01:160:307 Lecture Notes - Lecture 2: Organic Compound, Telomerase Reverse Transcriptase, Codeine
1
ORGANIC CHEMISTRY 307
Fall 2017
Lecture Notes II
Chapter 2
R. Boikess
I. Overall Organization and Systematization
As we have seen, focusing on functional groups is one of the approaches
we use to systemize the enormous amount of information about the chemical
behavior of organic compounds. But even more fundamentally, we must
have a way to describe, designate, and communicate about organic
compounds. How well you master these skills will be a major determinant in
how you do this year. We will present a slightly different approach than the
one in your textbook. Both approaches together will help you master these
skills.
a. Remember why there are so many compounds (C-C bonds and
chains). So one focus is to describe the carbon skeleton, which
consists of a “main” chain (the longest continuous chain) of C atoms
with various additional C atoms or groups of C atoms (smaller chains)
attached at various points.
b. Think of it as a “connect the dots puzzle” with “branches” allowed.
Each dot corresponds to a C atom. Let’s draw a big grid of dots (C
atoms) and then do some connecting.
Do for 3 dots, 4 dots, 5 dots and 6 dots. Each different pattern of
attachment corresponds to a different carbon skeleton, which is the
starting point for describing, identifying, and naming all organic
compounds. Note it’s the pattern of attachment that counts, how many
dots a given dot is connected to, not how we draw it, straight, zig-
zag, or bent. Note that we have drawn the same pattern of three
attached dots in three ways using the pink color. They look different,
but they are the same because the pattern of attachment is the same.
One dot is connected to two others and each of those two other dots is
2
connected to only the one. Similarly we use the green color to connect
four dots in the same pattern of attachment, drawn three different
ways. But we use the purple line to connect the four dots in a different
pattern. Be sure you understand why the purple pattern is different.
These observations tell us that there is only one skeleton of 3 C atoms,
but that there are two skeletons of 4 C atoms. Five dots (or C atoms)
can be connected in three different ways and six dots can be
connected in five different ways.
Notice that we could also connect dots in closed loops (called
rings), which are different because the pattern of attachment is
different. We have ignored that possibility in the analysis so far, but
we will come back to it later.
As you can see, the number of possible patterns increases very
rapidly as the number of dots increases. For 10 dots there are 75
patterns, for 25 dots almost 36.8 million patterns (not counting rings).
That’s why there are so many Organic compounds known.
3
c. How do we go from a carbon skeleton to a compound? Every C must
have 4 bonds. Most carbons in most compounds get to 4 bonds by
bearing a sufficient number of hydrogens.
d. We simplify our descriptions by defining 1, 2, and 3 carbons,
related to the number of C’s to which a given C is attached. A 1
carbon is attached to one other carbon; a 2 carbon is attached to two
other carbons, and a 3 carbon is attached to three other carbons. This
categorization is very widely used. You should be able to use it
automatically. We also use this categorization for hydrogens. A
1 hydrogen is attached to a 1 carbon, a 2 hydrogen is attached to a
2 carbon, and a 3 hydrogen is attached to a 3 carbon. So a 1
carbon has three 1 hydrogens attached to it. A 2 carbon has two
2 hydrogens attached to it; and a 3 carbon has one 3 hydrogen
attached.
e. Reminder you should be able to go from Kekule structures (Lewis
structures without the lone pairs) to condensed formulas and to even
more condensed formulas with parentheses. [such as CH3(CH2)4CH3
and CH3C(C2H5)2CH3]
e. Go from dots to skeletal structures, by focusing on the connections.
The point at which lines meet (usually the vertex of an angle)
represents a dot, which is a carbon atom. The end of a line is also a
carbon atom. Draw for propane, butane and isobutane. Note branches
off the main chain. Constitutional isomers are compounds with the
same composition, but different skeletal structures. Butane and
isobutane are simple examples.
f. Most org cpds have at least one C that gets to 4 in some way other
than maximum H’s. Any C of this type is part of a functional group.
(See Section 1.3 in BF). One obvious way for a C to have four bonds,
but not the maximum number of hydrogens is with a multiple (double
or triple) bond between 2 carbons. Notice that not all carbon
skeletons allow double or triple bonds. “Neopentane” (see last blue in
grid above) allows no multiple bonds and isobutane (see purple in grid
above) allows no triple bonds. Remember pentavalent carbon is
impossible under ordinary conditions and in Chemistry 307-308.
Document Summary
Chapter 2: boikess, overall organization and systematization. As we have seen, focusing on functional groups is one of the approaches we use to systemize the enormous amount of information about the chemical behavior of organic compounds. But even more fundamentally, we must have a way to describe, designate, and communicate about organic compounds. How well you master these skills will be a major determinant in how you do this year. We will present a slightly different approach than the one in your textbook. Both approaches together will help you master these skills: remember why there are so many compounds (c-c bonds and chains). Let"s draw a big grid of dots (c atoms) and then do some connecting. Do for 3 dots, 4 dots, 5 dots and 6 dots. Each different pattern of attachment corresponds to a different carbon skeleton, which is the starting point for describing, identifying, and naming all organic compounds.
