HYDROCARBONS are the compounds containing carbon and hydrogen only.
Depending upon the types of carbon-carbon bonds present, they can be classified into tree main categories:1) Saturated Hydrocarbon2) Unsaturated Hydrocarbon3) Aromatic Hydrocarbon
The hydrocarbon that contain only carbon-carbon single bond is called Saturated Hydrocarbon. These include open chain hydrocarbon as well as closed chain hydrocarbons. These compounds are called saturated because they have maximum number of bonded hydrogen If different carbon atoms are joined together to form an open chain of carbon atoms with single bonds, they are called Alkanes.For example: 2-Methylpropane (Isobutane)If carbon atoms form a closed chain or ring, they are called Cycloalkanes.For example: Cyclopentane
The hydrocarbons which contain carbon-carbon multiple bond (Double bonds or triple bond) are called unsturated hydrocarbon. Depending upon multiple bond they are further classified as alkenes and alkynes.o Alkenes : These are hydrocarbon which contain at least one carbon-carbon bond. For example: Etheneo Alkynes: These are hydrocarbons which contain at least one carbon-carbon triple bond. For example: Ethyle
The hydrocarbons which contain at least one special type of hexagonal ring of carbon atoms with three double bond in the alternate positions are called aromatic hydrocarbon. The ring is called aromatic ring.For example: i) Toluene ii) o-XyleneThe aromatic compounds may also contain more than one benzene rings. For example: i) Naphthalene ii) Anthracene
Hydrocarbon Type Characteristic Group ExampleSaturated Hydrocarbon: No double or Triple Bond CH3CH2CH3 Propane AlkanesUnsaturated Hydrocarbon: Double Bond CH3–CH═CH2 Alkenes Propene Alkynes Triple Bond CH3−C≡CH PropyneAromatic Hydrocarbons: Benzene ring Methyl Benzene
Alkanes are saturated hydrocarbon containing only carbon-carbon single bond in their molecule. They are also called Paraffins. At high temperatures and pressure do undergo some reaction. The alkanes may be divided as:1) Open chain or Acyclic alkanes .2) Cycloalkanes or cyclic alkanes.
These are simple alkanes without any close chains and have the general formula where CnH2n + 2 n is the number of carbon atoms.For example: i) Methane - CH4
These contain a closed chain or ring in their molecules. They have the general formula CnH2n.For example: i) Cyclopropane- or ii)Cyclobutane- or
Methane is the first member of the family. It has Tetrahedral Structure involving sp3 Hybridisation. The four sigma bond is formed by the overlapping of sp3 hybrid orbitals of carbon and 1s orbital of hydrogen. In this, carbon atom lies at the centre and the four hydrogen atoms lies at the corners of a regular tetrahedron. Making H-C-H bond angle of 109.5˚. a) b) c)
1.4 Nomenclature OfAlkanesNomenclature implies assigning proper name to the basis of certain standard rules so that the study of these compounds may become standard. The rules for naming them are as follows:i)First of all, select the longest continues chain of carbon atoms in a molecule. 1 2 3 4 5 6 7 8 9For eg. CH3– CH– CH2– CH2– CH2–CH– CH2– CH2–CH3 CH3 CH2−CH3In the example ,the longest chain has nine carbons and it is considered as parent root chain and carbon atoms which are not included in parent chain are called substituents.
