Organic Chemistry

Module 7. Carbohydrates

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

21.B.1 Introduction

The confirmation indicates arrangement of groups or element around anomeric (reference) carbon atom - ‘α’ and ‘β’ denotes conformation. (Fig. 21.5)

• In solution these forms are in equilibrium with each other via aldehyde from and have [α]^D_{20 ^oC} = 52.5° (Fig. 21.6 Mutarotation)
  

• One form (α or β) dissolved- changes to another till equilibrium established between the two – process- Mutarotation.

Configuration indicates arrangement of groups or atoms around number carbon atom – denotes by ‘D’ and ‘L’ (Fig. 21.7)

• Position of –OH group on asymmetric carbon atom farthest from the functional group (C-5 this case)

- on right hand side – ‘D’ configuration
-on left hand side – ‘L’ configuration

• Optical rotation: rotation of direction of plane polarized light

• towards right- clock-wise direction- dextrorotatory – denoted by (+) sign
• towards left- anticlock-wise direction- denoted by sign (-) = laevorotatory.
• Previously dextro and levo rotations were denoted by (d) and (l) signs but they create confusion with ‘D’ and ‘L’ configuration
• Therefore (d) and (l) signs are now replaced by (+) and (-) respectively for representing dextro and levo rotations
• e.g. D-(+)-glucose, D(-)-fructose etc.

1. Haworth formula: (cyclic form)

W.H. Haworth (1929) – an English chemist
Emphasized structural relationship of carbohydrates with furan and pyran (Fig. 21.8)

• Three dimensional representation of molecule

• Atoms of the ring lie in a single plane and perpendicular to the plane of the paper
• Groups or atoms (H, OH etc) attached to the carbon atoms are projected at right angles to the plane of the ring – above or below the plane
• Position of OH group- indicated by drawing only bond position of single H atom- not indicated (not to draw bonds)