HYDROCARBONS
“Hydrocarbons are the important sources of energy.”
The term ‘hydrocarbon’ is self-explanatory which means compounds of carbon and hydrogen only. Hydrocarbons play a key role in our daily life. You must be familiar with the terms ‘LPG’ and ‘CNG’ used as fuels. LPG is the abbreviated form of liquified petroleum gas whereas CNG stands for compressed natural gas. Another term ‘LNG’ (liquified natural gas) is also in news these days. This is also a fuel and is obtained by liquifaction of natural gas. Petrol, diesel and kerosene oil are obtained by the fractional distillation of petroleum found under the earth’s crust. Coal gas is obtained by the destructive distillation of coal. Natural gas is found in upper strata during drilling of oil wells. The gas after compression is known as compressed natural gas. LPG is used as a domestic fuel with the least pollution. Kerosene oil is also used as a domestic fuel but it causes some pollution. Automobiles need fuels like petrol, diesel and CNG. Petrol and CNG operated automobiles cause less pollution. All these fuels contain mixture of hydrocarbons, which are sources of energy. Hydrocarbons are also used for the manufacture of polymers like polythene, polypropene, polystyrene etc. Higher hydrocarbons are used as solvents for paints. They are also used as the starting materials for manufacture of many dyes and drugs. Thus, you can well understand the importance of hydrocarbons in your daily life. In this unit, you will learn more about hydrocarbons.
13.1 CLASSIFICATION
Hydrocarbons are of different types. Depending upon the types of carbon-carbon bonds present, they can be classified into three main categories - (i) saturated (ii) unsaturated and (iii) aromatic hydrocarbons. Saturated hydrocarbons contain carbon-carbon and carbon-hydrogen single bonds. If different carbon atoms are joined together to form open chain of carbon atoms with single bonds, they are termed as alkanes as you have already studied in Unit 8. On the other hand, if carbon atoms form a closed chain or a ring, they are termed as cycloalkanes. Unsaturated hydrocarbons contain carbon-carbon multiple bonds double bonds, triple bonds or both. Aromatic hydrocarbons are a special type of cyclic compounds. You can construct a large number of models of such molecules of both types (open chain and close chain) keeping in mind that carbon is tetravalent and hydrogen is monovalent. For making models of alkanes, you can use toothpicks for bonds and plasticine balls for atoms. For alkenes, alkynes and aromatic hydrocarbons, spring models can be constructed.
13.2 ALKANES
As already mentioned, alkanes are saturated open chain hydrocarbons containing carbon - carbon single bonds. Methane
These hydrocarbons are inert under normal conditions as they do not react with acids, bases and other reagents. Hence, they were earlier known as paraffins (latin: parum, little; affinis, affinity). Can you think of the general formula for alkane family or homologous series? If we examine the formula of different alkanes we find that the general formula for alkanes is
In alkanes, tetrahedra are joined together in which
13.2.1 Nomenclature and Isomerism
You have already read about nomenclature of different classes of organic compounds in Unit 8. Nomenclature and isomerism in alkanes can further be understood with the help of a few more examples. Common names are given in parenthesis. First three alkanes - methane, ethane and propane have only one structure but higher alkanes can have more than one structure. Let us write structures for
Structures I and II possess same molecular formula but differ in their boiling points and other properties. Similarly structures III, IV and
Based upon the number of carbon atoms attached to a carbon atom, the carbon atom is termed as primary (1), secondary (2), tertiary (3) or quaternary (4). Carbon atom attached to no other carbon atom as in methane or to only one carbon atom as in ethane is called primary carbon atom. Terminal carbon atoms are always primary. Carbon atom attached to two carbon atoms is known as secondary. Tertiary carbon is attached to three carbon atoms and neo or quaternary carbon is attached to four carbon atoms. Can you identify
In structures II, IV and V, you observed that
Let us recall the general rules for nomenclature already discussed in Unit 8. Nomenclature of substituted alkanes can further be understood by considering the following problem:
If it is important to write the correct IUPAC name for a given structure, it is equally important to write the correct structure from the given IUPAC name. To do this, first of all, the longest chain of carbon atoms corresponding to the parent alkane is written. Then after numbering it, the substituents are attached to the correct carbon atoms and finally valence of each carbon atom is satisfied by putting the correct number of hydrogen atoms. This can be clarified by writing the structure of 3-ethyl-2, 2-dimethylpentane in the following steps :
i) Draw the chain of five carbon atoms:
ii) Give number to carbon atoms:
iii) Attach ethyl group at carbon 3 and two methyl groups at carbon 2
iv) Satisfy the valence of each carbon atom by putting requisite number of hydrogen atoms :
Thus we arrive at the correct structure. If you have understood writing of structure from the given name, attempt the following problems.