The carbon atoms of the parent chain are numbered to identify the parent alkane and to locate the positions of the carbon atom at which branching take place due to the substitution of alkyle group in place of hydrogen atom. The numbering is done in such a way that the branched carbon atoms get the lowest possible number.For eg: 9 8 7 6 5 4 3 2 1 C−C−C−C−C−C−C−C−C C C−C
When two or more substituents are present, then end of the parent chain which gives the lowest set of the locants is preferred for numbering. This rule is called lowest set of locants. This means that when two or more different sets of locants are possible, that set of locants which when compared term with other sets, each in order of increasing magnitude, has the lowest term at the first point of difference. For eg: 6 5 4 3 2 1 H3C−CH−CH3−CH−CH−CH3 CH3 CH3 CH3 Set of locants: 2,3,5
If the same substituent or side chain occurs more than once, the prefix di(for 2), tri(for 3), tetra(for 4), penta(for 5),hexa(for 6)…etc., are attached to the names of the substituents. The positions of the substituents are indicated separately and the numerals representing their positions are separated by commas.For eg: 1 2 3 4 5 CH3–CH–CH2–CH–CH3 CH3 CH3 2,4-Dimethylpentane
If two or more different substituents or side chains are present in the molecule, they are named in the alphabetical order along with their appropriate positions. Prefix are ignored while comparing the substituents.For eg: CH3CH3 5 4 3 2 1 CH3−CH3−C−CH3−CH3 CH3CH3 3 -Ethyl-2,3-dimethylpentane
If two different substituents are in equivalent positions from the two ends of the chain, then the numbering of the chain is done in such a way that the group which comes first in the alphabetical order gets lower down.For eg: 1 2 3 4 5 6 7 7 6 5 4 3 2 1 CH3−CH2−CH−CH2−CH−CH2−CH3 CH3−CH2−CH−CH2−CH−CH2−CH3 CH3 CH2CH3 CH3 CH3CH3 ( Methyl at C-3) (Ethyl at C-3)The carbon bearing ethyl group gets lower position because it is cited first in the name according to alphabetical order of substituents. So correct name of compound is :3-Ethyl-5-methylheptane CH3−CH2−CH−CH2−CH2−CH− CH2 −CH3 (3-Ethyl-6-methyloctane) CH2CH3 CH3
If the substituent on the parent chain is complex it is named as substituted alkyl group by numbering the carbon atom of this group attached to the parent chain as 1.the name of such substituents is given in brackets in order to avoid confusion with the numbering of the parent chain.For eg: 1 2 3 4 5 6 7 8 9 CH3−CH3−CH3−CH3−CH3−CH3−CH3−CH3−CH3 1 CH3 2 CH3 Complex Substituent 3 CH3 5-(1,2- Dimenthylpropyl) nonane
Petroleum and natural gas are the main source of alkanes. However, alkanes can be prepared by three methods.
The unsaturated hydrocarbons (alkenes and alkynes) are converted into alkanes by catalytic hydrogenation. In this process dihydrogen is passed through alkenes or alkynes in the presence of finely divided catalysts such as Raney Ni, Pt or Pd. These metals absorb dihydrogen gas on their surfaces and activate the hydrogen- hydrogen bond. Platinum and palladium catalyse the reaction at room tempreture. However,higher tempreture (523-573k) and pressure are required with nickle catalysts. The hydrogenation reaction of unsaturated hydrocarbon using nickle at a tempreture of 523-573K is commonly known as Sabatier and Sender’s reaction or reduction. Methane cannot be prepared by this method because starting alkene or alkyne must contain at least two carbon atom.For eg:
i) Alkyl halides (except fluorides) on reduction with zinc and dilute hydrochloric acid give alkanes. For eg: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 atom.For eg:
Decarboxylation Kolbe’s electrolytic reaction methodi) Decarboxylation reaction :Sodium salts of carboxylic acids on heating with soda lime (mixture of sodium hydroxide and calcium oxide)gives 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.For eg:
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:
Alkanes Alkanes are almost non-polar molecules and therefore the molecules are holdonly by weak Van der Waals forces. The weak intermolecular forces depend onlyupon the size and the structure of the molecule. Due to weak forces, the C1 to C4are gases, the next thirteen alkanes from C5 to C17 are liquid and the highermember with more than 18 carbon atoms are solid at 298 K.
Alkanes have generally low boiling points because these are non-polar and the molecules are held together only by weak Van der Waals’ forces. With the increase in the number of carbon atoms, the molecular size increases and therefore, the magnitude of Van der Waals forces also increases. Consequently, the boiling points increase with increase in number of carbon atoms. It has been observed that each carbon added to the chain increases the boiling point by 20-30 k. the boiling point of n-alkanes with increase in number of carbon per molecule of the homologous series.
