Besides bonding patterns, functional groups also vary with respect to the oxidation states of carbon in those functional groups.
OXIDATION LEVELS
Besides
bonding patterns, functional groups also vary with respect to the oxidation
states of carbon in those functional groups. Thus another way to classify
functional groups is by the carbon oxidation level. Correspondingly, organic
reactions can be categorized as to whether an oxidation, a reduction, or no change
in oxidation level has occurred in the organic reactants in going from
reactants to products. This is a very useful distinction since the reagents
used in a given transformation must be compatible with the oxidation change
that occurs in the reaction. It is important to remember this fundamental
truth—that no oxidation can occur without a corresponding reduction and no
reduction can occur without a corresponding oxidation. As a consequence, if a
transformation of an organic compound involves a change in its oxidation level,
then the reagents necessary to cause that change must be able to undergo the
complementary change in oxidation level. To have an oxidation, an oxidizing
agent is required which itself gets reduced in the process. Similarly, to carry
out the reduction of an organic compound, a reducing agent is needed which
itself gets oxidized. Reagents can thus be categorized on the basis of their
oxidizing or reducing properties. If no change in oxidation state occurs during
a chemical reaction, then reagents used to effect the transformation should
themselves undergo no oxidation or reduction. Moreover, if a reagent is not
normally an oxidizing agent, then it is not easily reduced and cannot be used
to oxidize something else. Conversely, if a reagent is not normally a reducing
agent, then it is not easily oxidized and cannot be used to reduce something
else.
Oxidation
is defined as the loss of electrons. This concept is very straight-forward when
dealing with metal ions. Thus the change Mg0 → Mg2+ is an oxidation
because magnesium has lost two electrons in going from the element to the
positive ion. Similarly, oxidation of Cu1+ → Cu2+ involves loss of an
electron from Cu1+ to give the Cu2+ species.
Reduction
is defined as the gain of electrons. The conversion of Ag1+ → Ag0 involves a gain of
an electron by the silver ion and thus silver is reduced. Like-wise, the
permanganate ion MnO4− has Mn[VII] but MnO2
manganese dioxide has Mn[IV]. Thus the gain of three electrons by manganese
causes a reduction in oxidation level of from +7 to +4.
Because
organic compounds have an overwhelming preponderance of covalent bonds, changes
in oxidation state of carbon are not as easily determined by inspection as they
are for metal ions. While the definitions of oxidation and reduction for
organic compounds are the same as for metal ions (i.e., gain or loss of
electrons), the oxidation state of a carbon atom is determined by the types of
covalent bonds originating from it. A set of rules has been developed to assign
numerical values for the contributions of atoms covalently bonded to carbon to
the oxidation state of that carbon. Summation of the contributions of its
covalently bonded substituents gives the oxidation state of a particular carbon
in a molecule. Furthermore the oxidation levels of various carbons can be
compared just as +2 or +3 oxidation states in metal ions can
be compared.
These
rules are simple and are summarized as follows:
1. Bonds to hydrogen or other elements more electropositive
than carbon contribute −1
to the oxidation level.
2. Bonds to other carbon atoms contribute 0 to the oxidation
level.
3. Bonds to oxygen or other elements more electronegative
than carbon con-tribute +1
to the oxidation level.
4. Multiple bonds to an element count as multiple single
bonds to that ele-ment. That is, the carbon–oxygen double bond of carbonyl
group (C=O) is oxidatively
equivalent to a carbon atom with two single bonds to oxygen originating from it
(–O–C–O–).
5. A pair of electrons on carbon contribute −1 to the oxidation level.
6.
A positive charge on carbon contributes +1 to the oxidation level.
Given
these contributions, the oxidation level of a given carbon can be determined by
adding together the contributions of the four attached bonds.
Related Topics
TH 2019 - 2025 pharmacy180.com; Developed by Therithal info.