Chemistry · General chemistry II · Worked example

Find R or S in L-alanine

L-alanine, $H_{2} N - CH ({CH}_{3}) - COOH$ , is the form found in proteins. It is drawn with ${CH}_{3}$ at the upper left on a solid wedge, $COOH$ at the upper right and ${NH}_{2}$ straight down. The hydrogen is not drawn. Is its stereocenter R or S?

Find the four groups

The center carries ${NH}_{2}$ , $COOH$ , ${CH}_{3}$ and H.

Rank by the bonded atom

The bonded atoms are N, C, C and H. Nitrogen outranks carbon, so ${NH}_{2}$ is $1$ and hydrogen is $4$ . $COOH$ and ${CH}_{3}$ both start with carbon, so they tie.

Break the tie one atom out

The $COOH$ carbon holds three oxygens: its double bond to oxygen counts twice, and the $OH$ adds a third. The ${CH}_{3}$ carbon holds three hydrogens. Oxygen beats hydrogen, so $COOH$ is $2$ and ${CH}_{3}$ is $3$ .

Read the path from the far side

The ${CH}_{3}$ is on a solid wedge, so the hydrogen points away from you, behind the page, as the rule asks. Trace ${NH}_{2} (down)$ $\to COOH (upper right)$ $\to {CH}_{3} (upper left)$ . That path is counterclockwise, so the center is S.

(S)-alanine

Ask for the name

Chalk labels the center S. Ask for the molecule’s name and it answers (2S)-2-aminopropanoic acid, also known as L-alanine. The $- COOH$ group ranks above $- {NH}_{2}$ in naming, so it gives the name its ending and its carbon is carbon 1. The $- {NH}_{2}$ group becomes the prefix amino on carbon 2, the stereocenter, which is why the S carries a $2$ . The label does not depend on the name: it comes from the four groups and the wedge.

(2S)-2-aminopropanoic acid

Result

S. L-alanine is (S)-alanine, (2S)-2-aminopropanoic acid. D-alanine, its mirror image, is (R).

Your turn

1. One like the example

L-serine, $H_{2} N - CH ({CH}_{2} OH) - COOH$ , is drawn as L-alanine is in the example, with ${CH}_{2} OH$ in place of ${CH}_{3}$ at the upper left on a solid wedge. Is its stereocenter R or S?

Show the answer and explanation

S. L-serine is (2S)-2-amino-3-hydroxypropanoic acid.

${NH}_{2}$ is $1$ and H is $4$ . $COOH$ carries three oxygens and ${CH}_{2} OH$ carries (oxygen, H, H), so $COOH$ is $2$ and ${CH}_{2} OH$ is $3$ , the same order as in alanine. The groups sit where alanine’s did, so the path runs counterclockwise.

2. A variation

Write the full systematic name of D-alanine, the mirror image of L-alanine, with its descriptor.

Show the answer and explanation

(2R)-2-aminopropanoic acid.

A mirror image inverts every stereocenter, so the S of L-alanine becomes R. The rest of the name is unchanged, because the atoms and their connections are the same.

3. Somewhere new

L-cysteine, $H_{2} N - CH ({CH}_{2} SH) - COOH$ , is drawn as L-alanine is in the example, with ${CH}_{2} SH$ in place of ${CH}_{3}$ . Is its stereocenter R or S?

Show the answer and explanation

R, although it is an L-amino acid like alanine.

${CH}_{2} SH$ and $COOH$ both start with carbon. One bond out, ${CH}_{2} SH$ carries (sulfur, H, H) and $COOH$ carries three oxygens. Sulfur, $16$ , beats oxygen, $8$ , at the first comparison, so ${CH}_{2} SH$ is $2$ and $COOH$ is $3$ . Swapping two ranks reverses the path, so the center is R.

Keep exploring

The Import L-alanine button draws this molecule in the Skeletal tool, with a wedge at its center and its name as the target. Chalk may turn it differently from the description, and the letter stays the same. Choose Analyze → Stereochemistry to see an S beside the center. Analyze → Name answers (2S)-2-aminopropanoic acid and lists how the name is built, ending with the R or S step. Change the solid wedge to a hashed one and the name becomes (2R)-2-aminopropanoic acid, which Chalk tells you is the mirror image of L-alanine. Practice $1$ and Practice $2$ import their drawings without a name, for your own name to be checked.

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Sources and scope

Authored study material. Tool results depend on the stated inputs and model assumptions.

After reading

Study it, then try it

Practice the key terms as flashcards, then open the example in its tool, change a value and keep a useful result on your board.

Study the 5 key terms Draw L-alanine and read its center Import L-alanine and check its name Practice 1: import the drawing and check your name Practice 2: import the mirror image and check your name Open worked example on a board Stereochemistry rules in Chemistry Reference

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