Video Transcript
In this video, we will learn how to
describe the physical and chemical properties of phenol. In the first section of this video,
we’ll look at how we identify phenols.
Phenols are organic molecules that
contain a hydroxy group bonded directly to an aromatic ring. Aromatic hydrocarbons are
unsaturated molecules that are characteristic by the presence of the benzene
ring. Benzene has the molecular formula
C6H6, and its structure can be represented as a hexagon with a circle in the
center. The hydroxy group contains an
oxygen atom covalently bonded to a hydrogen atom. If we attach the hydroxy group
directly to the benzene ring, we get the molecule called phenol.
Phenols are distinctly different to
aliphatic alcohols, where we see the hydroxy group bonded to a carbon chain. Phenol is not to be confused with
cyclohexanol. Cyclohexanol contains a hydroxy
group bonded to a ring structure. In cyclohexanol, the ring structure
is saturated, so cyclohexanol is not a phenol.
Consider the two compounds shown
here as further examples. Here we see molecule A on the left
and molecule B on the right. Molecule A is classed as a phenol,
since the hydroxy group is bonded directly to the benzene ring. Molecule B is classed as an
alcohol. Although molecule B contains a
benzene ring, the hydroxy group is not bonded directly to it. Phenols and alcohols do share some
similar chemistry. As an example of the similar
chemistry of phenols and alcohols, they can both be converted into esters.
The molecule commonly known as
salicylic acid is easily converted into the ester aspirin by using a reagent called
ethanoic anhydride. Ethanoic anhydride is a carboxylic
acid derivative. It’s made quite simply by
condensing two molecules of ethanoic acid together. Aspirin is a common
over-the-counter medicine used for pain relief. The ester is formed via the phenol
hydroxy group in salicylic acid. The waste product in this reaction
is a molecule of ethanoic acid which originated from the ethanoic anhydride.
Alcohols react with carboxylic
acids in the presence of a concentrated sulfuric acid catalyst to make esters as
well. In this particular reaction, the
waste product is water. In this particular case, the
chemistry of alcohols and phenols appears to be fairly similar. In most cases, however, the
chemistry of alcohols and phenols differs greatly.
We will now take a look at some of
the physical properties of phenol. Phenol is a white solid at room
temperature. It has a rather sweet, tarry odor
that is similar to old-fashioned antiseptics and coal tar soap. If we heat solid phenol crystals to
around 41 degrees Celsius, we’d observe that it melts. In fact, phenol has a much higher
melting point than other aromatic compounds of a similar molecular size.
So how do we explain this unusually
high melting point? In solid phenol, the molecules can
hydrogen bond to each other. When we melt the phenol, we have to
overcome this intermolecular hydrogen bonding. More energy is required to overcome
these intermolecular forces than that required in other aromatic molecules where
hydrogen bonding does not exist.
Phenol is much more soluble in
water than benzene. At higher temperatures, it’s
possible to dissolve about 10 grams of phenol into 90 grams of water. This would result in a solution
that is about 10 percent phenol in water by mass. So why is phenol far more soluble
in water than benzene? Just as phenol molecules can
hydrogen bond to other phenol molecules, phenol molecules can also hydrogen bond to
water molecules. This hydrogen bonding allows phenol
molecules to fit into the extensive hydrogen-bonded network of water molecules in
liquid water. This explains the good solubility
of phenol in water.
Solutions of phenol in water were
commonly known as carbolic acid. In early operating rooms, phenol
solution was sprayed around the patient, as it was a known antiseptic.
In the next section, we’ll look at
the production of phenol and how it’s obtained industrially.
Phenol was traditionally obtained
by the fractional distillation of coal tar. Coal tar is obtained by heating
crushed coal in the absence of air. The vapors formed from the hot coal
are condensed in cold water. The condensate contains coal
tar. More volatile compounds are found
in the coal gas produced by this process. This gas was burnt as a waste
product or used to produce energy in the form of heat. Coal tar is in fact a mixture of
many aromatic compounds. As well as phenol, it contains
cresols or methylphenols. Since these molecules have
different boiling points, fractional distillation is used to separate them out.
Industrially, phenol is synthesized
by reacting chlorobenzene with aqueous sodium hydroxide. The process takes place at about
300 degrees Celsius, and the pressure is elevated to about 300 times atmospheric
pressure. The initial product in this
reaction is sodium phenoxide. The sodium phenoxide is then
treated with hydrochloric acid to produce phenol and sodium chloride as a waste
product.
In the next section, we’ll study
the acidity of phenol.
Phenol can behave as a weak acid in
aqueous solution. The hydroxy group can
deprotonate. In phenol, the oxygen-hydrogen
covalent bond breaks, and the phenoxide ion is formed. Water accepts the proton released
by the phenol molecule, and the ion H3O+ is formed. H3O+ is known as the hydronium ion,
sometimes called the hydroxonium ion. The phenoxide anion is a stable ion
due to the presence of the benzene ring. Other alcohols such as ethanol can
deprotonate in a similar way to phenol.
The reaction of ethanol with water
is very similar. In this case, the product is the
ethoxide anion. Since the ethoxide anion is not
stabilized in the same way as the phenoxide anion, alcohols are very weak acids
indeed. Phenols are therefore stronger
acids than alcohols. It’s important to realize that
acidity is a relative concept here. Phenols are in fact very weak acids
indeed. Even carboxylic acids, which are
classified as weak acids, are stronger acids than phenol.
