Vijendra Pal Singh

8/8/2019 Vijendra Pal Singh

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The simplest organic compounds containing

hydrogen are called hydrocarbons. They arewidely distributed in the form of petroleum,natural gas, and coal. Thus hydrocarbons areconsidered to be parent organic compounds

while all other compounds are thought tohave been derived from them by replacementof one or more of their hydrogen atomsby

functional groups.


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HYDROCARBON

ACYCLIC

SATURATED

(alkanes)

UNSATURATED

CYCLIC

ALICYCLIC

SATURATED

(cyclo alkanes) UNSATURATED

CYCLOALKENES

CYCLOALKYNES

AROMATIC


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Acyclic or open chain.

Cyclic or closed chain.

Acyclic or open chain :- These compounds containopen chains of carbon atom in their molecule. Theyare also called aliphatic hydrocarbons. These arefurther divided into following three categories :-


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1) This classification is primarily based upon the typeof carbon-carbon bonds present in their molecules.For example:-

1) CH3 - CH3 = Ethane

2) CH3 - CH3 - CH3 = Propane

2) An alkene has one carbon-carbon double bond. Forexample:-

1) CH2 = CH2 => Ethene

2) CH2 CH = CH2 =>Propane


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Cyclic or closed chain :- These compounds contains

rings of carbon atoms in their molecules.y They are further classified into following two cases:-

Alicyclic Hydrocarbons :- H ydrocarbons whichcontain a ring of three or more carbon atoms and have

properties similar to those of aliphatic hydrocarbonsare called alicyclic hydrocarbons.


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Thesearefurtherdividedintothreecategories:-

y

Cycloalkanes :- Saturated alicyclic hydrocarbons inwhich all carbon atoms are joined by single covalentbonds are called cycloalkanes. For examples :-

- cyclopropane - cyclopentane


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y Cycloalkenes :- Unsaturated alicyclic hydrocarbons which contain one carbon-carbon double bond arecalled cycloalkenes. For example:-y Cyclopropene

y Cyclobutene


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y Aromatic Hydrocarbons (Arenes) :-Hydrocarbonsand their alkly, alkenyl and alkynal derivatives which

contain one or more benzene rings either fused orisolated in their molecules are called aromatichydrocarbons. Since these compounds resemblebenzene in almost all of their properties. They are also

called benzenoid compounds.


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For example :-y Benzene - Toluene CH3

y Arenes may also contain two or more isolated of fusedrings. For example :-

- Diphenyl


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y Alkanes :- We have discuss about saturated

hydrocabons can be either acyclic or cyclic. The acyclichydrocarbons are called Paraffins, since they arerelatively unreactive towards most of reagents. In theIUPAC system they are called alkanes. Their generalformula is CnH2n+2 where n=1,2,3_ _ _ _ _ _ etc.


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No. of carbon atoms Formula IUPAC name

1 CH4 METHANE

2 C2H6 ETHANE

3 C3H8 PROPANE

4 C4H10 BUTANE

5 C5H12 PENTANE6 C6H14 HEXANE

10 C10H22 DECANE

11 C11H24 UNDECANE

12 C12H26 DODECANE

13 C13H28 TRIDECANE

14 C14H30 TETRADECANE

20 C20H42 EICOSANE

30 C30H62 TRIACONTANE


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y

Petroleum and natural gas are the main sources ofalkanes. However, alkanes can be prepared byfollowing method :y From Unsaturated Hydrocarbons :- Dihydrogen gas

adds to alkenes & alkynes in the presence of catalystslike platinum, palladium or nickel to form alkanes.Pt/Pd/Ni

CH2 =CH2 + H2 --------- CH3 CH3Pt/Pd/Ni

CH3 CH = CH3 + H2----------

CH3 CH2 CH3


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y From Alkly Halides :- (i) Alkly halides (exceptfluorides) on reduction with zinc and hydrochloric acidgives alkanes.

Zn.H+

CH3 Cl + H2 --------- CH4 + HCl

(ii) Alkly metals on treatment with sodium metal in dry

etheral. Thus reaction is called Wurtz reaction.dry ether

CH3Br + 2Na + BrCH3 --------- CH3 CH3 + NaBrbromomethane ethane


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y From Carboxylic Acid :- This I done by the following

(i) The process of elimination of carbon dioxide from acarboxylic acid is known Decarboxylic acid.

CaO

CH3COOHNa + NaOH ------- CH4 + Na2CO3

(ii) Kolbes electronic method An aqueous solution of

sodium or potassium salt of a carboxylic acid onelectrolysis gives alkane containing even number ofcarbon atoms at the anode.

2CH3COOHNa + 2H2O (on electrolysis )

CH3

CH3

+ 2CO2

+ H2

+ 2NaOH


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y

Physical :- (a)B

oiling point The boiling point ofstraight chain alkanes increase with increase in theirmolecular masses.

