I read that the executables for the commands issued using exec() calls are supposed to be stored in directories that are part of the PATH variable.
Accordingly, I fo
cd is a shell built-in, unfortunately.
$ type cd cd is a shell builtin
...from http://www.linuxquestions.org/questions/linux-newbie-8/whereis-cd-sudo-doesnt-find-cd-464767/
But you should be able to get it working with:
sh -c "cd /somedir; do something"