• UNIT 5: DERIVATIVES OF BENZENE

    Key unit competency

    The learner should be able to relate aromatic ketones, aldehydes, carboxylic acids and amines to their chemical activity.

    Learning objectives

    At the end of this unit , students will be able to:

    • Explain the effects of substituent groups on the benzene ring;

    • Give systematic names of aromatic compounds;

    • Describe the preparation and reactions of phenol, benzoic acid, benzaldehyde, phenyl ethanone and phenylamine;

    • State the uses of phenols;

    • Describe the reactions of phenol, aromatic carbonyl compounds and carbox-ylic acids;

    • Describe chemical properties of phenylamines;

    • Explain the azo-coupling reactions of phenylamine in manufacture of dyes and indicators;

    • Test and compare th acidity of phenol with alcohols and carboxylic acids;• Test for the presence of phenol in a given solution;

    • Compare and contrast the alkalinity of phenylamines with aliphatic amines and ammonia.;

    • Perform experiments on the reactions of phenol and phenylamine

    The simplest and most important member of aromatic hydrocarbons is benzene (C6H6). The benzene ring is particular because of its stability and certain properties.Many important chemical compounds are derived from benzene by replacing one or more of its hydrogen atoms with another functional group. It is a typical compound from which many of compounds of common properties derive.

    Some examples of derivatives of benzene are given below:

    5.1. Effect of substituent groups on the benzene ring

    The nature of a substituent already present in the benzene ring, not only determines the position of the next incoming group but also influences the rate of the second substitution reaction compared to the rate of substitution in benzene itself.A substituent might increase the rate of the second substitution, i.e. make the ring more reactive relative to benzene. Another group if present in benzene ring could decrease the rate of further substitution, i.e. make the ring less reactive compared to benzene.

    Groups already on the ring affect both the rate of the reaction and the site of attack. We say, therefore, that substituent groups affect both reactivity and orientation in electrophilic aromatic substitutions.

    5.1.1. Deactivating and activating substituents

    We can divide substituent groups into two classes according to their influence on the reactivity of the ring. The substituents which cause the compounds to undergo second substitution faster than benzene are called Activating Substituents (electron-releasing groups); they increase the electronic density on the benzene ring.On the other side, substituents which retard the rate of further substitution are referred to as Deactivating Substituents (electron-withdrawing groups); they decrease the electronic density on the benzene ring.

    5.1.2. Directing the incoming substituents

    During the formation of monosubstituted products in benzene, the electrophile can be attached at any position on the benzene ring. But, when the monosubstituted product is to be converted into disubstituted one, the existing substituent present in the ring directs the incoming group to a particular position. This is referred to as directive influence of the group. Depending on their directive influence, various groups (substituents) can be divided into two categories:

    • Ortho and Para Directors

    • Meta Directors

    a. Ortho and para directors

    These direct the new substituents to enter the ring primarily in Ortho and Para positions to themselves. These groups increase the electron density at the ring. Thus the reactivity of benzene ring towards electrophilic substitution reactions increases. For example if we carry out nitration of toluene, the mixture of ortho and paranitrotoluenes is formed

    b.Meta directors

    These direct the new substituents to enter the ring primarily in Meta position to themselves. For example, the nitration of benzoic acid produces m-nitrobenzene.

    These groups withdraw the electrons from benzene ring through resonance effect, reducing the electron density at the benzene ring. They decrease the reactivity of benzene ring towards electrophilic substitution reaction and make it less susceptible to the electrophilic attack.

    It has been found experimentally that in general ortho-para directing substituents activate the benzene ring and thus enhance the rate of reaction with electrophiles. On the contrary, the meta directing substituents deactivate the ring and retard the rate of reaction as compared to unsubstituted benzene.

    • Why activating substituents (Activators) have ortho and para directing properties?
    When the substituent present in the ring, has one or more lone pairs of electrons on the atom attached to the ring, it interacts with pi-electron system of the ring and it acts as electron donor (electron-donating substituent).

