Chapter 25 - Aromatic Chemistry

Question 1

How was benzene discovered?

Answer 1

Benzene, C6H6, was first discovered in 1825 by the English scientist Michael Faraday, who isolated and identified it in an oily residue from the gas that was used for street lighting.

Question 2

What is Benzene classified as?

Answer 2

a. Benzene is classed as an aromatic hydrocarbon or arene.
b. A benzene molecule consists of a hexagonal ring of six carbon atoms, with each carbon atom joined to two other carbon atoms and to one hydrogen atom

Question 3

What is the systematic name given to the Kekule structure?

Answer 3

The systematic name given would be cyclohexa-1,3,5-triene.

Question 4

What evidences disproved the kekule model?

Answer 4

a. Bond length (0.139nm) intermediate between the length of a C-C (long, 0.153nm) and C=C (short, 0.134nm).
b. delta H hydrogenation less eothermic than expected when compared to delta H hydrogenation for cyclohexene.
c. Only reacts with Br2 at high temperature or in presence of a halogen carrier catalyst / resistant to electrophilic attack.

Question 5

What are the main features of the delocalised model of benzene?

Answer 5

a. Benzene is a planar, cyclic, hexagonal hydrocarbon containing six carbon atoms and six hydrogen atoms.
b. Each carbon atom uses three of its available four electrons in bonding to two other carbon atoms and to one hydrogen atom.
c. Each carbon atom has one electron in a p-orbital at right angles to the plane of bonded carbon and hydrogen atoms.
d. Adjacent p-orbital electrons overlap sideways, in both directions, above and below the plane of the carbon atoms to form a ring of electron density.
e. The overlapping of the p-orbitals creates a system of pi-bonds which spread over all six of the carbon atoms in the ring structure.
f. The six electrons occupying this system of pi-bonds are said to be delocalised.

Question 6

How are aromatic compounds with one substituent group named?

Answer 6

a. In monosubstituted aromatic compounds, the benzene ring is considered the parent chain. Alkyl groups (CH3, C2H5), halogens (F, Cl, Br, I), and nitro (NO2), groups are all considered the prefixes to benzene.
i. i.e. ethylbenzene, chlorobenzene and nitrobenzene.
b. When a benzene ring is attached to an alkyl chain with a functional group or to an alkyl chain with seven or more carbon atoms, benzene is considered to be a substituent. Instead of benzene, the prefix phenyl is used in the name.

Question 7

In monosubstituted aromatic compounds, what are the exceptions?

Answer 7

a. Benzoic acid (benzenecarboxylic acid), phenyl amine, and benzaldehyde (benzenecarbaldehyde).
b. Benzoic acid is a benzene attached to a COOH group.
c. Phenylamine is a NH2 group attached to a benzene.
d. Benzaldehyde is a benzene attached to a CHO group.

Question 8

How are aromatic compounds with more than one substituent group named?

Answer 8

a. Some molecules may contain more than one substituent group on the benzene ring, for example, disubstituted compounds have two substituent groups.
b. The ring is now numbered, just like a carbon chain, starting with one of the substituent groups. The substituent groups are listed in alphabetical order using the smallest numbers possible.

Question 9

What is the reactivity of benzene and its derivatives?

Answer 9

a. Benzene and its derivatives undergo substitution reactions in which a hydrogen atom on the benzene ring is replaced by another atom or group of atoms.
b. Benzene typically reacts with electrophiles and most of the reactions of benzene proceed by electrophilic substitution.

Question 10

What is the nitration of benzene?

Answer 10

a. Benzene reacts slowly with nitric acid to form nitrobenzene.
b. The reaction is catalysed by sulfuric acid and heated to 50 degrees to obtain a good rate of reaction. A water bath is used to maintain the steady temperature.
c. In nitration, one of the hydrogen atoms on the benzene ring is replaced by a nitro, -NO2, group.
d. So, benzene reacts with nitric acid at 50 degrees with sulfuric acid to form nitrobenzene and water.

Question 11

What happens if the temperature of the reaction rises above 50 degrees?

Answer 11

Further substitution reactions may occur leading to the production of dinitrobenzene. i.e. increasing the temperature to 70 degrees may result in a disubstituted benzene, if excess nitric acid is used.

Question 12

What is nitrobenzene important for?

Answer 12

a. Nitrobenzene is an important starting material in the preparation of dyes, pharmaceuticals, and pesticides.
b. It can be used as a starting material in the preparation of paracetamol.

Question 13

What is the reaction mechanism for the nitration of benzene, what type of reaction is it and how is the catalyst regenerated?

Answer 13

It is an electrophilic substitution reaction.

Question 14

How can you undergo halogenation of benzene reactions?

Answer 14

a. Halogens do not react with benzene unless a catalyst called a halogen carrier is present.
b. Common halogen carriers include AlCl3, FeCl3, AlBr3, and FeBr3, which can be generated in situ (in the reaction vessel) from the metal and the halogen.

