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Introduction to Organic Chemistry
Organic chemistry is the study of the chemistry of carbon compounds. Over 16 million
carbon containing compounds are known and 90% of the new compounds discovered
or synthesized are also organic or carbon containing. All of the biomolecules essential
to life such as nucleic acids, amino acids, carbohydrates, lipids and fats are organic
compounds.
In the last 60 years the increased life expectancy is directly linked to life saving drugs
that minimize the dangers of microbial infections. These antibiotics were introduced
from 1940s onwards and radically changed the course of treatment of common
diseases.
Many of the most effective modern vaccines depend on organic chemistry.
What are the unique properties of carbon that leads to such a huge diversity of
compounds?
Stability of carbon-carbon bonds,
The existence of carbon-carbon multiple bonds
Carbon-oxygen, carbon-nitrogen, carbon-halogen bonds add to the huge variety of
compounds
Carbon-carbon bonds lead to linear and branched chain molecules as well as
Compounds with rings
REVISION OF CONCEPTS STUDIED IN CHEM101
Carbon is tetravalent
Methane
H
H
C
H
H
H
H
C
H
H
2
H
H
C
C
H
ethane
H
H
H
H
C
C
C
H
H
propane
H
H
H
H
H
H
H
H
H
C
C
C
C
H
H
butane
H
H
C
O
N
H
tetravalent
divalent
trivalent
monovalent
H
H
C
C
N
H
amine
H
H
H
H
H
H
H
H
C
C
H
H
O
H
H
alcohol
H
H
C
C
H
H
H
F
halide
Multiple Carbon-Carbon Bonds
H
H
C
C
H
ethane
H
H
H
H
H
C
C
H
H
ethane/ethylene
H
C
C
H
ethyne/acetylene
C C
C C
C C
single
double
triple
Chemical Bonding
Atomic orbitals
Electrons populate different energy levels, or electron shells, and the way in
which electrons fill the energy levels creates the atoms of the periodic table (rows
of periodic table)
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Orbitals 3-dimensional volume in which find electron (electron density)
Shape of orbitals are important
s orbitals are spherical
p orbitals are like dumbbell
Orbitals increase in energy in ranking
1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p.......
For Organic chemists 1s, 2s, 2p are most important!!
The inert gas configuration of electrons, a full outer energy level is especially stable.
(Octet rule)
The ground state electronic configuration is the lowest energy state of an atom and can
be written in either of two ways:
Sodium would be written as 1s2 2s2 2p6 3s1 or [Ne]3s1
When sodium loses an electron from its outer shell it achieves the electronic
configuration of neon; Na+ 1s2 2s2 2p6
Ionic Bonds
When atoms lose or gain electrons, they develop charges and
are called ions. Positively charged ions are cations, and
negatively charged ions are called anions.
Li +
.
lithium cation
Ffluoride anion
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Electronegativity
Electronegativity measures the tendency of an atom to attract
electrons. Electronegativity increases in going left to right across a row
in the Periodic Table. In some groups, it decreases in going down a
column.
Li
Row 2
Be B
C
N
O
F
increasing electronegativity
Ionic bonds are formed when atoms of
very different electronegativities interact.
Group 7
F
decreasing
Cl
electronegativity
Br
I
Atoms at extreme left or right of periodic table lose or gain electrons to achieve inert
gas configuration (very stable). Octet rule!!!
Example Na+ ClBoth atoms achieve the stable inert gas configuration by loosing and gaining an
electron.
Resultant compound is ionic
most electronegative
Linus Pauling (1901-1994)
Nobel Prize in Chemistry 1954
Nobel Peace Prize 1962
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Electronegativity and Types of Chemical Bonds
The electronegativity difference between interacting
atoms determines the type of bond that is formed.
