[KüssendistSpaß101]

When a molecule has a tail what does that generally mean for a chemical reaction?

For any kind of molecule, organic or not. I asked it in the Organic Chemistry Forum because I notice that a great deal of these molecules contain carbon. For example, THC has a tail on it.

Also, is there a special name for "tails" in chemistry?

[salteen]

It can depend on the context, but most commonly this refers to long hydrophobic chains (hence "tails").

So the tail portion on THC is the n-pentyl group attached to the aromatic ring.

[spirochete]

You need to be more specific in your question. But for a straight chain saturated (all single c-c and c-H Bonds) hydrocarbon "tail" there is very little effect from it in terms of chemical reactivity in a flask. Saturated hydrocarbons are relatively inert compared to more polar functional groups and they mainly do radical reactions. There's also little steric change when switching from say, a propyl group to a butyl group to a pentyl group, because the added carbons are further and further away from more reactive sites on the molecule.

The effect in the body could be significant, for example it could increase biological activity by making the molecule more non polar and therefore better able to cross membranes. Or it could even make it less biologically active by changing the shape of the molecule so that it doesn't fit into an enzyme as well.

[salteen]

Sorry, just realized I only read half of your question!  But spirochete summed it up nicely.

[Furanone]

In addition to a tail being a hydrophobic chain on a molecule, a tail is also used in the context of chromatographic peaks (GC, HPLC, etc) when quantifying an amount of a chemical, and the peak with a tail means it is no longer strictly Gaussian, but now is asymmetric distortion at the tail end of the peak. In GC, this is common with molecules with organic acid functional groups.