The melting points of alkanes do not shows regular variation with increase in molecular size. It has been observed that, in general, the alkanes with even number of carbon atoms have higher melting points as compared to the immediately next lower alkanes with odd number of carbon atoms. Alkane C3H8 C4H10 C5H12 C6H14 C7H16 C8H18 m.p.(K) 85.9 138 143.3 178.5 182.5 216.2This is because the alkanes with even number of carbon atoms have more symmetrical structures and result in closer packing in the crystal structure as compared to alkanes with odd number of carbon atoms. Therefore, the attractive forces in the former are more and the melting points are higher as compared to the alkanes with odd number of carbon atoms.
Alkanes being non-polar in nature, are expected to be insoluble in water(polar solvent). They dissolve in non-polar solvents such as ether, benzene, carbon tetrachloride etc. The solubility generally decreases with increase in molecular mass. As we know, petrol is a mixture of hydrocarbon and is used as a fuel for automobiles.Alkanes are lighter than water. The density increase with the increase in the number of the carbon atoms.
The reaction in which an atom or a group of atoms in a molecule is replaced by some other atom or group of atom. Alkanes undergo substitution reaction in which one or more hydrogen atoms are replaced or substituted by different atoms or groups such as halogen atom (Cl, Br or I), nitro group(-NO2) or sulphonic acid (-SO3H) group.
This involves the replacement of one or more atoms of alkanes by the corresponding number of halogens atoms. It is found that the rate of reaction of alkanes with halogen is F2>Cl2>Br2>I2. Rate of replacement of hydrogen of alkanes is:3˚>2˚>1˚.For eg:
The reaction is initiated by homolysis of chlorine molecule in the presence of light or heat, the Cl-Cl bond is weaker than the C-C and C-H bond and hence, is easiest to break.Chlorine free radicals attacks the methane molecule and takes the reaction in the forward direction by breaking the C-H bond to generate methyl free radical with the formation of H-Cl.The methyl radical thus obtained attacks the second molecule of chlorine to form CH3-Cl with the liberation of another chlorine free radical by homolysis of chlorine molecule.
The reaction stops after some time due to consumption of reactants and/or due to following side reaction: The possible chain terminating steps are:a)b)c)Though in (c) CH3-Cl, the one of the product is formed bur free radicals are consumed and the chain is terminated.
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
Alkanes on heating with a regulated supply of dioxygen or air at high pressure and in the presence of suitable catalyst give a variety of oxidation product:i)When a mixture of methane and oxygen in the molar ratio of 9:1 is compressed to about1100 atmospheres and passed through copper tubes at 575 K, methane is oxidised to methanol. 2CH4 + O2 Cu/575K/1100 atm. 2CH3OHii) When methane is mixed with oxygen and passed through heated molybdenum oxide (Mo2O3), under pressure it is oxidised to methanal. (CH3)3CH + O alk.KMnO4 HCHO + H2O
Alkane isomerise to branched chain alkanes when heated with anhydrous aluminium chloride (AlCl3) and hydrogen chloride at 573 K under a pressure of about 30-35 atmosphere. CH3 CH3CH2CH2CH3 anhy.AlCl3,HCl CH3−CH−CH3 n-butane isopropaneThe alkanes containing six or more carbon atoms when heated at about 773K under high pressure of 10-20 atm in the presence of catalyst on alumina gel get converted to aromatic compounds. This process is called aromatization. CH3−(CH2)4−CH3 773K, 10-20 atm Hexane Benzene
On passing a mixture of steam and methane over heated nickle (supported over alumina, Al2O3) catalyst at 1273 K, methane is oxidised to carbon monoxide and hydrogen is evolved. CH4 +H2O CO + 3H2When higher alkanes are heated to high tempreture in the presence of alumina or silica catalysts, the alkanes break down to lower alkanes and alkenes. For eg: C3H8 C2H4 + CH4 or C3H6 + H2This reaction is called Fragmentation or Cracking or Pyrolysis. Pyrolysis of hexane gives following product:
Chemist represent conformations in two simple ways:a)Sawhorse representation b)Newman projectionIn this projection, the molecule is viewed along the axis of the model from an oblique angle. The central carbon-carbon bond (C-C) s drawn as a straight line slightly tilted to right for the sake of clarity. The front carbon is shown as the lower left hand carbon and there are carbon is shown as the upper right hand carbon.
