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7/29/2019 hydrocarbonpreetibhatt-120104222324-phpapp01

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HYDROCARBON


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HYDROCARBONS are the compounds containing

carbon and hydrogen only.


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Depending upon the types of carbon-carbon bonds present, they can

be classified into tree main categories:

1) Saturated Hydrocarbon

2) Unsaturated Hydrocarbon

3) Aromatic Hydrocarbon


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The hydrocarbon that contain only carbon-carbon single bond is calledSaturated Hydrocarbon. These include open chain hydrocarbon as well asclosed 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


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The hydrocarbons which contain carbon-carbon multiple bond (Doublebonds 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: Ethene

o Alkynes: These are hydrocarbons which contain at least one carbon-carbon triple

bond. For example: Ethyle


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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-Xylene

The aromatic compounds may also contain more than one benzene rings.For example: i) Naphthalene ii) Anthracene


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Hydrocarbon Type Characteristic Group Example

Saturated Hydrocarbon:

Alkanes

No double or Triple Bond CH3CH2CH3Propane

Unsaturated Hydrocarbon:

Alkenes

Alkynes

Double Bond

Triple Bond

CH3CHCH2

Propene

CH3CCH

Propyne

Aromatic Hydrocarbons: Benzene ring

Methyl Benzene


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ALKANES


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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.


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These are simple alkanes without any close chains and havethe general formula where CnH2n+ 2n is the number of

carbon atoms.

For example: i) Methane - CH4


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These contain a closed chain or ring in their molecules. They have the general

formula CnH2n.

For example: i) Cyclopropane- or

ii)Cyclobutane- or


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Methane is the first member of the family. It hasTetrahedral Structureinvolvingsp3 Hybridisation. The four sigma bond is formed by the

overlapping ofsp3 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)


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1.4 Nomenclature Of AlkanesNomenclature implies assigning proper name to the basis of certain standard rules

so that the study of these compounds may become standard. The rules for namingthem 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 9

For eg. CH3CHCH2CH2CH2CHCH2CH2CH3CH3 CH2CH3

In 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.


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The carbon atoms of the parent chain are numbered to identify the

parent alkane and to locate the positions of the carbon atom atwhich branching take place due to the substitution of alkyle group

in place of hydrogen atom. The numbering is done in such a way thatthe branched carbon atoms get the lowest possible number.

For eg: 9 8 7 6 5 4 3 2 1

CCCCCCCCC

C CC


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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. Thisrule is called lowest set of locants.

This means that when two or more different sets of locants arepossible, 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

H3CCHCH3CHCHCH3

CH3 CH3 CH3

Set of locants: 2,3,5


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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 thenames 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

CH3CHCH2CHCH3

CH3 CH3

2,4-Dimethylpentane


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If two or more different substituents or side chains are present in the

molecule, they are named in the alphabetical order along with theirappropriate positions. Prefix are ignored while comparing the substituents.

For eg: CH3CH

3

5 4 3 2 1

CH3CH3CCH3CH3

CH3CH3

3 -Ethyl-2,3-dimethylpentane


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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

CH3CH2CHCH2CHCH2CH3 CH3CH2CHCH2CHCH2CH3

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 ofcompound is :3-Ethyl-5-methylheptane

CH3CH2CHCH2CH2CH CH2 CH3

(3-Ethyl-6-methyloctane)

CH2CH3 CH3


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If the substituent on the parent chain is complex it is named as substituted alkylgroup 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

CH3CH3CH3CH3CH3CH3CH3CH3CH3

1

CH3

2

CH3

Complex Substituent

3

CH3

5-(1,2- Dimenthylpropyl) nonane


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Petroleum and natural gas are the main source of alkanes. However,

alkanes can be prepared by three methods.


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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 Senders reaction

or reduction.

Methane cannot be prepared by this method because startingalkene or alkyne must contain at least two carbon atom.

For eg:


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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:


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i) 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. Thisprocess of elimination of carbon dioxide from a

carboxylic acid is known as decarboxylation.

For eg:

Decarboxylationreaction

Kolbes electrolyticmethod

ii) Kolbes electrolytic method:


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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:


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1.6 Properties of 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 C1to C4are gases, the next thirteen alkanes from C5to C17 are liquid and the higher

member with more than 18 carbon atoms are solid at 298 K.


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Alkanes have generally low boiling points because these are non-polar and themolecules 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.

Variations of boiling point of alkane with increase in number of C atoms.


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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 evennumber of carbon atoms have higher melting points as compared to the

immediately next lower alkanes with odd number of carbon atoms.

This is because the alkanes with even number of carbon atoms have more

symmetrical structures and result in closer packing in the crystal structure ascompared 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.

