Fundamental of Organic Chemistry

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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

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Homologous series

• Homologous series is a series of compound in
  which each 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
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Homologous series
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R Alkyl group
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Homologous series
CnH2n2O
CnmH2(nm)2O
CnH2n2O
CnH2n1CHO
CnH2nO
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Homologous series
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Isomerism

• Isomerism
• Isomer - same molecular formula but not
  identical.

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Structural isomerism

• Structural isomers having same molecular formula
  but differ in the sequence in which their atoms
  are linked together, i.e. different structural
  formula

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Unsaturation index

• Unsaturation index No. of carbon atoms No.
  of hydrogen atoms / 2 No. of halogen atoms / 2
  No. of nitrogen atoms / 2 1
• Unsaturation index (C) - (H)/2 - (X)/2 (N)/2
  1
• If unsaturation index 1 a double bond / ring
• If 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 2
• 1 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

• Shorten the longest continuous carbon chain, and
  consider the removed CH3 group

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C6H12O the aldehydes

• Continue to shorten the longest continuous carbon
  chain

• Put back the two carbon either two methyl
  groups or,

• One ethyl group

• The left one is repeated as the longest carbon
  chain also consists of 5 carbon

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C6H12O the aldehydes

• Can we continue to shorten the longest continuous
  carbon chain ?

• No, because this would increase the no. of carbon
  in the longest continuous carbon chain and this
  would give a structure we have drawn before.

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C6H12O the aldehydes

• In the above example, we havent consider the
  position of functional group because the aldehyde
  functional group must be in the terminal end of a
  chain.
• If it is ketone, the functional group a C O
  would be at the middle of the chain

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C5H10

• Unsaturation index 5 10/2 1 1
• It can bear a double bond or a ring.

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C4H7Cl

• Unsaturation index 4 7/2 1 1/2 1

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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

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Functional group isomers

• Alcohol and ether CnH2n2O
• Examples C3H8O

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Functional group isomers

• Carboxylic acid and ester CnH2nO2
• Example C3H6O2

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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 ? higher b.p.
• Intermolecular force Van der Waals force and H
  bond
• Compounds which can form H bond would 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.

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H bond formation in Organic compounds

• In alkanol In
  amine
• Most of them are liquids in room temperature and
  pressure.

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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.

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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.

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 the attractive 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 attractive forces
  formed 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 become
  stronger ? more energy required to break the
  bond.
• Example Ethanol is soluble in water in all
  proportion while hexan-1-ol is only slightly
  soluble.

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Solubility in water

• Formation of H bond
• Those compounds which can form extensive H bond
  with water has higher solubility in water

• Other homologous series includes carboxylic acid,
  alkanol and amine.

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Solubility in water

• For alkanals, ketones, etc. they are only
  slightly soluble in water unless the molecular
  size is small

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Physical properties of structural isomers

• Butyl alcohols

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Physical properties of structural isomers

• Boiling points decrease as branching increases
• Branching of carbon chain gives a more spherical
  shape to the molecules and reduces the extent of
  contact between molecules ?
• Weaker intermolecular force
• No regularity in the change in m.p. as the
  packing in solid state may vary
• Density of branching alcohol is also lower than
  its straight chain isomers.