8.17 Complete each synthesis by giving missing starting material, reagent or products

9.7 (c)]. This can be represented in the form This is often referred to as Hückel Rule.of two doughtnuts (rings) of electron clouds [Fig. 9.7 (d)], one above and one below the Some examples of aromatic compounds are plane of the hexagonal ring as shown below: given below: (electron cloud) Fig. 9.7 (c) Fig. 9.7 (d) The six π electrons are thus delocalised and can move freely about the six carbon nuclei, instead of any two as shown in Fig. 9.6 (a) or (b). The delocalised π electron cloud is attracted more strongly by the nuclei of the carbon atoms than the electron cloud localised between two carbon atoms. Therefore, presence of delocalised π electrons in benzene makes it more stable than the hypothetical cyclohexatriene. X-Ray diffraction data reveals that benzene is a planar molecule. Had any one of the above structures of benzene (A or B) been correct, 9.5.4 Preparation of Benzene two types of C—C bond lengths were expected. Benzene is commercially isolated from coalHowever, X-ray data indicates that all the tar. However, it may be prepared in thesix C—C bond lengths are of the same order laboratory by the following methods.(139 pm) which is intermediate between C— C single bond (154 pm) and C—C double (i) Cyclic polymerisation of ethyne: bond (133 pm). Thus the absence of pure (Section 9.4.4) double bond in benzene accounts for the (ii) Decarboxylation of aromatic acids: reluctance of benzene to show addition Sodium salt of benzoic acid on heating reactions under normal conditions, thus with sodalime gives benzene. explaining the unusual behaviour of benzene. 9.5.3 Aromaticity Benzene was considered as parent ‘aromatic’ compound. Now, the name is applied to all the ring systems whether or not having benzene (9.70) ring, possessing following characteristics. Reprint 2025-26 322 chemistry (iii) Reduction of phenol: Phenol is reduced (ii) Halogenation: Arenes react with halogens to benzene by passing its vapours over in the presence of a Lewis acid like anhydrous heated zinc dust FeCl3, FeBr3 or AlCl3 to yield haloarenes. (9.71) Chlorobenzene 9.5.5 Properties (9.73) (iii) Sulphonation: The replacement of aPhysical properties hydrogen atom by a sulphonic acid group inAromatic hydrocarbons are non- polar a ring is called sulphonation. It is carried outmolecules and are usually colourless liquids by heating benzene with fuming sulphuricor solids with a characteristic aroma. You are acid (oleum).also familiar with naphthalene balls which are used in toilets and for preservation of clothes because of unique smell of the compound and the moth repellent property. Aromatic hydrocarbons are immiscible with water but are readily miscible with organic solvents. They burn with sooty flame. Chemical properties (9.74) Arenes are characterised by electrophilic (iv) Friedel-Crafts alkylation reaction:substitution reactions. However, under When benzene is treated with an alkyl halidespecial conditions they can also undergo in the presence of anhydrous aluminiumaddition and oxidation reactions. chloride, alkylbenene is formed. Electrophilic substitution reactions The common electrophilic substitution reactions of arenes are nitration, halogenation, sulphonation, Friedel Craft’s alkylation and acylation reactions in which attacking reagent is an electrophile (E +) (i) Nitration: A nitro group is introduced (9.75) into benzene ring when benzene is heated with a mixture of concentrated nitric acid and concentrated sulphuric acid (nitrating mixture). (9.76) Why do we get isopropyl benzene on treating benzene with 1-chloropropane instead of n-propyl benzene? (v) Friedel-Crafts acylation reaction: The reaction of benzene with an acyl halide or (9.72) acid anhydride in the presence of Lewis acids (AlCl3) yields acyl benzene. Nitrobenzene Reprint 2025-26 Hydrocarbons 323 (9.77) In the case of nitration, the electrophile, nitronium ion, is produced by transfer of a proton (from sulphuric acid) to nitric acid in the following manner: (9.78) Step I If excess of electrophilic reagent is used, further substitution reaction may take place in which other hydrogen atoms of benzene Step II ring may also be successively replaced by the electrophile. For example, benzene on treatment with excess of chlorine in the presence of anhydrous AlCl3 can be Protonated Nitronium chlorinated to hexachlorobenzene (C6Cl6) nitric acid ion It is interesting to note that in the process of generation of nitronium ion, sulphuric acid serves