(b) Melting point Melting point also increases with

increase in carbon content but variation is not regular.(c) Solubility Alkanes are predominantly non-polarare insoluble in polar solvents such as water, alcoholetc. But are highly soluble in petroleum, ether,

benzene etc.(d) Density Density of alkanes increases withincreases in molecular masses.


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y Substitution reactions Alkanes because of havingCC and CH sigma bonds undergo only substitutionreactions. Some important substitution reactions are

given below :- Halogenation :- hv

CH4 + Cl2 ------- CH3Cl + HClhv chloromethane

CH3Cl + Cl2 ------- CH2Cl2 + HCl

dichloromethane


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Mechanism

Initiation The reaction is initiative by homolysis ofchlorine molecule in presence of light and heat. The ClCland CH and hence, is easiest to break.

Cl Cl -------------Cl + Clhomolysis chlorine free radicals

Propagation Chlorine free radical attacks the methanemolecule and takes reaction in forward direction by breakingCH bond to generate methyl force radical with formation ofHCl.

CH4 + Cl ----- CH3 + H Cl

CH3 = Cl Cl ----- CH3 Cl + Clchlorine free radical


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Termination The reaction stops after some time due toconsumptions of reactants and / or due to following side

reactions. The possible terminating steps are :Cl + Cl ------- Cl Cl

H3C + CH3 ------- H3C CH3

H3C + Cl --------H3C Cl

Combustion Alkanes on heating in presence of air or

dioxide are completely oxidized to carbon dioxide andwater with evolution of large amount of heat.

CH4(g) + 2O2(g) -------CO2(g) + 2H2O

CH = - 890 KJ/mol


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Controlled Oxidation Cu/523K/100atm

2CH4 + O2 --------------------- 2CH3OHMO2O3 methanol

CH4 + O2 ------------ HCOH + H2Omethanal

CH3(COO)2Mn2CH3CH3 + 3O2 -------------- 2CH3COOH + 2H2O

ethanoic acid


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Isomerisation

CH3(CH2)4CH3 ---------

n-Hexane

CH3CH (CH2)2 CH3 + CH3CH2 CH CH2 CH3

| |

CH3 CH32- Methylpentane 3- Methylpentane


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Aromatization

CH3

/ \ Cr2O3 or V2O5

CH3 CH3 or MO2O3

| | ----------------

CH2 CH2 773K

\ / 10-20 atmCH2


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Reaction with steam Ni

CH4 + H2O --- CO2 + 3H2 C6H12 + H2

Pyrolysis

C6H14 --------------------------------- C4H8 + C2H6

C3H6+C2H4+CH4


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y The result of rotation into different spatial

arrangements of atoms in space which can change intoone another. Such spatial arrangements of atoms

which can be converted in one another by rotationaround a CC single bond are called conformations.

y Conformations of ethane : there are infinite numberof conformations of ethane. However, there are twoextreme cases. One such conformation in whichhydrogen atoms are attached to two carbon are as

closed together as possible is called eclipsedconformations and the


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other in which hydrogens are as far apart as possible isknown as the staggered conformation. Any otherintermediate conformations is called skewconformations. Eclipsed and the staggeredconformations can be represented by Sawhorse andNewman projections.


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H

H H

H H

H

H H

H H H H

(i)Eclipsed (ii) Staggered


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H H H

H H

H H H H

H H

H(i)Eclipsed (ii) Staggered


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y These are unsaturated hydrocarbons containing atleast 1 double bond. These are also known as alefims(oil forming).

y Nomenclature

STRUCTURE IUPAC names

CH3 CH =CH2 Propene

CH3 CH2 CH = CH2 But 1 ene

CH3 CH = CH CH3 But 2 ene

CH2 = CH CH = CH2 Buta 1,3 - diene


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y Structural alkenes having C4H8 molecular formulacan be expressed by following 3ways.

1 2 3 4

CH2 = CH CH2 CH3 but 1 ene (C4H8)1 2 3 4

CH3 CH = CH CH3 but 2 ene (C4H8)1 2 3

CH2 = C CH3 2- methyprop 1 ene (C4H8)

|

CH3


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y Geometrical double bonded atoms have to satisfyremaining two valence by joining with 2 atoms orgroups. For example :

X Y X Y

\ / \ /

C C|| ||

C C

/ \ / \

X Y (a) X Y (b)


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y Geometrical Isomers the restricted rotation of

atoms or groups around doubly bonded carbon atomsgive rise to different geometric of such compounds.The stereoisomers of this types are called geometricalisomers.


