What is the absolute configuration of carbon 4 in glucopyranose?
therefore it will retain that chirality as it forms the hemiacetal.


This statement is incorrect. Many chiral centres retain their CIP-descriptor upon reactions that don’t influence them directly, many others do not.

How do you assign CIP priorities? You basically had the same problem in the linear form of D-glucose. You move along the chain in order of priorities, finding the first priority difference and using that to assign the higher-priority side. On C4, you have (read this from left to right, i.e. everything with the same indention at the same time):


an oxygen (a)
the carbon C3


an oxygen (a)


a hydrogen.

the carbon C2 (b)


an oxygen (a)
a carbon C1 (b) &lt;-- this one is higher priority than on the other side!
a hydrogen (c)

a hydrogen (c)

the carbon C5


an oxygen (a)


a hydrogen

the carbon C6 (b)


an oxygen (a)
a hydrogen &lt;-- less priority than carbon
another hydrogen.

a hydrogen (c)

a hydrogen (d)


Following the same method in the pyranose form we get:


an oxygen (a)
the carbon C5


an oxygen (a)


a carbon &lt;-- this one is higher priority than the other side!

the carbon C6 (b)
a hydrogen (c)

the carbon C3


an oxygen (a)


a hydrogen &lt;-- this is lesser than carbon!

the carbon C2 (b)
a hydrogen (c)

a hydrogen (d)


Therefore, the absolute configuration of said atom changes from (R) to (S).

This is actually a pretty common thing that can happen in highly functionalised molecules with very similarly modified carbons. Because we cannot distinguish at any proximate position we need to go far down the chain and a single distal modification can change a lot.

Other examples that would have changed the absolute configuration of carbon C4 without changing its geometry would be:


methylation at C5
silyl protection at C5
oxidation at C5


All of these would make C5’s priority higher than C3’s.