In this method, the molecule is viewed from the front along the carbon-carbon bond axis. The two carbon atoms forming the σ bond are represented by two circle; one behind the other so that only the front carbon is seen. The front carbon atom is shown by a point whereas the carbon further from the eye is represented by the circle. Therefore, the C-H bonds of the front carbon are depicted from the centre of the circle while C-H bonds of the back carbon are drawn from the circumference of the circle at an angle of 120˚ at each other.
Alkenes are unsaturated hydrocarbons containing carbon-carbon double bond (C═C)in their molecules. They have the general formula CnH2n. The simplest member of alkene family is ethene, C2H4. The alkenes are also called olefins (Greek olefiant meaning oil forming) because the larger member of the series (such as ethylene, propylene, etc react with chlorine to form oily products. Propylene
Carbon-Carbon double bond in alkenes consists of one strong sigma(σ) bond (bond enthalpy about 397kJ mol-1 due to head on overlapping of sp2 hybridised orbitals and one weak pi() bond(bond enthalpy about 284 kJ mol-1)obtained by lateral or sideways overlapping of the two 2p orbitals of the two carbon atom. The double bond is shorter in bond length (134pm) than the single bond (154pm). Alkenes are easily attacked by reagents or compounds which are in search of electron(electrophilic reagents)because they behave as source of loosely held mobile electron. The presence of weaker bond makes alkenes unstable molecules in comparison to alkanes and thus, alkenes can be changed into single bond compounds by combining with the electrophilic reagents.
According to IUPAC system alkenes are named similar to alkanes with the following modification:i)The longest continues chain should include both the carbon atoms of the double bond.ii)The suffix used for alkene is –eneiii)The chain is numbered from the end that gives the lower number to the first carbon atom of the double bond.iv)If there are two or more double bonds the ending ane of the alkane is replaced by adiene or atiene. 1 2 3 4 5 1 2 3 4For eg: CH3CH=CHCHCH3 CH2=CH−CH=CH2 Buta-1,3-diene CH3 4-Methylpent-2-ene
Alkenes show following types of structural isomerisms:The isomers differ with respect to the chain of carbon atoms. as in alkanes, ethene (C2H4) and propene(C3H6) can have only one structure but alkenes higher than propene have different structures.For eg: 4 3 2 1 But-1-ene CH3−CH2−CH=CH2The isomers differ in the position of the double bonds. For eg: CH2−CH=CH2−CH3 (But-1-ene) CH3−CH=CH−CH3 (But-2-ene)
The compounds which have the same structural formula but differ in the spatial arrangement of atoms or groups of atoms about the double bond are called geometrical isomers and the phenomena is known as geometrical isomerism. The isomers in which similar atoms or groups lie on the same side of the double bond is called cis-isomers while the other in which they are displaced on opposite sides, is called trans-isomerism.Cis-isomer is more polar than trans-isomers. These are distinguish on the basis of their physical properties such as melting point, boiling point etc.
Alkynes can be reduced to alkenes using palladium charcoal (palladised charcoal) catalyst partially deactivated with poison like sulphur compounds or quioline. Partially deactivated palladised charcoal is known as Lindlar’s catalyst. Alkynes can also be reduced to alkenes with sodium in liquid ammonia (called Birch reduction).For eg: CH3−C≡C−CH3 Pd- C, H2 CH3CH═CHCH3 But-2-yne But-2-ene CH3–C≡CH+H2 CH3–CH=CH2 Propyne Propene CH≡CH+H2 Pd/C CH2=CH2 Ethyne Ethene
Alkene can be prepared from alkyl halides(usually bromides or iodides) by treating with alcoholic potash(potassium hydroxide dissolved in ethanol). This reaction removes a molecule of HX and therefore, the reaction is called dehydrohalogenation. In this reaction, the hydrogen atom is eliminated from β carbon atom (carbon atom next to the carbon to which halogen is attached). Therefore, the reaction is also called β–elimination reaction.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 group it is Tertiary> Secondary>Primary.
