (H2). For example, reacting sodium metal with phenol produces sodium phenoxide and hydrogen.

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.

Reactions involving the phenyl group

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.

  • Phenol reacts with dilute nitric acid (HNO3) at room temperature to form nitrophenol and water. Increasing the concentration of the acid causes multiple substitutions to occur. This is an example of nitration.
  • Phenol also reacts with bromine water (Br2) at room temperature to form a tribromophenol, in which three of phenol's hydrogen atoms have been substituted for bromine atoms. A similar reaction occurs if you use chlorine or iodine instead. This is an example of halogenation.
  • Phenol also reacts with diazonium salts in alkaline conditions to form azo compounds.

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 reactions of phenol mind-map StudySmarterFig. 6: A mind-map of the reactions of phenol. We've highlighted the changes in structure of the molecules formed compared to phenol itself.StudySmarter Originals

Phenol - Key takeaways

  • Phenol (systematically known as hydroxybenzene) is an aromatic organic compound made up of the phenyl group (C6H5-) joined to a hydroxyl group (-OH). It contains an aromatic benzene ring and has the formula C6H5OH.
  • Phenol has high melting and boiling points, is slightly soluble in water, and is weakly acidic.
  • Phenol is formed by reacting phenylamine with nitrous acid.
  • Phenol's reactions are characterised by its hydroxyl group and its phenyl group.
    • Phenol's hydroxyl group reacts with strong bases and reactive metals.
    • Phenol's phenyl group takes part in nitration and halogenation reactions. It also reacts with diazonium salts.

Frequently Asked Questions about 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.

Test your knowledge with multiple choice flashcards

What are the products between a base and a phenol?

What are the products between a reactive metal and a phenol?

What are the products between a diazonium ion and a phenol?

Next

Join over 22 million students in learning with our StudySmarter App

The first learning app that truly has everything you need to ace your exams in one place

  • Flashcards & Quizzes
  • AI Study Assistant
  • Study Planner
  • Mock-Exams
  • Smart Note-Taking
Join over 22 million students in learning with our StudySmarter App Join over 22 million students in learning with our StudySmarter App

Sign up to highlight and take notes. It’s 100% free.

Start learning with StudySmarter, the only learning app you need.

Sign up now for free
Illustration

Entdecke Lernmaterial in der StudySmarter-App