Alkane C3H8 C4H10 C5H12 C6H14 C7H16 C8H18

m.p.(K) 85.9 138 143.3 178.5 182.5 216.2


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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, carbontetrachloride 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.


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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.


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This involves the replacement of one or more atoms of alkanes by the

corresponding number of halogens atoms. It is found that the rate ofreaction of alkanes with halogen is F2>Cl2>Br2>I2. Rate of

replacement of hydrogen of alkanes is:3>2>1.

For eg:


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The reaction is initiated by homolysis of chlorine molecule in the presence of lightor 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.


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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.


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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


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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+ O2Cu/575K/1100 atm. 2CH3OH

ii) When methane is mixed with oxygen and passed through heated molybdenumoxide (Mo2O3), under pressure it is oxidised to methanal.

(CH3)3CH + Oalk.KMnO4 HCHO + H2O


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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

CH3CH2CH2CH3anhy.AlCl3,HCl CH3CHCH3

n-butane isopropane

The 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)4CH3773K, 10-20 atm

Hexane Benzene


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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 + 3H2

When 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 + H2

This reaction is called Fragmentation or Cracking or Pyrolysis. Pyrolysis of hexanegives following product:


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Chemist represent conformations in two simple ways:

a)Sawhorse representation b)Newman projection

In this projection, the molecule is viewed along the axis of the model from anoblique 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.


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In this method, the molecule is viewed from the front along the carbon-carbon

bond axis. The two carbon atoms forming thebond are represented by twocircle; 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.


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Alkenes


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Alkenes are unsaturated hydrocarbons containing carbon-carbon double bond(CC)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


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Carbon-Carbon double bond in alkenes consists of one strong sigma() bond (bond

enthalpy about 397kJ mol-1 due to head on overlapping of sp2 hybridised orbitalsand 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.


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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 isene

iii)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 4

For eg: CH3CH=CHCHCH3 CH2=CHCH=CH2Buta-1,3-diene

CH34-Methylpent-2-ene


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Isomerism inAlkanes

StructuralIsomerism

Chain

Isomerism

PositionIsomerism

GeometricalIsomerism


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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

CH3CH2CH=CH2

The isomers differ in the position of the double bonds. For eg:

CH2CH=CH2CH3 (But-1-ene) CH3CH=CHCH3 (But-2-ene)


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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 calledgeometrical 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.


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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 Lindlars catalyst.

Alkynes can also be reduced to alkenes with sodium in liquid ammonia (called

Birch reduction).

For eg: CH3CCCH3Pd- C, H2 CH3CHCHCH3But-2-yne But-2-ene

CH3CCH+H2 CH3CH=CH2

Propyne Propene

CHCH+H2 Pd/C CH2=CH2

Ethyne Ethene


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Alkene can be prepared from alkyl halides(usually bromides or iodides) by treating

with alcoholic potash(potassium hydroxide dissolved in ethanol). This reactionremoves a molecule of HX and therefore, the reaction is called

dehydrohalogenation. In this reaction, the hydrogen atom is eliminated from

carbon atom (carbon atom next to the carbon to which halogen is attached).

Therefore, the reaction is also calledelimination 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.


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Dihalogen derivatives of alkanes in which two halogens atoms are attached to

adjacent carbon atoms (called vicinal dihalogen derivatives) are converted toalkenes by heating with zinc dust in ethyl alcohol. For eg:

CH3CHBrCH2Br+Zn CH3CH=CH2+ZnBr

Alkenes are prepared from alcohols by heating with protonic acids such as sulphuric

acid at about 443K. This reaction is called dehydration of alcohols

CH3CH2OHH

2SO

4or H

3PO

4 CH2=CH2+H2O

This reaction is also an example of-elimination reaction becauseOH group takes

out one hydrogen atom from the- carbon atom.


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In general, alkenes have higher melting point than the corresponding alkanes. This is dueto the reason that p-electrons of a double bond are more polarizable than s-electron ofsingle bonds. As a result, the intermolecular force of attraction are stronger in alkenesthan alkanes. The melting and boiling point of alkenes in general, increase withincrease in molecular mass.

The boiling points of alkene show a regular gradation with the increase in number ofcarbon atoms like alkanes. In general, for each addedCH2 group the boiling pointrises by 20-30.


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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 dipolemoments. 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.


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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 reddishorange 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


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Markovnikov, a Russian chemist made a generalisation in 1869. thesegeneralisation led Markovnikov to frame a rule call Markovnikov

rule. The rule stated that:

During the addition across unsymmetrical multiplebond, the negative part of the addendum (attackingmolecule)joins with the carbon atom which carriessmaller number of hydrogen atoms while the positivepart goes to the carbon atom with more hydrogen atom.


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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.


