In 11ac, if a channel is allocated, for 80MHz, example, 48, 44, 40 and 36. Here we can say 48 is the primary channel and remaining are secondary channels.
why can't we represent channel allocation as 48, 52, 56 and 60. so that the primary channel is 48 and remaining are secondary channels.
Are you referring to the dynamic bandwidth feature in 802.11ac?
Primary and secondary channels are fallbacks of narrower width for when the full width of a wider channel is not available. Actually 80MHz channels are labelled by their center frequency not their primary channel.
This link explains it: 802.11ac Channel Planning
For example, say you have the 80MHz channel 42, which covers the 40MHz channels 36+ and 44+, and the 20MHz channels 36, 40, 44 & 48. Most enterprise systems will let you designate the primary 40MHz channel as 36+ or 44+.
This enables two APs to operate on the same 80MHz channel but fallback to different primary channels when activity on their channel is detected.