订购UNION查询laravel 5

i have two tables, each table contain created_at time stamp column, after making union between them, i tried to make order by after union, but it's not work, so how i can order data descending according to created_at column of the two tables in Laravel ?

$vac = db::table("table_a")->select($arr);
    $m = db::table("table_b")->select($arr_1)->union($vac)->get();

in controller

use DB; 

in inside controller function

DB::select('select ? from table_a union  select ? from table_b order_by created_at desc',[$arr,$arr_1]);