Organic Chemistry I
CHEM 201
1st Term 2011-2012
Instructor:
Assoc. Prof. Khaled S. Abdel Halim
k.abdulhalem@uoh.edu.sa
Organic Chemistry I: Grading System
The marks will be allocated as follows:
*
*
30 %
15 %
Major Exams (M1 & M2)
Laboratory
*
40 %
Final Exam
*
15 %
Class work : [ Quizzes Attendance Homework ]
2
Organic Chemistry I: Contents
Chapter
Chapter 1:
Atoms & Molecules: Overview
Chapter II:
Orbitals & bonding in organic copmounds
Chapter III:
Structure & Nomenclature
Chapter IV:
Stereochemistry
Chapter V: Organic reaction mechanism
Chapter VI: Free radical reactions
Chapter VII: Alcohols
Chapter VIII: Alkenes & Alkynes
Content
Atomic structure - Chemical bonds. Formulas of
organic compounds - H- bond in organic
compounds – Acidity & Basicity.
Wave properties of electron motion.Bonding and
antibonding orbitals. Hybridization of atomic
orbitals.
Formation of sigma bond and Pi
bond.Functional groups. Conjugated double bond.
Resonance.
Saturated & Unsaturated Organic
Compounds.Isomerisations. Nomenclature of
organic hydrocarbon . Chemical properties of
Alkanes. Hydrocarbon Resources
Aspects of stereochemistry. Geometric Isomerism.
Rules of nomenclature. Cyclic compounds.
Alkyl halide; classification and nomenclature
Reaction kinetics & Mechanism –
Nucleophilic substitution reactions (SN1 SN2)
Elimination reactions
Organic radicals. Kinetics and mechanism of
radical reactions. Initiators & Inhibitors.
Polymerization
Classification & Nomenclature of alcohols
Properties of alcohols - Synthesis of alcohols
Reactions of alcohols.
Synthesis of alkenes & Alkynes
Electrophilic addition reactions
Oxidation reactions – Hydrogenation reactions
3
Chapter 1
Atoms & Molecules: Overview
What is organic chemistry?!
Is a branch of chemistry that involves the scientific study of the
structure, composition, properties, reaction and preparation of
hydrocarbons and their derivatives. Organic compounds may contain
any number of other elements such as N, O, S or halogen.
History of organic chemistry?!
Berzelius: organic compound is the substance isolated from living
system
Vital force theory: special force or life force is necessary to produce
organic compound.
Woehler theory: destroyed the vital force by producing organic
compound urea from ammonium cyanate (inorganic compound)
Organic chemistry is the chemistry of the compounds of carbon
although few carbon compounds, such as CO2, Na2CO3, and pot
cyanide, are inorganic compounds,
Study of living systems, plants, animals, microorganisms,
medical science, biochemistry, microbiology, agriculture,
plastics, synthetic fibers, petroleum, natural gas, coal, and many
others depends on the principles of organic chemistry.
Since organic chemistry involves the scientific study of
hydrocarbons; thus,
a background around the
structure of hydrogen & carbon atoms should be given
5
Electron structure of the atom
H: 1
C:6
N: 7
O: 8
6
The electronic configuration of an atom
N=4
M=3
L=2
K=1
Each electron shell is associated with a certain amount of energy.
Electrons close to the nucleus are more attracted by the protons in the nucleus .
The closer the electron to the nucleus, the lower its energy and the more difficult
to remove it in a chemical reaction.
7
Atomic Orbitals
Electrons surrounding atoms are concentrated into regions of space called
atomic orbitals. The Heisenberg uncertainty principle states that it is
impossible to know both the location and the momentum of an atomic
particle, but it is possible to describe the probability that the electron will be
found within a given region of space.
The electronic configuration of carbon is 1s2 2s2 2sp3.
Atomic orbitals with s-character have spherical
symmetry
The electron densities along the x, y and z axes
of the 2p orbitals are clearly shown in the figure;
the nodes are the points at the origin and at
these points, there is zero probability of finding
the electron.
8
Filling the orbitals
9
Properties of the atom
1. Atomic Radius
2. Electronegativity
Atomic radius is the distance from the center of the nucleus to the
valence electrons. It is determined by measuring the bond length
(the distance between nuclei ) and its value is given in nanometer
(1nm= 10 -9 m) or in angstrom (1 A0= 10 -10m).
What factors affect the value of atomic radii?!!
