Bonding and Structure of Organic Molecules

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Bonding and Structure of Organic Molecules

Suggested Reading
 Review general chemistry textbook as needed
 Brown and Foote (5th edition), Chapter 1: Sections 13 and 5
 Klein (2nd edition), Chapter 1: Sections 5 and 6

Lecture Supplement Handouts From lecture; also available at course web site
   Bonding and Structure of Organic Molecules

Optional Web Site Reading
  More on Lewis Structures (www.towson.edu/~ladon/lewis.html)

Suggested Text Exercises
 Brown and Foote (5th edition), Chapter 1: 112, 14, 2037, 4150, and 6466
 Klein (2nd edition), Chapter 1: 3368

Related Tutorials (web.chem.ucla.edu/~harding/tutorials/tutorials.html)
 Drawing Lewis Dot Structures
 Formal Charge
 Curved Arrows
 Molecular Model Kit


              Common Questions About Organic Chemistry Problems

I don't have time to do all these problems! There are many problems available to
enhance your understanding of organic chemistry, including the Concept Focus
Questions (CFQ; in this Thinkbook), the Practice Problems (PP; also in this Thinkbook),
as well as suggested problems from the text and other sources. Working all these
problems can be time-consuming, but this task is essential to your understanding of the
course material. Rarely do students who skimp on problem solving get good grades.
Most students who study a topic for the first time really do need to work all the problems.
Problem solving is an excellent way to reinforce the concepts in your mind. If you can
honestly say to yourself that you have a firm grasp on the concept, then feel free to skip
the problem. On the other hand if you are not 100% sure, then work the problem. Even
students who have a firm conceptual grasp should work a few "obvious" problems every
now and then to stay sharp.

Which problems are most important? Here is a suggested order of priority:

1. CFQ: Always read these before lecture, and then again when you study the textbook.
   They outline the fundamental concepts presented in lecture, and are the most
   important problems in this course.

2. OWLS and PP: Written by the same person who writes the exams. OWLS problems
   are designed to stimulate discussion during discussion section meetings.

Bonding and Structure of Organic Molecules                                              1
3. All other problems, including the optional ones: Different students get the most out of
   different problem sources, although most students seem to get more out of the
   textbook problems than from the other sources. Organic Chemistry as a Second
   Language and the course web site tutorials focus on specific topics, and should be
   examined if you are having trouble with these specific topics.



            Wikipedia: Another Good Organic Chemistry Learning Tool

The Wikipedia (www.wikipedia.org) is a huge online encyclopedia of knowledge created
and updated by thousands of people all over the world (including college students). It
does contain a few errors here and there, and shouldn't be used as the only source of
information for a research paper, but it is a great place to start learning about a topic.


Concept Focus Questions
1. Provide precise yet concise definitions for the following terms:
   (a) -                 (d) Electronegativity       (g) Lone pair
        +
   (b)                   (e) Formal charge           (h) Orbital
   (c) Covalent bond     (f) Functional group        (i) Polar covalent bond

2. Draw the Lewis dot structure for CH3OH.

3. How does electronegativity influence electron distribution within bonds? Illustrate
   with CH3OH as an example.

4. Why are functional groups important to a systematic study of organic chemistry?

5. Prepare a functional group list that includes the name and basic structure of each
   functional group, as well as an example molecule that contains exactly six carbons.
   Include all lone pairs and formal charges. Try to avoid using the same example
   molecules as given in the lecture supplement.

Concept Focus Questions Solutions
1. Illustrated definitions can be found at the Illustrated Glossary of Organic Chemistry
   available at the course web site.

    (a) -: An atom that bears this symbol has a slight electron excess, but not enough to
        give it a full negative formal charge.

    (b) +: An atom that bears this symbol has a slight electron deficiency, but not enough
        to give it a full positive formal charge.

    (c) Covalent bond: A chemical bond formed between two atoms by orbital overlap
        and sharing of an electron pair.

2                                           Bonding and Structure of Organic Molecules
   (d) Electronegativity: The measure of an atom's attraction for electrons in a chemical
       bond.

   (e) Formal charge: The charge on an atom in a Lewis structure if the bonding was
       perfectly covalent and the atom has exactly a half-share of the bonding electrons.

   (f) Functional group: A group of atoms whose bonding is the same from molecule to
       molecules.

   (g) Lone pair: A pair of electrons assigned to just one atom. Also called a nonbonded
       pair.

   (h) Orbital: The mathematical description of a volume of space in which there is a
       certain probability of finding an electron of a certain energy.

   (i) Polar covalent bond: A covalent bond between two atoms of different
       electronegativity, resulting in an uneven sharing of the bonding electron pair.

       H
2. H C O H
       H

3. The distribution of bonding electron density is influenced by the electronegativity of
   the atoms that comprise the bond. A more electronegative atom attracts more electron
   density toward itself. Thus, end of the bond with the more electronegative atom will
   have a small negative charge (-). The less electronegative atom loses electron
   density, as so has a small positive charge (+). In the CO bond, carbon is less
   electronegative than oxygen, so the carbon has a + charge and the oxygen atom has a
   - charge. The reasoning applies to the OH and CH bonds. The combination of
   short bond length and low electronegativity difference causes CH bonds to be
   nonpolar.
                                         H
                                          +   -   +
                                     H   C    O   H

                                         H    Nonpolar bond


4. A functional group is a set of atoms, bonded together, that gives a molecule particular
   chemical and physical properties. Because chemical reactions involve changes in
   electron distribution and bonds, functional groups with similar electronic structures
   will react in similar ways. This is just one reason why the study of functional groups
   is so critical to a systematic study of organic chemistry.




Bonding and Structure of Organic Molecules                                             3
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