Dihalogen derivatives of alkanes in which two halogens atoms are attached to adjacent carbon atoms (called vicinal dihalogen derivatives) are converted to alkenes by heating with zinc dust in ethyl alcohol. For eg: CH3CHBr−CH2Br+Zn CH3CH=CH2+ZnBrAlkenes are prepared from alcohols by heating with protonic acids such as sulphuric acid at about 443K. This reaction is called dehydration of alcohols CH3CH2OH H2SO4 or H3PO4 CH2=CH2+H2OThis reaction is also an example of β-elimination reaction because –OH group takes out one hydrogen atom from the β- carbon atom.
In general, alkenes have higher melting point than the corresponding alkanes. This is due to the reason that p-electrons of a double bond are more polarizable than s-electron of single bonds. As a result, the intermolecular force of attraction are stronger in alkenes than alkanes. The melting and boiling point of alkenes in general, increase with increase in molecular mass.The boiling points of alkene show a regular gradation with the increase in number of carbon atoms like alkanes. In general, for each added –CH2 group the boiling point rises by 20˚-30˚.
Alkenes are weakly polar. The p-electron of the double bond can be easily polarized. Therefore, their dipole moments are higher than those of alkanes. The dipole moment of alkene depends upon the position of the groups bonded to the two double bonded carbon atoms. The symmetrical trans alkenes are non-polar and hence have zero dipole moment. However, unsymmetrical trans-alkenes have small dipole moment because the two dipoles opposes each other but they do not cancel out each other exactly since they are unequal. On the other hand, both symmetrical and asymmetrical cis-alkenes are polar and hence have finite dipole moments. This is because the two dipoles of individual bonds are on the same side and hence have a resultant dipole moment.Alkenes are lighter than water. These are insoluble in water because they are non- polar. However, they readily dissolve in organic solvents like alcohol, benzene, ether, carbon tetrachloride, etc.
Alkenes add up on molecule of dihydrogen gas in the presence of finally divided nickle, palladium or platinum to form alkanes.Halogens like bromine or chlorine add up to alkene to form vicinal dihalides. 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 halogen to alkene is an example of electrophilic addition reaction.Hydrogen halides (HCl, HBr, HI) add up to alkenes to form alkyl halides. The order of reactivity of the hydrogen halides is HI>HBr>HCl. Like addition of halogens to alkenes, addition of hydrogen halides is also an example of electrophilic addition reaction
Markovnikov, a Russian chemist made a generalisation in 1869. these generalisation led Markovnikov to frame a rule call Markovnikov rule. The rule stated that:“During the addition across unsymmetrical multiple bond, the negative part of the addendum (attacking molecule)joins with the carbon atom which carries smaller number of hydrogen atoms while the positive part goes to the carbon atom with more hydrogen atom.”
Cold concentrated sulphuric acid adds to alkenes in accordance with Markovnikov rule to form alkyl hydrogen sulphate by the electrophilic addition reaction.In the presence of a few drops of concentrated sulphuric acid alkenes react with water to form alcohols, in accordance with the Markovnikov rule.
