With alkanes, a free-radical halogenation (substitution) is observed.
I was told that there will be no reaction in a similar situation with ethene and ethyne — and I am having trouble understanding why.
Is it because the halogen radical will not be able to abstract a hydrogen atom from carbon because it is influenced by the greater electron density in the carbon-carbon double/triple bonds? Or because the $\ce{C-H}$ bonds are shorter in these molecules compared to alkanes? (resonance?)
I've assumed that the problem arises in the propagation step of the most common reaction mechanism seen with alkanes.
Or is it due to sterics? ($\unicode[Times]{x3c0}$-bonds get in the way of the attacking bromine radical or restricted rotation in double/triple bonds get in way of ideal orientation for reaction to proceed to the transition state)
The necessary heat/$h \nu$ is provided for the reaction. Additionally, no other reactants/catalysts are present.
Please keep your answers on a novice college student level.