Question 15

How can benzene be brominated?

Answer 15

a. At room temperature and pressure and in the presence of a halogen carrier, benzene reacts with bromine in an electrophilic substitution reaction. In bromination, one of the hydrogen atoms on the benzene ring is replaced by a bromine atom.
b. The electrophile in the substitution reaction is Br+ (bromonium ion), which is generated when the halogen carrier catalyst reacts with bromine.

Question 16

How can benzene be chlorinated?

Answer 16

Chlorine will react with benzene in the same way as bromine and following the same mechanism. The halogen carrier used is FeCl3, AlCl3, or iron metal and chlorine, which react to make FeCl3.

Question 17

What are alkylation reactions?

Answer 17

a. The alkylation of benzene is the substitution of a hydrogen atom in the benzene ring by an alkyl group. The reaction is carried out by reacting benzene with a haloalkane in the presence of AlCl3 if chlorine is the halogen in the haloalkane used, which acts as a halogen carrier catalyst, generating the electrophile.
b. Alkylation increases the number of carbon atoms in a compound by forming carbon-carbon bonds.
c. The reaction is sometimes called a Friedel-Crafts alkylation after the two chemists who first carrier out the reaction.

Question 18

What are acylation reactions?

Answer 18

a. When benzene reacts with an acyl chloride in the presence of an AlCl3 catalyst, an aromatic ketone is formed. This is called an acylation reaction and is another example of an electrophilic substitution reaction. The reaction forms carbon-carbon bonds and is again useful in organic synthesis.
b. Ethanoyl chloride, CH3COCL, is the first member of the acyl chloride homologous series. Phenylethanone is produced in the reaction between benzene and ethanoyl chloride and is used in the perfume industry.

Question 19

What is the difference in reactivity between alkenes and arenes?

Answer 19

a. Alkenes decolourise bromine by an electrophilic addition reaction. i.e. the reaction between cyclohexene and bromine.
b. In this reaction, the pi-bond in the alkene contains the localised electrons above and below the plane of the two carbon atoms in the double bond. This produces an area of high electron density. The localised electrons in the pi-bond induce a dipole in the non-polar bromine molecule making one bromine atom of the bromine molecule slightly positive and the other slightly negative. The slightly positive bromine atom enables the bromine molecule to act like an electrophile.
c. Unlike alkenes benzene do not react with bromine unless a halogen carrier catalyst is present. This is because benzene has delocalised pi-electrons spread above and below the plane of the carbon atoms in the ring structure. The electron density around any two carbon atoms in the benzene ring is less than that in a C=C double bond in an alkene.
d. When a non-polar molecule such as bromine approaches the benzene ring there is insufficient pi-electron density around any two carbon atoms to polarise the bromine molecule. This prevents any reaction taking place.

Question 20

What is a phenol?

Answer 20

a. Phenols are a type of organic chemical containing a hydroxyl, -OH, functional group directly bonded to an aromatic ring.
b. The simplest member of the phenols, C6H5OH, has the same name as the group – phenol.

Question 21

What are phenols used in?

Answer 21

a. Phenols are used in many everyday antiseptics.

b. It is also an important chemical used in the production of disinfectants, detergents, plastics, paints and aspirin.

Question 22

How was phenol originally manufactured?

Answer 22

a. Phenol used to be made from benzene, C6H6, using sulfuric acid and sodium hydroxide in a multi-stage process.
i. C6H6 + H2SO4 + 2NaOH ==> C6H5OH + Na2SO3 + 2H20.

Question 23

How is phenol currently manufactured?

Answer 23

a. The majority of phenol is manufactured from benzene and propene, C3H6, in a multistep reaction.
i. C6H6 + C3H6 + O2 ==> C6H5OH + CH3COCH3
b. This reaction has an 86% yield of phenol from benzene. The other product is propanone, CH3COCH3 (acetone).
c. Although, acetone is useful, it is in less demand than phenol, and companies find it difficult to make use of all the acetone produced.

Question 24

Which method are research chemists investigating to manufacture phenol in the future?

Answer 24

a. Research chemists are investigating manufacturing phenol using benzene and nitrogen (I) oxide, N20 (also called nitrous oxide).
i. C6H6 + N2O ==> C6H5OH + N2.
b. This reaction has a 95% yield of phenol from benzene.
c. Nitrous oxide is a gaseous waste product from the production of nylon and cannot be allowed to escape into the atmosphere, as it is a greenhouse gas.

Question 25

What type of acid is phenol classified as?

Answer 25

Phenol is classified as a weak acid. When dissolved in water, phenol partially dissociates forming the phenoxide ion and a proton. It is classified as a weak acid because of this ability to partially dissociate to produce protons.

Question 26

What are the properties of phenol?

