Organic chemistry note of aromatic compounds - benzene, nitrobenzene, aniline and phenol based on CTEVT health science first year syllabus
Unit 2.11 – Aromatic compounds
Definition
Benzene and those cyclic compounds that chemically behave as benzene are called aromatic compounds. Eg.
Structure of benzene
Kekule’s structure of benzene :
Kekule, a German scientist proposed the structure of benzene for the first time. According to Kekule, all the 6 carbon atoms of benzene molecule are joint to each other by alternate single and double bond forming a hexagonal ring and a hydrogen atom is bonded to each carbon atom.
Resonance structure of benzene :
The double bonds may be localized in any position and therefore following resonating structures are possible :
According to these structures, there should be three single bonds (bond length 154 pm) and three double bonds (bond length 134 pm) between carbon atoms in the benzene molecule. But actually it has been found by X- ray diffraction studies that all the carbon-carbon bonds in benzene are equivalent and have bond length 139 pm , which is intermediate between C – C (154 pm) and C = C (134 pm). Thus, the actual structure of benzene is different from both ‘A’ and ‘B’ and is a resonance hybrid of these two resonating forms.
Note: pm = picometre, 1pm = 10 -12 m
Nomenclature of aromatic compounds
1. Naming of mono-substituted benzene :
Monosubstituted benzenes are usually named by prefixing the name of the substituent before the word ‘ benzene’. Eg.
However, some of the monosubstituted benzenes are known by their special names.
2. Naming of disubstituted benzene :
- 2 and 6 positions are also indicated by prefix ortho (o-)
- 3 and 5 positions are also indicated by prefix meta (m-)
- Position ‘4’ is also indicated by prefix para (p-). Eg.
If two different substituents are present then, ortho, meta and para is followed by the substituent in an alphabetical order. Eg.
If one of the substituent present in the disubstituted benzene gives the special name then disubstituted benzene is named as derivatives of that special molecule. Eg.
Necessary conditions for any compound to show aromaticity
The necessary conditions for any compound to show aromaticity are as follows :
- The compound must be cyclic and planar and allow cyclic overlap of p-orbitals.
- There must be complete delocalization of π – electrons.
- The compounds must contains π – electrons according to Huckel’s rule, i.e. (4n+2) π – electrons, where n = 0, 1, 2, 3, 4, etc.
Huckel’s rule :
Huckel’s rule states that a cyclic, planar and conjugated molecule is aromatic if it contains 4n+2 delocalized π electrons, where n = 0, 1, 2, 3, 4, etc.
Example : Benzene :
Benzene is cyclic and planar and has cyclic overlap of p-orbitals. There are 3 double bonds i.e. 6 delocalized π – electrons, which is consistant with Huckel’s rule.
i.e. 4n+2 = 6
4n= 4
n = 1(which is an integer)
Therefore, benzene is an aromatic compound. It will show aromaticity.
Preparation of benzene
1. From ethyne ( manufacture of benzene) :
When ethyne gas is passed through a red hot iron or copper tube, the three molecules of ethyne (acetylene) polymerize to give benzene.
2. By decarboxylation of sodium salt of benzoic acid ( Laboratory method) :
Benzene can be prepared by heating sodium salt of benzoic acid ( i.e. sodium benzoate) with sodalime. This reaction is called decarboxylation reaction.
3. By reducing phenol with zinc :
When vapours of phenol are passed over heated zinc dust, benzene is produced.
Chemical properties of benzene
Electrophilic substitution reactions of benzene :
The most important/common reaction of benzene is electrophilic substitution reaction. In this reaction, an electrophile attacks the benzene and substitutes one of the hydrogen atoms of benzene ring. Eg.
1. Halogenation : Benzene reacts with bromine in presence of ferric bromide as catalyst to give bromobenzene.
Similarly, chlorine reacts with benzene in presence of ferric chloride or AlCl3 as catalyst to give chlorobenzene.
2. Nitration : When benzene is heated with conc. HNO3 in the presence of conc. H2SO4 at about 600C gives nitrobenzene.
3. Sulphonation : When benzene is heated with conc. H2SO4 , benzene sulphonic acid is formed.
4. Friedel – Craft’s reaction :
- Friedel-Craft’s alkylation : Introduction of an alkyl group ( – R ) in the benzene ring by treating benzene with an alkyl halide (R-Cl or R-Br) in the presence of anhydrous AlCl3 is known as Friedel – Craft’s alkylation. Eg.
- Friedel-Craft’s acylation : Introduction of an acyl group (i.e. acyl group) (
) in the benzene ring by treating benzene with an acylating agent like acid chloride (RCOCl) in the presence of anhydrous AlCl3 is known as Friedel- Craft’s acylation. Eg.
Uses of benzene
1. It is used as a starting material for the preparation of varieties of aromatic compounds which are used for the manufacture of dyes, drugs, perfumes, explosives, etc. Eg. benzene is used for making toluene which is needed for making TNT.
2. It is used as a solvent for the extraction of fat and oil.
3. It is used as a fuel for automobiles in the name of benzol.
4. It is used for dry cleaning of woolen clothes.
5. It is used for making phenol needed for producing Bakelite.
Nitrobenzene
Laboratory Preparation of Nitrobenzene
It is prepared in lab by heating benzene with conc. HNO3 and conc. H2SO4 at 600C.
