In this explainer, we will learn how to describe the physical and chemical properties of phenol.
Phenols are a class of aromatic molecules that contain a hydroxy () group bonded directly to an aromatic ring. The presence of an group suggests that phenols are a type of alcohol. However, the physical and chemical properties of phenols differ from those of aliphatic alcohols.
Often, phenol refers to the simplest hydroxy aromatic molecule where a single hydroxyl group is bonded to a benzene ring. The structure of phenol, also known as carbolic acid or hydroxybenzene, is shown below.
Phenols are of great importance in pharmaceutical industries. For example, salicylic acid (the precursor to aspirin), paracetamol, and vanillin, shown below, are all examples of phenol compounds. Phenolic compounds are commonly used as the starting point for synthesizing a wide array of important compounds.
A class of organic molecules with a hydroxy group bonded directly to an aromatic ring.
Phenol, with the chemical formula , is a white solid that has a smell similar to that of a disinfectant. Care must be taken when handling solid phenol as it can be corrosive to the skin. The melting point of phenol is much higher than those of benzene and methylbenzene (toluene). Phenol is reasonably soluble in water, much more soluble than benzene and methylbenzene. Both these properties of phenol can be mostly attributed to hydrogen bonding that can occur between molecules of phenol, and between molecules of phenol and water.
Example 1: Identifying Phenol from Its Physical Properties
The table shows the physical properties of three aromatic compounds: benzene, phenol, and toluene. Which column corresponds to phenol?
|Melting Point ()||40.5||5.5|
|Boiling Point ()||181.7||80.1||111|
|Appearance||Crystalline solid||Colorless liquid||Colorless liquid|
|Solubility in Water at||8.30||1.80||0.52|
The table shows the properties of three different aromatic compounds. Benzene, phenol, and toluene all contain a benzene ring. However, phenol and toluene contain an additional group. Phenol has a hydroxy () group, while toluene has a methyl () group. The presence of the hydroxy group in phenol suggests the presence of hydrogen bonding between molecules of phenol. The effect of this will be much higher melting and boiling points compared with similar compounds. We would therefore expect the column for phenol to be the one with much higher melting and boiling points. The ability of phenol to form hydrogen bonds with water molecules also increases its solubility. We would therefore expect the solubility of phenol in water to be the greatest out of the three molecules. Using this information, we can see that column A shows the highest boiling point, melting point, and solubility. The high melting point also results in column A’s compound being a solid under normal conditions, which indeed phenol is. We can therefore conclude that column A corresponds to phenol.
Phenol can be synthesized in several ways. The most direct is the fractional distillation of coal tar, a process that separates phenol from cresols (methylphenols). Another possible way is the direct oxidation of benzene; however, this process is not commonly in use.
One popular method for synthesizing phenol is the hydrolysis of halogenated benzene with sodium hydroxide. For example, the reaction of chlorobenzene with sodium hydroxide at and 300 atmospheres of pressure produces sodium phenoxide. The sodium phenoxide can then be treated with to produce phenol and sodium chloride, as shown in the reaction scheme below.
A similar reaction between chlorobenzene and steam, over a silicon catalyst, produces phenol and hydrogen chloride.
Example 2: Determining the Reactants and Reaction Type in the Preparation of Phenol
Phenol can be prepared from a benzene derivative. The scheme shows the products and reaction conditions for the preparation of phenol:
- What are the reactants X and Y?
- Bromobenzene and
- Sodium benzoate and
- Chlorobenzene and
- Bromobenzene and
- Chlorobenzene and
- Which of the following best describes this reaction?
Inspection of the reaction scheme given in the question shows that the reaction produces phenol and sodium chloride. From this, we can assume that the reactants must contain sodium, chlorine, a benzene ring, and a hydroxy group. Using this knowledge, we can exclude options A and D. A does not contain any reactants with chlorine and D does not contain any reactants with sodium. Similarly, option C does not contain sodium, so we can discount this. The reactant in option B, sodium benzoate, actually contains a group connected to the benzene ring. The addition of would likely protonate sodium benzoate, producing benzoic acid and sodium chloride, but not phenol. This leaves option E, chlorobenzene and . One way to produce phenol is the reaction of halogenated benzenes under extreme conditions. This corresponds well to the conditions given in the reaction scheme, and so we can conclude the correct answer to be option E.