13.2.2 Preparation
Petroleum and natural gas are the main sources of alkanes. However, alkanes can be prepared by following methods :
1. From unsaturated hydrocarbons
Dihydrogen gas adds to alkenes and alkynes in the presence of finely divided catalysts like platinum, palladium or nickel to form alkanes. This process is called hydrogenation. These metals adsorb dihydrogen gas on their surfaces and activate the hydrogen - hydrogen bond. Platinum and palladium catalyse the reaction at room temperature but relatively higher temperature and pressure are required with nickel catalysts.
2. From alkyl halides
i) Alkyl halides (except fluorides) on reduction with zinc and dilute hydrochloric acid give alkanes.
ii) Alkyl halides on treatment with sodium metal in dry ethereal (free from moisture) solution give higher alkanes. This reaction is known as Wurtz reaction and is used for the preparation of higher alkanes containing even number of carbon atoms.
Bromoethane n-Butane
What will happen if two different alkyl halides are taken?
3. From carboxylic acids
i) Sodium salts of carboxylic acids on heating with soda lime (mixture of sodium hydroxide and calcium oxide) give alkanes containing one carbon atom less than the carboxylic acid. This process of elimination of carbon dioxide from a carboxylic acid is known as decarboxylation.
ii) Kolbe’s electrolytic method An aqueous solution of sodium or potassium salt of a carboxylic acid on electrolysis gives alkane containing even number of carbon atoms at the anode.
The reaction is supposed to follow the following path :
i)
ii) At anode:
iii)
iv) At cathode :
Methane cannot be prepared by this method. Why?
13.2.3 Properties
Physical properties
Alkanes are almost non-polar molecules because of the covalent nature of
Boiling point (b.p.) of different alkanes are given in Table 13.2 from which it is clear that there is a steady increase in boiling point with increase in molecular mass. This is due to the fact that the intermolecular van der Waals forces increase with increase of the molecular size or the surface area of the molecule.
You can make an interesting observation by having a look on the boiling points of three isomeric pentanes viz., (pentane, 2 -methylbutane and 2,2-dimethylpropane). It is observed (Table 13.2) that pentane having a continuous chain of five carbon atoms has the highest boiling point (309.1K) whereas 2,2 - dimethylpropane boils at
Chemical properties
As already mentioned, alkanes are generally inert towards acids, bases, oxidising and reducing agents. However, they undergo the following reactions under certain conditions.
1. Substitution reactions
One or more hydrogen atoms of alkanes can be replaced by halogens, nitro group and sulphonic acid group. Halogenation takes place either at higher temperature (573-773 K) or in the presence of diffused sunlight or ultraviolet light. Lower alkanes do not undergo nitration and sulphonation reactions. These reactions in which hydrogen atoms of alkanes are substituted are known as substitution reactions. As an example, chlorination of methane is given below:
Halogenation
Table 13.2 Variation of Melting Point and Boiling Point in Alkanes
Molecular formula | Name | Molecular mass/u | b.p./(K) | m.p./(K) |
---|---|---|---|---|
Methane | 16 | 111.0 | 90.5 | |
Ethane | 30 | 184.4 | 101.0 | |
Propane | 44 | 230.9 | 85.3 | |
Butane | 58 | 272.4 | 134.6 | |
2-Methylpropane | 58 | 261.0 | 114.7 | |
Pentane | 72 | 309.1 | 143.3 | |
2-Methylbutane | 72 | 300.9 | 113.1 | |
2,2-Dimethylpropane | 72 | 282.5 | 256.4 | |
Hexane | 86 | 341.9 | 178.5 | |
Heptane | 100 | 371.4 | 182.4 | |
Octane | 114 | 398.7 | 216.2 | |
Nonane | 128 | 423.8 | 222.0 | |
Decane | 142 | 447.1 | 243.3 | |
Eicosane | 282 | 615.0 | 236.2 |
It is found that the rate of reaction of alkanes with halogens is
Halogenation is supposed to proceed via free radical chain mechanism involving three steps namely initiation, propagation and termination as given below:
Mechanism
(i) Initiation : The reaction is initiated by homolysis of chlorine molecule in the presence of light or heat. The
(ii) Propagation : Chlorine free radical attacks the methane molecule and takes the reaction in the forward direction by breaking the
(a)
The methyl radical thus obtained attacks the second molecule of chlorine to form
(b)
The chlorine and methyl free radicals generated above repeat steps (a) and (b) respectively and thereby setup a chain of reactions. The propagation steps (a) and (b) are those which directly give principal products, but many other propagation steps are possible and may occur. Two such steps given below explain how more highly haloginated products are formed.