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Alkenes react with cold dilute aqueous or alkaline potassium permanganate

solution to form 1,2-diols called glycols. The glycols contain twoOH groupson 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: CH3CH=CHCH3 KMnO4/H+ 2CH3COOH

But-2-ene Ethanoic acid


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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.


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Alkynes


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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 HC CHAcetylene

(ethyne)

C3H4 CH3CCH Methylacetylene

Propyne


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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 thesimilar sp hybrid orbital of the other carbon atom to formbond. 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 onebond and twobonds.


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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 isyne.

iii) The chain is numbered from the end which gives the lower number to the firstcarbon 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

CH3CH2CCH CH3CH2CCCH2CH3But-1-yne Hex-3-yne


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Alkynes exhibit the following structural isomerisms:

The isomers differ in the chain of carbon atoms. For example, the molecule having

molecular formula C5H8shows chain isomers as:

5 4 3 2 1

CH3CH2CH2CCH

Pent-1-yen

Alkynes having more than four carbon atoms show position isomerism. For

example: 4 3 2 1

CH3CH2CCH CH3CCCH3But-1-yne But-2-yne


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Acetylene is prepared in the laboratory as well as an industrial scale by the action

of water on calcium carbide.

CaC2 + 2H2O HCCH + Ca(OH)2

Calcium carbide required for this purpose is obtained by heating calcium oxide

(from limestone) and coke in an electric furnace at 2275 K.

CaCO3Heat CaO + CO2

Preparation OfAlkynes

From calcium carbide From Vicinal Dihalides


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Vicinal dihalides on treatment with alcoholic potassium hydroxide

undergo dehydrohalogenation. One molecule of hydrogen halides iseliminated to form alkenyl halide which on treatment with sodamide

gives alkyne.


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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 alkynesare colourless. However, ethyne has characteristic odour of garlic smell.

Alkynes are weakly polar in nature. They are lighter than water and immisciblewith water but are soluble in organic solvents such as petroleum ether, carbon

tetrachloride, benzene, etc.


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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 havelinear 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 pointincrease 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


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Alkynes react readily with hydrogen in the presence of finely divided Ni, Pt or Pdas a catalyst. The reaction is called hydrogenation.

HCCH+H2Pt/Pd/Ni [H2C=CH2]

H2 CH3CH3

Reddish orange colour of the solution of bromine in carbon tetrachloride is

decolourised. This is used as a test for unsaturation.


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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:


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Linear polymerisation of ethyne takes place to produce polyacetylene of polythyne

which is a high molecular weight polyene containing repeating units of(CH=CHCH=CH).

Alkynes have larger tendency to polymerize then alkenes and, therefore these givelow 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.


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Aromatic

Hydrocarbon


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These hydrocarbons are also known as arenes. Since most of them possess

pleasant odour (Greek; aroma means pleasant smelling), the class of compoundswas 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:


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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 stabilisedand 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- hybridisedand one orbital remains unhybridised Out of the three hybrid orbitals two


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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-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, benzenehas a planar structurewith 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( h i fi ) A lt th t ti i f


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(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


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Aromatic compounds are those which resembles benzene in chemical

behaviour. These compounds contain alternate double and singlebonds 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:

l l


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Delocalisation: the molecule should contain a cyclic cloud of delocalizedelectron above and below the plane of the molecule

Planarity: for the delocalisation of -electron the ring must be planar toallow cyclic overlap of p-orbitals. Therefore, for a molecule to be aromatic, thering 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 . Thisis known as Huckel Rule.

Benzene, 6 e- Naphthalene, 10 e- Anthracene, 14 e-

(n=1) (n=2) (n=3)


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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.


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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 forcesof 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.


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ChemicalProperties

Electrophilicsubstitution reaction

Mechanism ofelectrophilic

substitution reactionAddition reaction


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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 mixtureconsisting 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 ( SO H) group


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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


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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 , duringchlorination of benzene, an electrophile (Cl+) is generated by reacting with

anhydrous AlCl3 used as catalyst.

Cl2 + AlCl3 Cl+ + AlCl4


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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 resonance

The resulting carbocation has three important contributing structures which

spread the positive charge over the remaining carbon atom.


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The carbocation formed loses a proton to the nucleophile (Nu) 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.


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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.


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When monosubstituted benzene is subjected to further substitution,

three possible disubstituted products are not formed in equalamounts. 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 groups

b)Meta directing group


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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 directiveinfluence ofOH (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.


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The groups which direct the incoming group to meta position are called meta

directing groups. Some examples of meta directing groups areNO2, -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.


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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 combustionof organic materials like tobacco, coal and petroleum. They enter into human

body and undergo various biochemical reaction and finally damage DNA and

cause cancer.


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Presented By: G. L. KapdePGT (Chem) KV-1, Colaba.

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