# 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 projection of threonine. How many other possible stereoisomers of threonine are there?

##### 1 Answer

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

**Step 3.** Attach atoms to the bonds.

Arbitrarily put

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

**Step 4.** Determine the configuration at

The order of priority of the functional groups is

The sequence *R*).

But the *R*).

We want (*S*), so we must must interchange

**Step 4.** Determine the configuration of

The order of priority of the functional groups is

The sequence *S*).

But the *R*). That's correct!

The Fischer projection of threonine is therefore

The number of possible stereoisomers is

Since threonine has 2 stereocentres, there are

So there are **three** other stereoisomers of threonine.