A History of the Fenton Reactions (Fenton Chemistry for Beginners)

*Rafael Ovalle*

### **Abstract**

A deceptively simple mixture, ferrous sulfate (FeSO4), hydrogen peroxide (H2O2), tartaric acid (C4H6O6), and water (H2O), initiated a century-long argument and a convoluted hunt to understand the oxidation mechanism(s) initiated by the combination of these components. Fenton's discovery rallied a legion of scientists, including two Nobel Winners, to find an explanation for the chemistry discovered when a graduate student mixed a couple of random chemicals, producing a molecule that became purple in strong base. Those investigation uncovered three separate branches of iron/oxygen chemistry, the Hydroxyl Radical [HO•], the Ferryl-Oxo Ion [Fe = O]+2, and the Perferryl-Oxo Ion [Fe = O]+3. Today their uses include chemical modifications [either untargeted and random [HO•] or targeted and selective [Fe = O]+2, [Fe = O]+3 dehydrogenations and/or oxygen additions] to effective and green oxidation and mineralization of persistent organic wastes.

**Keywords:** Fenton chemistry, ferryl-oxo ion, perferryl-oxo ion, hydroxyl radical, hydrocarbons (alkanes), alcohols, polyols, carbohydrates, reactions, history, biology

### **1. Introduction**

The Initial Experiments. In 1876, Henry John Horstman Fenton first discovered the enhanced oxidizing power of ferrous ions (Fe+2), hydrogen peroxide (H2O2) on tartaric acid (C4H6O6). When Fenton added sodium hydroxide (NaOH) to the mixture, the solution became bright purple [1]. Fenton made a decision to find out what that purple molecule was. That goal became his career and immortalized his name in the annals of chemistry [2].

Eighteen years later, Fenton repeated the experiment, again adding to a tartaric acid solution, a catalytic amount of FeSO4, followed by H2O2 with the molar ratio of each factor: C4H6O6 / H2O2 / Fe+2 = 1.0: 1.0: 'catalytic'. Fenton then isolated the reaction product by sequentially precipitating the acid with heavy metal ions, weighing the salt to calculate the molar formula, re-purifying the acid, then repeating the process with a different cation, thus calculating the empirical formula of the new acid. The new acid bound one divalent cation ion or two monovalent cations ions per molecule, and thus was a di-acid. Fenton determined: 1) the molecule had the empirical formula C4H4O6; 2) was a 4-carbon di-acid; 3) produced by abstraction of two hydrogen atoms from tartaric acid [3].

Fenton (1896) assumed that the 4-chain backbone was not severed, limiting his options to three possible structures: 1) loss of two hydrogen from a single middle carbon, forming a hydroxy-, keto-, di- carboxylic acid: (2-hydroxy-3-oxosuccinic acid); or 2) loss of two hydrogen from the internal adjacent carbon atoms forming a double bond with the HO- groups either in: 2a) *trans-* conformation: 2-, 3-, di-hydroxyfumaric acid, or 2b) *cis-* conformation: 2-, 3-, di-hydroxymaleic acid (Eq. (1)).

The first structure was eliminated when the di-acid failed to form a hydrazone with phenylhydrazine (an aldehyde/ketone reactive agent). The assumption that the molecule had two internal hydroxyl groups was verified when the molecule formed a 4-carbon di-ester, di-anhydride with either acetyl chloride or acetic anhydride.

s Initial Guess forC6H4O6*:* (1)

Fenton<sup>0</sup>

The structure of the molecule was finalized by reaction with aniline. Fenton knew (from literature) that the 1:1 product of aniline and fumaric acid (C4H2O4: -C2H = HC3- in *trans-*) was soluble in water, whereas the 1:1 product of aniline and maleic acid (C4H2O4, -C2H=HC3- in *cis*) was insoluble in water. The aniline derivative of the unknown acid was also insoluble in water. Fenton concluded that Fe+2/H2O2 oxidized tartaric acid to 2-, 3-, di-hydroxy-maleic acid: a loss of two H• atoms in *cis*orientation, forming a double bond and creating a previously unknown molecule [4].
