UNIT 5

ORGANIC COMPOUNDS
 Outline
1.1 Organic Chemistry
1.2 Structure of Organic Compounds
1.3 Functional Groups
1.4 Structural Effects
1.4.1 Resonance
1.4.2 Hyperconjugation
1.4.3 Inductive effect
1.4.4 Steric effect
1.5 Acidity and Basicity
1.6 Stability of Reaction Intermediates
 Organic Chemistry
• It is the study of the compounds of carbon.
• Organic compounds are made up of C covalently linked to atoms of other
elements, most commonly H, O, or N. (some contains S, P, halogens)
 Organic Chemistry
Why do we study organic chemistry?
History
• It was believed before that a “vital force” in living organisms was
necessary to produce an organic compound.
• 1828, Wöhler showed that it was possible to produce organic
substance urea from inorganic compounds.
 Organic Chemistry
The sheer number of organic compounds
• Chemists have discovered or made over 10 million organic compounds
and an estimated 100,000 new ones are discovered or made each
year.
• Approximately 85% of all known compounds are organic.

The link to biochemistry

• Carbohydrates, lipids, proteins, enzymes, nucleic acids, hormones,
vitamins, and almost all other chemicals in living systems are organic
compounds.
 Organic Chemistry
Organic molecules (biomolecules) present in
living organisms.
 Organic Chemistry
A comparison of organic and inorganic compounds
 Outline
1.1 Organic Chemistry
1.2 Structure of Organic Compounds
1.3 Functional Groups
1.4 Structural Effects
1.4.1 Resonance
1.4.2 Hyperconjugation
1.4.3 Inductive effect
1.4.4 Steric effect
1.5 Acidity and Basicity
1.6 Stability of Reaction Intermediates
 Structure of Organic Compounds
• Carbon has 4 valence electrons and form 4 covalent bonds.
• Able to bond with one another to form long chains and rings
• Only element that has the ability to form immense diversity of
compounds

Common elements found in organic compounds. The figure shows the number of
covalent bonds form by each atom of the element in neutral organic compounds.
 Structure of Organic Compounds
• Molecular formula: Shows actual number of atoms of each element in a
molecule.
• Structural formula: Shows the atoms present in a molecule and the bonds
that connect them.
• Expanded structural formula: (Kekulé or Line-bond structure): Shows
two-electron covalent bond as a line drawn between atoms
• Condensed structural formula: Shows symbols of atoms which are listed
in order as they appear in the molecule's structure with bond dashes (C-
H or C-C) omitted.
• Skeletal structure (Line-angle structure): shows the carbon skeleton in the
structure, with H atoms omitted and line ends & vertices represents C.
Atoms other than C or H are still shown.
Structure of Organic Compounds
Structures of some organic compounds

Molecular formula

Expanded
structural formula

Condensed formula

Skeletal formula
 Outline
1.1 Organic Chemistry
1.2 Structure of Organic Compounds
1.3 Functional Groups
1.4 Structural Effects
1.4.1 Resonance
1.4.2 Hyperconjugation
1.4.3 Inductive effect
1.4.4 Steric effect
1.5 Acidity and Basicity
1.6 Stability of Reaction Intermediates
 Functional Group
It is an atom or group of atoms within a molecule that shows a
characteristic set of predictable physical and chemical properties.

Why are functional groups important?

• Serve as sites of predictable chemical reactions
• To a large measure, they determine the chemical and physical
properties of a molecule
• Serve as units to classify organic compounds into families
• Serve as basis in naming organic compounds
 Structures of Some Functional Groups

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 Structures of Some Functional Groups

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 Structures of Some Functional Groups

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 Structures of Some Functional Groups
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 Exercise
Identify class of the following organic compounds based on the functional
group present in the given structures. CH3
CH3CH2C CH2CCHCH3
CH3C CCH2CHCH3
CH2 O
CH2CH2CH3
CH3

H3C CH3 CH2CH3

CH3CH2CHCH2OH
H3C O CH3

NH2
O
CH3
CH 3CH 2CH 2C OCH 2CH 3
 Outline
1.1 Organic Chemistry
1.2 Structure of Organic Compounds
1.3 Functional Groups
1.4 Structural Effects
1.4.1 Resonance
1.4.2 Hyperconjugation
1.4.3 Inductive effect
1.4.4 Steric effect
1.5 Acidity and Basicity
1.6 Stability of Reaction Intermediates
 Structural Effects
• Effect of the structure on stability and reactivity of the organic compound
• Resonance
• σ electron delocalization (C-H hyperconjugation)
• Inductive effect
• Steric effect
 Outline
1.1 Organic Chemistry
1.2 Structure of Organic Compounds
1.3 Functional Groups
1.4 Structural Effects
1.4.1 Resonance
1.4.2 Hyperconjugation
1.4.3 Inductive effect
1.4.4 Steric effect
1.5 Acidity and Basicity
1.6 Stability of Reaction Intermediates
Types of Electron Delocalization

• π Delocalization
• Lone pair delocalization
• σ Delocalization (Hyperconjugation)
 Resonance
• Some molecules have structures that cannot be shown with a single
representation (they differ in the placement of electron pairs).
• Electron pair(s) are shared between more than two atoms (electrons are
delocalized).
• Structural forms are called contributing or resonance or canonical
structures.