    The presence of an electron-donating group such as –OH or -NH2 causes further electrophilic substitution in ortho-para positions and also activates the ring to electrophilic attack.

    Let us take the example of phenol (C6H5-OH) and aniline (C6H5-NH2) which have available electron pairs on the atom directly attached to benzene ring. Thus phenol and aniline exhibit resonance and can be represented as hybrid of the following forms:

    In the above two examples, positions 2 and 4 are relatively richer in electrons than position 3 and this makes them susceptible to electrophilic attack. The electrophile would attack the ring preferentially at ortho and para positions where the electron density relatively is greater as compared to the meta positions. The second electrophile will be directed where sites are negatively charged, i.e ortho and para positions.

    From the above considerations, we conclude that all groups which are electron-donating are ortho-para directing and facilitate electrophilic substitution in the benzene ring.

    • Why Deactivating Substituents (Deactivators) have meta-directing prop-erties?

    When the substituent has at least one strongly electronegative atom and a multiple bond in conjugation with benzene ring, the substituent acts as electron- withdrawing substituent.

    Consider the nitrobenzene which contains –NO2 is able to exist as the following resonance forms:

    In the above example, it may be noted that resonance causes the decrease of electron density in the ring of nitrobenzene, and specifically at the ortho and para positions.

    In general, the electron withdrawing substituents decrease the electron density of benzene ring and thereby act as deactivators and meta-directors.

    • Anomalous Behaviour of Halogen Substituents

    The resonance effect enables the halogen substituents to act as ortho and para director. It is also expected to activate the ring to electrophilic attack, but on the contrary it is a ring deactivator. This is attributed to the very high electronegativity of the halogens due to which they withdraw electrons so strongly that they deactivate the benzene ring.

    While the resonance effect accounts for the ability of halogen to donate electrons to ortho and para positions, the combination of the two effects makes the halogenated benzene deactivated.

    5.2. Phenol

    The phenols are organic compounds with one or more  -OH-OH-  groups directly attached to a carbon atom in a benzene ring. The following are examples of phenols:

    Phenols occupy an important position in the modern synthetic organic chemistry for the preparation of dyes, antioxidants, phenolic resins and certain pharmaceutical products.

    The most important member in this family is phenol (hydroxybenzene):

    Phenol (hydroxybenzene) is a colorless crystalline solid which melts at 43oC and boils at 182oC. On exposure to air or light, it becomes coloured due to oxidation.

    Phenol is soluble in organic solvents and slightly soluble in water at room temperature, but infinitely soluble above 66 °C.

    Phenol exhibits intermolecular hydrogen bonding and this makes its melting point higher than that of hydrocarbons of comparable molecular mass.

    5.2.1. Sources and preparations of phenol

    Phenols are common in nature; examples include tyrosine, one of the standard amino acids found in most proteins. Many of the more complex phenols used as flavourings and aromas are obtained from essential oils of plants. Other phenols obtained from plants include thymol, isolated from thyme, and eugenol, isolated from cloves.

    Phenol, the cresols (methylphenols), and other simple alkylated phenols can be obtained from the distillation of coal tar or crude petroleum

    Phenol can be prepared:

    a. From benzenesulfonic acid

    In this method, benzenesulphonic acid obtained from sulphonation of benzene reacts with sodium hydroxide to produce phenol.

    5.2.2. Acidity of phenol

    The O-H bond is weaker in phenol than in alcohol. This is because the lone pair of electrons on the oxygen atom becomes associated with delocalized electrons of the ring. Because of this partial double bond develops between carbon and oxygen with the result that C-O bond is strengthened and the O-H weakened as the electronic density is displaced towards the ring. This gives to phenol a more acidic nature than alcohols.

