What`s meant by organic chemistry?
• Organic chemistry is the study of "living"
things not in the same way that biology is
the study of life. Rather, organic chemistry
takes a look at what composes the living
things, and how they’re structured.
• It focuses mainly on carbon, which is highly
essential to maintaining life
• Chemistry of Life [ proteins, amino acids
DNA, RNA,Sugar……]
Atomic Theory
•Electrons exist in energy levels that surround the nucleus
of the atom.
•The energy levels are called shells, and within these
shells are other energy levels, called subshells or orbitals,
that contain up to two electrons.
the first two energy levels (shells 1 and 2) are the most
important for bonding in organic chemistry.
Orbitals
Shell
s
p
1
1
2
2
3
3
3
3
d
5
Total Electrons Possible
2
8
18
*energy level 1 contains up to two electrons in a spherical orbital called a
1s orbital.
*energy level 2 contains up to eight electrons; two in an 2s-orbital and two in
each of three orbitals designated as 2p-orbitals. The p-orbitals
have a barbell type shape and are aligned along the x, y, and
z axes. They are thus called the px, py, and pz orbitals.
*energy level 3 contains up to eighteen electrons, two electrons in a
3s orbital, six electrons in the three 3p orbitals, and ten electrons
in the five 3d orbitals.
The most important element that we must know its electron configuration is the Carbon
C
its Atomic number= 6 electron
arranged as
1s2, 2s2, 2p2
• Carbon has 2 electrons in its outermost shell
• From this it was expected to be……Divalent.
• Carbon was found to be ……………..Tetravalent
HYBRIDIZATION OF CARBON
Carbon may form single, double and triple bonds. Bonding in any
element will take place with only the valence shell electrons.
In order to determine the hybridization on a carbon
atom, one must first draw the Lewis structure.
The first example Methane
CH4
H
H
C
H
H
In order to form four hybrid orbitals, four atomic orbitals
have been mixed. The s orbital and all three p orbitals
have been mixed, thus the hybridization is sp3 .
Let's show this using the atomic orbitals of excited state carbon
found in the valence shell:
The geometry that achieves this is tetrahedral
geometry, where any bond angle is 109.5o.
Each hybrid orbital contains 1 electron. A hydrogen 1s
orbital will come in and overlap with the hybrid orbital to
form a sigma bond (head-on overlap)
2) ethene, C2H4 .
In order to form three hybrid orbitals, three atomic orbitals
have been mixed. The s orbital and two of the p orbitals for
each carbon have been mixed, thus the hybridization for
each carbon is sp2
•The three sp2 hybrid orbitals will arrange themselves in three dimensional
spaceto get as far apart as possible.
• The geometry that achieves this is trigonal planar geometry.
• where the bond angle between the hybrid orbitals is 120o.
•The unmixed pure p orbital will be perpendicular to this plane.
• Keep in mind, each carbon atom is sp2, and trigonal planar.
3) acetylene, C2H2
two hybrid orbitals have formed. In order to form two hybrid
orbitals, two atomic orbitals have been mixed.
•The two sp hybrid orbitals arrange themselves in three
dimensionalspace to get as far apart as possible.
•The geometry which achieves is linear geometry with a bond
angle of 180o.
•The two pure p orbitals which were not mixed are perpendicular
to each other.
It is the ability of an atom to attract electrons to itself, and
generally increases as one moves from left to the right
across the periodic table. Elements that easily lose electrons
and attain a positive charge are called electropositive
elements. Alkali metals are electropositive elements.
Electronegativities of Selected Elements
H
2.2
Li
1.0
Na
0.9
K
0.8
Be
1.6
Mg
1.3
B
2.0
Al
1.6
C
N
O
F
2.6
3.0
3.4
4.0
Si
1.9
P
2.2
S
2.6
Cl
3.2
Br
3.0
I
2.7
• Electronegativity increasea from left to right
and from top to bottom.
• From this the most electronegative atom is….
Types of Chemical Bonds
• Ionic bond
• Covalent bond
1- Ionic bond:
formed between atoms widely different in EN (> 2)
The bond results from one atom giving up an electron while another
atom accepts the electron. The ions are held together by the
electrostatic attraction of the positive and negative ions.
-
2-Covalent Bond
•Formed by a sharing of two electrons by two atoms
e.g
H
+
H
H
H
Two types:
A] non-polar covalent: ( No difference in EN)
C-C , H-H, Cl-Cl
B] polar covalent:
is formed when the difference in the EN is < 2.
