Comparing the oxidation levels of various carbon atoms is excellent for illustrating what oxidation state change must occur at a particular carbon in a given reaction of that compound.
OXIDATION LEVEL CHANGES DURING
REACTIONS
Comparing
the oxidation levels of various carbon atoms is excellent for illustrating
what oxidation state change must occur at a particular carbon in a given
reaction of that compound. For example,
The
oxidation level of the primary alcohol (−1) is less than the aldehyde product
(+1); thus this
conversion requires an oxidation of the alcohol function to the aldehyde. Any
reagent capable of effecting this change must necessarily be an oxidizing agent
which is itself reduced. In the preceding reaction the need for an oxidant is
noted above the arrow by the [O]. The reverse process, conversion of an
aldehyde to a 1◦
alcohol, is a reduction. Any reagent capable of effecting this change must be a
reducing agent and is itself oxidized. A reduction is commonly indicated by a
bracketed [H] above the arrow.
By
the same analysis conversion of an aldehyde to an acetal involves neither
oxidation nor reduction. As a consequence no oxidant or reductant is necessary
to carry out this reaction.
Similarly,
for the process shown below, the oxidation state of C-1 remains at −1 throughout the sequence; thus the
overall sequence involves no change in oxida-tion level at C-1, nor does either
step. Modifications of substituents and substitu-tion of one electronegative
group for another are generally not redox processes.
One
often must consider the balanced reaction in order to be certain of any net
changes in oxidation state, and similar procedures for determining the
oxidation level can be followed for other covalently bound elements. For
example, the conversion of methane into ethane is an oxidation of the carbon
atoms since the carbons in methane are at the −4 level while in ethane they are at
the −3 oxidation state.
However, the oxidation level of hydrogen changes from being bound to carbon (0) in the reactant to being bound to another hydrogen (−1) in the product. Thus hydrogen is formally reduced. The sum of oxidation levels in the reactants (−8) is the same as that in the products (−8), and the overall process is neither an oxidation nor reduction. This transformation can be thought of as an internal redox process since part of the reactant (carbon) is oxidized and part (hydrogen) is reduced. Generally, such internal redox processes require only a catalyst, not an oxidant or reductant.
On
the other hand, if the by-product of the conversion of methane to ethane is H+, then the balanced
reaction is written as shown below and a net oxidation is required. An
oxidizing agent is thus needed to effect this process. Again the recognition
that the organic reactant (methane) and product (ethane) are both alkanes is
not sufficient to determine that an oxidant is necessary.
The
Grignard reaction is often one of the first reactions encountered for the
preparation of organometallic compounds. As such it provides a method for the
conversion of an alkyl bromide to an alkane. From the example shown below it is
seen that the overall oxidation level change from the organic reactants to the
products is from 0 to −2,
so a reduction has occurred. Magnesium is the reductant and is itself oxidized
from 0 to +2 oxidation state.
The actual reduction takes place in the first step of the process in which the
C–Br bond is converted to a C–Mg–Br bond. The reaction with water is merely a
hydrolysis that does not change the oxidation state of carbon.
Reactions
of olefins and acetylenes illustrate that the overall change in oxida-tion
level of an organic functional group must be considered when deciding if a net
oxidation level change has occurred in a chemical reaction. For example,
addition of hydrogen across an acetylene gives a net reduction of each carbon
and thus is a reductive process with respect to the alkyne. The same is true
for the hydrogenation of an alkene. From the point of view of the alkyne and
the alkene, the hydrogen can be considered a reducing agent since it undergoes
oxidation during the process.
The
conversion of an alkyne to a trans-alkene
can be accomplished by heating with lithium aluminum hydride (LAH), by reaction
with lithium in liquid ammo-nia (Li, NH3). Thus all of these
reagents (H2/P-2 Ni, LAH, and Li, NH3) are reducing
agents for alkynes and give alkenes as the reduced products.
In
general, any reaction which results in the addition of two hydrogen atoms
across a π bond of any type are
reductions. Conversions of aldehydes and ketones to alcohols are reductions;
thus any reagents which are capable of effecting that conversion must function
as reducing agents. Thus NaBH4, LAH, and a large variety of other
reagents reduce aldehydes and ketones
to alcohols by the net addition of hydrogen across the C–O π bond. By the same logic, con-version of a primary alcohol to an
aldehyde (the reverse process) must be an oxidation, and reagents which are
capable of effecting this conversion, such as dimethyl sulfoxide (DMSO) and
acetic anhydride (Swern oxidation) or pyri-dinium chlorochromate (PCC), are
oxidants. Similar considerations hold for other π-bonded functional groups,
including acid derivatives and nitriles.
