Hi Sandip,
your suggestion was helpful. Through httpwatch we could able to figure out what URL it is using.
Finally the thing is there was a wrong URL placed in SAP_SRM backend connection under system configuration --> system landscape.
URL was pointing to wrong port.
Instead of hostname:8001 it was pointing hostname: 3201(which is wrong)
we have changed it and purchasing tab started working.
Thanks Guys..
KN