Monosaccharides Haworth convention

Figure 1.12 The a- and P-anomers of monosaccharides drawn using the Haworth convention. In this convention solid lines represent bonds above the plane of the ring whilst dotted lines are used to indicate bonds below the plane of the ring. Reproduced from G Thomas, Chemistry for Pharmacy and the Life Sciences including Pharmacology and Biomedical Science, 1996, by permission of Prentice Hall, a Pearson Education Company...

William Mills described a similar convention to depict the structures of monosaccharides. While the ring atoms of the Haworth projections are oriented perpendicular to the paper, Mills chose to depict the carbon skeleton in the plane of the paper (Fig. 1.5). Although Fischer, Haworth, and Mills projections are useful tools for depicting the structures of carbohydrates, the planar nature of these representations does not provide an accurate picture of the actual geometry of the molecules. In order to understand carbohydrate function and reactivity, recognition of each distinct conformation and the properties associated with it is required [15]. 

Haworth projection A conventional planar representation of a cyclized monosaccharide molecule. The hydroxyls that are represented to the right of the chain in a Fischer projection are shown below the plane in a Haworth projection. 

When drawing either a Haworth projection or a chair conformation, the convention is to place the oxygen atom in the upper, rear-right position, as highlighted in red. Notice that in a cycfic form of D-glucose all substituents can occupy equatorial positions, which renders the compound particularly stable and explains why D-glucose is the most common naturally occurring monosaccharide. 

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