Carboxylic acids will react with
relatively weak bases, such as carbonates, to produce carbon dioxide gas. Since phenols are weaker acids,
they will not react with weak bases such as carbonates, and no carbon dioxide gas is
produced. Phenol will react with very strong
bases. Phenol reacts with sodium hydroxide
to give sodium phenoxide. On the other hand, alcohols such as
ethanol, do not react with sodium hydroxide to any extent. If sodium hydroxide is added to
ethanol, sodium ethoxide is not formed in any appreciable amount.
Another way to produce sodium
phenoxide is to heat sodium metal directly with phenol. In this reaction, hydrogen gas is
produced, which escapes, leaving solid sodium phenoxide behind. The chemistry of alcohols is
similar here. If sodium metal is heated with
ethanol, sodium ethoxide and hydrogen gas are produced.
In the next section, we’ll study
the nitration reactions of phenol.
Phenol reacts readily with dilute
nitric acid. Nitric acid has the formula
HNO3. In these reactions, a hydrogen atom
attached to the benzene ring is substituted for a nitro group. These reactions are known as
monosubstitutions. With dilute nitric acid, the
products formed are 2-nitrophenol and 4-nitrophenol. If phenol is reacted with
concentrated nitric acid in the presence of a concentrated sulfuric acid catalyst,
further substitution reactions may occur. The product formed in this reaction
is 2,4,6-trinitrophenol. 2,4,6-Trinitrophenol is also known
as picric acid. Picric acid can decompose very
suddenly and exothermically. Large volumes of carbon monoxide
steam and nitrogen gas are produced. It’s been used as a high
explosive.
In the next section, we’ll see how
a plastic, called Bakelite, can be made from phenol.
Phenol can be heated with methanal
or formaldehyde in the presence of an acid or base catalyst to form a hard brittle
plastic. In the first step, the phenol
molecule becomes substituted. The substituted phenol product then
undergoes a condensation reaction with another phenol molecule. In the condensation reaction, a
small molecule, in this case water, is eliminated. The substitution and condensation
reactions then continue until a rigid network of molecules develops. The polymer formed is known as
Bakelite.
Bakelite contains a
three-dimensional, cross-linked network of aromatic rings based upon phenol. Bakelite can be molded, but it sets
upon heating, so it’s described as a thermosetting plastic. As Bakelite is an electrical
insulator, it was used widely in electrical equipment cases, such as radios and
light fittings.
In the next section, we’ll see how
phenol solutions can be detected chemically using some simple chemical tests in the
lab.
Phenols react with ferric chloride,
also known as iron(III) chloride, to produce colored complex ions. A solution of phenol in water is
generally colorless, whereas ferric chloride has a distinct orange-to-brown
color. The ferric chloride reacts with the
phenol solution to give a violet-to-purple solution. Other phenol derivatives will react
with the ferric chloride in a similar way. The actual color of the complex ion
formed will depend precisely on the phenol compound used.
An alternative test is to add a few
drops of bromine water to a solution of phenol. Bromine water has a distinct
orange-to-brown color. When bromine water is shaken gently
with a solution of phenol, a white precipitate develops. The white precipitate causes a
medicinal or antiseptic smell. The white precipitate contains
2,4,6-tribromophenol. Notice that in this reaction, the
orange-to-brown bromine water will be decolorized.
Now it’s time to look at a question
to test our understanding of the reactions of phenol.
Fill in the blank. Phenol reacts with concentrated
nitric acid in the presence of concentrated sulfuric acid, forming blank acid. (A) Phthalic, (B) salicylic, (C)
citric, (D) picric, (E) benzoic.
In this question, our starting
molecule is phenol. Phenol is an aromatic molecule that
contains a benzene ring. When we draw the structure of
phenol, we do not normally include the hydrogen atoms that are bonded to the carbon
atoms in the benzene ring. Phenol tends to undergo
substitution reactions with other reagents, where these hydrogen atoms are
substituted for some other group.
If the reagent concerned is
concentrated nitric acid, we get substitutions of the hydrogen atoms in positions
two, four, and six on the benzene ring. We also see that concentrated
sulfuric acid is included here. This is present as a catalyst. So for this particular reaction,
nitro groups are substituted for hydrogen atoms at positions two, four, and six. The product is therefore named
2,4,6-trinitrophenol. The common name for this molecule
is picric acid. Historically, it was used as a high
explosive. Although picric acid does not
contain an easily identifiable acidic group, we find that the hydroxy group in this
molecule can release a proton. This does not happen to any great
extent, so picric acid is a weakly acidic molecule.
Phthalic acid, salicylic acid,
citric acid, and benzoic acid all contain carboxylic acid functional groups, which
picric acid does not. So picric is the correct
answer.
We will now review the key points
about phenol. Phenols contain a hydroxy group
bonded directly to an aromatic ring. An example of an aromatic ring is a
benzene ring. Some of the chemistry of phenol is
similar to that of alcohols. However, phenol is a stronger acid
than alcohols; it reacts with strong bases like sodium hydroxide. Phenol can be nitrated with
concentrated nitric acid to make 2,4,6-trinitrophenol. This is also known as picric
acid. Phenol is used to make plastics,
such as Bakelite. Phenol forms a violet solution with
iron(III) chloride. This reaction is one possible test
for phenols.
