How can i send a mail to specific user when workflow state is changed or changed to particular state?
This can be achieved with the help of Rules module.


  The Rules module allows site administrators to define conditionally executed actions based on occurring events (known as reactive or ECA rules). It's a replacement with more features for the trigger module in core and the successor of the Drupal 5 workflow-ng module.
  Example use cases
  
  Build flexible content publishing workflows changes
      Send customized mails to notify your users about important
      Create custom redirections, system messages, breadcrumbs, ...
      Build an eCommerce store using Drupal Commerce
  
  And many more...


Check Tutorial: editorial workflow with rules, which has good documentation on the how to's.