Sir, I am not able to understand on which basis do we see whether compound is R or S. and also how to give the prioity order to such compounds.

for assigning r-s configuration please have a look at  Cahn–Ingold–Prelog priority rules or CIP system then assign priority according  this rule then move from 1 to 3 via 2 If this is clockwise, then the center is R (Latin: rectus = right) and If this is counter clockwise, then it is S (Latin: sinister = left). 

there are many rules like in fisher projection the lowest priority group i. e. the 4th one should be in vertical position then you need to move clockwise or anti-clockwise. for bringing 4th on vertical position even no. of exchange are allowed because each exchange changes the configuration from R to S or from S to R. so after even no. exchange it will be back to its own configuration.

similarly in wedge dash projection 4th needs to be on dashed bond and then proceed.I would suggest that you go through any organic book which gives the exact rules for R-S nomenclature and practice some questions.

Approved

for assigning r-s configuration please have a look at  Cahn–Ingold–Prelog priority rules or CIP system then assign priority according  this rule then move from 1 to 3 via 2 If this is clockwise, then the center is R (Latin: rectus = right) and If this is counter clockwise, then it is S (Latin: sinister = left). 

there are many rules like in fisher projection the lowest priority group i. e. the 4th one should be in vertical position then you need to move clockwise or anti-clockwise. for bringing 4th on vertical position even no. of exchange are allowed because each exchange changes the configuration from R to S or from S to R. so after even no. exchange it will be back to its own configuration.

similarly in wedge dash projection 4th needs to be on dashed bond and then proceed.I would suggest that you go through any organic book which gives the exact rules for R-S nomenclature and practice some questions.

Approved