Organic Chemistry

Module 7. Carbohydrates

Lesson 21A
STRUCTURE OF GLUCOSE - OPEN CHAIN AND RING STRUCTURE AND EVIDENCES FOR THE RING STRUCTURE

21.A.1 Introduction

The structure of carbohydrates (taking glucose as an example) is given below

21.A.1.1 Open chain structure (Bacyer’s formula)

• Arrangement of six carbon atoms- a straight chain
• Aldehyde group – in terminal position
• One of the five –OH groups as –CH₂OH and in terminal position

Fig. 21.1 Open chain structure of glucose

21.A.1.2 Fisher’s projection formula

• Emil Fisher- German organic chemist
• In glucose- for asymmetric carbon atoms- therefore 2⁴ =16 stereoisomeric structures possible
• Therefore essential to define the configuration about each of the four asymmetric carbon atoms
• Glucose- assigned the configuration as shown here
• Horizontal lines represent bonds coming out of the plane- i.e. towards us.
• Vertical lines represent bonds going behind the plane – i.e. away from us.

Fig. 21.2 Fishers projection formula

21.A.1.3 Ring structure

• Chemistry of glucose- anomalies in behavior
• Lacks some characteristic reactions of aldehyde

- Does not give schiff’s test
- Does not form a bisulphate product

• Kiliani cyanohydrins synthesis is not rapid (difficult)
• Exists in two isomeric forms – ‘α’ and ‘β’ – both undergo mutarotation
• These anomalies- explained on the basis of cyclic (ring) structure – resulting from intramolecular hemiacetal formation
• A hemiacetal – a compound formed by reaction between aldehyde and alcohol

Fig. 21.3

• Hemiacetals- quite reactive compounds- give many, but not all the reactions of aldehyde
• Hemiketals and ketals- formed from ketones
• “An intramolecular hemiacetal is formed in glucose by reaction between aldehyde group and one of the hydroxyl group of the molecule – resulting into cyclic (ring) structure”
• The angles of tetrahedral carbon atoms tend to bend the glucose molecule in such a way that the -OH group on carbon number five forms the hemiacetal by reaction with –CHO group of the molecule in two stereoisomeric forms known as Anomers.

Fig. 21.4 Anamors, glucopyranose structures

• The carbon atoms which held aldehyde or ketone group- anomeric carbon atom/ The glycosidic carbon.
• The hydroxyl group formed from the carbonyl oxygen due to hemiacetal (or hemiketal) formation – anomeric hydroxyl group.
• The hemiacetal linkage, formed between first and fifth carbon atoms- amylene oxide ring.
• The first carbon atom becomes asymmetric/chiral due to hemiacetal formation- therefore two modifications of this form (structure) exist = namely ‘α’ and ‘β’ as shown.