Always use Url.Action
helper method to generate url so that there is no chance of Url mistake.
do like this:
$.ajax({
type: "POST",
// async: false,
url: '@Url.Action("SetIntroductionData","Introduction")',
data: {
projectName:'@Session["projectName"]',
intro: _intro
},
success: function (){
alert("success");
},
});