<input autocomplete="off" type="email" name="email" id="g-email" class="form-control required order" placeholder="Email address">

  <button type="submit" name="submit" class="btn btn-md btn-default" id="messageButton" value="Place Order" onclick="$(this).attr('disabled', true);">Send message</button></div>

$(document).ready(function(){
  $("input[name='email']").change(function(){
    if ($(this).val() != $(this).val())
    {
       $("input[name='submit']").removeAttr('disabled');
    }
  });
});