C-Li, C- Cl
+
C
-
Cl
+ -
H F
Attraction between molecules: (van der Waals forces):
These are very weak forces but responsible for holding uncharged
molecules together in liquid or solid state.
These forces can be classified into:
•Dipole-dipole interactions:
Bonds that contain a separation of charge possess
a dipole moment,
a property the contributes to the overall polarity of the molecule
H
H C OH
H
H
H C Cl
H
C O
2) Hydrogen Bonds and Bond Polarity
The bonds O-H, N-H and F-H are highly polar covalent
 The hydrogen atom has a partial positive charge.
 The hydrogen atom is attracted to the basic site in other
molecules, such as the non-bonding electrons on oxygen and
nitrogen This attraction is called hydrogen bonding
 Hydrogen bonding and is useful for explaining high boiling
points and high melting points of fairly low mass molecules.
Extra energy is required to break the hydrogen bonds during
the boiling process.
C H3
..- O :
C H3

H - O :
+
H
In te r m o le c u la r H -b o n d
H
O
C H3- C
O
C - O Et
C
H
In tr a m o le c u la r H -b o n d
Bond Polarization
Effects influencing the electron displacement
at the time of formation of covalent bond.
Some of these effects are permanent while others are temporary.
The effects that remain permanently within the molecule are
termed as polarization effects and are permanent in nature.
They include
the inductive effect and
the mesomeric effect.
Inductive Effect
 It is an effect that causes permanent displacement of electrons involved in
covalent bond formation towards a more electronegative element.
 formation of a covalent bond between a carbon atom and other atom which is
highly electronegative causes the electrons involved in the covalent bond
formation to be pushed towards the high EN atom. This imparts a small negative
charge on it and small positive charge on carbon. This is termed as –I effect
S+
C H3- C H2
C H2
C H2
SX
x has (- I) effect
Examples of groups capable of exerting negative inductive effect include
F, Cl, Br, I, OH, OCH3 etc.
These groups are termed as electron attracting or with-drawing groups.
 if a covalent bond is formed between carbon and an atom that is electropositive in
nature, then the electrons involved in the bond formation will be pushed towards carbon
imparting a small negative charge on carbon. This is termed as +I effect
C H3
SC H2
S+
Li
L i h a s (+ I) effect
They can be said as electron releasing groups.
Examples of groups showing the +I effect include the
(CH3)3C, (CH3)2CH, CH3 CH2, CH3.
The inductive effect may be caused by some molecules also.
Relative inductive effects have been experimentally measured with reference to
hydrogen.
Resonance effect or mesomeric effect
The effect in which π electrons are transferred from a multiple bond to an atom, or
from a multiple bond to a single covalent bond or lone pair (s) of electrons from an atom
to the adjacent single covalent bond is called mesomeric effect or simply as M-effect.
In case of the compound with conjugated system of double bonds, the M effect is
transmitted through whole of the conjugated system and thus the effect may better be
known as conjugative effect.e.g
+
(1,3-butadiene)
4
2
1
-
3
 Groups which have the capacity to increase the electron density of the rest of the
molecule are said to have +M effect. Such groups possess lone pairs of electrons.
+M effect groups :
 Groups which decrease the electron density of the rest of the molecule by
withdrawing electron pairs are said to have –M effect, e.g.,
The negative resonance effect -M of carbonyl group is shown below.
It withdraws electrons by delocalization of π electrons and reduces the electron density
particularly on 3rd carbon.
 The inductive and mesomeric effects, when present together, may act in the
same direction or oppose each other.
 The mesomeric effect is more powerful than the inductive effect.
For example, in vinyl chloride due to – I effect the chlorine atom should
develop a negative charge but on account of mesomeric effect it has positive
charge.
Difference between I & M
I
M
1) Electrons move in σ bond
1) Electrons move in π
bond
2) Must be difference in EN
2) Not a must
3) Decrease by distance
3) Not affected by
distance
Types of bond cleavage
1)Homolytic
2)Heterolytic
•Homolytic cleavage:
.
1) Occurs in a non-polar bond involving two atoms of similar electronegativity.
2) A single bond breaks symmetrically into two equal parts, leaving each atom
with one unpaired electron.
3) Formed free radicals.
U .V
A.
A . .B
.B
+
U .V
C l-C l
C l.
+
C l.
Free radicals are electrically neutral highly reactive species.
 Free radical reactions occur in the sunlight, at high temperature
•Heterolytic cleavage:
1)Occurs in a polar bond involving unequal sharing of electron pair between two
atoms of different electronegativities.