Having identified X and Y in the first part of the question to be chlorobenzene and , we are now asked to name this type of reaction. In the reaction, the atom of chlorine on the benzene is replaced by an group, with the formation of also occurring. As the chlorine has been removed, the has not simply been added to the chlorobenzene and therefore we can discount A. Similarly, the loss of chlorine from the benzene ring means that this reaction is not a halogenation, which usually refers to the addition of a halogen to a molecule. This means we can exclude option B. The lack of as a reactant and the fact that the benzene ring is not reduced to a cycloalkene means we can exclude option C, hydrogenation. The final two options refer to the addition (hydrolysis) or removal (condensation) of water during the reaction. As the reaction does not produce water, we can exclude option E, leaving option D as the correct answer. This makes sense as the benzene ring has gained a hydroxy group, essentially a deprotonated water molecule.
Despite the presence of a hydroxy group, phenols differ in their physical and chemical behavior from alcohols. One defining characteristic is how the acidity of phenol differs from that of alcohols.
Phenol is weakly acidic; in an aqueous solution, deprotonation can occur to form a phenoxide anion and a hydronium ion. This is shown below:
Phenol is, however, more acidic than a typical alcohol such as ethanol. This can be explained by the presence of the benzene ring. The acidity of a molecule is related to the stability of the corresponding anion. When phenol loses a hydrogen ion from the hydroxy group, the corresponding phenoxide anion can gain stabilization from the presence of the benzene ring.
Such stabilization does not occur for alcohols such as ethanol, the deprotonation of which is shown below:
This makes phenol a stronger acid than alcohols. However, because the stabilization from the benzene ring is minor, and the majority of charge remains on the oxygen, phenol is only weakly acidic.
The difference in acidity can be observed by the reactions with sodium metal and sodium hydroxide. Both phenol and ethanol react with sodium metal to produce the corresponding anion and hydrogen gas. The equations for these reactions are
For phenol, this reaction produces the phenoxide anion, while for ethanol the ethoxide anion is formed. However, sodium hydroxide will only react with phenol and not appreciably with ethanol:
Example 3: Identifying the Statement That Correctly Compares Phenol with Ethanol
Which of the following correctly compares phenol and ethanol?
- Ethanol is more highly acidic than phenol.
- Phenol can react with hydrochloric acid, while ethanol cannot.
- Phenol can react with sodium hydroxide, while ethanol cannot.
- Both of them can react with hydrochloric acid.
- Both of them can react with sodium hydroxide.
In this question, we are asked to identify which of the given statements correctly describes the difference between phenol and ethanol. Despite both containing a hydroxyl group (), the chemistries of phenol and ethanol can differ. One difference is in their acidity.
The strength of an acid is related to the stability of the corresponding anion. The more stable this is, the stronger the acid. When phenol loses a hydrogen ion, the negative charge on the phenoxide anion can be stabilized somewhat by the benzene ring. However, when ethanol loses a hydrogen ion, the negative charge on the ethoxide anion does not have such stabilization. It is this additional stabilization that makes phenol a stronger acid than ethanol. Therefore, statement A is not correct.
Statements B–E relate to the reactivity of phenol and ethanol with hydrochloric acid and sodium hydroxide. Sodium hydroxide is a strong base and will therefore react with an acid or deprotonate a molecule. Phenol will react with sodium hydroxide to produce sodium phenoxide and water. However, ethanol is not acidic enough to react with sodium hydroxide, and so, no appreciable reaction occurs. This suggests statement C to be correct and statement E to be wrong, but let’s check the other statements.
Statements B and D refer to the reaction of ethanol and phenol with hydrochloric acid. Under the right conditions, ethanol will react with hydrochloric acid to produce a halogenated alkane and water. Phenol does not normally react with hydrochloric acid. This confirms that statements B and D are not correct.
The correct answer is therefore statement C.
Like most hydrocarbons, the combustion of phenol in excess oxygen produces carbon dioxide and water. However, the presence of the benzene ring often means that incomplete combustion occurs, producing both carbon monoxide and carbon soot. This results in phenol burning with a very smoky flame.