Answer 26

a. Phenol is a weak acid.
b. Phenol is an aromatic compound.
c. Phenols undergo electrophilic substitution reactions.
d. The reactions of phenol take place under milder conditions and more readily than the reactions of benzene.
e. Phenol is less soluble in water than alcohols due to the presence of the non-polar benzene ring
f. Phenol is more acidic than alcohols but less acidic than carboxylic acids. As the Ka of an alcohol (1 x 10^-16, ethanol) is less than that of phenol (1.26 x 10^-11) which is in turn less than that of a carboxylic acid (1.00 x 10-5, ethanoic acid).

Question 27

How can you distinguish between alcohols, phenols and carboxylic acids?

Answer 27

a. Alcohols such as ethanol does not react with sodium hydroxide (a strong base) or sodium carbonate (a weak base).
b. Phenols and carboxylic acids react with solutions of strong bases such as aqueous sodium hydroxide.
c. Only carboxylic acids are strong enough acids to react with the weak base, sodium carbonate.
d. A reaction with sodium carbonate can be used to distinguish between a phenol and a carboxylic acid – the carboxylic acid reacts with sodium carbonate to produce carbon dioxide, which is evolved as a gas.

Question 28

What is the reaction of phenol with sodium hydroxide?

Answer 28

Phenol reacts with sodium hydroxide to form the salt, sodium phenoxide, and water in a neutralisation reaction.

Question 29

Outline the bromination of phenol.

Answer 29

a. Phenol reacts with an aqueous solution of bromine (bromine water) to form a white precipitate of 2,4,6 – tribromophenol and 3 moles of hydrogen bromide.
b. The reaction decolourises the bromine water (orange to colourless). With phenol, a halogen carrier catalyst is not required, and the reaction is carried out at room temperature.

Question 30

Outline the nitration of phenol.

Answer 30

Phenol reacts readily with dilute nitric acid at room temperature. A mixture of 2-nitrophenol and 4-nitrophenol is formed and water is formed as a by-product.

Question 31

What is the difference in reactivity between phenol and benzene?

Answer 31

a. In phenol, lone pair of electrons on O is partially delocalised into the ring.
b. This increases the electron density in the ring.
c. As a result, the electrophile is more polarised.

Question 32

Can phenol undergo further substitution?

Answer 32

a. Phenol as well as many other substituted aromatic compounds can undergo a second substitution – disubstitution.
b. Some of these reactions take place more readily than benzene itself whereas other reactions take place less easily and require extreme conditions.

Question 33

How does bromine react with phenylamine in comparison to benzene?

Answer 33

a. Bromine requires a halogen carrier catalyst to react with benzene, whereas bromine will react rapidly with phenylamine.
b. Bromine reacts with phenylamine to form 2,4,6 – tribromophenylamine and 3 moles of hydrogen bromide.

Question 34

How does bromine react with nitrobenzene in comparison to benzene?

Answer 34

a. Nitrobenzene reacts slowly with bromine, requiring both a halogen carrier catalyst and a high temperature. The benzene ring in nitrobenzene is less susceptible to electrophilic substitution than benzene itself.
b. Nitrobenzene reacts with bromine to form 3-bromonitrobenzene and hydrogen bromide.

Question 35

What is meant by activation and deactivation of the aromatic ring?

Answer 35

a. For example, the -NH2 group in phenylamine activates the ring as the aromatic ring reacts more readily with electrophiles.
b. The -NO2 group deactivates the aromatic ring as the ring reacts less readily with electrophiles
c. All 2- and 4- directing groups (ortho and para- directors) are activating groups, with the exceptions of halogens.
d. All 3-directing groups are deactivating groups.

Question 36

What positions are ortho, meta and para?

Answer 36

a. Ortho is position 2, meta is position 3 and para is position 4.
b. For example, 3-bromonitrobenzene is also known as meta-bromonitrobenzene or more simply as m-bromonitrobenzene

Question 37

What are all the 2- and 4- directing (ortho- and -para directing) substituents?

Answer 37

• NH2 or -NHR
• OH
• OR
• R or -C6H5
• F, -Cl, -Br, -I

Question 38

What are all the 3-directing (meta directing) substituents?

Answer 38

-RCOR
-COOR
-SO3H
-CHO
-COOH
-CN
-NO2
NR3+

Question 39

How can you make TNT (2,4,6-trinitrotoluene)?

Answer 39

a. The reaction of methylbenzene (toluene) with an excess of concentrated nitric and sulfuric acid.
b. At 50 degrees, toluene (methylbenzene) forms a monosubstituted product, 2-nitrotoluene.
c. The product of this reaction is less reactive than toluene because the -NO2 group deactivates the benzene ring. However, if the temperature of the reaction mixture is increased to 70 degrees a disubstituted product, 2,4-dintrotoluene is formed.
d. The second -NO2 group deactivates the benzene ring further. Further reaction will produce 2,4,6-trinitrotoluene – TNT. However, this requires extreme conditions – a good job really as any accidental over-heating could result in an explosion.