2) A single bond breaks unsymmetrically.
3) Both the bonding electrons are transferred to the more electronegative
atom.
4) Formed cation and anion.
A..B
-
+
A: + B
anion cation
Reaction Intermediates
a)Carbocation
b) Carbanion
c) Free Radical
They are unstable and highly reactive.
Carbocations:
They are positively charged species containing a carbon atom having only 6
electrons in 3 covalent bonds.
Carbocation carbon is sp2 hybridized C and its shape is planar.
+I effect of R groups stabilizes the carbocations.
+
120
Order of stability of carbocations:
R
R
R
R- C
+
R
R- C
+
H
o
3
o
2
R- C
+
H
o
1
+
CH3
m eth yl carb o catio n
(least stab le)
2-Carbanions:
They are negatively charged species containing carbon atom with 3 bonds
and unshared pair of electrons.

Carbanion carbon is sp3 hybridized C and its shape is tetrahedral.
+I effect of R group destabilizes the carbaninon.
Order of stability of carbanions:
: C H3
-
..
RC H 2
m ost stabale
1
o
..
CH
R
R
C:
R
R
R
2
o
3
o
carbanions (least stable)
3-Free radicals:
They are electrically neutral species with one odd electron.
.
C
Order of stability of free radicals:
R
.
R
.
R- C
R- C
R
H
.
R C H2
.
CH3
Note that allyl carbocations and allyl free radicals are highly stabilized by
resonance.
+
+
.
.
allyl carbocation
allyl free radical
Reagents of organic reactions
A) Electrophile (E+)
Means ‘electron loving’.
It is an electron-deficient species and accepting electron from an attacking
nucleophile(high electron density species)
Electrophiles may be
•Positively charged atom or group (Cations)
•Neutral molecules with vacant orbitals e.g. AlCl3, FeCl3, SO3.
•Carbenes and free radicals may also considered as electrophiles.
C:
+
+
E
C
E
B)Nucleophiles (Nü):
-
-
C
N
•means ‘nucleus loving’
•An electron-rich species and electron-pair donor.
•A nucleophile can be either neutral.or negatively charged.
Nucleoophiles may be
neutral molecules having unshared electron pairs. e.g. ROH, NH3 and
amines
, H 2O , H 2S

negatively charged atom or group e.g. H- , X - , OH - RO , NH2, carbanions.
Nucleophiles attack the carbon of low electron density (carbocations).
C +
+
..
Nu
C - Nu
Types of Organic Reactions:
1- Addition.
2- Substitution.
3-Elimination.
1- Addition reactions:
A reaction in which atoms or groups add to adjacent atoms of a multiple
bond e.g C = C, C ≡ C, C≡N, C = O and sometimes at small size rings.
Two types of addition :a) Electrophilic Addition
b) Nucleophilic Addition
A)Electrophilic addition
•Initiated by an electrophile accepting electron from an attacking nucleophile
•Typical reaction of unsaturated compounds such as alkenes and alkynes.
CH2=CH2 + Cl2
CH2-CH2
Cl
Cl
b)Nucleophilic Addition
•Initiated by a nucleophile, which attacks an electrophilic site of a molecule.
•Typical reaction of carbonyl compounds.
+
C =O
+HCN
OH
C
CN
2)Substitution reactions:
A reaction in which an atom or group in a molecule is replaced by another atom.or group.
There are three types of substitution :-
a) free radical substitution.
Substitution which involves free radicals as intermediate species
A–B + Y.
A – Y + B.
e.g.
CH4 + 2Cl .
CH3Cl + HCl
b) electrophilic substitution.
Typical reaction of aromatic compounds. The aromatic nucleus has high electrondensity,
thus it is nucleophilic and is prone.to electrophilic attack.
A–B
+ Y+ (E+)
e.g. C6H6 (benzene) + +NO2
A–Y+ B+
SE
C6H5NO2 + H+
Nitrobenzene
C)Nucleophilic Substitution(SN):
Typical reaction of saturated organic compounds bearing polar bond as
functional group, such as haloalkane with alcohol
A–B
+ Y- (Nu)
A–Y + B-
OH-
e.g. CH3I +
SN
CH3OH + I-
3) Elimination reactions (E):
This is the reverse of addition reactions, An atoms or groups are removed
from djacent carbon atoms of a molecule to form a multiple bond (double or
triple bond). Results in the formation of unsaturated molecules . e.g
-
CH3 CH2Cl + NaOH
CH = CH2