Alkenes
also undergo a variety of other addition reactions in which a reagent is added
across the double bond. Hydration and hydrohalogenation are classic examples.
Consideration
of the oxidation level reveals that while one carbon is reduced (the one to
which hydrogen adds), the other is oxidized (the one to which the oxygen adds).
There is no net change in oxidation level of the alkene functional group.
Likewise the reverse processes of these addition reactions, namely, elimination
of HX from alkyl halides and dehydration of alcohols to give alkenes, are not
redox processes. Additions of water to alkynes is analogous. In this case,
however, the product is a ketone, the oxidation level of the ketone is seen to
be the same as the alkyne, and so no net change in oxidation level has
occurred.
The conversion of alkenes to 1,2-diols by osmium tetroxide is also an olefin addition reaction. In this case a hydroxy group is added to each carbon of the olefin group, and the addition is termed an oxidative addition since the diol product is at a higher oxidation level than the alkene reactant. Oxidation of the carbon atoms of the alkene takes place in the first step, which is the reaction with OsO4 to produce the intermediate osmate ester.
Zinc
serves to further reduce osmium and free the diol product. Similar oxidative
additions to alkenes occur with bromine, chlorine, IN3, peracids,
and many other electrophiles.
Peracids
such as m-chloroperbenzoic acid
(MCPBA) clearly illustrate the redox nature of oxidative addition. In this
reaction the olefin is oxidized and the MCPBA is reduced to meta-chlorobenzoic acid, which
precipitates slowly from solution.
Another
common reaction process is one in which one atom or group replaces another atom
or group. These are known as substitution reactions. When one electronegative
group is substituted for another, no change in oxidation level occurs; thus the
reagents which carry out such substitutions are neither oxidants nor
reductants.
Such
substitutions in saturated compounds can be carried out by a variety of strategies
involving different nucleophiles and leaving groups, but the oxida-tion states
remain the same. Acyl substitutions are analogous. For this reason carboxylic
acid derivatives are treated as a common family of compounds. All have the same
oxidation level and all can be converted from one to another by substitution
reactions not requiring oxidation or reduction.
Many
useful functional group transformations occur in more than one step, and it is
not uncommon to find that different redox processes can be found in different
steps of the process. However, from the methods of determining oxidation states
it is clear that substitution of an electronegative group by a carbon group or
a hydrogen atom is a reduction and requires a reducing agent. For example,
conversion of an acid chloride to a ketone by a lithium organocuprate reagent
involves a reduction of the acid chloride to the ketone oxidation level.
Consequently
the copper is oxidized from a cuprate species to an organocop-per. By
classifying organocuprates as reducing agents toward acid chlorides, we should
expect that they could act as reducing agents toward other functional groups.
It is not surprising therefore that as Michael addition reagents they can be
used to give net reduction of an α, β -unsaturated ketone.
Reaction
of the organocuprate intermediate with water gives the fully reduced product.
If the organocuprate intermediate is reacted with bromine, the α-brominated product is formed. This
product has the equivalent oxidation level as the starting enone but differs in
that an additional carbon substituent is present. Functionally this is
equivalent to the addition of HBr to an enone. Thus functionally no net redox
has taken place. If individual steps are considered, it is clear that the first
step (addition of the organocuprate to the enone) is a reduction and the second
step (reaction of the cuprate with bromine) is an oxidation.
No
net change in the oxidation level has occurred for the overall process;
how-ever, each step in the sequence can involve an oxidation or reduction. This
is an important idea to keep in mind—even though no net change in the oxidation
level occurs, individual steps in the sequence may have an oxidation or
reduction and thus would require oxidants or reductants consistent with the
individual step being undertaken.
The
realization that many reactions or steps in reactions involve an oxidation or
reduction is an important consideration when these reactions are being studied
and learned. The change in oxidation level produced is indicative of the
transfor-mation and provides an additional organizational category by which
reactions can be classified. Reagents can also be classified by their ability
to cause oxidation or reduction. For example, from its addition reactions with
alkenes and alkynes, bromine can be considered an oxidizing reagent for organic
molecules. It is not surprising, therefore, to find that bromine also serves as
an oxidant toward other functional groups such as enols, hydrocarbons,
aldehydes, and organometallic compounds. Lithium aluminum hydride is well known
as a reductant; thus, if it reacts with an organic compound, it is a good bet
that some functional group is being reduced by the addition of a hydride. By
applying the concepts of oxida-tion and reduction, a new view of organic
reactions is possible which is often neglected but extremely important.
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