Subject: Chemistry-CIX :Organic Chemistry-III Lesson:Polynuclear Aromatic Hydrocarbons Unit-I Naphthalene Lesson Developer:Dr. S.P Bhutani Formerly Associate Professor Department of Chemistry, Rajdhani College University of Delhi Lesson Editor:Dr. N.K Gautam Advisor Sciences, ILLL University of Delhi

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Contents 1. Introduction 2. Naphthalene 3. Aromatic Character-General 4. Structure And Aromatic Character Of Naphthalene 5. Nomenclature Of Naphthalene Derivatives 6. Reactions Of Naphthalene 1. Electrophilic Substitution A. Nitration B. Halognation C. Sulphonation D. Friedel-Crafts Acylation 2. Oxidation 3. Reduction 4. Dehydrogenation—Aromatisation 5. Nucleophilic Substitution Reactions Of Naphthalene Derivatives A. Preparation Of Naphthols B. Formation Of -Naphthylamine And Other - Substituted Derivatives C. Preparation Of -Naphthylamine And - Substituted Derivatives 6. Electrophilic Substitution In Naphthalene Derivatives 7. Synthesis Of Naphthalene And Its Derivatives 1. Haworth Synthesis A. Synthesis Of Naphthalene B. Synthesis Of Naphthalene Derivatives 2. Diels-Alder Method 8. References

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1. INTRODUCTION Polycyclic aromatic hydrocarbons may be divided into two broad groups; the biaryls and the ―fused polycyclic‖ aromatic compounds. The biaryls are obtained when two rings are joined by a single bond. The simplest compound of this class is biphenyl, in which two benzene rings are joined together by a single bond. While numbering, the rings are considered to be joined at the 1-position and the two rings are distinguished by the use of primes. Simple derivatives of biphenyl are named by the use of ortho, meta and para nomenclature. More complex compounds are named by using numbers. For example,

The ―fused polycyclic‖ aromatic compounds are generally referred to as polynuclear aromatic compounds. These are characterised by two or more benzene rings fused together at ortho positions in such a way that each pair of rings share two carbons. Naphthalene is the simplest polynuclear hydrocarbon in which two benzene rings are fused together. When a third benzene ring fuses with naphthalene, we get anthracene if the fusion is linear and phenanthrene if the fusion is angular. In this chapter, we shall study the chemistry of these polynuclear aromatic hydrocarbons.

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2. NAPHTHALENE Naphthalene is the parent compound of the series of polynuclear aromatic compounds. We have already learnt that benzene is isolated from coal tar; the low boiling range of coal tar contains benzene, toluene and xylenes. Naphthalene is isolated from a high boiling fraction, although other polynuclear hydrocarbons are also present. A fraction boiling at 195 – 230oC, called naphthalene oil, yields crude naphthalene on cooling. Naphthalene is the most abundant single component of coal tar. It is also the major constituent of high boiling petroleum fraction. Naphthalene is a colourless crystalline solid. We are aware of the characteristic smell of naphthalene. The moth balls, which we use as insect repellants, contain naphthalene. It is also a constituent of many insecticides. A large number of synthetic dyes contain naphthalene nucleus.

3. AROMATIC CHARACTER-GENERAL We know that all compounds that resemble benzene in their chemical properties are called aromatic compounds. Before we discuss the aromatic character of naphthalene, let us review the various characteristics of an aromatic compound. An aromatic compound has got a high degree of unsaturation. The chemical properties of such compounds are unusual. They do not undergo addition reactions as expected of an unsaturated compound. They, however, undergo electrophilic substitution reactions as in the case of benzene which undergoes nitration, halogenation, sulphonation and FriedelCrafts alkylation or acylation. We find that aromatic compounds have an unusual stability as is evident from their low heats of hydrogenation (For benzene H = –49.8 kcal/mole) and low heats of combustion.

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Aromatic compounds are cyclic in nature containing 5, 6 or 7-membered rings. Applying the resonance concept we get a number of canonical forms for an aromatic compound, e.g., benzene has two canonical structures (1, 2).

