Why can’t we just keep one perspective in organic chemistry?
Condensed, expanded, line drawing, Newman projections, Fischer projections…geez!! What’s the deal here, OChem whisperer?
This is a good question…and we’ll save it for later. So – instead of me rambling on about why this and why that, let’s go over how to do it.
What do the horizontal and vertical lines mean?
I know it is hard to believe, but it is written this way for a reason. Here is the typical way you will see the Fischer projection written in your textbook.
However, we know that molecules are not flat. They have a 3-dimensional structure. Let’s now look at a more accurate Fischer projection drawing.
As you can see, the horizontal lines are coming out at you. And the vertical lines are going away from you. You can think of the horizontal lines as bowties.
It is possible some of you may have no idea what a bowtie is. Don’t worry – just remember wedge!
It gets pretty messy to draw it this way. That is why you will typically see only the bowties/wedges.
Start by tipping your molecule to the right
Alright…no matter your carbohydrate structure, start by tipping it over to the right. When you do this, this will put your aldehyde (CHO) group on the right side.
Now, let me show you why the dash/wedge combo is important. Let’s rewrite the structure as follows:
We can see that the structure is actually wrapped around like a “broken” circle. And to go from Fischer projection to Haworth projection, we need to form “that” circle.
Is your OH group pointing up or down?
So – here is the cool part. Once you have tipped your structure, look at your OH groups.
If you have an OH group down, it will be down in the ring. And if you have an OH group up, well….you know…it will be up.
The only thing left is to close our ring. And that is the easy part.
You might be asking…
What about the OH group on carbon 1? Is that up or down?
Good question! And the answer is both.
When this forms a ring, the OH group can go up or down. And the percentage of each one can vary.