Fundamentals of Organic Chemistry 2

8/2/2019 Fundamentals of Organic Chemistry 2

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Fundamental of Organic Chemistry

-Functional group and homologous series

-Isomerism-Structure and shape of organic

compounds

-Nomenclature

-Structure determination

-Stereoisomerism


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Functional groups

Functional group is an atom or a group of atoms

which determine the properties of a

homologous series.Examples : alkanoic acid / carboxylic acid

C O H

O


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Homologous seriesHomologous series is a series of compound in whicheach member differs from the next by a

CH2 group

All members:

Can be prepared by similar methods

e.g. preparation of ethanoic acid

Have similar chemical properties and graded

physical properties.e.g. Both ethene and propene decolourize Br2 quickly.

Boiling point of methane and ethane are gradually

increased.


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Homologous series

All members

Can be represent by a

General formula (which also shows the

functional groups the members bear)

Examples : (in handouts)


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Homologous series

FunctionalGroup

GeneralFormulae

Name Examples

Saturatedhydrocarbon

CnH2n+2 alkane CH4methane

CnH2n alkene ethene

-CC- Cn

H2n-2

alkyne ethyne


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Homologous series

arene benzene

saturatedring

CnH2n cycloalkane cyclohexane

-X RXCnH2n+1X

haloalkane chloroethane


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R : Alkyl group

CnH2n+2 Alkane CnH2n+1 Alkyl group

CH4 methane CH3 Methyl

C2H6 ethane C2H5 ethyl

C3H8 propane C3H7 propyl

CH3CHCH3 methylethyl


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Homologous series

-OH ROHCnH2n+1OH

alkanol ethanol

-O- R-O-R

CnH2n+1OCmH2m+1

ether ethoxyethane

(-CHO) RCHO alkanal ethanal

C H

O

CnH2n+2O

Cn+mH2(n+m)+2O

CnH2n+2O

CnH2n+1CHO

CnH2nO


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Homologous series

alkanone

butan-2-one

-COOH RCOOH alkanoic

acidethanoic

acid

-COOR RCOOR ester methylethanoate

C

O

C R'R

O


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-COX RCOX alkanoyl

halide

ethanoyl

chloride

acidanhydride

ethanoic

propanoicanhydride

-NH2 RNH2CnH2n+1NH2CnH2n+3N

amine methanamine

C

O

ClCH3

CO

O

C

O

CO

O

C

O

R R'

CO

O

C

O

CH3 CH2CH3

NH2C

H

H

H


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-NO2 RNO2 nitro-compounds

CH3CH2NO2nitroethane

-CN R-CN cyano-compound

(carbonitriles)

ethanenitrile

-CONH2 Acid amide CH3CONH2

ethanamide


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Isomerism

Isomerism

Isomer - same molecular formula but

not identical.

Isomerism

Structural Isomerism Stereoisomerism


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Structural isomerism

Structural isomers having same

molecular formula but differ in the

sequence in which their atoms are linkedtogether, i.e. different structural

formula :


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Unsaturation index

Unsaturation index = No. ofcarbonatoms No. ofhydrogen atoms / 2

No. ofhalogen atoms / 2 + No. ofnitrogen atoms / 2 +1

Unsaturation index = (C) - (H)/2 - (X)/2 +(N)/2 +1

If unsaturation index = 1 : a double bond / ringIf unsaturation index = 2 : two double bonds /rings OR one triple bond OR one d.b. + one ring


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Examples

C6H6 : 6 6/2 + 1 = 4

3 d.b. + 1 ring

C6H12O : 6 12/2 + 1 = 1

1 d.b.

C3H5N : 3

5/2 + 1/2 + 1 = 21 t.p.


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C6H12O

Unsaturation index = 1

Identify possible functional groups :

OH,

O ,

CHO,CO , etc

Try aldehyde CHO


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C6H12O the aldehydes

Start with longest continuous carbon chain :

H

O

H

O

Shorten the longest continuous carbon chain,

and consider the removed CH3 group :

H

O

H

O

H

O


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Continue to shorten the longest continuous carbon chain :

H

O

Put back the two carbon either two methyl groups or,

One ethyl group

H

O

H

O

H

O

H

O

H

O The left one is repeated as

the longest carbon chain

also consists of 5 carbon



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Can we continue to shorten the longestcontinuous carbon chain ?