as an acid and nitric acid as a base. Thus, it is a simple acid-base equilibrium. (b) F o r m a t i o n o f C a r b o c a t i o n (arenium ion): Attack of electrophile results in the formation of σ-complex or (9.79) 3 arenium ion in which one of the carbon is sp Mechanism of electrophilic substitution hybridised. reactions: According to experimental evidences, SE (S = substitution; E = electrophilic) reactions are supposed to proceed via the following three steps: (a) Generation of the eletrophile sigma complex (arenium ion) (b) Formation of carbocation intermediate The arenium ion gets stabilised by resonance:(c) Removal of proton from the carbocation intermediate (a) Generation of electrophile E ⊕: During chlorination, alkylation and acylation of benzene, anhydrous AlCl3, being a Lewis acid helps in generation of the elctrophile Cl⊕, R ⊕, RC⊕O (acylium ion) respectively by combining with the attacking reagent. Reprint 2025-26 324 chemistry Sigma complex or arenium ion loses its chemical equation: aromatic character because delocalisation of 3 CxHy + (x + ) O2 → x CO2 + H2O n (9.83)electrons stops at sp hybridised carbon. (c) Removal of proton: To restore the 9.5.6 Directive influence of a functional aromatic character, σ -complex releases group in monosubstituted benzene proton from sp3 hybridised carbon on attack – When monosubstituted benzene is subjected by [AlCl4] (in case of halogenation, alkylation – to further substitution, three possible and acylation) and [HSO4] (in case of disubstituted products are not formed in nitration). equal amounts. Two types of behaviour are observed. Either ortho and para products or meta product is predominantly formed. It has also been observed that this behaviour depends on the nature of the substituent already present in the benzene ring and not on the nature of the entering group. This is known as directive influence of substituents. Reasons for ortho/para or meta directive nature of groups are discussed below: Addition reactions Ortho and para directing groups: The Under vigorous conditions, i.e., at high groups which direct the incoming group to temperature and/ or pressure in the presence ortho and para positions are called ortho and of nickel catalyst, hydrogenation of benzene para directing groups. As an example, let us gives cyclohexane. discuss the directive influence of phenolic (–OH) group. Phenol is resonance hybrid of following structures: Cyclohexane (9.80) Under ultra-violet light, three chlorine molecules add to benzene to produce benzene hexachloride, C6H6Cl6 which is also called gammaxane. Benzene hexachloride, It is clear from the above resonating (BHC) structures that the electron density is more on (9.81) o – and p – positions. Hence, the substitution Combustion: When heated in air, benzene takes place mainly at these positions. However, burns with sooty flame producing CO2 and it may be noted that –I effect of – OH group also H2O operates due to which the electron density on 15 ortho and para positions of the benzene ring C H6 + O2 → 6CO2 +3H2 O is slightly reduced. But the overall electron 6 2 (9.82) density increases at these positions of the General combustion reaction for any ring due to resonance. Therefore, –OH group hydrocarbon may be given by the following activates the benzene ring for the attack by Reprint 2025-26 Hydrocarbons 325 an electrophile. Other examples of activating In this case, the overall electron density groups are –NH2, –NHR, –NHCOCH3, –OCH3, on benzene ring decreases making further –CH3, –C2H5, etc. substitution difficult, therefore these groups are also called ‘deactivating groups’. TheIn the case of aryl halides, halogens are moderately deactivating. Because of their electron density on o – and p – position strong – I effect, overall electron density on is comparatively less than that at meta benzene ring decreases. It makes further position. Hence, the electrophile attacks on substitution difficult. However, due to comparatively electron rich meta position resonance the electron density on o– and resulting in meta substitution. p– positions is greater than that at the 9.6 Carcinogenicity and Toxicitym-position. Hence, they are also o– and p – directing groups. Resonance structures of Benzene and polynuclear hydrocarbons chlorobenzene are given below: containing more than two benzene rings fused together are toxic and said to possess cancer producing (carcinogenic) property. Such polynuclear hydrocarbons are formed on incomplete combustion of organic materials like tobacco, coal and