Converting Variables to Name Variables in jQuery Ajax Calls

Question

I can work around this problem with some extra code but would very much like to know why the following code does not work.

The first bit of code responds to a click ( I have about 6 check boxes) and returns a string value of id = "discover_ID" from statement var id = $(this).val(); I can verify this with an alert (id) statement.

 $(document).ready(function() {
    $(".chk").click(function() {
       var val = ($(this).prop("checked") == true) ? 1 : 0;
       var id = $(this).val();
        setPhaseID(id);
    });
});

The code for setPhaseID is shown next which just sets a $_SESSION variable, $_SESSION (id) using a jQuery ajax call.

 function setPhaseID(id) {

    $.ajax({
        type : "POST",
        async : true,
        cache : false,
        timeout : 5000, // sets timeout to 10 seconds
        url : 'resources/library/setsession.php',
        data : {
                id : '1' 
        },
           success : function(data) {
        }
        });
     }

Now this does not appear to work however I can get the code will work if I implicity force the data to be:

data :{
      'discover_ID': '1';
}