• “Actual” structure is a hybrid of all resonance structures (resonance hybrid)

 Resonance
How to show the movement of electrons?
• Curved arrow indicates movement of an electron pair (π bond or
lone pair).

Allowed movement
of electrons
 Resonance
Electron delocalization can be possible.
• In the presence of conjugated system
• Presence of pi-bond next to positive charge on an adjacent atom.
• The presence of an atom bearing lone pairs of electrons next to a
positive charge.
• The presence of a pi bond with an adjacent atom bearing lone pairs
of electrons.
Resonance
• Resonance hybrid of allyl anion
Resonance
• Resonance hybrid of benzene
 Drawing Resonance Structures
• The curved arrows show the movement of electron pairs not of
atoms nor charges.
• Multiple curved arrows point will flow in the same direction.
• Electrons should flow toward positive charges and away from
negative charges.

A B

• Beware of exceeding the octet rule.

 Outline
1.1 Organic Chemistry
1.2 Structure of Organic Compounds
1.3 Functional Groups
1.4 Structural Effects
1.4.1 Resonance
1.4.2 Hyperconjugation
1.4.3 Inductive effect
1.4.4 Steric effect
1.5 Acidity and Basicity
1.6 Stability of Reaction Intermediates
 Hyperconjugation
• The electrons from the σ bond (C-C or C-H) are involved in the delocalization.
• It occurs when the σ bond (C-C or C-H) is adjacent to carbon with a π bond
or an empty p orbital.
• Electrons delocalized due to partial overlapping of between sigma bond
orbital and pi-bond orbital or empty p-orbitals.

Hyperconjugation in propene, CH2=CHCH3

 Hyperconjugation
• Electron delocalization
results to an increased
stability of the molecule.
• As the number of C-H bonded
in double-bonded carbon
atom increases, the
possibility of
hyperconjugation increases
which results to more
stability.
 Hyperconjugation Effect
Stability of Alkenes
• Overlap of σ orbital of C – H bond with π orbital of adjacent C – C double
bond
• C – H possesses partial ionic character and its electrons get delocalized
into the adjacent π system.
• Stability of alkenes will increase with increase opportunity for
hyperconjugation.

2-butene is more stable

than 1-butene
 Outline
1.1 Organic Chemistry
1.2 Structure of Organic Compounds
1.3 Functional Groups
1.4 Structural Effects
1.4.1 Resonance
1.4.2 Hyperconjugation
1.4.3 Inductive effect
1.4.4 Steric effect
1.5 Acidity and Basicity
1.6 Stability of Reaction Intermediates
 Inductive Effect
• It is defined as the displacement of electrons forming a covalent bond
towards the more electronegative atom/group.
• It occurs when the electrons shared between two atoms/groups of
different electronegativities are withdrawn to the more
electronegative atom/group through the σ bond.
• This results in separation of charge or dipole.

C + – X -
 Types of Inductive Effect
1. Electron-withdrawing(attracting) inductive effect
• When the atom/group (X) is more electronegative than C: electrons
attracted to X
C+– X-

2. Electron-releasing(donating) inductive effect

• When the atom/group (Z) is less electronegative than C: electrons
attracted to C
C-– Z+
 Types of Inductive Effect
with excess positive charges

Electron-withdrawing R N
+
R
+
NH3
+
NO2
(attracting) substituents R
those with electronegative atoms
F
NH2 OH OC H3 Cl
Br
groups exhibiting orbital electronegativity

C N N N

those with easily polarizable valence electrons

-
I
 Types of Inductive Effect
Electron-releasing (donating/repelling) substituents
alkyl groups
C H3

negative groups
- - - H3C
C OO S O CH C N
H3C
O
-
H3C C -- H3C S
O
Types of Inductive Effect
 Outline
1.1 Organic Chemistry
1.2 Structure of Organic Compounds
1.3 Functional Groups
1.4 Structural Effects
1.4.1 Resonance
1.4.2 Hyperconjugation
1.4.3 Inductive effect
1.4.4 Steric effect
1.5 Acidity and Basicity
1.6 Stability of Reaction Intermediates
 Steric Effect
• It refers to the spatial environment around a functional group, where
many atoms/groups situated near the functional group hindering the
accessibility of other molecules to the functional group.
• Groups “crowding” into one another are engage in steric repulsion.
 Outline
1.1 Organic Chemistry
1.2 Structure of Organic Compounds
1.3 Functional Groups
1.4 Structural Effects
1.4.1 Resonance
1.4.2 Hyperconjugation
1.4.3 Inductive effect
1.4.4 Steric effect
1.5 Acidity and Basicity
1.6 Stability of Reaction Intermediates
 Acids and Bases
• Acid: A proton donor (Bronsted-Lowry), electron pair acceptor (Lewis).
• Base: A proton acceptor (Bronsted-Lowry), electron pair donor (Lewis).
• pKa is a measure of acid strength.
pKa  acidity  basicity
• Structural affects which stabilizes the conjugate base will increase
acidity.