petroleum. They enter into human body and undergo various biochemical reactions and finally damage DNA and cause cancer. Some of the carcinogenic hydrocarbons are given below (see box). Meta directing group: The groups which direct the incoming group to meta position are called meta directing groups. Some examples of meta directing groups are –NO2, –CN, –CHO, –COR, –COOH, –COOR, –SO3H, etc. Let us take the example of nitro group. Nitro group reduces the electron density in the benzene ring due to its strong–I effect. Nitrobenzene is a resonance hybrid of the following structures. Reprint 2025-26 326 chemistry SUMMARY Hydrocarbons are the compounds of carbon and hydrogen only. Hydrocarbons are mainly obtained from coal and petroleum, which are the major sources of energy. Petrochemicals are the prominent starting materials used for the manufacture of a large number of commercially important products. LPG (liquefied petroleum gas) and CNG (compressed natural gas), the main sources of energy for domestic fuels and the automobile industry, are obtained from petroleum. Hydrocarbons are classified as open chain saturated (alkanes) and unsaturated (alkenes and alkynes), cyclic (alicyclic) and aromatic, according to their structure. The important reactions of alkanes are free radical substitution, combustion, oxidation and aromatization. Alkenes and alkynes undergo addition reactions, which are mainly electrophilic additions. Aromatic hydrocarbons, despite having unsaturation, undergo mainly electrophilic substitution reactions. These undergo addition reactions only under special conditions. Alkanes show conformational isomerism due to free rotation along the C–C sigma bonds. Out of staggered and the eclipsed conformations of ethane, staggered conformation is more stable as hydrogen atoms are farthest apart. Alkenes exhibit geometrical (cis-trans) isomerism due to restricted rotation around the carbon–carbon double bond. Benzene and benzenoid compounds show aromatic character. Aromaticity, the property of being aromatic is possessed by compounds having specific electronic structure characterised by Hückel (4n+2)π electron rule. The nature of groups or substituents attached to benzene ring is responsible for activation or deactivation of the benzene ring towards further electrophilic substitution and also for orientation of the incoming group. Some of the polynuclear hydrocarbons having fused benzene ring system have carcinogenic property. EXERCISES 9.1 How do you account for the formation of ethane during chlorination of methane ? 9.2 Write IUPAC names of the following compounds : (a) CH3CH=C(CH3)2 (b) CH2=CH-C≡C-CH3 (c) (d) –CH2–CH2–CH=CH2 (f) CH3(CH2)4 CH (CH2)3 CH3 (e) CH2 –CH (CH3)2 (g) CH3 – CH = CH – CH2 – CH = CH – CH – CH2 – CH = CH2 | C2H5 9.3 For the following compounds, write structural formulas and IUPAC names for all possible isomers having the number of double or triple bond as indicated : (a) C4H8 (one double bond) (b) C5H8 (one triple bond) 9.4 Write IUPAC names of the products obtained by the ozonolysis of the following compounds : (i) Pent-2-ene (ii) 3,4-Dimethylhept-3-ene (iii) 2-Ethylbut-1-ene (iv) 1-Phenylbut-1-ene Reprint 2025-26 Hydrocarbons 327 9.5 An alkene ‘A’ on ozonolysis gives a mixture of ethanal and pentan-3-one. Write structure and IUPAC name of ‘A’. 9.6 An alkene ‘A’ contains three C – C, eight C – H σ bonds and one C – C π bond. ‘A’ on ozonolysis gives two moles of an aldehyde of molar mass 44 u. Write IUPAC name of ‘A’. 9.7 Propanal and pentan-3-one are the ozonolysis products of an alkene? What is the structural formula of the alkene? 9.8 Write chemical equations for combustion reaction of the following hydrocarbons: (i) Butane (ii) Pentene (iii) Hexyne (iv) Toluene 9.9 Draw the cis and trans structures of hex-2-ene. Which isomer will have higher b.p. and why? 9.10 Why is benzene extra ordinarily stable though it contains three double bonds? 9.11 What are the necessary conditions for any system to be aromatic? 9.12 Explain why the following systems are not aromatic? (i) (ii) (iii) 9.13 How will you convert benzene into (i) p-nitrobromobenzene (ii) m- nitrochlorobenzene (iii) p - nitrotoluene (iv) acetophenone? 9.14 In the alkane H3C – CH2 – C(CH3)2 – CH2 – CH(CH3)2, identify 1°,2°,3° carbon atoms and give the number of H atoms bonded to each one of these. 9.15 What effect does branching of an alkane chain has on its boiling point? 9.16 Addition of HBr to propene yields 2-bromopropane, while in the presence of benzoyl peroxide, the same reaction yields 1-bromopropane. Explain and give mechanism. 