Thus, the unusual stability of an aromatic compound is due to its high resonance energy. In the case of benzene, it is equal to 36 kcal/mole. When examined by physical methods the aromatic compounds are found to be flat i.e., they have planar structures. According to the molecular orbital approach an aromatic compound must have a delocalized -electron cloud above and below the plane of the molecule. This delocalisation is responsible for unusual stability of an aromatic compound. It sustains a ring current, which is indicated in the NMR spectra. Lastly an aromatic compound must obey Hückel’s rule. According to this rule, for a compound to be aromatic, the number of -electrons in its planar cyclic system must be equal to 4n + 2, where n = 0 or a positive integer (n = 1, 2, 3, 4 etc.). In other words,

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coplanar monocyclic rings with 2, 6, 10, 14 delocalised -electrons should be aromatic. The benezene molecule has got six -electrons, which we call as the aromatic sextet.

4. STRUCTURE AND AROMATIC CHARACTER OF NAPHTHALENE The molecular formula of naphthalene is C10H8 indicating a high degree of unsaturation. That naphthalene has got five double bonds is evident from addition reactions of naphthalene e.g., it adds five molecules of hydrogen. Oxidation of naphthalene gives phthalic acid. This shows that at least one benzene ring is present in naphthalene and there may be another ring fused at the ortho positions. It was proved by Graebe that naphthalene consists of two benzene rings fused together at the ortho positions on the basis of oxidation reactions. Nitration

of

naphthalene

gives

nitronaphthalene

(3),

which

can

be

reduced

to

aminonaphthalene (4). Oxidation of nitronaphthalene and aminonaphthalene give different products. From aminonaphthalene we get phthalic acid and from nitronapthalene, 3nitrophthalic acid is obtained. This indicates clearly that in naphthalene molecule there are two benzene rings present, which are fused together. We can designate these rings as A and B and give the sequence of reactions as follows:

Applying the various characteristics of an aromatic compound to naphthalene molecule, we find that naphthalene fulfills the requirements for being an aromatic compound. As mentioned above the molecular formula of naphthalene indicates a high degree of unsaturation. Like benzene, naphthalene undergoes electrophilic substitution reactions

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namely, nitration, halogenation, sulphonation and Friedel-Crafts acylation. Naphthalene is resistant to addition reactions but less so than benzene. Naphthalene has unusual stability. That is clear from the low values of heat of hydrogenation and heat of combustion. The resonance energy of naphthalene, obtained from either its heat of hydrogenation or heat of combustion is 61 kcal/mole. Naphthalene is considered to be a resonance hybrid of three canonical structures (5a–5c).

If we assume that each canonical structure contributes equally to its resonance hybrid, then the above two structures (5a) and (5b) have double bond at 1, 2-positions. That means in naphthalene 1, 2 and 2, 3-bonds have 2/3-and 1/3-double bond character respectively. Therefore, 1, 2-bond should be shorter than 2, 3-bond, which is proved by its X-ray analysis. All the bonds are not of equal length but are close to the benzene value of 1.39Å (6).

From the chemical investigations, it is clear that naphthalene molecule has two benzene rings fused together. If we consider one benzene ring at a time, we find that each ring contains six -electrons, an aromatic sextet. Considering the orbital picture of naphthalene we get ten carbons lying at the corners of two fused hexagons. Each carbon is sp 2hybridised and is bonded with other two carbon atoms and one hydrogen atom. Therefore, all these atoms must lie in one plane. With this each carbon atom is left with one pzelectron in an orbital perpendicular to the plane of the rings. These orbitals overlap to give a -cloud above and below the plane of the molecule, thus resulting in delocalisation of the electrons (Fig. 1). The delocalisation is responsible for the observed ring current on the protons in the NMR spectra of naphthalene.

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Fig. 1 Orbital picture of naphthalene. We cannot apply Hückel’s rule to naphthalene and other higher polynuclear hydrocarbons because of the restriction of the rule to monocyclic systems. However, the total number of -electrons in this case is equal to ten. The aromatic sextets in the fused benzene rings of naphthalene are shown by circles as indicated in (7). However, for convenience we shall represent naphthalene in one of the resonating structures in this text.

