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Exercise 2OXIDATION OF A SECONDARY ALCOHOL

STEVENS-CHAPMAN REACTIONCyclohexanone from cyclohexanol

Inorganic oxidation and reduction processes are readily recognized as the loss and gainof electrons, respectively. It is often less clear whether electrons are being gained or lost in a

reaction with organic compounds. Thus, more useful definitions were used to define oxidation

and reduction in other terms. Oxidation is a process that adds electronegative atoms such as

halogen or oxygen to or removes hydrogen from a molecule. On the other hand, reduction is the

removal of halogen or oxygen or the addition of hydrogen (Vollhardt and Schore, 2011).

Oxidation of organic compounds normally involve transfer of electron but since the

valence of carbon remains at four, it is more convenient to define it as either loss of hydrogen

atoms or addition of oxygen to the other electronegative atoms. Conversely, reduction of

organic compounds results in gain of electron density of the carbon atom, bond formation

between a carbon and a less electronegative atom or the bond breaking between a carbon and a

more electronegative atom.

Oxidation of an alcohol yields a carbonyl compound which may be an aldehyde, a

ketone, or a carboxylic acid. This depends on the alcohol and the oxidizing agent used. Primary

alcohols are oxidized resulting either to an aldehyde or to a carboxylic acid. Secondary alcohols

are oxidized to ketones while tertiary alcohols have no hydrogen on their hydroxyl-bearing

carbon and do not usually undergo oxidation reaction readily (Carey and Giuliano, 2011).

The most commonly used oxidizing agents to synthesize ketones from secondary

alcohols are H 2CrO 4 or KMnO 4. However, NaOCl in acetic acid, an alternative oxidizing agent

was introduced by Stevens, Chapman, and Willer in 1980. This reagent oxidizes secondary

alcohols rapidly and gives high yield. It is also less toxic and costly. It can be disposed through

the sink unlike the traditional oxidizing agents that contains heavy metals which causes

problems in handling and waste disposal.

In this experiment, cyclohexanone, a ketone, will be synthesized from the oxidation of

cyclohexanol, a secondary alcohol. The general equation is:

OH O

+ ClO - + 2H + ---- > + H 3O + Cl -

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The oxidation number of chlorine of the hypochlorite is +1 while the chloride ion is -1. Thus, 2 moles of electrons were involved in going from ClO - to Cl -. Chlorine is reduced.

The mechanism involved in the reaction is

(1) Formation of alcohol hypochlorite(2) Elimination of HCl

Glacial acetic acid, a weak acid, is used as a catalyst in the dissolution of cyclohexanol. This is done to control the formation of Cl 2 gas and avoid a vigorous reaction.

Cl-O - + 2H + ----- > Cl-O +H 2 + Cl - ---- > Cl 2 + H 2O

The hypochlorite oxidant needs a proton as a catalyst. It is not advisable to use strongacids such as HCl or H 2SO 4 since it causes the formation of Cl 2 , causing green coloration of thereaction mixture.

In the experiment, sodium bisulfate (NaHSO 3) is added in the preparation of thecyclohexanone from cyclohexanol and acts as a reducing agent. It reacts with excess Ca(ClO) 2 .

HSO 3 + ClO - --- > HSO 4 - + Cl -

To neutralize the acidic distillate, NaHCO 3 was used since with the use of this reagent,neutralization usually stops when the bubbles formation stops.

HCO 3 - + H 3O + --- > H 2O + CO 2

Addition of sodium chloride (NaCl) aids in the isolation of the product to be obtainedthrough salting-out technique. This technique allows the separation of layers in the distillatefrom steam distillation.

(show reaction here)

The ion-dipole interaction between NaCl and water is stronger than the dipole-dipoleinteraction between water and cyclohexanone. Therefore, the solubility of the compound inwater decreases.

Chemical tests will be performed to compare the product obtained and the startingmaterial. Jones test is a rapid method for distinguishing primary and secondary alcohols fromtertiary alcohols. It is a suspension of chromic anhydride (CrO 3) in concentrated H 2SO 4 inwater. The reaction involved is

(show reaction here)

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A positive result for this is the production of an opaque suspension with blue greencolor.

Another test is the reaction with warm, dilute, neutral KMnO 4 . Formation of brownprecipitate and disappearance of purple color indicates a positive result.

(show reaction here)

Derivatization of cyclohexanone was performed to confirm if the desired product wassuccessfully synthesized using 2,4-dinitrophenylhydrazine. The reaction involved is

(show reaction here)

In the synthesis, the possible side reactions are:

1. Ether formation. This side reaction can be minimized by mixing the reaction mixturethoroughly and also the use of a weak acid.

(show reaction here)

2. Enol formation. This side reaction is prevented by using a weak acid catalyst. This isalso minimized during distillation since acetic acid is being distilled off and also, byneutralizing the product with NaHCO 3 .

(show reaction here)

3. Dehydration of alcohol. This is minimized at lower temperature and with the use of aweak acid catalyst.

(show reaction here)

4. Ester formation. This is not favored since the mixture is slightly acidic.

(show reaction here)

In this experiment, the student should be able to: (1) synthesize cyclohexanone from

cyclohexanol using , (2) to apply the different techniques for the isolation and purification ofthe product, (3) to perform different tests for the comparison of the product and the startingmaterial, and (4) perform derivatization.