deshawn isabelle allegedly punched the woman in the head and dragged her to the ground by her hair before repeatedly beating her in chicago train .
isabelle stole $ 2,000 of the woman 's cash and her iphone , prosecutors said .
teen allegedly spent the money on junk food and air jordan track suits .
woman suffered a concussion and cuts and bruises all over her body . prosecutors said she was still vomiting two days later from her injuries .
isabelle confessed to robbing , beating and sexually assaulting the woman on the cta after his mother turned him in , according to prosecutors .