2. Propagation of the Radical Chain Reaction:

a. Once an alkyl radical has been formed, this reacts irreversibly with oxygen to form an alkyl peroxy radical, which is extremely fast, and is independent of temperature:

b. Hydrogen abstraction by a peroxy radical from another hydrocarbon, which leads to a hydroperoxide and an alkyl radical:

The alkyl radical can again react with oxygen, as step (a).

The peroxy radical may undergo hydrogen abstraction via interamolecular propagation:

The alkyl-hydroperoxide radical undergo a propagation steps (a, b):

Again the hydroperoxide-peroxy radical may undergo hydrogen abstraction via interamolecular propagation:

The alkyl dihydroperoxide radical undergo a propagation steps (a, b):

3. Chain Branching:

a. At low concentrations; hydroperoxides may be cleaved homolytically to yield an alkoxy and a hydroxy radical:

Once formed, hydroxy and especially primary alkoxy radicals are so active that they abstract hydrogen atoms in non-selective reactions:

b. At high concentrations; hydroperoxides may react via a bimolecular mechanismhydroperoxides may be cleaved homolytically to yield an alkoxy and a hydroxy radical:

4. Termination of the radical chain Reaction:

Termination may be effected by the combination of radical species such as peroxy radicals to yield ketones and alcohols:

If the oxygen concentration in the bulk liquid phase is limited two additional ways of radical recombination result:

B. Oxidation of hydrocarbon at high temp. > 120°:

Above 120°C the degradation process can be divided into a primary and a secondary oxidation phases.

1. Primary Oxidation Phase:

Initiation and propagation of the radicals’ chain reaction are the same as discussed under low-temperature conditions.

Acids are formed by the following two reactions:

When the rate of oxidation becomes limited by diffusion, ethers are formed:

2. Secondary Oxidation Phase:

1^st Step: aldehydes or ketones formed in the primary oxidation phase combine via an acid- or base-catalysed aldol condensation to form α,β-unsaturated aldehydes or ketones:

Further aldol condensations lead to high molecular weight but still oil-soluble polycondensation products (molecular weight about 2000 amu).

2^nd Step: co-polymerization of two different polycondenzation species, This leads to sludge and deposit formation as well as to additional oil soluble high molecular weight products which contribute to the viscosity increase:

NOTE

Under high-temperature conditions there is always the possibility of thermal cleavage of a hydrocarbon chain, especially when the availability of oxygen is limited, leads to unsaturated molecules with lower molecular weight and higher volatility.