Alkenes react with cold dilute aqueous or alkaline potassium permanganate solution to form 1,2-diols called glycols. The glycols contain two –OH groups on adjacent carbon atoms. This reaction of addition of two hydroxyl groups to each end of double bond is called hydroxylation of the double bond. 2KMnO4+H2O 2KOH+2MnO2+3[O]When alkene is treated with hot acidic potassium permanganate or potassium dichromate solution the alkene gets split up at the double bond forming carboxylic acids or ketones. This is also called oxidative cleavage of alkanes.For eg: CH3−CH=CH−CH3 KMnO4/H+ 2CH3COOH But-2-ene Ethanoic acid
Alkenes are oxidised with ozone to form ozonides which are unstable compounds. These are reduced with zinc and water forming aldehydes and ketones. The reaction is called ozonolysis.Polymerisation is a process in which a large number of simple (same or different) molecules combine to form a bigger molecule of higher molecular mass. The small molecule are called monomers while the bigger molecule are called macromolecules or polymers.
Alkynes are unsaturated hydrocarbon having carbon-carbon triple bonds in their molecules. There general formula is CnH2n-2. The simplest member of this class is ethyne (C2H2) which is properly known on acetylene. C2H2 H:C:::C:H H—C C—H Acetylene (ethyne) C3H4 CH3C CH Methylacetylene Propyne
Ethyne is the simplest molecule of alkyne series. In the triple bond formation, one sp hybridised orbital of one carbon atom overlaps axially (head on) with the similar sp hybrid orbital of the other carbon atom to form σ bond. Each of the two unhybridised orbitals of one carbon overlaps sidewise with the similar orbital of the other carbon atom to form two weak bonds. The remaining sp hybrid of each carbon atom overlaps with 1s orbital of hydrogen to form C-H bond. Thus, carbon to carbon triple bond is made up of one σ bond and two bonds.
In IUPAC system they are named as derivatives of corresponding alkanes replacing ‘ane’ by the suffix ‘yne’. The following rules should be followed:i)The longest continues chain should include both the carbon atoms of the triple bond.ii) The suffix used for alkyne is – yne.iii) The chain is numbered from the end which gives the lower number to the first carbon atom of the triple bond.iv) The positions of the substituents are indicated.For eg: 4 3 2 1 1 2 3 4 5 6 CH3CH2C≡CH CH3CH2C≡C−CH2CH3 But-1-yne Hex-3-yne
Alkynes exhibit the following structural isomerisms:The isomers differ in the chain of carbon atoms. For example, the molecule having molecular formula C5H8 shows chain isomers as: 5 4 3 2 1 CH3−CH2−CH2−C≡CH Pent-1-yenAlkynes having more than four carbon atoms show position isomerism. For example: 4 3 2 1 CH3−CH2−C≡CH CH3−C≡C−CH3 But-1-yne But-2-yne
Preparation Of Alkynes From calcium carbide From Vicinal DihalidesAcetylene is prepared in the laboratory as well as an industrial scale by the action of water on calcium carbide. CaC2 + 2H2O HC≡CH + Ca(OH)2Calcium carbide required for this purpose is obtained by heating calcium oxide (from limestone) and coke in an electric furnace at 2275 K. CaCO3 Heat CaO + CO2
Vicinal dihalides on treatment with alcoholic potassium hydroxide undergo dehydrohalogenation. One molecule of hydrogen halides is eliminated to form alkenyl halide which on treatment with sodamide gives alkyne.
The first three members (ethyne, propyne, butyne) of the family are gases at room tempreture, the next eight are liquid while the higher ones are . All alkynes are colourless. However, ethyne has characteristic odour of garlic smell.Alkynes are weakly polar in nature. They are lighter than water and immiscible with water but are soluble in organic solvents such as petroleum ether, carbon tetrachloride, benzene, etc.
The melting and boiling point of the members of the family are slightly higher as compared to those of the corresponding members of alkane and alkene families. This is due to the fact that the alkynes have linear structure and therefore, their molecules are more closely packed in space as compared to alkanes and alkenes. The magnitude of attractive forces among them are higher and therefore, the melting and boiling point are also higher. The melting and boiling point increase with increase in molecular mass of the alkynes. Hydrocarbon Ethane Ethene Ethyne m.p. (K) 101 104 191 b.p. (K) 184.5 171 198
Alkynes react readily with hydrogen in the presence of finely divided Ni, Pt or Pd as a catalyst. The reaction is called hydrogenation. HC≡CH+H2 Pt/Pd/Ni [H2C=CH2] H 2 CH3−CH3Reddish orange colour of the solution of bromine in carbon tetrachloride is decolourised. This is used as a test for unsaturation.
