Side Chain Oxidation: Toluene to Benzoic Acid

[Skyblitz]

Hello I had a few questions if anyone could help me out.

I wanted to convert toluene into benzoic acid via side chain oxidation using potassium permanganate. However, I'm a little unsure of the mechanism.

If I'm right, you convert toluene into benzyl alcohol using NaOH, and then from benzyl alcohol to benzoic acid via KMnO4 and HCl. However, how do these reactions take place?

Also, is the haloform reaction (on acetophenone) better than the side chain oxidation at producing benzoic acid? My lab results tell me yes but the only thing I can come up with is that KMnO4 isn't a strong oxidizer.

Thanks for your help!

 

[chem_tr]

I think that things are going somewhat different on these reactions. Toluene is oxidized to benzoic acid in one step, as permanganate is a powerful oxidant, so it is nearly not possible to isolate the intermediate stages. However, there are mild oxidants like selenium dioxide to provide just one step of oxidation; if you apply this to toluene, what you'll get is plain benzaldehyde, along with ametallic selenium.

Permanganate works best in strong acidic solutions, as obvious in the following redox half reaction:

$$Mn^{7+} + 5e^- \longrightarrow Mn^{2+}$$
$$MnO_4^- + 5e^- + 8H^+ \longrightarrow Mn^{2+}+4H_2O$$

In the second redox half reaction, it is seen that the redox is directly dependent on the pH value of the solution; the more acidic it is, the more favorable and faster the oxidation proceeds.

The mechanism probably involves manganese-oxygen-carbon bonding with subsequent reduction of manganese and oxidation of carbon to yield aldehyde, which is much more vulnarable to oxidation with permanganate.

In haloform reaction, we don't use permanganate, we prefer hypoiodite (prepared in situ from iodine and a powerful base like NaOH). It oxidizes the side chain composed of an acidic alpha proton-containing terminal methyl group in a stepwise manner, yielding iodoform (CHI₃) as the main product with formation of a carboxylate anion. If you start with toluene, you'll get iodoform and benzoate. Permanganate oxidation is harder, since you must provide enough acidity and supply it during the reaction as it is consumed in the reaction. However, hypoiodite oxidation is easier, proceeding with a different mechanism. Iodoform is the side product, so in my opinion, it must give less yield when compared with permanganate.

 

[Skyblitz]

Thank you for your reply :)