The nitration of phenol can also be achieved and is the major source of picric acid, otherwise known as 2,4,6-trinitrophenol. Recall that the hydroxy group has an ortho and para directing effect on where the nitro group substitutes onto the benzene ring.
The reaction between phenol and nitric acid requires concentrated sulfuric acid and varies depending on the concentration of nitric acid. Dilute nitric acid typically results in monosubstitution, whereas concentrated nitric acid is needed to produce the trinitro molecule. The reaction also produces water.
The vast majority of picric acid produced is used for ammunition and explosives. This is not surprising given its similarity to 2,4,6-trinitrotoluene, otherwise known as TNT. Historically, picric acid was used as a yellow dye. More recently, it found use as an antiseptic and even in the treatment of burns. Many first aid boxes contain bandages soaked in picric acid.
Example 4: Determining the Product from the Nitration of Phenol
Fill in the blank: Phenol reacts with concentrated nitric acid in the presence of concentrated sulfuric acid, forming acid.
When reacted with concentrated nitric acid, phenol will form a trisubstituted product. The hydroxy group of phenol directs the substitution of the nitro group to the ortho and para positions of the benzene ring. Therefore, the reaction of phenol with concentrated nitric acid will produce 2,4,6-trinitrophenol, a molecule otherwise known as picric acid. Therefore, the correct answer is D, picric acid.
The reaction of phenol with formaldehyde (methanal) is the starting point for a condensation polymerization that produces a plastic known as Bakelite. The properties of Bakelite made it hugely popular and widely used. Besides being heat-resistant, this polymer is a good electrical insulator, which made it the ideal choice for the coverings of electrical appliances such as radios and phones. The ability to easily mold Bakelite saw it being used in many aspects of our lives from kitchen appliances to children’s toys and even jewelry.
There are many slightly different ways of forming Bakelite. The first step for most is the reaction of one molecule of formaldehyde with two molecules of phenol, usually with an acid or basic catalyst. This condensation reaction produces a phenol formaldehyde molecule with the loss of . The process is then repeated to form the cross-linked polymer known as Bakelite.
How To: Detecting Phenol
There are two tests that can be used to suggest the presence of phenol:
- The first of these is the reaction with iron(III) chloride, also known as
ferric chloride. The details of the reaction are complicated with the product
being a phenol iron complex. However, it is the observations that are most
Adding a few drops of an iron(III) chloride solution to a solution containing phenol will result in a violet-purple color being formed. Phenol derivatives will also change color with iron(III) chloride, but the exact color depends on the compound. This is illustrated in the image below:
- The second test is the addition of bromine water, which reacts with phenol
to produce a white precipitate. This reaction also decolorizes the bromine
This reaction differs slightly from the detection of alkene and alkynes where a colorless solution is formed without the precipitate.
Example 5: Determining a Reagent to Test for Phenol
Which of the following reagents could be used for the detection of phenol?
There are two common tests that can be used to detect phenol. One is bromine water, which will turn colorless if phenol is present along with the formation of a white precipitate. However, bromine water is not one of the reagents given in the question. Another test is the addition of a solution of iron(III) chloride. If phenol is present, a color change to a violet-purple solution will occur. Iron(III) chloride has the chemical formula , which is option E in the list of given reactants. So the correct answer is E, .
- Phenols are a class of molecules containing a hydroxy () group bonded directly in an aromatic ring.
- The presence of the hydroxy group in phenol gives rise to much higher melting and boiling points and to solubility compared with those of benzene and toluene.
- Phenol can be synthesized from the distillation of coal tar or from the hydrolysis of chlorobenzene with sodium hydroxide.
- Phenol is weakly acidic, but stronger than alcohols, and reacts with sodium hydroxide.
- The nitration of phenol with concentrated nitric acid produces 2,4,6-trinitrophenol, also known as picric acid.
- Phenol can react with formaldehyde in a condensation polymerization to produce a polymer known as Bakelite.
- The presence of phenol can be detected through color change upon the addition of either an iron(III) chloride solution or bromine water.