F2
compound
electronegativity
difference
type of bond
4.0 - 4.0 = 0
HF
4.0 - 2.1 = 1.9
polar
covalent
nonpolar
covalent
LiF
4.0 - 1.0 = 3.0
ionic
Lithium Fluoride
Lithium, a metal, has a very low electronegativity. Fluorine, a
nonmetal, has a very high electronegativity. When they react,
lithium gives up its single outermost (valence-level) electron,
while fluorine takes on a single electron to fill the second energy
level (valence-level).
Li
+ F
Li
+ F
elect ron
configurat ion
of neon
elect ron
configurat ion
of helium
In the solid state, lithum fluoride forms a very stable ionic crystal
lattice structure where each Li+ is surrounded by six F- , and each Fis surrounded by six Li+. Strong electrostatic attractive forces among
the ions of opposite charge stabilize the solid state structure.
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Carbon and Covalent Bonds
Carbon has 4 outer shell electrons 1s2 2s2 2p2
Loosing 4 electrons to become C4+ or C4- would require far too much energy.
Carbon shares electrons with the atom to which it is bonded and achieves an octet of
electrons
Example methane: each hydrogen atom has a share of two electrons and the carbon a
share of 8.
Covalent Bonding
Covalent Bonding
When two atoms of the same or similar electronegativities react, they
achieve a noble gas electron configuration by sharing electrons in
covalent bonds. A two electron bond is shown by a dash or line.
Examples
H2
H.
.
. C . + H.
.
H :H
or
H-H
H
H
or
:
H :C H
HCH
H
H
: :
CH 4
H. +
Note: Carbon, Group 4, needs 4 electrons to reach the electron
configuration of neon. Hydrogen achieves the electron configuration
of helium by forming a single two electron bond.
Sharing of electrons results in the covalent bond
Simple examples H2, O2
(Lewis)
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Covalent Bonding
When two atoms of the same or similar electronegativities react, they
achieve a noble gas electron configuration by sharing electrons in
covalent bonds. A two electron bond is shown by a dash or line.
Examples
H2
H.
.
. C . + H.
.
H :H
or
H-H
H
H
or
H :C :H
HCH
H
H
: :
CH 4
H. +
Note: Carbon, Group 4, needs 4 electrons to reach the electron
configuration of neon. Hydrogen achieves the electron configuration
of helium by forming a single two electron bond.
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The Octet Rule
By sharing 6 electrons, each nitrogen achieves the electron
configuration of neon with an octet of electrons in the valence level.
This tendency to reach 8 electrons in the valence level is called the
octet rule.
Examples of the Octet Rule
N
N
nitrogen
: :
: :
H
H:C :H
H
methane
H:O:H
H :F :
water
hydrogen
fluoride
The above structures are called Lewis structures in honor of G.N.
Lewis. In a Lewis structure, all the valence-level (outermost)
electrons are shown as dots. The Lewis structure of an atom is the
chemical symbol with the valence-level electrons shown as dots.
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Example CH3F methyl fluoride
The Lewis structures of the atoms are:
: :
.
. C . .F H . H . H .
.
or
H
HC F
H
: :
H
H
H :C : F : or H C F :
H
H
14 valence electrons
: :
: :
:
The remaining 6 valence
electrons are nonbonding
electrons around the
fluorine atom. The octet
rule applies to both the
carbon and fluorine atoms.
H
H :C : F
H
: :
The four covalent bonds
to the central carbon
atom account for 8 of the
valence electrons:
There are 4 + 7 + 3 = 14
valence electrons available for
chemical bonding and as
nonbonding electron pairs.
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Theory of Chemical Bonding
3 Key ideas of the 19th Century
1
4 bonds to Carbon - Tetravalent (Kekulé and Couper)
2
Tetrahedral Shape of Substitution at Carbon
Spatial direction of 4 bonds from Carbon is Tetrahedral
(van’t Hoff and Le Bel)
3
Covalent bonding occurs by sharing electrons
Covalent Bonds and Lewis Structures
Covalent bonding occurs by sharing electrons
Fluorine
Oxygen
Water H2O
Ammonia NH3