Let’s introduce the Haworth Projection, a frequently encountered convention for depicting sugars.
“What!?” you may ask. Yet another drawing convention? At this late stage of the course?
And isn’t drawing glucose just like drawing cyclohexane, more or less? Didn’t we learn how to draw cyclohexanes back in… Org 1?
Yes to all of those questions. It’s still important, however, so read on.
The Haworth Projection
Haworth projections didn’t originate as some dumbed-down, simpified depiction of sugars, even if they largely serve in that capacity today. In fact the Haworth projection pre-dates (1929) the chair depiction of cyclohexane (1943) and represented a vast improvement from the most common depiction of cyclic sugars at that time: the bloodyawful cyclic Fischer projection:
The Haworth projection didn’t go away with the advent of the cyclohexane chair, however. Here’s why.
Let’s look at glucose. The most stable chair conformation of glucose has the C-2, C-3, and C-4 hydroxyl groups all on equatorial positions. [Note 1] Of course, with organic chemistry fresh in your mind, you can tell which hydroxyl groups are up and which are down in the diagram below (right?):
But something often happens to people a few years after their last organic chemistry class: they start to forget the details of interpreting this whole cyclohexane chair business. Plus, conformational details are usually TMI to people who just want to look at the structure of a sugar and be able to clearly spot the differences in configuration.
[See, one of the slightly confusing aspects beginners encounter in learning cyclohexane chair conformations is getting used to the fact that “up” and “down” refers not merely to the axial groups on the chair (which indeed point “straight up” and “straight down”) but the equatorial groups as well – even though they only look like they are “somewhat up” or “somewhat down”. [See: The Ups and Downs of Cyclohexanes]. The key point is that since five and six-membered rings can’t turn themselves inside-out without self-destruction, no amount of conformational twisting (or chair flips) will convert, say, cis-1,2-dimethylcyclohexane to trans-1,2-dimethylcyclohexane, or for that matter, D-glucose to D-galactose; each group is locked on its particular face.]
A Haworth projection simply depicts a pyranose as a hexagon, and a furanose as a pentagon, and the substituent groups are placed straight up and down. Simple as that. [Note 2] By analogy with the Haworth, the oxygen is usually placed at the upper-right position with C-1 as the chair’s “foot-rest” for ease of comparison between sugars. [Note 3]
Plus, it’s also effective for five-membered ring sugars (furanoses), such as ribose.
Converting a Chair to a Haworth
Let’s say we wanted to convert a sugar drawn in a chair form to a Haworth projection. How would we do that?
The convention for drawing a pyranose in the chair form dictates that the ring oxygen is placed in the upper right hand position like that depicted below (the sugar in question is β-D-mannose, drawn in its pyranose form). The orientation of the chair is important: the “foot-rest” of the chair is on the right hand side, which is to say that the axial substituent on carbon 1 points down.
To convert a chair form to a Haworth projection, number the carbons 1 through 6 and then draw the Haworth template next to it, similarly numbered.
The next step is to identify which groups on the chair are “up” and which are “down”. Once this has been achieved, map the up and down groups onto the appropriate up and down positions on the Haworth template.
With all the hydrogens in place, things can look crowded. It’s OK to make the hydrogens “implicit”, which makes for a cleaner diagram:
[It’s extremely rare that you’d start with a sugar not in the “correct” chair form, but note that this works no matter what chair conformation you start with, as during a chair flip, “up” groups remain “up”, and “down” groups remain “down”.]
Now, let’s do a furanose. β-D-ribose is drawn below. Use the same protocol of numbering and mapping, we can convert the “flat” form to the Haworth projection:
Converting a Haworth to a Chair
Of course, we can also go in the opposite direction. The Haworth of α-D-galactofuranose is drawn below. To convert it to a chair, we start by drawing in a chair template (foot-rest on right) with ring oxygen at back-right and number each carbon.
In the next post, we’ll cover how to convert a cyclic Fischer projection to a Haworth projection, and vice versa.
- This is an easy way to remember how to draw glucose: it’s the sugar with all-equatorial C-2, C-3, and C-4 hydroxyl groups when drawn according to the standard convention. Interestingly, the sugar with all-axial hydroxyl groups, D-idose, is largely unknown in nature.
2. What one might call the “classic” Haworth for a pyranose employs a full hexagon with equally spaced sides and the front three C-C bonds in bold. The Haworth used in this post is based on a template in Chemdraw that elongates the horizontal lines and bolds the three frontal C-C bonds. Either is acceptable.
4. We are going to completely avoid the subject of the conformations of sugars here. Levy’s book (partly available on Google Books) contains useful discussion.