Q:

how to set id to an element inside tiny mce

I want to add an Id to the element which is being edited using a tine mce instance ?  Is there any way ? 

A:

Yes, there is a way.  You need to get the selections parent node and apply the attribute id there:
ed.onDesiredEvent.add(function(editor, event) {
    node = editor.selection.getNode();
    node.setAttribute('id','the_id_to_be_assigned');
}

