Question

In a combustion reaction of an hydrocarbon, what occurs if `C-C` bonds remain unreacted?

Answer 1

`"It don't.........??"`

Explanation:

Under the conditions of complete combustion, the `C-C` bonds of an alkane do indeed break to give water and carbon dioxide:

`H_3C-CH_3(g) + 7/2O_2(g) rarr 2CO_2(g) + 3H_2O(l)`

As the carbon chain gets longer, incomplete combustion is possible, and likely, and its products include carbon monoxide, and sometimes, soot. Certainly this occurs in the internal combustion engine, and, more commonly, in diesel engines.

We could represent this by the reaction:

`C_7H_14(g) + 9O_2(g) rarr 5CO_2(g) + CO(g) + C(s) + 7H_2O(l)`

Of course this is an ad hoc representation; the point is that some of the hydrocarbon is oxidized incompletely (note that diesels are typically longer chain hydrocarbons with one degree of unsaturation).

So the take-home message is that `C-C` bonds are completely oxidized under strongly oxidizing conditions.