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QUESTION

# The systematic (IUPAC) name for the amino acid threonine is (2S,3R)-2-amino-3-hydroxybutanoic acid. The systematic name indicates that threonine has two stereocenters, at positions 2 and 3, with S and R configurations, respectively. What is ths the Fischer...

Here's how I would draw the Fischer projection of threonine.

Step 1. Draw the structural formula of threonine.

Step 2. Draw the longest continuous chain of carbon atoms vertically, with "COOH" at the top. Draw horizontal lines to make crosses at "C-2" and "C-3".

Step 3. Attach atoms to the bonds.

Arbitrarily put "NH"_2 on the right of "C-2" and "OH" on the right of "C-3". Put "H" atoms on the left of "C-2" and "C-3".

I have a 25 % chance of being right. If I am wrong, I will simply reverse the locations of the "H" atoms.

Step 4. Determine the configuration at "C-2".

The order of priority of the functional groups is "NH"_2 = 1; "COOH" = 2; "C-3 = 3"; H = 4.

The sequence "NH"_2 → "COOH" → "C-3" is clockwise (R).

But the "H" atom is pointing forward, so we must reverse the assignment to (R).

We want (S), so we must must interchange "H" and "NH"_2.

Step 4. Determine the configuration of "C-3".

The order of priority of the functional groups is "OH = 1"; "C-2 = 2"; "CH"_3 = 3; "H" = 4.

The sequence "OH → C-2 → CH"_3 is counterclockwise (S).

But the "H" atom is pointing forward, so we must reverse the assignment to (R). That's correct!

The Fischer projection of threonine is therefore

The number of possible stereoisomers is 2^n, where n is the number of stereocentres.

Since threonine has 2 stereocentres, there are 2^n = 4 stereoisomers.

So there are three other stereoisomers of threonine.