The above structure of naphthalene has been confirmed by synthesis (See page 20)

5. NOMENCLATURE OF NAPHTHALENE DERIVATIVES The various positions in naphthalene are designated by use of the numbering system as given in (8). For monosubstituted naphthalenes we usually use the prefixes  and  (9). This system of  and  nomenclature is not used for other polynuclear hydrocarbons.

Thus, when a monosubstituted naphthalene is named we usually describe it as -substituted or -substituted naphthalene. For example,

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For di-and polysubstituteed naphthalenes the numbers are chosen in such a manner so as to give the lowest cumulative total.

6. REACTIONS OF NAPHTHALENE 1. Electrophilic Substitution Naphthalene undergoes the usual electrophilic subsitution reactions such as nitration, halogenation, sulphonation and Friedel-Crafts acylation. Before we discuss these reactions with naphthalene, let us see how the electrophilic substitution occurs in benzene. In case of nitration of benzene, the nitronium ion attacks the substrate to give an intermediate carbocation which can stabilise itself by resonance as shown below. Abstraction of a proton from this intermediate takes place with the help of the conjugate base

(H S O 4)

to give the final product.

The rate controlling step in this is the stability of the intermediate transition state.

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Orientation of Electrophilic Substitution in Naphthalene We apply the same approach to naphthalene. It has two non-equivalent positions - and - as shown below:

Electrophilic substitution can occur either at the - or at the -position. The -position, however, is preferentially substituted. We can explain this on the basis of relative stabilities of intermediate transition states. When an electrophile attacks the -position, the intermediate carbocation has got seven resonating structures, out of which four structures have benzene rings intact. These structures contribute much more to the overall resonance hybrid. In the case of attack only two out of six resonating structures for the intermediate carbocation have benzene rings intact which contribute more to the stability of this cation. Therefore, the carbocation in the -attack is more stable than that obtained from the -attack. Hence, electrophilic substitution at the -position is preferred (Fig. 2)

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Fig. 2 Electrophilic attack at - and -positions in naphthalene.

A. Nitration As already shown naphthalene has got more reactivity than benzene. So reaction conditions for the electrophilic substitution are milder than those used for substitution in case of benzene. Nitration of naphthalene can be carried out using the mixed acid (HNO 3 + H2SO4) at 50 – 60°C to give predominantly the -isomer (yield >90%). Only a small amount of the isomer (<10%) is formed which is removed readily by recrystallisation.

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B. Halognation Chlorination of naphthalene is carried out in the presence of iron as catalyst. It gives good yields of the -isomer.

Bromine in refluxing carbon tetrachloride without any Lewis acid is sufficient to brominate naphthalene to give -bromonaphthalene in good yields. No such reaction takes place with benzene under similar conditions. This indicates that napthalene is more reactive towards electrphilic substitution than benzene. Bromine in 50% acetic acid-water, however, gives sufficiently pure -isomer.

C. Sulphonation Sulphonation of naphthalene under mild conditions (conc. H 2SO4 at 80°C) gives mainly - naphthalenesulphonic acid. If the reaction is carried out at a higher temperature (160°C), the predominant product is -naphthalenesulphonic acid. When -naphthalenesulponic acid is heated in concentrated sulphuric acid at 160°C, - naphthalenesulphonic acid is formed.

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The predominant formation of -isomer at low temperature suggests that this reaction is rate controlled and -substitution takes place at a faster rate than -substitution as has been observed in the case of nitration and halogenation since this involves the more stable intermediate transition state. At higher temperature (160°C) desulphonation of the -isomer

takes

place

readily

with

subsequent

sulphonation

of

-position.

-Naphthalenesulphonic acid once formed resists desulphonation because -isomer is more stable

thermodynamically.

-naphthalenesulphonic

acid

Although

-position

is

hindered

more

is and

more less

reactive, stable

than

but the

-naphthalenesulphonic acid because the bulky sulphonic acid group is within the van der Waal’s radius of the 8-hydrogen.

