Oxidation of Tertiary Alcohols

Tertiary alcohols are resistant to oxidation. rcr -Butyl alcohol is frequently used as a solvent in oxidations. However, some tertiary alcohols are converted into tertiary hydroperoxides on treatment with hydrogen peroxide in sulfuric acid [177, 179]. Dimethylphenylcarbinol added to a mixture of 87% hydrogen peroxide and sulfuric acid at a temperature below 0 °C gives a 94% yield of cumyl hydroperoxide after 3.5 h [777]. Similarly, acetylenic alcohols with the tertiary hydroxyl group adjacent to the triple bonds are converted into the corresponding hydroperoxides in high yields [179] (equation 272). 

In acid media, for example, in chromic acid in sulfuric acid, tertiary alcohols that can suffer dehydration form cilkenes, which are degraded to ketones and carboxylic acids Barbier-Loquin and Wieland degradation of carboxylic acids) 1150 (equation 273). 

Rather rare examples of the oxidation of tertiary alcohols are the degradation of 3-ethyl-3-pentanol to 3-pentanone and iodoethane in respective yields of 90 and 84% by acetyl hypoiodite in acetic acid and chlorobenzene under irradiation for 1 h at 20-25 °C [780 and the conversion of trflnj-9,10-dihydroxy-l,4,5,8,9,10-hexahydronaphthalene into 3,8-cyclodecadiene-l,6-dione and its tetramethylacetal by lead tetraacetate (equation 274) [447.  

Tertiary alcohols can not be oxidised because they do not have a hydrogen atom on the carbon atom bearing the — OH group. 

However, if tertiary alcohols are heated up to very high temperatures in the presence of a catalyst, they can decompose into unsaturated hydrocarbons and water. 

In the first step, methanol oxidises to an aldehyde (methanal). 

3CH3OH + K2Cr207 + 4H2S04 3HCHO + K2S04 + Cr2(S04)3 + 7H20 

In the second step, the formed aldehyde oxidises to carboxylic acid (methanoic acid) 

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In the presence of strong oxidizing agents at elevated temperatures oxidation of tertiary alcohols leads to cleavage of the various carbon-carbon bonds at the hydroxyl bearing carbon atom and a complex mixture of products results... 

The oxidation of tertiary alcohols by chromic acid is comparatively slow and shows a zero-order dependence of the rate upon oxidant concentration For 1-methylcyclohexanol the kinetics are... 

This type of fission has been observed in a detailed examination of the oxidation of tertiary alcohols by Co(ril). The kinetics are similar to those reported for cyclohexanol vide supra) although the rate is about 40 times less. The possibility of alkoxyl radical formation seems attractive, for Co(III) is known to oxidise... 

The mechanism of this oxidation is shown in Figure 4.29. The preferred cofactor for this reaction is nicotinamide adenine dinucleotide (NAD+). It can be seen from this mechanism that oxidation of tertiary alcohols does not occur because there is no hydrogen on the OH-substituted carbon. 

Oxidation of tertiary alcohols by lead tetraacetate gives alkyl radicals by /3-scission of the initially formed alkoxy radicals. The reaction has been used to alkylate protonated heteroaromatic bases using 1-methyl-cyclohexanol. (Scheme 4). 

Although allyl-arenes are prone to olefin isomerization, several successful reactions have been performed, for example in the chemoselective oxygenation of 22 to aryl-acetone 23 (Table 2) [38]. Allyl alcohols sometimes react sluggishly, but examples with high ketone selectivity are known, for example the oxidation of tertiary alcohol 24 to a-hydroxyketone 25 [39]. 

Oxidation of tertiary alcohols is not an important reaction in organic chemistry. Tertiary alcohols have no hydrogen atoms on the carbinol carbon atom, so oxidation must take place by breaking carbon-carbon bonds. These oxidations require severe conditions and result in mixtures of products. 

There are a variety of products, depending upon the alcohol. Allylic and benzylic alcohols are easily oxidized under mild conditions. Secondary alcohols are oxidized under rather stronger conditions. Simple primary alcohols (i.e., not activated benzylic alcohols) are not oxidized. The oxidation of tertiary alcohols is accomplished with C-C bond fission. 

Oxidation of Tertiary Alcohols.—In the case of tertiary alcohols it is a fact that on oxidation they yield neither aldehydes nor ketones. This agrees with our ideas as there is no hydrogen linked to the carbon which has the hydroxyl. Oxidation, therefore, does not take place readily nor without breaking the carbon chain. 

Oxidation of tertiary alcohols is difficult because the breaking of a carbon-carbon bond is required. Such oxidations are of little use in s)mthesis. 

Tertiary alcohols are not readily oxidized under comparable mild conditions, as there is no hydrogen atom attached to the carbon atom to which the hydroxyl group is attached. Any oxidation of tertiary alcohols requires more drastic conditions as it is necessary to break the carbon skeleton of the molecule. Therefore we do not see a colour change in the acidified potassium dichromate(vi) oxidizing agent when it is heated with a tertiary alcohol (Figure 10.77). 

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