Organic Chem 1

Topics: Atom, Chemical bonding, Chemical bond Pages: 18 (2314 words) Published: June 17, 2015
Organic Chemistry, Fourth Edition
Janice Gorzynski Smith
University of Hawai’i

Chapter 1
Lecture Outline
Prepared by Layne A. Morsch
The University of Illinois - Springfield

Copyright © 2014 The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

1

Bonding
•  Bonding is the joining of two atoms in a stable
arrangement.
•  Through bonding, atoms attain a complete outer shell
of valence electrons (stable noble gas configuration).
•  Atoms can form either ionic or covalent bonds to
attain a complete outer shell (octet rule for second row
elements).
• Ionic bonds result from the transfer of
electrons from one element to another.
• Covalent bonds result from the sharing of
electrons between two nuclei.
2

Valence Electrons
•  Second row elements can have no more than eight
electrons around them. For neutral molecules, this has
two consequences:
•  Atoms with one, two, three, or four valence electrons form one, two, three, or four bonds, respectively, in
neutral molecules (e.g., BF3, CH4).
•  Atoms with five or more valence electrons form enough bonds to give an octet (e.g., NH3). This results in the
following equation:

3

Lewis Structures
Lewis structures are electron dot representations for
molecules.
General rules for drawing Lewis structures:
1.  Draw only the valence electrons.
2.  Give every second-row element no more than eight
electrons.
3.  Give each hydrogen two electrons.

A solid line indicates a twoelectron covalent bond.

4

How to Draw a Lewis Structure
Step [1] Arrange atoms next to each other that you think
are bonded together.
•  Always place hydrogen and halogens on the periphery
because they form only one bond each.

•  Place no more atoms around an atom than the number of bonds it usually forms.

5

How to Draw a Lewis Structure
Step [2]
• 
• 
• 
• 

Count the electrons.

Count the number of valence electrons from all atoms.
Add one electron for each negative charge.
Subtract one electron for each positive charge.
This gives the total number of electrons that must
be used in drawing the Lewis structure.

Step [3] Arrange the electrons around the atoms.
•  Place a bond between every two atoms, giving two
electrons to each H and no more than eight to any
second-row atom.
•  Use all remaining electrons to fill octets with lone pairs. •  If all valence electrons are used and an atoms does not have an octet, form multiple bonds.

Step [4] Assign formal charges to all atoms.
6

Multiple Bonds
•  If all valence electrons are used and an atom does not have an octet, form multiple bonds.

•  To give both C’s an octet, change one lone pair into one bonding pair between the two C’s, forming a double bond.
7

Problem 1.4
•  Draw a valid Lewis structure for each species:
A)  CH3CH3
B)  CH5N
C)  CH3D)  CH3Cl

8

Formal Charge
•  Formal charge is the charge assigned to individual atoms in a Lewis structure.
•  Formal charge is calculated as follows:

•  The number of electrons “owned” by an atom is determined by its number of bonds and lone pairs.
•  An atom “owns” all of its unshared electrons and half of its shared electrons.
9

Electron Ownership
The number of electrons “owned” by different atoms is
indicated in the following examples:

10

11

Problem 1.6
Calculate the formal charge on each second-row
atom:

12

Isomers
•  Sometimes more than one arrangement of atoms (Lewis
structure) is possible for a given molecular formula.

•  These two compounds are called isomers.
•  Isomers are different molecules having the same
molecular formula. Ethanol and dimethyl ether are
constitutional isomers.
13

Resonance
•  Some molecules cannot be adequately represented by a
single Lewis structure. For example:

•  These structures are called resonance structures. A
double-headed arrow is used to separate the two
resonance structures.
•  Resonance...
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