    Therefore, phenol is more acidic than phenylmethanol and ethanol or cyclohexanol and this explains why phenol and not cyclohexanol nor phenylmethanol is soluble in sodium hydroxide solution. Because of this acidic property, phenol unlike alcohols reacts with alkali metal and sodium hydroxide or potassium hydroxide solution. Alcohols are not acidic enough to react with sodium hydroxide or potassium hydroxide solution but react only with alkali metal.

    Note: Phenol is a weaker acid it does not react with sodium carbonate or sodium hydrogen carbonate unlike carboxylic acids. The reaction with sodium carbonate or sodium hydrogen carbonate is also used to distinguish a carboxylic acid like benzoic acid from a phenol. Carboxylic acids give effervescence (liberate CO2) with sodium carbonate or sodium hydrogen carbonate while phenols do not.

    5.2.3. Reactions of phenols

    The reactions of phenol are of two types:

    • Reactions in which the O-H is broken;

    • Those involving the aromatic ring (Electrophilic substitution reactions).

    a. Esterification

    Phenols are weaker nucleophiles compared to alcohols because their lone pairs of electrons are partially delocalized over the benzene ring; that is why they do not form esters by direct reaction with carboxylic acids. However, the phenoxide ion (C6H5O-) is a better nucleophile and it reacts with acid derivatives such as acid chloride or acid anhydride, which are themselves more reactive than the parent acid:

    c. Electrophilic substitution reactions of phenol

    These reactions involve the replacement of hydrogen atoms at Para and Ortho positions of the ring, since the –OH is a para and ortho directing. The hydroxyl group increases the availability of electrons in the aromatic ring especially at para and ortho positions.

    Because of this ring activation, phenols react more readily with electrophiles than benzene itself.

    An aromatic hydrocarbon or arene (or sometimes aryl hydrocarbon)is a hydrocarbon with sigma bonds and delocalized pi electrons between carbon atoms forming a circle. In contrast, aliphatic hydrocarbons lack this delocalization

     

    5.3.1. Structure and nomenclature of aromatic hydrocarbons

    The trivial name of the parent monocyclic arene is benzene. The other members of this class are to a large extent assigned the systematic IUPAC names. However IUPAC have adopted the trivial names of lower arenes particularly, which have become popular by long usage. This has been done for brevity and convenience. Thus methylbenzene is invariably named as Toluene.

    a. Structure and nomenclature of aromatic alkanesIn the IUPAC system, arenes of this class are named in straight forward manner as substituted-benzenes.

    For example,


    When there are two substituents on the benzene ring, then positions are indicated by numbers, or by the prefixes ortho (o-), meta (m-) and para (p-). Thus the isomeric dimethylbenzenes are named as:

    if there are three or more substituent groups present on the ring, the arenes are preferably designated by IUPAC names. One of the groups is written at the to position of the hexagon, which becomes number 1. The six carbon atoms of the benzene are then numbered from 1 to 6 around the ring so that the substituents groups get the lower numbers. The substituent groups are preferably named in the alphabetical order. Thus,


    Note: The hydrocarbon group left after the removal of a hydrogen atom of the benzene itself is called phenyl group (C6H5-). The group left after the removal of a hydrogen atom of the CH3- group of toluene is called benzyl (C6H5-CH2-).

    b.Structure and nomenclature of some aromatic alkenes


     

    5.3.2. Reactions of alkylbenzenes

    Alkylbenzenes involve two parts of their structures in chemical reactions:

     

    • The side chain which can be oxidized by strong oxidizing agents like KMnO4 and Na2Cr2O7

    • The benzene ring which can take place in electrophilic substitution. In electro-philic substitution alkylbenzenes react more strongly than benzene itself since the alkyl groups donate electrons. Alkyl groups are ortho and para directing.

    a.   Oxidation of the side chain

    When alkylbenzenes are oxidized by powerful oxidizing agents (such as hot acidified or alkaline KMnO4 and Na2Cr2O7), the entire side chain, regardless of length, are oxidized to benzoic acid.