Procedure : 50 ml of benzene is taken in a round bottomed flask. To this flask, 60 ml conc. HNO3 and 60 ml conc. H2SO4 (i.e. nitrating mixture) is added a little at a time, shaking and cooling after each addition. Then the mixture is heated (refluxed) in water bath at 600C for about one and half hour till the yellow oily layer appears on the surface. The flask is then cooled and the layer of nitrobenzene is separated by using separating funnel. 
Purification : It is first washed with dil. Na2CO3 to remove the acidic impurities and then with water severaltimes. It is then dried over fused calcium chloride. It is finally distilled at 2110C to get pure nitrobenzene.
Chemical Properties of Nitrobenzene
Reduction of nitrobenzene in different medium :
Nitrobenzene gives different products in different medium by using different reducing agent.
1. Reduction of nitrobenzene in acidic medium :
Nitrobenzene on reduction with Zn/HCl or Sn/ HCl gives aniline.
2. Catalytic reduction of nitrobenzene :
Nitrobenzene when reduced by hydrogen in presence of nickel or platinum as a catalyst gives aniline.
3. Reduction of nitrobenzene in neutral medium :
Nitrobenzene on reduction with Zn and aq. NH4Cl gives phenyl hydroxylamine.
4. Reduction of nitrobenzene with LiAlH4 :
Lithium aluminium hydride reduces nitrobenzene to azobenzene.
5. Reduction of nitrobenzene in alkaline (basic) medium :
6. Electrolytic reduction of nitrobenzene :
Nitrobenzene when reduced electrolytically, first gives phenyl hydroxylamine which immediately rearranges to give p- aminophenol.
Aromatic amine (Aniline)
Laboratory preparation of aniline
Aniline is prepared in laboratory by reducing nitrobenzene with tin (Sn) and conc. HCl.
10 ml nitrobenzene and 20 gm of granulated tin are placed in the 250 ml round bottom flask fitted with a reflux condenser. 50 ml of conc. HCl is added gradually with constant shaking. After each addition, the round bottom flask is cooled so that temperature may not go above 900C. Then the reaction mixture is heated on a boiling water bath for about one hour until the reaction is completed which is indicated by the smell of nitrobenzene, the disappearance of smell indicates the completion of the reaction. The flask is then cooled and a crystalline solid mass of double salt is separated out.
The crystalline solid mass is then treated with conc. NaOH until the solution is cleared and becomes strongly alkaline. Aniline is separated out and is floated on the surface as a dark brown oil.
The mixture obtained is then subjected to the process of steam distillation until clear distillate is obtained.
Purification : Aniline is extracted by shaking the distillate several times with ether. The ethereal layer is separated each time with the help of separating funnel. Now, the ethereal aniline is placed for the evaporation where ether evaporates out. Aniline thus obtained is finally purified by redistillation at 182-1840C.
Chemical properties of aniline
1. Basic nature of aniline : Aniline is basic in nature. It reacts with acid to give salt. Eg.
2. Halogenation : Aniline reacts with aq. Bromine ( i.e. bromine water) to give white ppt. of 2,4,6-tribromoaniline.
3. Nitration : Aniline reacts with conc. HNO3 in presence of conc. H2SO4 under proper conditions to give o- and p- nitro aniline.
4. Sulphonation : When aniline is heated with conc. H2SO4 gives p-amino benzenesulphonic acid.
Phenols
Preparation of phenol
From diazonium salt (Laboratory method) :
Phenol is prepared in the laboratory by warming an aqueous solution of diazonium chloride.
Chemical properties of phenol
1. Action with zinc dust : When phenol is heated with zinc dust benzene is formed.
2. Action with NaOH : Phenol behaves as weak acid and reacts with NaOH to form salt and water.
3. Action with ammonia : In the presence of anhydrous ZnCl2, phenol reacts with ammonia at high temperature to give aniline.
4. Action with PCl5 : Phenol reacts with PCl5 to give chlorobenzene.
5. Halogenation : Phenol reacts with aq. bromine ( i.e. bromine water) to give 2,4,6-tribromophenol.
⇐ See note of previous chapters
See the note of next chapter ⇒
References
- Sthapit, M.K., Pradhananga, R.R., Foundations of Chemistry, Vol 1 and 2, Fourth edition, Taleju Prakashan, 2005.
- Finar, I. L., Organic Chemistry, Vol. I and Vol. II, Prentice Hall, London, 1995.
- Ghosh, S.K., Advanced General Organic Chemistry, Second Edition, New Central Book Agency Pvt. Ltd., Kolkatta, 2007.
- Morrison, R.T. , Boyd, R.N., Organic Chemistry, Sixth edition, Prentice-Hall of India Pvt. Ltd., 2008.
- https://www.atsdr.cdc.gov/toxfaqs/tfacts140.pdf
- https://chemicalnote.com/nitrobenzene-laboratory-preparation-properties-and-uses/
- https://chemicalnote.com/aromatic-compounds-structure-preparation-properties-and-uses-of-benzene/
- https://emergency.cdc.gov/agent/benzene/basics/facts.asp#:~:text=Benzene%20is%20a%20chemical%20that,float%20on%20top%20of%20water.