(iii) Termination: The reaction stops after some time due to consumption of reactants and / or due to the following side reactions :
The possible chain terminating steps are:
(a)
(b)
(c)
Though in (c),
2. Combustion
Alkanes on heating in the presence of air or dioxygen are completely oxidized to carbon dioxide and water with the evolution of large amount of heat.
The general combustion equation for any alkane is :
Due to the evolution of large amount of heat during combustion, alkanes are used as fuels.
During incomplete combustion of alkanes with insufficient amount of air or dioxygen, carbon black is formed which is used in the manufacture of ink, printer ink, black pigments and as filters.
3. Controlled oxidation
Alkanes on heating with a regulated supply of dioxygen or air at high pressure and in the presence of suitable catalysts give a variety of oxidation products.
(iv) Ordinarily alkanes resist oxidation but alkanes having tertiary
4. Isomerisation
5. Aromatization
Toluene
6. Reaction with steam
Methane reacts with steam at
7. Pyrolysis
Higher alkanes on heating to higher temperature decompose into lower alkanes, alkenes etc. Such a decomposition reaction into smaller fragments by the application of heat is called pyrolysis or cracking.
Pyrolysis of alkanes is believed to be a free radical reaction. Preparation of oil gas or petrol gas from kerosene oil or petrol involves the principle of pyrolysis. For example, dodecane, a constituent of kerosene oil on heating to
13.2.4 Conformations
Alkanes contain carbon-carbon sigma (
Conformations of ethane : Ethane molecule
1. Sawhorse projections
In this projection, the molecule is viewed along the molecular axis. It is then projected on paper by drawing the central
2. Newman projections
In this projection, the molecule is viewed at the
Relative stability of conformations: As mentioned earlier, in staggered form of ethane, the electron clouds of carbon-hydrogen bonds are as far apart as possible. Thus, there are minimum repulsive forces, minimum energy and maximum stability of the molecule. On the other hand, when the staggered form changes into the eclipsed form, the electron clouds of the carbon - hydrogen bonds come closer to each other resulting in increase in electron cloud repulsions. To check the increased repulsive forces, molecule will have to possess more energy and thus has lesser stability. As already mentioned, the repulsive interaction between the electron clouds, which affects stability of a conformation, is called torsional strain. Magnitude of torsional strain depends upon the angle of rotation about
13.3 ALKENES
Alkenes are unsaturated hydrocarbons containing at least one double bond. What should be the general formula of alkenes? If there is one double bond between two carbon atoms in alkenes, they must possess two hydrogen atoms less than alkanes. Hence, general formula for alkenes is
13.3.1 Structure of Double Bond
Carbon-carbon double bond in alkenes consists of one strong sigma (
13.3.2 Nomenclature
For nomenclature of alkenes in IUPAC system, the longest chain of carbon atoms containing the double bond is selected. Numbering of the chain is done from the end which is nearer to
the double bond. The suffix ’ene’ replaces ‘ane’ of alkanes. It may be remembered that first member of alkene series is:
13.3.3 Isomerism
Alkenes show both structural isomerism and geometrical isomerism.
Structural isomerism : As in alkanes, ethene
Structures I and III, and II and III are the examples of chain isomerism whereas structures I and II are position isomers.
Geometrical isomerism : Doubly bonded carbon atoms have to satisfy the remaining two valences by joining with two atoms or groups. If the two atoms or groups attached to each carbon atom are different, they can be represented by YX C = C XY like structure.