H

O

No, because this would increase the no. ofcarbon in the longest continuous carbon chainand this would give a structure we have drawnbefore.


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In the above example, we havent consider the positionof functional group because the aldehyde functionalgroup must be in the terminal end of a chain.

If it is ketone, the functional group a C = O would beat the middle of the chain :

O

O

O=


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C5H10

Unsaturation index = 5 10/2 + 1 = 1

It can bear a double bond or a ring.

C C C CC

H

H

HH

H

H

H

H

H

H =

=


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C4H7Cl

Unsaturation index = 4 7/2 + 1 1/2 = 1

Cl

Cl

Cl

Cl

Cl

Cl

Cl

Cl

Cl

Cl

Cl

Cl


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Functional group isomers

Isomerism occurs between members of two

different homologous series (which have same

general formula).Examples : Ketone and Aldehyde CnH2nO

C3H6O

O

H

HH

HH

H H

HH

H

O

HH


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Alcohol and ether CnH2n+2O

Examples : C3H8O


OH

O


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Carboxylic acid and ester CnH2nO2

Example : C3H6O2


O

O

OH

O

H O

O


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Effect of relative sizes of alkyl group

and functional group on physical

properties of compounds

Boiling point Boiling point depends on intermolecular force.

Stronger intermolecular forces

higherb.p.

Intermolecular force : Van der Waals force

and H bond

Compounds which can form H

bondwould have higher boiling point :


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Boiling point of a homologous series

Within the same homologous series, b.p. increased

gradually with the carbon atoms increased.

But the difference in b.p. between consecutive

members of the homologous series become smaller.

B.p.

No. of carbon atoms


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In alkanol In amine

Most of them are liquids in room temperature and pressure.

H bond formation in Organic compounds

O H OH

N H

H

NH

H


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H bond formation in Organic

compounds

In carboxylic acid In acid amide Liquids and solids

Benzenecarboxylic acid is a white crystalline solid.

O

O H

O

O

H

O

O

H

O

N H

H

O

N

H

H

O

N

H

H


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Melting point

It has a similar trend in b.p.

But in case of alkanes and carboxylic acids, an

even member of the series has a higher m.p.

than the odd member immediately below (or

even above) it.

Melting

point

No. of carbon atoms


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Melting point

This is because in even member, different

chains packed closer together than for the

odd member, thus larger theattractive force.


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Solubility in water

It involves two kinds of energy when a substance

dissolves in water :

Energy required to break the bond /overcome the force of attraction

between solute particles

(intermolecular force).

Energy released when the attractiveforcesformed by the water molecule and the

solute particles.


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Solubility in water

Size of molecule

Solubility decreases with increasing

chain length as the intermolecular force becomestronger more energy required tobreak the bond.

Example : Ethanol is soluble in water in allproportion while hexan-1-ol is only slightlysoluble.


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Formation of H bond

Those compounds which can form extensive H

bond with water has higher solubility in water :

Solubility in water

O

N H

H

O

H

H

O

HH

O

HH

O

H

H

O

H

HOther homologous

series includescarboxylic acid,

alkanol and amine.


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For alkanals, ketones, etc. they are only slightly

soluble in water unless the molecular size is

small :

Solubility in water

H H

O

O

H

H

O

H

H

CH3

CH3

O

O

H

H

O

H

H


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Physical properties of structural isomers

Butyl alcohols :

Density B.P. /o

CCH3CH2CH2CH2OH 0.810 118

(CH3)2CHCH2OH 0.802 108

CH3CH2CH(CH3)OH 0.806 100

(CH3)3COH 0.789 83


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Boiling points decrease as branching increases

Branching of carbon chain gives a more sphericalshape to the molecules and reduces the extent ofcontact between molecules

Weaker intermolecular force

No regularity in the change in m.p. as the packing in

solid state may varyDensity of branching alcohol is also lower than itsstraight chain isomers.

Physical properties of structural isomers

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Fundamentals of Organic Chemistry 2

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