Chemistry Reference
In-Depth Information
FIGURE 2.3
Three-dimensional representations around atomic centers.
substituents in the plane of the paper with normal bond lines. The other two
substituents are drawn going into the paper with a dashed wedged bond, or
coming out of the paper with a solid wedged bond.
The Fischer projection is a less common alternative. By definition in these draw-
ings, the vertical bonds go into the paper, and the horizontal bonds come out
of the paper.
You do not always have to show the full
stereochemistry
(3-D shape) of a mol-
ecule. However, as you will see in Chapter 3, it is important not to forget that
molecules have 3-D shapes.
2.2.2 Oxidation States for Carbon
This concept helps to create a link between the various classes of carbon com-
pounds. The type and electronegativity of the atoms which are bonded to a carbon
lets us assign nominal
oxidation numbers
to the various carbon atoms. These oxi-
dation numbers indicate the relative gain or loss of electrons at the carbon in each
compound type. This shows the relative equivalence of particular carbon oxidation
states. From this, we can compare the oxidation levels of different functional groups.
The series of oxygen-containing functional classes in
Figure 2.4
shows the prin-
ciple. We can extend this process to other functional classes that involve other
heteroatoms such as nitrogen, sulfur, and the halogens.
FIGURE 2.4
Nominal carbon oxidation numbers in functional classes.
Hydrogen is given the oxidation number of +1. Therefore, methane has carbon
in its most reduced form of −4, which is its most stable, least reactive state. If a
hydrogen atom is replaced with a bond to another carbon, the nominal oxida-
tion number of the original carbon changes to −3. This is because we consider
the carbons to have no effect on each other. The replacement of another hydro-
gen atom with a carbon, or the formation of a carbon-carbon double bond,
then changes the oxidation number to −2, and so on.
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