Organic chemistry for medicine and biology students
Chem 2311Chapter 1
Introduction
By Prof. Dr.Adel M. Awadallah
Islamic University of Gaza1
Chapter 1
Bonding and Isomerism
Atoms consist mainly froma) Nucleus: (containing Protons and Neutrons)
Protons (positive particles, Atomic Number)
Neutrons (Neutral particles)
Protons + Neutrons (Atomic weight)
b) Electrons: negatively charged particles
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Electronic Configuration (review)1) Shells (n = 1,2,3,4, ….)2) (Subshells , s, p, d, f)3) (orbitals: rgions of space around the nucleus containing
electrons)4) each orbital contains only 2 electrons with different spins
SubshellNumber of orbitals (electrons)Name of orbitals
S1 (2)s
P3 (6)Px, py, pz
d5 (10)Dxy, dyz, dxz, dz2,
dX2
-y2
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Examples:Filled Shells
Play almost no role in chemical reaction
Valence Electrons
They are the outermost electrons and they are mainly involved in chemical bonding
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Ionic CompoundsThey are formed by the transfer of one or more valence
electrons from one atom to another
Electropositive atoms: give up electrons and form cations.
Electronegative atoms: accept electrons and form anions
Ionic compounds: are composed of positively charged cations and negatively charged cations
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Covalent compoundsA covalent bond: is formed when two atoms share one or more
electron pairs. A molecule consists of two or more atoms joined by covalent bonds
Bond energy: is the energy necessary to break a mole (6.022 x 1023)of covalent bonds.
Bond length: is the average distance between two covalently bonded atoms
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• Isomers: different compounds having the same molecular formula
CH3CH2OH CH3OCH3
ClI
Br
H
ICl
Br
HCH3 CH3CH3CH3
CH3
Et
Cl
Cl HH
CH3
Et
Cl
ClH
H
ClH
Cl H
ClH
H Cl
Cl H
ClH
Cl Cl
HH
IsomersDifferent compounds that have the same molecular formula
Constitutional isomers Stereoisomers
Atoms are attached to one another in different ways
Atoms have the same constitution but different arrangement in space
Enantiomers Diasteriomers
Non superimposable mirror- image stereoisomers
Non mirror-image stereoisomers
Conformational Configurational
Interconvertable by rotation
Interconvertable by bond breaking
SR
cis- trans-
cis trans
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• Formal Charges: are the charges that each atom carries, and can be calculated as follows
• Formal charge = Valence electrons – bonds – electrons
• Example:
• Resonance: arises whenever we can write two or more structures for a molecule with different arrangements of electrons but identical arrangement of atoms
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• Arrows:
CH3 CH3
Reaction
equilibrium reaction
Resonance
Movement of two electrons
Movement of one electron
straight arrow
double headed straight arrow
Pair of straight arrows with half heads
Curved arrow
Fishhook
2CH3.
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• Orbital picture of Methane and ethane
•
The bond formed by end-to-end overlap is called a sigma bond. 16
A pi bond is one in which the electrons in the p orbitals are held above and below the plane of the molecule.The sigma bond is stronger than the pi bond.A double bond is formed from a sigma bond and a pi bond, and so it is stronger than a single bond.
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