10
Electronegativity
It is a measure of an atom’s attraction for its outer bonding electrons. It is used for
predicting the chemical reactivity of the atom. Like atomic radii, EN is affected by the
number of protons in the nucleus and by the number of shells containing electrons.
N.B. An element with a very low EN is called electropositive elements (as Li).
11
Chemical Bonds
(Ionic & covalent bonds)
-Ionic bond results from transfer of electrons from one atom to another.
- Ionic bond is formed when electronegativity difference between two atoms is
larger than 1.7
- A covalent bond results from the sharing of a pair of electrons by two atoms.
- Carbon forms covalent bonds with other carbon atoms and with atoms of other
elements.
-Atoms transfer or share electrons so as to gain a nobel gas electron configuration
(Octet rule)
12
13
Number of covalent bonds
The number of covalent bonds that atom forms depends upon the number of
additional electrons needed for the atom to attain a noble- gas configuration.
Carbon forms four covalent bonds.
14
Polar Covalent bonds
•Atoms with equal or nearly equal electronegativities form pure covalent bond
or nonpolar covalent bond as C - C and C - H bonds.
•In some compounds like H2O, HCl, CH3OH or H2C=O, one atom has
electronegativity greater than the other, so it attracts the electron density and
form Polar covalent bonds.
•The distribution of electron density in polar bond is represented by partial
positive and partial negative charge.
15
Bond length & bond angle
Bond length is the distance between the nuclei of two atoms covalently bonded. It range
from 0.74 A0 to 2A0
If there are more than two atoms in a molecule, the bonds form an angle which vary from
60 up to 180 o
16
Bond dissociation energy
It is the energy required to breakdown or cleavage the chemical bond. It is
measured by the change in enthalpy (change in heat content or energy), ∆H
Two types of cleavage can be observed in organic compounds; Heterolytic and
Homolytic cleavage
17
Chemical Formulas in Organic Compounds
3 formulas to represent organic compounds:
Structural formulas are the most useful of the different types of formulas.
Substance
Molecular Empirical
formula
formula
Water
H2O
H2O
Methane
CH4
CH4
Benzene
C6H6
CH
Sulfur
S8
S
Glucose
C6H12O6
CH2O
18
Condensed structural formulas
19
Cyclic compounds and polygon formulas
A compound such as CH3CH2CH2CH3 is said to have its carbon atoms connected in a chain.
Carbon atoms can be joined together in rings as well as in chains; a compound with one
or more rings is called a cyclic compound which is always represented by polygon
formulas (condensed structure).
20
Attraction between organic molecules
* Dipole- Dipole Interaction
* Hydrogen bond
Molecules attract and repel one another because of dipole- dipole interactions between
opposite charges (attraction) and like charges (repulsion). The dipole-dipole interaction is
called Van der Waals forces.
Nonpolar molecules are attracted to one another by weak dipole-dipole interaction
called London forces which arise from dipole induced in one molecule by another.
Electrons in one molecule are weakly attracted to the nucleus of the second molecule.
Also, electron of the second molecule is repelled by electrons of the first.
21
Hydrogen bond
*H-bond is strong type of dipole-dipole interaction occurs between molecules containing a
hydrogen atom bonded to N , O or F which are electronegative atoms and have unshared
valence electrons.
*H-bonds are not all the same strength. O---HO > N---HN Why?!
*H-bonds may form between two different compounds as CH3OH and H2O.
22
Effects of H-bonds
•H-bond acts as glue between organic compounds.
•H-bond increase the boiling points of the compound.
•H- bond increase the solubility of the organic compounds.
•H-bond affects the shape of large biomolecules organic compounds.
23
Acidity and basicity of organic compounds
Three major definitions of acids and bases :
* Arrhenius (1884)
* Brønsted and Lowry (1923)
* Lewis (1916)
An Arrhenius acid is a source of H+ ion
An Arrhenius base is a source of OH- ion
A Brønsted-Lowry acid is a proton donor
A Brønsted-Lowry base is a proton acceptor
A Lewis acid is an electron-pair acceptor
A Lewis base is an electron-pair donor
24
Strength of acid and base
Strong acid/base …………………..completely ionized
Weak acid/base………………………incompletely ionized
Some organic compounds act as acids and bases. Amine which is similar to ammonia is
weak base. Carboxylic acids are weak acids. The strength of acid and/or base of organic
compounds is attributed to the polarity!