In (a), the two identical atoms i.e., both the
Cis form of alkene is found to be more polar than the trans form. For example, dipole moment of cis-but-2-ene is 0.33 Debye, whereas, dipole moment of the trans form is almost zero or it can be said that
trans-but-2-ene is non-polar. This can be understood by drawing geometries of the two forms as given below from which it is clear that in the trans-but-2-ene, the two methyl groups are in opposite directions, Threfore, dipole moments of
In the case of solids, it is observed that the trans isomer has higher melting point than the cis form.
Geometrical or cis-trans isomerism is also shown by alkenes of the types
13.3.4 Preparation
1. From alkynes: Alkynes on partial reduction with calculated amount of dihydrogen in the presence of palladised charcoal partially deactivated with poisons like sulphur compounds or quinoline give alkenes. Partially deactivated palladised charcoal is known as Lindlar’s catalyst. Alkenes thus obtained are having cis geometry. However, alkynes on reduction with sodium in liquid ammonia form trans alkenes.
Will propene thus obtained show geometrical isomerism? Think for the reason in support of your answer.
2. From alkyl halides: Alkyl halides (R-X) on heating with alcoholic potash (potassium hydroxide dissolved in alcohol, say,
ethanol) eliminate one molecule of halogen acid to form alkenes. This reaction is known as dehydrohalogenation i.e., removal of halogen acid. This is example of
Nature of halogen atom and the alkyl group determine rate of the reaction. It is observed that for halogens, the rate is: iodine > bromine > chlorine, while for alkyl groups it is : tert
3. From vicinal dihalides: Dihalides in which two halogen atoms are attached to two adjacent carbon atoms are known as vicinal dihalides. Vicinal dihalides on treatment with zinc metal lose a molecule of
4. From alcohols by acidic dehydration: You have read during nomenclature of different homologous series in Unit 12 that alcohols are the hydroxy derivatives of alkanes. They are represented by
13.3.5 Properties
Physical properties
Alkenes as a class resemble alkanes in physical properties, except in types of isomerism and difference in polar nature. The first three members are gases, the next fourteen are liquids and the higher ones are solids. Ethene is a colourless gas with a faint sweet smell. All other alkenes are colourless and odourless, insoluble in water but fairly soluble in non-polar solvents like benzene, petroleum ether. They show a regular increase in boiling point with increase in size i.e., every -
Chemical properties
Alkenes are the rich source of loosely held pi
1. Addition of dihydrogen: Alkenes add up one molecule of dihydrogen gas in the presence of finely divided nickel, palladium or platinum to form alkanes (Section 9.2.2) 2. Addition of halogens : Halogens like bromine or chlorine add up to alkene to form vicinal dihalides. However, iodine does not show addition reaction under normal conditions. The reddish orange colour of bromine solution in carbon tetrachloride is discharged when bromine adds up to an unsaturation site. This reaction is used as a test for unsaturation. Addition of halogens to alkenes is an example of electrophilic addition reaction involving cyclic halonium ion formation which you will study in higher classes.
3. Addition of hydrogen halides: Hydrogen halides
Addition reaction of
Addition reactions of
Addition reaction of
How will
Markovnikov, a Russian chemist made a generalisation in 1869 after studying such reactions in detail. These generalisations led Markovnikov to frame a rule called Markovnikov rule. The rule states that negative part of the addendum (adding molecule) gets attached to that carbon atom which possesses lesser number of hydrogen atoms. Thus according to this rule, product I i.e., 2-bromopropane is expected. In actual practice, this is the principal product of the reaction. This generalisation of Markovnikov rule can be better understood in terms of mechanism of the reaction.
Mechanism
Hydrogen bromide provides an electrophile,
(i) The secondary carbocation (b) is more stable than the primary carbocation (a), therefore, the former predominates because it is formed at a faster rate.
(ii) The carbocation (b) is attacked by
Anti Markounikov addition or peroxide effect or Kharash effect
In the presence of peroxide, addition of
Mechanism : Peroxide effect proceeds via free radical chain mechanism as given below:
The secondary free radical obtained in the above mechanism (step iii) is more stable than the primary. This explains the formation of 1 -bromopropane as the major product. It may be noted that the peroxide effect is not observed in addition of
4. Addition of sulphuric acid : Cold concentrated sulphuric acid adds to alkenes in accordance with Markovnikov rule to form alkyl hydrogen sulphate by the electrophilic addition reaction.
5. Addition of water : In the presence of a few drops of concentrated sulphuric acid alkenes react with water to form alcohols, in accordance with the Markovnikov rule.