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y From alkynes

R R`

Pd/C \ /

RC CR` + H2 ------------ C = C/ \

H H


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y From alkyl halides

H H H H

| | alc. KOH \ /

H C C H ---------------- C = C

| | / \

H X H H

(X= Cl. Br. I)


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y From vicinal dihalides

CH2Br CH2Br + Zn -----

CH2 = CH2 + ZnBr2CH3CHBr CH2Br + Zn -----CH3CH = CH2 + ZnBr2

y From alcohols by acidic dehydration

H H| | Conc.H2SO4

H C C H -------------- CH2 = CH2 + H2O

| | Ethene

H OH


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y Physical properties (a)Alkenes as a class resemblealkenes in physical properties except i n t y pes ofisomerism and difference in polar nature.

(b) These show regular increase in boiling point with

increase in size.(c)All the gases are colourless and odourless insolublein non-polar solvents except ethene.


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y Chemical properties (a)Alkenes are rich source ofloosely held Pi () electrons due to which the showreactions.

(b) Oxidation and ozonlysis reactions are also quiteprominent in alkenes.

y

Addition reaction of HB

r to symmetrical alkenes it takes place by electrophilic addition mechanism.

CH3 CH = CH CH3 + HBr----CH3CH3- CHCH3

|

Br


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y Addition reaction of HBr to unsymmetricalalkenes

CH3 CH CH3

|

Br

CH3 CH = CH2 + H Br 2-Bromopropane

CH3CH2CH2Br


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y It states that negative part of adding molecules getsattached to that carbon atom which passes lessernumber of hydrogen atoms. This generation of rulecan be understood in terms of mechanism of reaction.

y Mechanism hydrogen bromide provides anelectrophile, H+, which attacks double bond to formcarbocation as given below :-


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H3C CH = CH2= H BrH+

H3C CH2 CH2 + Br H3C CH CH2 + Br

(a)lessstable (b)morestable

primarycarbocation secondarycarbocation


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y These are also unsaturated hydrocarbons. Theycontain at least one triple bond between two carbonatoms. Their general formula is CnH2n-2.

y Nomenclature and Isomerism In common system,

alkynes are named as derivatives of acetylene.y Position isomers (i) but-1-yne (ii) bute-2-yne these

2 compounds differ in their structures due to positionof their triple bond, known as position isomers.


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STRUCTURE IUPAC names

HC C CH2 CH2 CH3

PENT 1YNE

H3C C C CH2 CH3 PENT 2YNE

H3C CH C CH|

CH3

3 METHYL BUT 1YNE


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y From calcium carbide on industrial scale, ethyneis prepared by treating calcium carbide with water.

CaCO3 --------- CaO + CO2

CaO +3C ------

CaC2 + COcalcium

carbide

CaC2 +2H2O --------- Ca(OH)2 + C2H2


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y From vinicial dihalides Vinicial dihalides ontreatment with alcoholic potassium hydroxide undergodehydrogenation.

H H H

| \ /

H2C C H + KOH ------------------

C = C| | -KBr / \

Br Br -H2O H Br

NaNH2 -NaBr,-NH3

CH CH


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y All alkynes are colourless.

y All members are odourless except ethene.

y These are weaklypolar in nature.

y These are also lighter than water and immencible withwater but soluble in organic solvents.


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y These are also known as arenes. Since most of thempasses pleasant odour. The class of compounds wasnamed as aromatic compounds. Most of suchcompounds were found to contain benzene ring.

Benzene is highly unsaturated but in majority ofreactions of aromatic compounds, the unsaturated ofbenzene ring is retained.


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y Benzenoids aromatic compounds containingbenzene ring called benzenoids. For examples-

CH3

(i)Benzene

(ii)Toluene

- (iii) Naphthalene


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y Aromacity benzene was considered as parent

aromatic compound. The name is applied to all ringsystems whether or not having benzene rings passingfollowing characters

y Planarity

y Complete delocalization of electron in ring.y Presence of (4n+2) electron in ring where n is

integer.


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ExamplesofAromaticcompounds:-

Benzene

Cyclopentdienyl

( n = 1, 6 )


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y Benzene is commercially isolated from coal tar.y Cyclic polymerization of ethyne.

y Decarboxylation of aromatic acids.

y Reduction ofphenol.


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y Generation of electrophile Cl Cl +AlCl3 ---------- Cl + [AlCl4]

chloronium ion

CH3 Cl +AlCl3 -----------

CH3 + [AlCl4]

CH3 C Cl +AlCl3 ----------- CH3 C + [AlCl4]

|| ||

O O


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y Formation of carbocation intermediate

sp3 hybridized carbon

H

+ E+ --------- E

+H


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y Removal of proton

H E

E [AlCl4] + HCl +

H -------- AlCl3

H [HSO4] E

E ----------

+ H2SO4

H


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y Benzene and polynuclear hydrocarbons containingmore than two benzene rings fused together are toxicand said to passes cancer producing (carcinogenic)property. Such polynuclear HC are formed byincomplete combustion of tobacco, coal andpetroleum. They enter into human body and damagesDNAand causes cancer.

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Vijendra Pal Singh