     For example,


    With weak oxidising agents such as acidic manganese dioxide (MnO2) or chromylchloride (CrO2Cl2), the side chain is oxidised to aldehyde (-CHO) group.


    b.Radical substitution

     Radical substitution takes place in three steps as for aliphatic alkanes: initiation, propagation and termination. The side chain substitution of hydrogen atom (s) occurs when chlorine or bromine is bubbled through boiling alkylbenzene in the presence of ultraviolet light or strong sunlight.

    Note that the above reaction may continue until all hydrogen atoms are replaced by halogen atoms.


    Note: Bromine gives similar products under similar conditions.

    5.4. Aromatic carbonyl compounds

    The compounds with carbonyl group attached to the benzene ring are known as aromatic aldehydes and aromatic ketones.

     5.4.1. Structure and nomenclature of aromatic carbonyl compounds

    a. Aromatic Aldehydes

    These carbonyl compounds contain a phenyl group in their structures. Aromatic aldehydes are of two types: (a) those in which the aldehyde group (CHO) is directly attached to a carbon of the aromatic ring; and (b) those in which the aldehyde group (CHO) is directly attached to a carbon of the side chain. Aldehydes of type (a) are called aromatic aldehydes, while those of type (b) are best regarded as aryl-substituted aliphatic aldehydes.

    Benzaldehyde is a typical aromatic aldehyde, while phenylacetaldehyde and cinnamic aldehyde are to be designated as aryl-substituted aliphatic aldehydes.


    The IUPAC name of an aromatic aldehyde is derived by dropping the ending –ene of the name of the parent hydrocarbon and appending the suffix –al. Thus

    Aromatic aldehydes are generally called by their common names which are derived from the names of the corresponding carboxylic acid by replacing the –ic or –oic acid by –aldehyde. Thus the name benzaldehyde is derived from benzoic acid; ortho-tolualdehyde is derived from ortho-toluic acid, and salicylaldehyde from salicylic acd.

    b.Aromatic ketones

    Ketones containing the carbonyl group attached to a benzene ring are named phenones. The individual name of such a ketone is derived by adding phenone to the stem formed by removing –ic from the name of the corresponding acid.

    The common names of aromatic ketones are obtained as usual by naming the alkyl or aryl groups attached to the carbonyl group, followed by the word ketone. These are given in the brackets above

    5.4.2. Preparation of aromatic carbonyl compounds

    a. Preparation of aromatic aldehydes

    Benzaldehyde is the simplest member in this family and it may be prepared by the following methods which are applicable to aromatic aldehydes in general. Benzaldehyde can be prepared using different methods. However, the main meth-od is oxidation of methylbenzene. The methods of preparation include:

    b.Preparation of aromatic ketones

    Although aromatic ketones may be prepared by any of the methods used for aliphatic ketones, they are generally prepared by Friedel-Crafts reaction between an aromatic hydrocarbon and acylchloride or acid anhydride. The benzene is treated with acylchloride or acid anhydride in the presence of a halogen carrier.

    • From acylchloride

    5.4.3. Reactions of aromatic carbonyl compounds

    The most typical reactions of the carbonyl groups are nucleophilic addition. In the carbon-oxygen double bond of the carbonyl group, oxygen is more electronegative than carbon, hence it has a strong tendency to pull electrons towards itself. This makes the carbon-oxygen double bond highly polar.

    The partially positive carbon atom can be attacked by nucleophiles. During the reaction, the carbon-oxygen bond gets broken and the net effect is that the carbonyl group undergoes addition reaction.