6. Oxidation: Alkenes on reaction with cold, dilute, aqueous solution of potassium permanganate (Baeyer’s reagent) produce vicinal glycols. Decolorisation of
b) Acidic potassium permanganate or acidic potassium dichromate oxidises alkenes to ketones and/or acids depending upon the nature of the alkene and the experimental conditions
7. Ozonolysis : Ozonolysis of alkenes involves the addition of ozone molecule to alkene to form ozonide, and then cleavage of the ozonide by
8. Polymerisation: You are familiar with polythene bags and polythene sheets. Polythene is obtained by the combination of large number of ethene molecules at high temperature, high pressure and in the presence of a catalyst. The large molecules thus obtained are called polymers. This reaction is known as polymerisation. The simple compounds from which polymers are made are called monomers. Other alkenes also undergo polymerisation.
Polymers are used for the manufacture of plastic bags, squeeze bottles, refrigerator dishes, toys, pipes, radio and T.V. cabinets etc. Polypropene is used for the manufacture of milk crates, plastic buckets and other moulded articles. Though these materials have now become common, excessive use of polythene and polypropylene is a matter of great concern for all of us.
13.4 ALKYNES
Like alkenes, alkynes are also unsaturated hydrocarbons. They contain at least one triple bond between two carbon atoms. The number of hydrogen atoms is still less in alkynes as compared to alkenes or alkanes. Their general formula is
The first stable member of alkyne series is ethyne which is popularly known as acetylene. Acetylene is used for arc welding purposes in the form of oxyacetylene flame obtained by mixing acetylene with oxygen gas. Alkynes are starting materials for a large number of organic compounds. Hence, it is interesting to study this class of organic compounds.
13.4.1 Nomenclature and Isomerism
In common system, alkynes are named as derivatives of acetylene. In IUPAC system, they are named as derivatives of the corresponding alkanes replacing ‘ane’ by the suffix ‘yne’. The position of the triple bond is indicated by the first triply bonded carbon. Common and IUPAC names of a few members of alkyne series are given in Table 13.2.
You have already learnt that ethyne and propyne have got only one structure but there are two possible structures for butyne (i) but-1-yne and (ii) but-2-yne. Since these two compounds differ in their structures due to the position of the triple bond, they are known as position isomers. In how many ways, you can construct the structure for the next homologue i.e., the next alkyne with molecular formula
Structures I and II are position isomers and structures I and III or II and III are chain isomers.
13.4.2 Structure of Triple Bond
Ethyne is the simplest molecule of alkyne series. Structure of ethyne is shown in Fig. 13.6.
Each carbon atom of ethyne has two
orbitals of the other carbon atom, which undergo lateral or sideways overlapping to form two pi
13.4.3 Preparation
1. From calcium carbide: On industrial scale, ethyne is prepared by treating calcium carbide with water. Calcium carbide is prepared by heating quick lime with coke. Quick lime can be obtained by heating limestone as shown in the following reactions:
2. From vicinal dihalides: Vicinal dihalides on treatment with alcoholic potassium hydroxide undergo dehydrohalogenation. One molecule of hydrogen halide is eliminated to form alkenyl halide which on treatment with sodamide gives alkyne.
13.4.4 Properties
Physical properties
Physical properties of alkynes follow the same trend of alkenes and alkanes. First three members are gases, the next eight are liquids and the higher ones are solids. All alkynes are colourless. Ethyene has characteristic odour. Other members are odourless. Alkynes are weakly polar in nature. They are lighter than water and immiscible with water but soluble in organic solvents like ethers, carbon tetrachloride and benzene. Their melting point, boiling point and density increase with increase in molar mass.
Chemical properties
Alkynes show acidic nature, addition reactions and polymerisation reactions as follows :
A. Acidic character of alkyne: Sodium
metal and sodamide
These reactions are not shown by alkenes and alkanes, hence used for distinction between alkynes, alkenes and alkanes. What about the above reactions with but-1-yne and but-2-yne ? Alkanes, alkenes and alkynes follow the following trend in their acidic behaviour :
i)
ii)
B. Addition reactions: Alkynes contain a triple bond, so they add up, two molecules of dihydrogen, halogen, hydrogen halides etc. Formation of the addition product takes place according to the following steps.
