How to get a object to slowly rotate to the left and then right.

I am creating an enemy that when it reaches a destination it stops, looks to the left for the player, and then to the right. I've tried the following code but the rotation is instant in both directions:

private IEnumerator LookForPlayer() { for (float f = 0; f < 30; f += Time.deltaTime) { transform.Rotate(0, f, 0); }

     yield return new WaitForSeconds(0.1f);

     for (float f = 0; f < 30; f += Time.deltaTime)
     {
         transform.Rotate(0, f, 0);
     }
 }