Oxidation states of Alcohols and Related Functional Groups

Oxidation states of Alcohols and Related Functional Groups

– Oxidation states of Alcohols leads to ketones, aldehydes, and carboxylic acids.

– These functional groups, in turn, undergo a wide variety of additional reactions.

– For these reasons, alcohol oxidations are some of the most common organic reactions.

– In inorganic chemistry, we think of Oxidation as a loss of electrons and Reduction as a gain of electrons.

– This picture works well for inorganic ions, as when Cr6+ is reduced to Cr3+

– Most organic compounds are uncharged, however, and gain or loss of electrons is not obvious.

– Organic chemists tend to think of oxidation as the result of adding an oxidizing agent ( O2, Br2, etc.), and reduction as the result of adding a reducing agent (H2, NaBH4, etc.).

Difference between Oxidation and Reduction

– Most organic chemists habitually use the following simple rules, based on the change in the formula of the substance:

Oxidation: addition of O or O2  – addition of X2 (halogens) – loss of H2

Reduction: addition of H2 (or H) –  loss of O or O2 – loss of X2

Neither: addition or loss of H+, OH , H2O, HX, etc. is neither an oxidation nor a reduction.

Oxidation of Alcohols

– We can tell that an oxidation or a reduction of an alcohol has taken place by counting the number of C-O bonds to the carbon atom.

– Oxidation usually converts C-H bonds to C-O bonds.

– The first row of structures in the Figure below shows that a primary alcohol is more oxidized than an alkane because the carbinol (C-OH) carbon atom has one bond to oxygen, while the alkane has no bonds to oxygen.

– Oxidation of a primary alcohol gives an aldehyde with a carbonyl carbon having two bonds to oxygen.

– Oxidation of the aldehyde to an acid adds another bond to oxygen, for a total of three.

– Further oxidation would require breaking a carbon–carbon bond to give four bonds to oxygen, the oxidation state of carbon dioxide.

– The following Figure compares the oxidation states of primary, secondary, and tertiary alcohols with those obtained by oxidation or reduction.

– The symbol [O] indicates an unspecified oxidizing agent.

– Notice that oxidation of a primary or secondary alcohol forms a carbonyl group by the removal of two hydrogen atoms: one from the carbinol carbon and one from the hydroxyl group.

– A tertiary alcohol cannot easily oxidize because there is no hydrogen atom available on the carbinol carbon.

Oxidation states of Alcohols

Summary: Oxidation states of alcohols

An alcohol is more oxidized than an alkane, yet less oxidized than carbonyl compounds such as ketones, aldehydes, and acids.

– Oxidation of a primary alcohol leads to an aldehyde, and further oxidation leads to an acid.

– Secondary alcohols are oxidized to ketones.

– Tertiary alcohols cannot be oxidized without breaking carbon–carbon bonds

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