The addition product formed depends upon stability of vinylic cation. Addition in unsymmetrical alkynes takes place according to Markovnikov rule. Majority of the reactions of alkynes are the examples of electrophilic addition reactions. A few addition reactions are given below:
(i) Addition of dihydrogen
(ii) Addition of halogens
Reddish orange colour of the solution of bromine in carbon tetrachloride is decolourised. This is used as a test for unsaturation.
(iii) Addition of hydrogen halides
Two molecules of hydrogen halides
(iv) Addition of water
Like alkanes and alkenes, alkynes are also immiscible and do not react with water. However, one molecule of water adds to alkynes on warming with mercuric sulphate and dilute sulphuric acid at
(v) Polymerisation
(a) Linear polymerisation: Under suitable conditions, linear polymerisation of ethyne takes place to produce polyacetylene or polyethyne which is a high molecular weight polyene containing repeating units of
Thin film of polyacetylene can be used as electrodes in batteries. These films are good conductors, lighter and cheaper than the metal conductors.
(b) Cyclic polymerisation: Ethyne on passing through red hot iron tube at
13.5 AROMATIC HYDROCARBON
These hydrocarbons are also known as ‘arenes’. Since most of them possess pleasant odour (Greek; aroma meaning pleasant smelling), the class of compounds was named as ‘aromatic compounds’. Most of such compounds were found to contain benzene ring. Benzene ring is highly unsaturated
but in a majority of reactions of aromatic compounds, the unsaturation of benzene ring is retained. However, there are examples of aromatic hydrocarbons which do not contain a benzene ring but instead contain other highly unsaturated ring. Aromatic compounds containing benzene ring are known as benzenoids and those not containing a benzene ring are known as non-benzenoids. Some examples of arenes are given below:
13.5.1 Nomenclature and Isomerism
The nomenclature and isomerism of aromatic hydrocarbons has already been discussed in Unit 8. All six hydrogen atoms in benzene are equivalent; so it forms one and only one type of monosubstituted product. When two hydrogen atoms in benzene are replaced by two similar or different monovalent atoms or groups, three different position isomers are possible. The 1, 2 or 1, 6 is known as the ortho
Friedrich August Kekulé,a German chemist was born in 1829 at Darmsdt in Germany. He became Professor in 1856 and Fellow of Royal Society in 1875. He made major contribution to structural organic chemistry by proposing in 1858 that carbon atoms can join to one another to form chains and later in 1865, he found an answer to the challenging problem of benzene structure by suggesting that these chains can close to form rings. He gave the dynamic structural formula to benzene which forms the basis for its modern electronic structure. He described the discovery of benzene structure later as:
“I was sitting writing at my textbook, but the work did not progress; my thoughts were elsewhere. I turned my chair to the fire, and dozed. Again the atoms were gambolling before my eyes. This time the smaller groups kept modestly in the background. My mental eye, rendered more acute by repeated visions of this could now distinguish larger structures of manifold conformations; long rows,sometimes more closely fitted together; all twisting and turning in snake like motion. But look! What was that? One of the snakes had seized hold of it’s own tail, and the form whirled mockingly before my eyes. As if by a flash of lightning I woke;… I spent the rest of the night working out the consequences of the hypothesis. Let us learn to dream, gentlemen, and then perhaps we shall learn the truth but let us beware of making our dreams public before they have been approved by the waking mind.”( 1890).
One hundred years later, on the occasion of Kekule’s centenary celebrations a group of compounds having polybenzenoid structures have been named as Kekulenes.
13.5.2 Structure of Benzene
Benzene was isolated by Michael Faraday in 1825. The molecular formula of benzene,
The Kekulé structure indicates the possibility of two isomeric 1, 2-dibromobenzenes. In one of the isomers, the bromine atoms are attached to the doubly bonded carbon atoms whereas in the other, they are attached to the singly bonded carbons.
However, benzene was found to form only one ortho disubstituted product. This problem was overcome by Kekule by suggesting the concept of oscillating nature of double bonds in benzene as given below.
Even with this modification, Kekule structure of benzene fails to explain unusual stability and preference to substitution reactions than addition reactions, which could later on be explained by resonance.
Resonance and stability of benzene
According to Valence Bond Theory, the concept of oscillating double bonds in benzene is now explained by resonance. Benzene is a hybrid of various resonating structures. The two structures, A and B given by Kekulé are the main contributing structures. The hybrid structure is represented by inserting a circle or a dotted circle in the hexagon as shown in (C). The circle represents the six electrons which are delocalised between the six carbon atoms of the benzene ring.
