The amino acid threonine, (2 S, 3 R)-2...

The amino acid threonine, $(2 S, 3 R)-2$ -amino-3-hydroxybutanoic acid, has two chirality centers.
(a) Draw threonine, using normal, wedged, and dashed lines to show dimensionality.
(b) Draw a diastereomer of threonine, and label its chirality centers as $R$ or $S$.

Key Concepts

Chirality

Chirality refers to a property of a molecule where it is not superimposable on its mirror image. In organic compounds, particularly in amino acids, chirality typically arises when an atom, usually carbon, is bonded to four different groups, leading to non-identical configurations that can have important biological implications.

Stereocenters and R/S Configuration

Stereocenters or chiral centers are atoms at which the interchange of two groups produces a stereoisomer. The absolute configuration of these centers is assigned using the Cahn-Ingold-Prelog (CIP) priority rules, which label the center as either R (rectus, right) or S (sinister, left) based on the spatial arrangement of the substituents.

Diastereomers

Diastereomers are stereoisomers that are not mirror images of each other. They occur in molecules with more than one stereocenter and have different physical and chemical properties. Unlike enantiomers (which are non-superimposable mirror images), diastereomers can have different reactivity and behavior, making their identification and characterization important in stereochemistry.

Wedge and Dashed Bond Representation

Wedge and dashed bond notations are used in structural drawings to represent the three-dimensional geometry of molecules on a two-dimensional medium. Solid wedges indicate bonds coming out of the plane towards the observer, dashed lines denote bonds going behind the plane, and normal lines represent bonds in the plane of the paper. This method is essential for depicting chiral centers and understanding molecular stereochemistry.

Transcript

00:01 All right, so the molecule that's shown here is 3a, which is one of the 20 alpha amino acids.

00:07 All right, so it's asking us to draw a fisher projection.

00:11 So the way we start a fisher projection is we basically start with carbon 1 at top.

00:15 At this point, this carbon right here attached to the carbonylic acid, if it's that one right there.

00:23 That's going to be carbon 1.

00:25 All right, so i'm going to start with just c -o -o -h.

00:27 So that's my entire carbonylic acid.

00:29 I'm going to draw my line straight down, and carbon 4, the very last carbon will be ch3.

00:36 Now, i have two other carbons in the middle of the molecule, so i draw two lines straight through.

00:40 And that kind of represents all four of my carbons.

00:44 Okay, so it's specified that the chirality at the second carbon is the s configuration.

00:52 And in order to have the s configuration, that means we need our hydrogen here and our amid group on this side.

00:59 You'll notice that if i write out kind of the priorities, that goes one, two, three, four, like that.

01:06 Right.

01:07 So now normally when i'm determining r and s, that would just be, oh, well, that's clockwise, so that must be r.

01:13 Now, this is a little different because you can remember that if i draw a fisher projection, the projection itself looks something like this, right? where the things going left and right are on wedges and the things going up and down are on dashes.

01:28 So remember that when we're determining absolute configuration or chirality using the r &s, that our lowest priority needs to be on a wedge...

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