Two molecule of hydrogen halides(HCl, HBr and HI) add to alkynes to form gem dihalides (in which two halogens are attached to the same carbon atom). For example:Alkenes react with water in the presence of mercuric sulphate (HgSO4) and sulphuric acid at 337K. The product are carbonyl compounds (aldehydes and ketones). For eg:
Linear polymerisation of ethyne takes place to produce polyacetylene of polythyne which is a high molecular weight polyene containing repeating units of (CH=CH−CH=CH).Alkynes have larger tendency to polymerize then alkenes and, therefore these give low molecular mass polymers alkynes when passed through a red hot iron tube at 873k polymerize to give aromatic hydrocarbons. For eg:This is the best route for entering from aliphatic to aromatic compounds.
These hydrocarbons are also known as ‘arenes’. Since most of them possess pleasant odour (Greek; aroma means pleasant smelling), the class of compounds was named as ‘aromatic compounds’. The parent member of the family is benzene having the molecular formula C6H6. it has hexagonal ring of six carbon atoms with three double bond in alternate position.Aromatic compounds containing benzene ring are known as benzenoids and those not containing a benzene ring are known as non-benzenoids. For eg:
The stability of benzene can be explained on the basis of concept of resonance. Kekule in1865 gave a ring structure for benzene in which the positions of the three double bonds are not fixed. He suggested that the double bond keep on changing their positions an this is called Resonance. The resonance structure of benzene is supported by the following facts:i)The carbon-carbon bond length in benzene is 139 pm which is intermediate between bond lengths for C-C bond (154 pm)and C=C bond (134 pm) and the value is the same for all the bonds.ii)Due to resonance the -electron charge in benzene gets distributed over greater area i.e., it gets delocalised. As a result of delocalisation the energy of the resonance hybrid decreases as compared to contributing structure by about 50kJ mol-1. the decrease in energy is called resonance energy. Therefore, it is stabilised and behaves as a saturated hydrocarbon.iii)If the positions of double bonds are fixed. We expect two isomers of 1,2- dichlorobenzene as shown below (one having Cl atoms attached to C-C bond and the other having Cl atoms attached to C=C bond).
According to the orbital concept, each carbon atom in benzene is sp2- hybridised and one orbital remains unhybridised. Out of the three hybrid orbitals, two overlap axially with the orbitals of the neighbouring carbon atoms on both side to form σ-bond. The third hybridised orbital of the carbon atom overlaps with the half-filled orbital of the hydrogen atom resulting in C-H bonds. Thus, benzene has a planar structure –with bond angle of120˚ each.There is still one unhybridised 2p-orbital left on each carbon atom. Each one of these orbitals can overlap sidewise with similar orbital of the carbon atoms on either sides to form two sets of -bonds. (Shown in fig a. and b. respectively) a) b)
The resultant -orbital cloud is spread over all the six carbon atoms (shown in fig c.). As a result, there are two continuous rings of - electron clouds, one above and the other below the plane of the carbon atoms(shown in fig d.). c) d) electron cloud
Aromatic compounds are those which resembles benzene in chemical behaviour. These compounds contain alternate double and single bonds in a cyclic structure. They undergo substitution reaction rather than addition reaction. This characteristic be behaviour is called aromaticity. The aromaticity depends upon the electronic structure of the molecule. Cyclopentadienyl anion
The main essential for aromaticity are: Delocalisation: the molecule should contain a cyclic cloud of delocalized electron above and below the plane of the molecule Planarity: for the delocalisation of -electron the ring must be planar to allow cyclic overlap of p-orbitals. Therefore, for a molecule to be aromatic, the ring must be planar. (4n+2) electron: for aromaticity, the -electron could must contain a total of (4n+2) electrons where n is an integer equal to 0,1,2,3……..n . This is known as Huckel Rule. Benzene, 6 e- Naphthalene, 10 e- Anthracene, 14 e- (n=1) (n=2) (n=3)
Decarboxylation of aromatic acidbenzene is prepared in the laboratory by heating sodium benzoate with soda lime. Reduction of phenolBenzene can be prepared from phenol by distillation with zinc.
