Question

How do we represent the combustion of alkanes, olefins, and acetylenes? How do they react in other scenarios?

Answer 1

They differ in terms of formula........

Explanation:

We specify the `"degree of saturation"` in organic molecules by reference to their chemical formulae. See this old answer.

So for hydrocarbons.......a saturated example is `C_nH_(2n+2)`; an olefin, with one degree of unsaturation is `C_nH_(2n)`, and an alkyne, with TWO degrees of unsaturation is `C_nH_(2n-2)`.

And we thus compare `"ethane"` to `"ethylene"` to `"acetylene"`: `H_3C-CH_3; H_2C=CH_2; HC-=CH`. Does the given formulation hold? Every degree of unsaturation corresponds to a double bond OR a ring junction. Compare `"cyclohexane"`, `C_6H_12`. If we want to include nitrogen, we subtract `NH` from the formula before assessing saturation: compare SATURATED `"methylamine"`, `H_3C-NH_2` to `3^@` `"unsaturated"` `HC-=N`

All hydrocarbons, saturated or not combine with oxygen....to give carbon dioxide and water under conditions of complete combustion:

`C_nH_(2n+2) + (3n+1)/2O_2 rarr nCO_2 + (n+1)H_2O`

`C_nH_(2n) + (3n)/2O_2 rarr nCO_2 + nH_2O`

`C_nH_(2n-2) + (3n-1)/2O_2 rarr nCO_2 + (n-1)H_2O`

You should not have to reproduce these formulae (I don't guarantee they are right). You should, however, be able to represent COMPLETE combustion of `"ethane, ethylene, and acetylene"` at the very least. So try it!

And alkanes typically react by substitution reactions, olefins and acetylenes by addition reactions. You should check your organic text.......