Why does this code not work in Scheme?

(define a 42)
 (set! 'a 10)


(define a 42)
(define (symbol) 'a)
(set! (symbol) 10)


(define a (cons 1 2))
(set! (car a) 10)

>(define pair (cons 1 2))
>pair
(1 . 2)
>(set-car! pair 3)
(3 . 2)