Oxidation Reactions of Alcohols and Aldehydes Class Notes

Oxidizing and Reducing Agents

Carbon atoms that have a great number of oxygen bonds are said to be highly oxidized while those with a great number of hydrogen bonds are said to be highly reduced.

Oxidizing agents increase the number of C - O bonds and decrease the number of C - H bonds.

The reaction is said to be oxidation and the oxidizing agent may be specified or indicated with the general symbol [O].

Reducing agents decrease the number of C - O bonds and increase the number of C - H bonds.

In organic chemistry it is generally desired that the parent not be torn apart, so oxidizing agents are chosen which affect the functional groups only.

Primary alcohols have an OH attached to the primary carbon which is a carbon with only one other carbon attached to it.

Secondary alcohols have the OH attached to the secondary carbon (there are two carbons attached to the secondary carbon).

Tertiary alcohols have the OH attached to the tertiary carbon ( three carbon atoms directly attached to that carbon)

What The Molecule Will Oxidize To

In order to oxidize, there must be at least one C - H bond in the functional group.

1^o Alcohols are oxidized to carboxylic acids.

2^o Alcohols are oxidized to ketones.

3^o Alcohols CANNOT be oxidized. There are no C-H bonds to remove.

Aldehydes are easily oxidized to carboxylic acids.

Carboxylic Acids and Ketones CANNOT be oxidized. There are no C-H bonds to remove.

Oxidation with Potassium Dichromate

Potassium dichromate oxides 1^o and 2^o alcohols and aldehydes.

The bright orange dichromate ion is the oxidizing agent and in the process of oxidizing it is reduced to the green Cr³⁺ ion. This color change can be used to indicate a reaction. If the orange color changes to green or blue green an oxidation reaction has occurred. If the color stays orange no reaction has occurred.

Tollens Reagent

To prepare Tollens reagent, mix silver nitrate and sodium hydroxide until you get a brown precipitate (silver oxide). Add ammonium hydroxide drop by drop until the brown precipitate just disappears. It is okay to have a few brown specks left, because we do not want to add too much ammonium hydroxide. You now have Tollens Reagent.

When Tollens reagent oxidizes an organic compound, the silver ion complex is the oxidizing agent. During the oxidation the silver ion is reduced to metallic silver resulting in a silver mirror. This silver mirror is the indicator that an oxidation has occurred.

Tollens Reagent is a mild oxidizing agent. It cannot oxidize alcohols, but it can oxidize aldehydes because aldehydes are easily oxidized to carboxylate anions.

NOTE:Occasionally the mirror fails to form, and instead a gray precipitate of Ag develops - this should also be considered a positive test for oxidation.

Image of silver mirror from http://chemmovies.unl.edu/chemistry/smallscale/SS075.html

Benedicts Test for Reducing Sugars

Reducing sugars are glucose, fructose and any other sugars that can be reduced, by increasing the number of C-H bonds, using Benedicts reagent. Reducing sugars have either a ketone group or a free aldehyde group in their molecular structure.

Benedicts reagent is a mild oxidizing agent. It is special because it oxidizes formaldehyde and sugars with a hemiacetal or hemiketal group. It will not oxidize alcohols or other aldehydes.

Hemiacetals form when an alcohol is added to an aldehyde group. The hemiacetal is a molecule that always has a carbon atom holding both an OH and an O - R group. Both of these must be attached to the carbon.

Hemiketals form when alcohols are added to ketones.

Benedicts reagent has Cu²⁺ ions which gives it a deep blue color. During the oxidation, Cu²⁺ is reduced to Cu₂O (a precipitate). The formation of a precipitate is the indicator for reducing sugars. Colors of the precipitate can vary from green, yellow, to brick red. If the blue color does not change no oxidation occurs. If the blue color stays and you have a precipitate, oxidation occurs. The indicator is the PRECIPITATE. The color sequence as follows: green, yellow, orange, red, brown, is in order of increasing concentration of reducing sugar in the test solution.

Benedicts test before boiling.              Benedicts test after boiling 2 minutes.

Benedict's Test for Reducing sugar images from http://samson.kean.edu/~breid/enzyme/enzyme.html#BENEDICTS

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