Fischer Projections; R–S and D–L Configurations

Transcript

0:00Introduction

Hello everyone! Here is a small puzzle. Alanine and cysteine are two natural amino acids. Both are in the L series, drawn the same way, with the amino group on the left. Yet alanine is labelled S, and cysteine is labelled R, the other letter! Same family, same drawing, but opposite letters? That sounds strange, right? Today we learn to draw a molecule flat on paper, and read R, S, D and L from it. Questions on R and S come up often in JEE, so let's begin.

0:34Reading a Fischer projection

So, look at the drawing on the left: glyceraldehyde, as a Fischer projection. Just two crossing lines, with a group at each end. The carbon atom is not even written. The crossing point is the chiral carbon, the carbon with four different groups. Its label, D glyceraldehyde with an R in brackets, is what we learn to read today. The tartaric acids on the right come in a later lesson. Okay? Now the key rule. The horizontal bonds point towards you, out of the paper. So the hydrogen and the O H come towards you, like two arms reaching out. Right? And the vertical bonds point away from you, behind the paper. So CHO at the top, and C H two O H at the bottom, both go back. The small two is a subscript, two hydrogens. Clear? One more rule: the carbon chain is written vertically, with the most oxidised carbon, C one, at the top. C dash one on screen means carbon number one. In glyceraldehyde, that is the CHO carbon, on top. With me?

1:39What you may and may not do

Now, strict rules for moving a Fischer projection. First, you may rotate it by one hundred and eighty degrees in the plane of the paper; the small circle means degrees. Nothing changes, because horizontal groups stay horizontal, and vertical ones stay vertical. Okay? So let me ask you this. What if we rotate it by only ninety degrees? Is it still the same molecule? No, and this is a trap: a ninety degree turn gives the enantiomer, the mirror image. The horizontal groups become vertical, so groups that came towards you are now drawn going away. Lifting it out of the plane and turning it over also gives the enantiomer. Right? Next, exchanging any two groups on the chiral carbon gives the enantiomer too. But exchange twice, and you get the original back, because the first swap flips it and the second swap flips it back. Got it?

2:35Assigning R or S

Now the main skill: R or S, step by step. Step one: rank the four groups, one to four, by the sequence rules from chapter three, so group one is the highest and group four the lowest. Okay? Step two: view the molecule with group four, the lowest, pointing away from you. Clear? Step three: follow the arrows on screen, from group one, to two, to three. If it runs clockwise, the configuration is R, for rectus. If it runs anticlockwise, it is S, for sinister. With me? In a Fischer projection, it is really simple. If group four is on a vertical line, it already points away from you, so just read one, two, three directly. Right? But what if group four, usually hydrogen, sits on a horizontal line? Which way is it pointing now? Towards you, yes, exactly the wrong way! So read one, two, three as usual, then reverse the answer: clockwise gives S, and anticlockwise gives R, okay? Forgetting this reversal is a very common mistake. Got it?

3:45D and L: compare with glyceraldehyde

Now the second system, the D and L system. It compares a compound with glyceraldehyde. Draw the Fischer projection with C one at the top, and look at the lowest chiral carbon, the one farthest from C one. Okay? If the O H on that carbon, or the N H two in amino acids, is on the right, the compound belongs to the D series; if on the left, to the L series. The dash before each group is its bond. In our figure, glyceraldehyde has its O H on the right, so it is D glyceraldehyde. Right? And remember, natural sugars belong to the D series, and natural amino acids to the L series. So alanine and cysteine from our puzzle, both natural amino acids, are in the L series. Clear? Now let me ask you something. Does D mean the compound is dextrorotatory, turning plane polarised light to the right? No, and many students get this wrong. D, L, R and S describe the configuration, while the sign of rotation is measured. Got it?

4:49Example 1: R or S for 2-chlorobutane in a Fischer projection

Now, let's try one. Two chloro butane is drawn with C H three at the top, C two H five at the bottom, chlorine on the right, and hydrogen on the left. Is this R, or is it S? Pause the video and try it first. Done? Okay, let's check. Chlorine has the highest atomic number, so it is group one. Next, C two H five, the ethyl group, beats C H three, the methyl group: carbon, hydrogen, hydrogen beats three hydrogens. Hydrogen is last. The greater than signs show this order. Clear? Now trace the path. Chlorine on the right, to ethyl at the bottom, to methyl at the top. On a clock, that is three o'clock, to six, to twelve, going clockwise. Right? So it runs clockwise, and the answer is R, isn't it? Oh no, careful, look where the hydrogen is! It is on the left, on a horizontal line, so it points towards you. So we reverse, and this two chloro butane is S, okay?

5:56Example 2: why L-alanine is S but L-cysteine is R

Now, our puzzle from the start. L alanine has COOH at the top, C H three at the bottom, N H two on the left, and hydrogen on the right. L cysteine is the same, but with C H two S H in place of C H three. Find R or S for each. Pause the video and try it first. Done? Okay, alanine first. Nitrogen beats carbon, so N H two is group one. COOH, with oxygen, oxygen, oxygen, beats C H three, and hydrogen is last. Clear? The path goes from left, to top, to bottom: nine o'clock, to twelve, to six, which is clockwise. But hydrogen is on a horizontal line, so we reverse. So L alanine is S, just as the puzzle said. Right? Now cysteine. COOH has three oxygens, and C H two S H has one sulphur and two hydrogens. So COOH still wins, doesn't it? No, no, wait, compare the highest atoms first! Sulphur has a higher atomic number than oxygen, so C H two S H wins at the first point of difference. The brackets on screen list the atoms attached. Got it? So the new order is N H two, then C H two S H, then COOH, then hydrogen. Now the path is left, to bottom, to top: nine o'clock, to six, to twelve. That is anticlockwise. With me? Hydrogen is horizontal again, so we reverse: L cysteine is R, while L alanine is S, opposite letters! Puzzle solved: the drawing did not change, only the priorities did. D and L compare the drawing with glyceraldehyde, but R and S come from priorities. Two different systems, clear?

7:57Quick revision: Fischer rules, R or S reversal, D or L

So, quick revision. In a Fischer projection, horizontal bonds come towards you, vertical bonds go away. For R or S, trace one, two, three, and if group four is horizontal, reverse it. For D or L, look at the O H or N H two on the lowest chiral carbon: on the right it is D, on the left it is L, okay? Now go and try the practice problems on this page. All the best!