9.17 Write down the products of ozonolysis of 1,2-dimethylbenzene (o-xylene). How does the result support Kekulé structure for benzene? 9.18 Arrange benzene, n-hexane and ethyne in decreasing order of acidic behaviour. Also give reason for this behaviour. 9.19 Why does benzene undergo electrophilic substitution reactions easily and nucleophilic substitutions with difficulty? 9.20 How would you convert the following compounds into benzene? (i) Ethyne (ii) Ethene (iii) Hexane 9.21 Write structures of all the alkenes which on hydrogenation give 2-methylbutane. 9.22 Arrange the following set of compounds in order of their decreasing relative reactivity with an electrophile, E+ (a) Chlorobenzene, 2,4-dinitrochlorobenzene, p-nitrochlorobenzene (b) Toluene, p-H3C – C6H4 – NO2, p-O2N – C6H4 – NO2. 9.23 Out of benzene, m–dinitrobenzene and toluene which will undergo nitration most easily and why? 9.24 Suggest the name of a Lewis acid other than anhydrous aluminium chloride which can be used during ethylation of benzene. 9.25 Why is Wurtz reaction not preferred for the preparation of alkanes containing odd number of carbon atoms? Illustrate your answer by taking one example. Reprint 2025-26

8.4 Classification of Organic (homocyclic). Compounds The existing large number of organic compounds and their ever -increasing numbers has made it necessary to classify them on the basis of their structures. Organic Cyclopropane Cyclohexane Cyclohexene compounds are broadly classified as follows: Sometimes atoms other than carbon are also present in the ring (heterocylic). Tetrahydrofuran given below is an example of this type of compound: Tetrahydrofuran These exhibit some of the properties similar to those of aliphatic compounds. (b) Aromatic compounds Aromatic compounds are special types of compounds. You will learn about these compounds in detail in Unit 9. These include benzene and other related ring compounds (benzenoid). Like alicyclic compounds, aromatic comounds may also have hetero atom in the ring. Such compounds are called I. Acyclic or open chain compounds hetrocyclic aromatic compounds. Some of the examples of various types of aromatic These compounds are also called as aliphatic compounds are: compounds and consist of straight or branched chain compounds, for example: Benzenoid aromatic compounds CH3CH3 Ethane Isobutane Benzene Aniline Naphthalene Non-benzenoid compound Acetaldehyde Acetic acid II Cyclic or closed chain or ring compounds (a) Alicyclic compounds Tropone Alicyclic (aliphatic cyclic) compounds contain carbon atoms joined in the form of a ring Reprint 2025-26 262 chemistry Heterocyclic aromatic compounds so because it is found in citrus fruits and the acid found in red ant is named formic acid since the Latin word for ant is formica. These names are traditional and are considered as trivial or common names. Some common Furan Thiophene Pyridine names are followed even today. For example, Organic compounds can also be classified Buckminsterfullerene is a common name on the basis of functional groups, into families given to the newly discovered C60 cluster (a or homologous series. form of carbon) noting its structural similarity to the geodesic domes popularised by the8.4.1 Functional Group famous architect R. Buckminster Fuller. The functional group is an atom or a group Common names are useful and in many of atoms joined to the carbon chain which is cases indispensable, particularly when the responsible for the characteristic chemical alternative systematic names are lengthy and properties of the organic compounds. The complicated. Common names of some organic examples are hydroxyl group (–OH), aldehyde compounds are given in Table 8.1. group (–CHO) and carboxylic acid group (– COOH) etc. Table 8.1 Common or Trivial Names of Some Organic Compounds 8.4.2 Homologous Series A group or a series of organic compounds each containing a characteristic functional group forms a homologous series and the members of the series are called homologues. The members of a homologous series can be represented by general molecular formula and the successive members differ from each other in molecular formula by a –Ch2 unit. There are a number of homologous series of organic compounds. Some of these are alkanes, alkenes, alkynes, haloalkanes, alkanols, alkanals, alkanones, alkanoic acids, amines etc. It is also possible that a compound contains two or more identical or different functional groups. This gives rise to polyfunctional compounds.