Benzene and its containing up to eight carbon atoms are colourless liquids with characteristic smell. Aromatic hydrocarbons are immiscible with water but are soluble in organic solvents. They are inflammable and burn with sooty flame. They are toxic and carcinogenic in nature. The melting and boiling point of aromatic hydrocarbon increase with increasing molecular mass. This is due to increase in magnitude of van der Waals’ forces of attraction with increase in molecular size. Amongst isomeric arenes, (i.e., o- ,m- and p- xylenes), the p- isomer has the highest melting point because it is most symmetrical.
Chemical Properties Mechanism of Electrophilic electrophilic Addition reactionsubstitution reaction substitution reaction
The replacement of a hydrogen atom in the ring by a nitro (-NO2) group is called nitration. It is carried out by heating benzene with the nitrating mixture consisting of concentrated nitric acid and sulphuric acid to about 330K.The replacement of a hydrogen atom in the ring by a halogen atom (F, Cl, Br or I) is called halogenation. Arenes react with halogen in the presence of a Lewis acid like anhydrous FeCl3, FeBr3 or AlCl3 to yield haloarenes.
The replacement of a hydrogen atom in the ring by a sulphonic acid (-SO3H) group is called sulphonation. It is carried out by heating benzene with fuming sulphuric acid and oleum.When benzene is treated with an alkyl halide in the presence of anhydrous aluminium chloride, alkylbenene is formed.The reaction of benzene with acyl halide or acid anhydride in the presence of lewis acid (AlCl3) Yields acyl benzene
According to experimental evidences, SE (S= substitution; E= electrophilic) reaction are supposed to proceed via the following three steps:a)Generation of the electrophile.b)Formation of carbocation intermediate.c)Removal of proton from the carbonation intermediate.The attacking reagent may not be strong electrophile. Therefore, first of all an electrophile is generated by some preliminary reaction. For example , during chlorination of benzene, an electrophile (Cl +) is generated by reacting with anhydrous AlCl3 used as catalyst. Cl2 + AlCl3 Cl+ + AlClˉ4
The electrophile E+ approaches the -electron cloud of the aromatic ring and forms a bond with carbon, creating a positive charge on the ring. This results in the formation of a sigma complex (called arenium ion).The arenium ion gets stabilized by resonanceThe resulting carbocation has three important contributing structures which spread the positive charge over the remaining carbon atom.
The carbocation formed loses a proton to the nucleophile (Nuˉ) present in the reaction mixture to form a substitution product. During this step, the aromatic character of the benzene ring is restored and this step is fast. The loss of proton allows the two electrons from the carbon- hydrogen bond to move to regenerate the aromatic ring and thus restoring the aromatic character.
Benzene reacts with hydrogen in the presence of a catalyst such as nickel, or platinum at 473 to 573 K under pressure to form cyclohexane.Benzene reacts with chlorine or bromine in the presence of sunlight and absence of halogen carrier to form benzene hexachloride.On completely burning with oxygen, benzene gives carbon dioxide and water with the evolution of a large amount of energy.
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. 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.a)Ortho and para directing groupsb)Meta directing group
The groups which direct the incoming group to ortho and para position are called ortho and para directing groups. As an example, let us discuss the directive influence of –OH (phenolic) group.The resonance structures of phenol show that the overall electron density on the benzene ring increases in comparison to benzene. Therefore, it is an activating 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 an example of Nitro group.Nitro group reduces the electron density in the benzene ring due to its strong-I effect. Nitrobenzene is the resonance hybrid of the following structures.
Benzene and polynuclear hydrocarbon 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 reaction and finally damage DNA and cause cancer.
Presented By: G. L. KapdePGT (Chem) KV-1, Colaba.