The orbital overlapping gives us better picture about the structure of benzene. All the six carbon atoms in benzene are
The unhybridised
Structures shown in Fig. 13.7(a) and (b) correspond to two Kekulé’s structure with localised
The six
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, two types of
13.5.3 Aromaticity
Benzene was considered as parent ‘aromatic’ compound. Now, the name is applied to all the ring systems whether or not having benzene ring, possessing following characteristics. (i) Planarity
(ii) Complete delocalisation of the
(iii) Presence of
This is often referred to as Hückel Rule.
Some examples of aromatic compounds are given below:
13.5.4 Preparation of Benzene
Benzene is commercially isolated from coal tar. However, it may be prepared in the laboratory by the following methods.
(i) Cyclic polymerisation of ethyne: (Section 13.4.4)
(ii) Decarboxylation of aromatic acids: Sodium salt of benzoic acid on heating with sodalime gives benzene.
(iii) Reduction of phenol: Phenol is reduced to benzene by passing its vapours over heated zinc dust
13.5.5 Properties
Physical properties
Aromatic hydrocarbons are non- polar molecules and are usually colourless liquids or solids with a characteristic aroma. You are 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
Arenes are characterised by electrophilic substitution reactions. However, under special conditions they can also undergo addition and oxidation reactions.
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
(i) Nitration: A nitro group is introduced into benzene ring when benzene is heated with a mixture of concentrated nitric acid and concentrated sulphuric acid (nitrating mixture).
(ii) Halogenation: Arenes react with halogens in the presence of a Lewis acid like anhydrous
(iii) Sulphonation: The replacement of a hydrogen atom by a sulphonic acid group in a ring is called sulphonation. It is carried out by heating benzene with fuming sulphuric acid (oleum).
(iv) Friedel-Crafts alkylation reaction: When benzene is treated with an alkyl halide in the presence of anhydrous aluminium chloride, alkylbenene is formed.
Why do we get isopropyl benzene on treating benzene with 1-chloropropane instead of
(v) Friedel-Crafts acylation reaction: The reaction of benzene with an acyl halide or acid anhydride in the presence of Lewis acids
If excess of electrophilic reagent is used, further substitution reaction may take place in which other hydrogen atoms of benzene ring may also be successively replaced by the electrophile. For example, benzene on treatment with excess of chlorine in the presence of anhydrous
Mechanism of electrophilic substitution reactions:
According to experimental evidences,
(a) Generation of the eletrophile
(b) Formation of carbocation intermediate
(c) Removal of proton from the carbocation intermediate
(a) Generation of electrophile
In the case of nitration, the electrophile, nitronium 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) Formation of Carbocation (arenium ion): Attack of electrophile results in the formation of
Sigma complex or arenium ion loses its aromatic character because delocalisation of electrons stops at
(c) Removal of proton: To restore the aromatic character,
Addition reactions
Under vigorous conditions, i.e., at high temperature and/ or pressure in the presence of nickel catalyst, hydrogenation of benzene gives cyclohexane.
Under ultra-violet light, three chlorine molecules add to benzene to produce benzene hexachloride,
Combustion: When heated in air, benzene burns with sooty flame producing
General combustion reaction for any hydrocarbon may be given by the following chemical equation:
13.5.6 Directive influence of a functional group in monosubstituted benzene
When monosubstituted benzene is subjected to further substitution, three possible disubstituted products are not formed in 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:
Ortho and para directing groups: The groups which direct the incoming group to ortho and para positions are called ortho and para directing groups. As an example, let us discuss the directive influence of phenolic
It is clear from the above resonating structures that the electron density is more on
In the case of aryl halides, halogens are moderately deactivating. Because of their strong - I effect, overall electron density on benzene ring decreases. It makes further substitution difficult. However, due to resonance the electron density on
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
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.
In this case, the overall electron density on benzene ring decreases making further substitution difficult, therefore these groups are also called ‘deactivating groups’. The electron density on
13.6 CARCINOGENICITY AND TOXICITY
Benzene and polynuclear hydrocarbons 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
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
Benzene and benzenoid compounds show aromatic character. Aromaticity, the property of being aromatic is possessed by compounds having specific electronic structure characterised by Hückel