When an acid becomes a conjugate base, the stability of the new lone pair on
the conjugate base is a factor in determining how favorable the reaction will be.
 Acids and Bases
Factors affecting Acidity/Basicity
• Charge
• Electronegativity and Polarizability
• Resonance effect
• Inductive effect
• Hybridization effect
Factors Affecting Acidity/Basicity
Charge
•  Acidity basicity  positive charge on an atom
Factors Affecting Acidity/Basicity
Charge
•  Acidity basicity  positive charge on an atom
 Factors Affecting Acidity/Basicity
Electronegativity and Polarizability
• Down the group, acidity polarizability
 Factors Affecting Acidity/Basicity
Polarizability: ease of
distorting the electron
distribution in the
atom/molecule.
• High charge densities
tend to be less stable
than low charge
densities. The more
“spread out” a
charge can be, the
more stable it will be.
Factors Affecting Acidity/Basicity
Electronegativity and Polarizability
• Across the period, acidity electronegativity
 Factors Affecting Acidity/Basicity
Resonance Effect
• Resonance increases the
stability of the conjugate
base (therefore,
acidity) because the
negative charge can be
delocalized.
 Factors Affecting Acidity/Basicity
Resonance Effect
•  electron density
on the N of amine
(lone pair more
available),
basicity.
• Resonance
stabilization
decreases basicity.
 Factors Affecting Acidity/Basicity
Inductive Effect
• Electron-withdrawing substituent stabilizes the
negative charge of the conjugate base. (acidity)
Factors Affecting Acidity/Basicity
Inductive Effect
• Electron-releasing substituent destabilizes the
negative charge of the conjugate base. (acidity)
pKa
HCOOH 3.75
CH3COOH 4.76
CH3CH2COOH 4.87
CH3CH2CH2COOH 4.82
Factors Affecting Acidity/Basicity

Inductive Effect

Number, type and the distance of the substituent(s) also affect the acidity.
Factors Affecting Acidity/Basicity
Inductive Effect

Number, type and the distance of the substituent(s) also affect the acidity.
Factors Affecting Acidity/Basicity
Inductive Effect

Number, type and the distance of the substituent(s) also affect the acidity.
Factors Affecting Acidity/Basicity
Inductive Effect
• Electron-releasing substituent increases electron
density on the N of amine, basicity
• Electron-withdrawing substituent, basicity
pKb

CH3NH2 3.36

CH3CH2NH2 3.27

CH3CH2CH2NH2 3.16
Factors Affecting Acidity/Basicity
Inductive Effect
• Electron-releasing substituent increases electron
density on the N of amine, basicity
• Electron-withdrawing substituent, basicity
 Factors Affecting Acidity/Basicity
Hybridization Effect
• Increasing the “s-character” of the orbital, the closer the electrons will
be to the nucleus, and the lower in energy (= stable) they will be.
Exercise
Exercise
 Outline
1.1 Organic Chemistry
1.2 Structure of Organic Compounds
1.3 Functional Groups
1.4 Structural Effects
1.4.1 Resonance
1.4.2 Hyperconjugation
1.4.3 Inductive effect
1.4.4 Steric effect
1.5 Acidity and Basicity
1.6 Stability of Reaction Intermediates
 Reaction Intermediates
• If the reaction occurs in more than one step, it must involve species that
are neither the reactant nor the final product. (reaction intermediate)

• Reaction intermediate maybe a free radical, a carbocation or a

carbanion.
 Reaction Intermediates
• Free radical: unpaired electron

• Carbocation

• Carbanion
 Stability of Reaction Intermediates
• Structural effects influence the stability of reaction intermediates
Hyperconjugation effect
• The empty p orbital of this sp2 C can overlap with σ orbital of C – H bond
of adjacent
• The positive charge/free radical is dispersed over large volume of space
and is stabilized.
 Stability of Reaction Intermediates
• Stability of carbocation/free radical will
increase with increase opportunity for
hyperconjugation.
• The order of stability of carbocations can
be given as:

• Similarly the increasing order of stability of free radical is primary

< secondary < tertiary free radical.
 Stability of Reaction Intermediates
Inductive effect
• As the number of alkyl groups increases, they push more and more
electrons towards positively charged carbon atom of carbocation. This
causes dispersion of positive charge, resulting in increased stability.
 Stability of Reaction Intermediates
Inductive effect
• More is number of alkyl groups more they push electrons to negatively
charged carbon atom lesser is stability of carabanion.
 Tapos na po!
Thanks be to God for creating functional groups that
bring order, not chaos, to how organic compounds
“behave” in nature (and in the lab)!