At low temperatures desulphonation is slow and we get predominantly the -isomer which is formed quickly. The -isomer formed at a slower rate is desulphonated less rapidly.

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Synthetically sulphonation is an important reaction because we get -substituted products very easily.

D. Friedel-Crafts Acylation Friedel-Crafts alkylation of naphthalene is of no use. It results in the formation of polyalkylated products because of the high reactivity of substituted products. Friedel-Crafts acylation, however, is quite useful. Like sulphonation, in this case also we get either the -isomer predominating or -isomer depending on the polarity of the solvent used. -substitution is favoured when we use non-polar solvents like tetra chloroethane or carbon disulphide e.g., acylation of naphthalene with acetyl chloride in the presence of aluminium chloride with carbon disulphide as solvent gives a mixture of - and -isomers having the ratio 3 : 1. The -isomer is difficult to remove in such a case. In polar solvents, the -isomer is the favoured product. The bulkier electrophile prefers the less-hindered -position. Thus, when nitrobenzene is used as the solvent in the above reaction, 2-acetylnaphthalene is the main product.

2. Oxidation Under mild conditions naphthalene is oxidised to 1, 4-naphthoquinone.

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Alkyl substituted naphthalenes also undergo ring oxidation under these conditions. 2- Methylnaphthalene on oxidation gives 2-methy1-1, 4-naphthoquinone, known as menadione, which has marked vitamin K activity.

Quinone is the name we give to a compound having the structure.

The quinone structure exists in a number of naturally occurring compounds. Two forms of Vitamin K, which aid in the clotting of blood, are derivatives of naphthoquinone.

We already know that alkyl benzenes are oxidized to benzoic acid using alkaline potassium permanganate. Under similar conditions naphthalene is oxidised to phthalic acid. Sodium dichromate at higher temperatures also oxidises naphthalene to phthalic acid.

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Air oxidation of naphthalene using vanadium pentoxide as catalyst gives a commercial method for the preparation of phthalic anhydride.

When a substituted naphthalene is oxidised, the substituent present plays an important role. An electron donating group (–NH2) activates the substituted ring to oxidation whereas an electron withdrawing group (–NO2) protects the ring from oxidation.

3. Reduction Reduction of naphthalene under different conditions gives different products. The Birch reduction using sodium metal/liquid ammonia in ethanol affords 1,4dihydronaphthalene.

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Catalytic hydrogenation of naphthalene can also be carried out. Tetralin (1,2,3,4tetrahydronaphthalene) and decalin (decahydronaphthalene) are obtained depending upon the catalyst or conditions. Tetralin is also produced from naphthalene when reduction is carried out by sodium in n-amyl alcohol. cis-Decalin is the predominant product of complete hydrogenation. Tetralin and decalin are liquids and are used as solvents.

4. Dehydrogenation—Aromatisation Hydrogenation of naphthalene takes place to give a number of products as shown in Sec.3. Dehydrogenation is the reverse of hydrogenation. This process is also called aromatisation, which occurs with relative ease for compounds that can form the stable aromatic ring. Dehydrogenation can be accomplished by using the same catalysts which are used for catalytic hydrogenation like platinum, palladium or nickel. Hydrogenation is favoured by excess of hydrogen under pressure whereas dehydrogenation is favoued by sweeping away the hydrogen in a stream of an inert gas. For example, tetralin can be dehydrogenated to naphthalene by heating with palladium at refflux temperature.

Aromatisation can also be affected by heating the hydroaromatic compound with sulphur, selenium or organic disulphides. When sulphur or selenium is used for dehydrogenation it is reduced to H2S or H2Se respectively. Aromatisation is an important process in the synthesis of many polynuclear hydrocarbons, which are obtained by ring closure of open chain compounds.

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5. Nucleophilic Substitution Reactions of Naphthalene Derivatives We have already studied the nucleophilic substitution reactions of benzene derivatives e.g., chlorobenzene can be converted to phenol and aniline very easily.

