organic chemistry

[person]

Why will the chain length of an alcohol affect its reactivity with an ion (say Na for example)?

[Cervantes]

The chain is hydrophobic. Non-polar bonds. That region doesn't like electrically charged stuff.

What kind of scenario are you asking about? Dumping an alcohol onto salt crystal? Or pouring an alcohol into salt water?

[person]

Not a real scenario, but something on the lines of:

OH2(CH2)CH3 + Na --> ONaCHCH2CH3 + H2

versus something longer, say the same reaction with 1-octanol instead of isopropanol

[Delos]

Oi-vay!

Legible forms perhaps...I'm not even sure what your reactants are...is "OH2(CH2)CH3" supposed to be:
HO-C=C-CH3? That's what it looks like. In which case you're bringing in alkene chemistry too. That's not terribly bad, it's just not very illustrative for these purposes.

Let's take a simple alcohol and react with sodium, like propanol:

2(HO-CH2-CH2-CH3) + 2Na --> 2[(Na+ve)(O-ve)-CH2-CH2-CH3] + H2

Note the numbers!
Now, this reaction can proceed for 2 primary reasons: HO- is chemically reactive with Na, and Na is unhindered. Slap on a huge hydrocarbon chain, let's say: HO-(CH2)21-CH3 and you're no longer working too well. The HO- is still reactive, but in terms of the rest of the molecule is quite insignificant.
Your first problem would be getting to the HO- itself. There's far too much steric hinderence from the rest of the hydrocarbon chain. And yes, a straight chain will cause hinderence - it will bend and contort into an entropically favoured form.
The other problem is as Cervantes mentioned the polarity. This substance is far more non-polar than propanol, hence inherently less reactive towards an metal.

Ah, this brings me to an important point. You said in your first post that the alcohol was reacting with an ion, but in your subsequent post you spoke of a metal.
Na == metal; (Na+ve) == ion. Plain and simple, we all know this. Alcoholic interactions with the former are explosive and extremely exothermic (ka-boom!), where as the same reactions with the latter are much more mild and might even be endothermic (H-bonds or ionic bonds are formed). A slight slip of nomenclature can land you in serious trouble in a test. Best to be hard on yourself now and save yourself later.