25
Conjugate acids and bases
When acid loses its hydrogen ion form conjugate base. The conjugate acid of a base is
the product of the reaction of base and hydrogen ion (base protonated). The conjugate
acid of NH3 is NH4 +
26
* Atomic size
Factors affecting acidity:
* Electronegativity
The ease of lose a proton depends on the strength of the bond to proton!
27
Acidity and basicity constants
For the ionization of acid (or base) in water, the equilibrium constant K is called acidity (or
basicity) constant Ka (or Kb). The more ionized an acid, the larger the value of Ka . The
smaller the value of pKa.
28
29
Organic Chemistry I
CHEM 201
1st Term 2011-2012
Instructor:
Assoc. Prof. Khaled S. Abdel Halim
k.abdulhalem@uoh.edu.sa
1
Chapter II:
Orbitals and Bonding in organic compounds
In this chapter, we shall study the following topics:
•Wave properties of electron motion.
•Bonding and antibonding orbitals.
•Hybridization of atomic orbitals.
•Formation of sigma bond and Pi bond.
•Functional groups.
•Conjugated double bond.
•Resonance.
2
Wave properties of electron motion
• Electrons have properties of waves as well as properties of particles.
•One atomic orbital can overlap an atomic orbital of another atom resulting linear
combination of atomic orbitals. When orbitals overlapping in phase, the result is
reinforcement and a bonding molecular orbital. But if orbitals overlapping out of phase,
the result is interaction creating a node between the two nuclei and forming antibonding
orbitals.
3
Example: The bonding in Hydrogen:
*Any orbital (molecular or atomic ) can hold a maximium of two electrons, which must be
of opposite spin.
•The number of MO equal the number of AO used for their formation.
•In the filling of MO with e , the lowest- energy orbitals are filled first. .
• Sigma bond is formed from sigma MO (orbital that is symmetrical about the axis
connecting the nuclei).
4
Hybridization of orbitals
Carbon atom blends or hybridize its atomic orbitals in one of three different
ways for bonding:
SP3
SP2
SP
5
6
7
8
Importance of Pi bond in organic compounds:
• Pi bond has slightly higher energy (less stable) than sigma bond. The bond
dissociation energy of sigma bond in ethylene molecule is account to be 95 kcal/mol,
while Pi bond is 68 kcal/mol.
• The Pi bond is polarized more easily, it’s delocalized bond (mobile bond). The pi
electrons are more easily promoted to higher energy (antibonding) orbital. Pi bond is
considered the source of chemical reactivity (Function group).
9
10
Effects of hybridization
•Hybrid orbitals are greatly affect the bond length particularly the hybrid orbital with a
greater proportion of S character such as sp hybrid orbitals which have lower energy and
closer to the nucleus than sp2 and sp3 . So, sp orbital forms shorter and stronger bonds
than sp3.
*SP3 hybrid orbital usually do not play a major role in organic reactions because it forms sigma
bond which is very strong to be broken!
SP2 and SP hybrid orbitals form Pi bond which is very easily to be broken and so it can be
considered functional group! (What is meaning of functional group in organic compounds?!
11
Hybrid orbitals in functional group compounds
Many important functional groups in organic compounds contain N and O, they form
hybrid orbitals like C atoms:
12
The carbonyl group is part of a variety of functional groups. The carbonyl group is
more polar than the C-O group in alcohol or ether because the pi electrons are more
easily drawn toward oxygen (highly electronegative atom).
13
Conjugated double bonds and Resonance phenomena
When two or more double bond exist in organic compound, the double bonds can be
positioned in two ways: conjugated double bond (delocalized) and isolated double bond
14
Benzene & Resonance
Benzene is a compound that cannot be accurately represented by a single line-bond
formula. The electron delocalization causes the pi electrons to encompass more than two
atoms. So, benzene is a resonance hybrid of two structures. The resonance symbols are
not true structures but the true structure is a composite of all resonance symbols.
In resonance structures, the position of nucleus can not change, only electrons (Pi
electrons or unshared valence electrons) are delocalized or shifted.
15
Rules for writing resonance structures
Resonance greatly affects the stability and acidity of organic compounds. Resonance
increase the stability (lower energy) and acidity of organic compounds. Delocalization of pi
electrons stabilizes the structure.
Explain why?!
16
Homework (Ch. II)
•
•
•
•
•
•
•
•
•
•
2.9
2.18
2.19
2.20
2.22
2.29
2.30
2.34
2.38
2.41
Write a report on :” Stability of aromatic compounds”
17
18