Similar nucleophilic substitution reactions apply to naphthalene derivatives. A. Preparation of Naphthols Like phenols, naphthols can be prepared from the corresponding sulphonic acids by fusion with alkali.

Naphthols can also be obtained by direct replacement of the amino group by —OH group. Such a reaction is not feasible in the benzene series.

Naphthols give the usual reactions of phenols. They also couple with diazonium salts to give azo dyes. B. Formation of -Naphthylamine and other - Substituted Derivatives -Naphthylamine

can

be

prepared

directly

aby

reduction

of

the

nitrocompound obtained by nitration of naphthalene.

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corresponding

-Naphthylamine can now be used for carrying out a number of reactions to give the substituted naphthalene derivatives as listed below:

C. Preparation of -Naphthylamine and - Substituted Derivatives -Naphthylamine can not be obtained from - nitronaphthalene because this nitro isomer is only a minor product in the nitration of naphthalene. A method for the direct formation of naphthylamines from naphthols has been developed. In this reaction, known as Buchrer reaction, naphthol is directly treated with ammonia in the presence of sodium bisulphite, which acts as a catalyst. Both - and -naphthols give the corresponding amines in this reaction.

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The reaction is reversible and also provides a route for the synthesis of naphthols from naphthylamines. This rection does not apply to benzene derivatives.

-Naphthol is readily available from -naphthalenesulphonic acid. Hence, we get a convenient method for preparing -naphthylamine. Having arrived at its synthesis we can now convert -naphthylamine to a large number of naphthalene derivatives through the formation of corresponding diazonium salts. -Nitronaphthalene can also be prepared by applying the Sandmeyer reaction i.e., treating the diazonium salt with cuprous nitrite.

6. Electrophilic Substitution in Naphthalene Derivatives

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The following generalisations can be made when further electrophilic substitution of mono-substituted naphthalene is carried out. (i) Ortho and para directing groups at the 1-position give mainly 4-substituted products with a small amount of 2-isomer. For example,

(ii) When the ortho and para directing groups are present in the 2-position, they generally direct the incoming group to 1-position, e.g., diazo coupling with -naphthol takes place at 1-position.

(iii) Meta directing groups either at 1- or 2-position generally direct the second substituent to 5- and 8-positions. For example,

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7. SYNTHESIS OF NAPHTHALENE AND ITS DERIVATIVES 1. Haworth Synthesis This involves basically three steps: (i) Friedel-Crafts acylation of a derivative of benzene. (ii) Ring closure to give a hydroaromatic compound. (iii) Aromatisation or dehydrogenation of the hydroaromatic compound to give finally the naphathalene derivative. Naphthalene can be synthesised by following the above scheme, but in actual practice it is not obtained in this way. The synthesis, however, establishes the structure of naphthalene. A. Synthesis of Naphthalene Friedel-Crafts acylation of benzene with succinic anhydride gives -benzoylpropionic acid. The keto group in it is reduced by Clemensen’s reduction to give 4-phenylbutyric acid which on ring closure affords -tetralone. -Tetralone on further Clemensen’s reduction gives tetralin, which can be aromatised to naphthalene.

B. Synthesis of Naphthalene Derivatives Starting from toluene we get 2-methylnaphthalene as given in Fig. 3.

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Fig. 3. Synthesis of 2-methyl naphthalene. From the above scheme it is clear that the route is open to synthesis of a variety of naphthalene derivatives.

2. Diels-Alder Method Naphthalene derivatives can also be obtained via the Diels-Alder reaction. Benzoquinone reacts with butadiene to give an addition product which on heating with HCl gives dihydroxydihydronaphthalene. This can be converted in good yields to 1, 4-naphthoquinone, which can then be converted to other derivatives of naphthalene.

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References: 1. Organic Chemistry by Morrison, R.T. and Boyd, R.N. Darling Kindersley (India) Pvt. Ltd. (Pearson Education) 2. Introduction to Organic Chemistry by Andrew Streitwieser, Jr. and Claylon H. Heathcock Macmillan Publishing Company, New York.

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