     

    The carbonyl group of aromatic aldehydes and ketones withdraws electrons from the benzene ring by inductive effect and resonance effect. Hence this group deactivates the benzene ring towards electrophilic substitution. The presence of the carbonyl group directs the substitution in meta-position. The electrophilic substitution of aromatic aldehydes and ketones is more difficult than electrophilic substitution of non substituted benzene.

    a.   Reactions of aromatic aldehydes

    Aromatic aldehydes present the same properties as aliphatic aldehydes. They give a positive test with Brady’s reagent (2, 4- Dinitrophenyl Hydrazine; observation: yellow or orange precipitate), with Fehling solution (observation: red solution brown precipitate), with Tollen’s reagent (observation: silver mirror) and with Schiff reagent (pink colour will be observed).

    Benzaldehyde is the typical and the simplest of aromatic aldehyde and will be used for illustrating the properties of this class of aromatic aldehydes. Benzaldehyde undergoes chemical reactions involving the side chain and benzene ring.

     

    i.   Benzaldehyde is not oxidized as readily as aliphatic aldehydes of oxidizing agents. While it reduces ammoniacal silver nitrate forming a silver mirror, it does not reduce Fehling’s solution. Nevertheless, benzaldehyde undergoe oxidation by atmospheric oxygen at ordinary temperature to form benzoic acid. This process known as autoxidation is catalyzed by light.


    vi. Benzaldehyde on treatment with concentrated sodium hydroxide solution gives benzyl alcohol and sodium benzoate this reaction is called Cannizzaro reaction:

    Note: Benzaldehyde condenses with hydroxylamine (NH2OH) and phenylhydrazine (C6H5NHNH2) to form benzaldoxime and phenylhydrazone respectively. Similarly, benzaldehyde reacts with hydrazine and semicarbazide to give hydrazine and semicarbazone respectively.

    Benzaldehyde is used for flavouring foods, scenting soaps, in the manufacturing of perfumes, in the preparation of dyes and in the synthesis of antibiotics.

    b. Reactions of aromatic ketones

    The reactivity of the carbonyl group in aromatic ketones is not greatly affected by the aryl groups attached to it. In consequence, they undergo the same general reactions as aliphatic ketones. However, they do not form the bisulphite compound, and in addition give the usual substitution reactions in the aromatic ring.


    Aromatic acids contain one or more carboxyl groups (–COOH) attached directly to the benzene ring. The acids in which the –COOH group is attached to the side-chain group may be regarded as aryl-substituted aliphatic acids. However, there are no characteristics differences in the behavior of the ring and side-chain acids. The term “aromatic acids” includes both classes of compounds.

    5.5.1. Structure and nomenclature of aromatic carboxylic acids

    Aromatic carboxylic acids are called by their common names or after the name of the parent hydrocarbon. Thus:

    5.5.2. Preparation of aromatic carboxylic acids

    Aromatic acids can be prepared by the same general methods which are available for aliphatic acids. In addition they may be obtained by oxidation of aromatic hydrocarbons having a side-chain.

    5.5.3. Reactions of aromatic carboxylic acids

    Aromatic acids are white crystalline solids, having higher melting points than aliphatic acids. They are slightly soluble in cold water but dissolve readily in hot water to form a colorless solution and on cooling, the acids recrystallise.

    Aromatic carboxylic acids with unsubstituted benzene ring are slightly stronger acids than the aliphatic acids. Thus benzoic acid is stronger acid than acetic acid.

    For the most part, the reactions of aromatic carboxylic acids are identical with those of aliphatic acids, the more important differences being in their rates.

    Benzoic acid, the simplest aromatic carboxylic acid is more acidic than phenol since it can react with Na2CO3 or NaHCO3 to liberate carbon dioxide gas. Its reactivity is attributed to its two parts: the carboxylic acid groupand the benzene ring.

    5.5.4 Uses benzoic acid and its derivatives

    • Benzoic acid is used in medicine, in the dye industry for making aniline blue.

    • Sodium benzoate being less toxic is used for preserving food products such as tomato ketchup and fruit juices.

    5.6. Aromatic amines

    The amino derivatives of the aromatic hydrocarbons are of two types:

    a. Aromatic amines or aryl amines in which the –NH2 group (or substituted –NH2 group) is attached directly to a carbon of the benzene ring.

    b. Aryl-alkyl or aralkyl amines in which the –NH2 group is attached to a carbon of the side-chain.

