How to make a 2d object back flip when a key is pressed

So im making a game where you have to jump over obstacles that come at you. But i want to include a option where you can backflip when pressing the Q key. The script I have makes the character flip a very tiny bit but not all the way. i have to keep pressing the key for it to flip

here is my script: using UnityEngine; using System.Collections;

public class Flip : MonoBehaviour {

 public float myRotationSpeed = 100.0f;

 public bool isRotateZ = false;


 private bool positiveRotation = false;

 private int posOrNeg = 1;

 
 void Start ()
 {
     
     GetComponent<Collider>().isTrigger = true;
     if(positiveRotation == false)
     {
         posOrNeg = -1;
     }
 }


 void Update ()
 {

     if (Input.GetKeyDown (KeyCode.Q)) {
 
         if (isRotateZ) {
                         
             transform.Rotate (0, 0, myRotationSpeed * Time.deltaTime * posOrNeg);//rotates coin on Z axis
             
         }
     }
 }

}