Unity 2D collision detector not working

I'm making a 2D sidescrolling game in Unity where the player shoots at obstacles while running. But nothing I try seems to get the collision detection to work. This is the obstacle script:

void onCollisionEnter2D(Collision2D col)
{
    if (col.gameObject.tag == "circle")
                    Destroy (gameObject);
    }

the circle tag refers to the projectile being shot at the obstacle