Rotation on input question

Hello everyone. Im working on a script where a cube would rotate 90 degrees, at a set speed, whenever a key is pressed. The current code im working with is:

float rotationAmount = 1f;
// Use this for initialization
void Start()
{
}
// Update is called once per frame
void Update()
{
if (Input.GetKey(KeyCode.A))
{
if (transform.rotation.eulerAngles.y >= 89 && transform.rotation.eulerAngles.y <= 91)
{
while (transform.rotation.eulerAngles.y <= 180)
{ transform.Rotate(0, rotationAmount * Time.deltaTime, 0); }

Then would put else ifs for the other degrees (0, 90, 180, 270). Currently when I press A it snaps 90 degrees instead of rotating the 90 at a slower rate. I have lowered and played with my variable of rotationAmount to no avail. Is there something I am missing? Or is there a better way to do this?
Thanks in advance

 

Code (CSharp):

1. float rotationAmount = 1f;
2. // Use this for initialization
3. void Start()
4. {
5. }
6. // Update is called once per frame
7. void Update()
8. {
9. if (Input.GetKey(KeyCode.A))
10. {
11. if (transform.rotation.eulerAngles.y >= 89 && transform.rotation.eulerAngles.y <= 91)
12. {
13. while (transform.rotation.eulerAngles.y <= 180)
14. { transform.Rotate(0, rotationAmount * Time.deltaTime, 0); }

Use code tags please ty. Anyway I would say get the gameobject than divide 90 by the speed than change to quaternion of the gameobject or you can also lerp to make it smoother(check the documentation). Good luck.