    Like the aliphatic amines, the aromatic amines may also be divided into primary, secondary and tertiary amines.

    5.6.1. Structure and nomenclature of aromatic amines

    in systematic naming, the numbering of carbon depends on the whole structure of the aromatic compound and the nature of the group attached to the benzene ring. In general, when more than one group are attached to the benzene ring, some groups are given priority as functional group.

    Here is a simple list for guidance on precedence (priority) of groups: COOH > COO- > COOR >COCl> CONH2 > CN > CHO > COR > OH >NH2 which means that from the list given, the amino is the least important, whereas the carboxylic acid group is the most important.


    5.6.2. Preparation of Phenylamine and uses of its derivatives

    a. Preparation of phenylamine

    i. Phenylamine, like other arylamines, can be prepared by the reduction of nitro compounds. The nitro compounds are treated with granulated tin, zinc or iron and HCl.

    b.Uses of aniline and its derivatives

    Derivatives of phenylamine or aniline are known as “Anilines”. These are employed in various fields of science and everyday life as given below:

    i. Anilines are used in the rubber industry for the processing of rubber material such as car tyres, balloons, gloves, etc.

    ii. It is used as a dyeing agent in the manufacture of clothes such as jeans, etc

    iii. It is employed in the production of drugs such as Paracetamol, Tylenol, Acetaminophen.

    iv. It is used as a pesticide and fungicides in the agricultural industry.

    v. It is utilized in the manufacture of polyurethane, which is in turn used in the making of plastics.

    5.6.3. Alkalinity of phenylamine

    Amines, both aliphatic and aromatic are basic. They are soluble in water. Phenylamine is weaker base than ammonia and aliphatic amines because the non-bonded electrons on the nitrogen atom are delocalized into the pi-system of the benzene ring. This makes the lone pair of electrons less available for reaction with a proton.

    The delocalization of the lone pair of electrons on the nitrogen atom in phenylamine can be represented by the following resonance forms:

    Thus aniline structure is greatly resonance stabilized.

    5.6.4. Reactions of phenylamine

    Aniline, also called phenylamine or aminobenze (C6H5NH2), is a colorless oily liquid, slightly soluble in water and soluble in organic solvents. It turns brown on exposure to sunlight.

    The aromatic amines in general give all the reactions of the amine group of aliphatic amines. However, the reactivity of the amine group is modified by delocalization of the non-bonded electrons on nitrogen atom into the pi-system of the aromatic ring.

    5.6.5. Reactions of diazonium salts

    Aromatic amines when treated with nitrous acid in cold mineral acid solution, give a very important class of compounds known as aryldiazonium salts. For example, aniline reacts with nitrous acid in hydrochloric acid solution at 0-50C to form a solution of benzenediazonium chloride. The reaction producing these salts is referred to as Diazotization.

    Diazonium salts give two types of reactions: (a) those in which the –N2X (where X is any anion, such as Cl-, Br-, NO3-, etc) is replaced by another univalent atom or group, with the liberation of nitrogen gas; (b) those in which the two nitrogen atoms are retained.

    c. Coupling reactions

    Diazonium salts act as electrophiles and react with highly activated benzene rings (like phenols and aromatic amines) to form brightly colored compounds called azo-compounds with the general formula: Ar-N=N-Ar, -N=N- is a linkage called azo linkage and responsible for the colour of the dye. These reactions are referred to as coupling reactions and they proceed via electrophilic substitution mechanism.

    A large number of benzene derivatives can be synthesized via diazonium salts. This will be illustrated by taking example of benzenediazonium chloride.

    Reactions of benzenediazonium chloride are summarized in the following scheme

    UNIT 4 :BENZENEUNIT 6: POLYMERS AND POLYMERIZATION