Chapter 1 Electronic Structure and Covalent Bonding Dr. Sujatha Krishnaswamy Chemistry Faculty Chandler Gilbert Community College

What is organic chemistry? “Organic” – until mid 1800’s referred to compounds from living sources (mineral sources were “inorganic”) Wöhler in 1828 showed that urea, an organic compound, could be made from a minerals 2

3 Today, organic compounds are those based on carbon structures. Organic chemistry is a study of their structures and reactions Includes biological molecules, drugs, solvents, dyes Does not include metal salts and materials (inorganic) Does not include materials of large repeating molecules without sequences (polymers)

Atomic Structure Structure of an atom  Positively charged nucleus (very dense, protons and neutrons) and smal ( m)  Negatively charged electrons are in a cloud ( m) around nucleus  Neutral neutrons  Electrons forms chemical bonds.  Atomic number – number of protons in the nucleus  Mass number – the sum of protons and neutrons in an atom  Isotopes have the same atomic number but different mass number  Atomic weight – average weighted mass of all atoms in an element  Molecular weight – sum of atomic weights of all atoms in a molecule 4

Distribution of Electrons in an Atom Quantum mechanics uses the mathematical equation of wave motions to characterize the motion of an electron around a nucleus. Wave functions or orbitals tell us the energy of the electron and the volume of space around the nucleus where an electron is most likely to be found. The atomic orbital closer to the nucleus has the lowest energy. Degenerate orbitals have the same energy. Give an example of degenerate orbitals. 5

Distribution of Electrons in an Atom 6

7 Electronic Configuration of Carbon Atomic Number = 6

8 Electronic Configuration of Carbon Atomic Number = 6

Chemical Bonding Theory Van't Hoff and Le Bel proposed that the four bonds of carbon have specific spatial directions Atoms surround carbon as corners of a tetrahedron 9

10 The Covalent Bond Some atoms form a chemical bond by sharing pairs of electrons between them. A covalent bond consists of a pair of electrons shared between two atoms.

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Polar Covalent Bonds 12

13 Electronegativity is a measure of the tendency of an atom to attract a bonding pair of electrons.

Bond Polarity 14

Which bond is more polar? 16

Problem 6

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Dipole Moments Dipole moment (  ) - Net molecular polarity, due to difference in summed charges dipole moment (D) =  = e x d (e) : magnitude of the charge on the atom (d) : distance between the two charges 19

Absence of Dipole Moments In symmetrical molecules, the dipole moments of each bond has one in the opposite direction The effects of the local dipoles cancel each other 20

21 Useful information for writing Lewis structures Page 13 Problem 13

22 Write Lewis structures for - Ethane, C 2 H 6 Ethene, C 2 H 4 Ethyne, C 2 H 2 Acetic acid, CH 3 COOH Ethyl amine, CH 3 CH 2 NH 2

23 Formal Charges

Problem 11

Formal charge observed with common bonding patterns 25

26 sp 3 Hybridization in Methane

27 sp 3 Hybridization in Ethane

28 sp 2 Hybridization in Ethylene

29 sp 2 Hybridization in Acetylene

Why cyclopentyne does not exist? 30

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Problem 28

Problem 29

Bond Lengths and Bond Strengths 34

Drawing Organic Structures Expanded structure Condensed structure Line angle structure 35

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Problem 14

Problem 18

Draw line angle structures for 41

Problem 17