Cyclisation of Sugar and Haworth projections- Structural Formula

In a process known as Cyclisation, monosaccharides form cyclic structures solution. The carbonyl group of the aldehyde or ketone reacts with one of the hydroxyl groups further down the sugar molecule.

·       The bond that is formed between the carbonyl group and the hydroxyl group is an ether linkage with a C-O-C bond in the cyclic structure

·       Ketose sugars form hemiketals, while aldose sugars form hemiacetal

·       The molecular formula remains the same

·       The cyclisation is not a chemical reaction, but is a rearrangement in the bonds- isomers (compounds with the same chemical formula, but different structures) 

·     Longer sugars the amount of cyclisation under normal biological conditions is high (5 or 6 carbons)

·      When these sugars cyclise, they form stable five- or six-membered rings

·     Smaller sugars tend not to cyclise as there is too much strain on the ring structure (3 or 4 carbons)

·     Less likely to cyclise and more unstable 

Haworth projections

Fisher projections are used to show the straight-chain forms of monosaccharides and Haworth projections are used to show the cyclic form of monosaccharides. Showing the conversion from straight-chain to cyclic forms is simple.

1.    Number all carbons in the straight chain, giving the carbonyl carbon the highest priority (lowest number).

2.    Identify the location of the carbonyl group and the hydroxyl group involved in forming the ether linkage. The hydroxyl group will not be the one at the very end of the chain, but the second from the last carbon. 

3.    The oxygen from the hydroxyl group bonds directly to the carbonyl carbon, forming the ester linkage.

4.    The hydrogen from the hydroxyl group will join the oxygen from the carbonyl group, forming a new hydroxyl group.

5.    Check to be sure all other substituents are in the correct location and that the general formula is still the same.