Is 2 and half feet apart ok for planting lilacs?

Two and half feet apart is a little too close because (assuming they are the common lilac) they can spread out to 8 or more feet. You might want to take out every other plant and transplant them somewhere else. You could leave the plants in place, but remove every other plant when they start to become crowded.

Better to be too far away than too close when planting.