int dir = 1; // somewhere global

Quaternion r = Quaternion.Euler(0, Timer.deltaTime * dir, 0);

transform.rotation *= r;

// I want to: if the "angle is >= 5f", i want to do dir *= -1 to reverse it

if (/* angle delta is >= 5f or <= -5f */)
{
    dir *= -1;
}

public class rotator : MonoBehaviour {

public float _Angle;
public float _Period;

private float _Time;

// Update is called once per frame
void Update () {
    _Time = _Time + Time.deltaTime;
    float phase = Mathf.Sin(_Time / _Period);
    transform.localRotation = Quaternion.Euler( new Vector3(0, phase * _Angle, 0));
}
}