Chemistry · General chemistry II · Worked example

Find R or S in butan-2-ol

Carbon 2 of butan-2-ol is drawn with the ${CH}_{3}$ group straight up on a solid wedge, the ${CH}_{2} {CH}_{3}$ group down to the left and the $OH$ group down to the right. The hydrogen is not drawn. Is this (2R)- or (2S)-butan-2-ol?

Find the stereocenter

Carbon 2 carries H, $OH$ , ${CH}_{3}$ and ${CH}_{2} {CH}_{3}$ , four different groups. Carbons 1, 3 and 4 each carry at least two hydrogens, so carbon 2 is the only stereocenter.

Rank by the bonded atom

The bonded atoms are oxygen, carbon, carbon and hydrogen. Oxygen has the highest atomic number, $8$ , so $OH$ is $1$ , and hydrogen, at $1$ , is $4$ . The ethyl and methyl groups both start with carbon, so they tie.

Break the tie one atom out

The ${CH}_{2}$ of the ethyl group carries (C, H, H) and the methyl carbon carries (H, H, H). Carbon beats hydrogen at the first difference, so ethyl is $2$ and methyl is $3$ .

Read the path from the far side

The hydrogen is not drawn, so it points away from the solid wedge, behind the page. That is already the view the rule asks for. Trace $OH (lower right)$ $\to {CH}_{2} {CH}_{3} (lower left)$ $\to {CH}_{3} (top)$ . The path runs clockwise, so the center is R.

Write the name

The letter goes in parentheses before the name, with the number of its carbon. Change the solid wedge to a hashed one and the hydrogen comes toward you. The same path still looks clockwise, but from the wrong side, so it reverses. That drawing is the mirror image, (2S)-butan-2-ol.

(2R)-butan-2-ol (2S)-butan-2-ol

\begin{array}{c} (2R)-butan-2-ol \\ (2S)-butan-2-ol \end{array}

Result

(2R)-butan-2-ol. Its mirror image, with the ${CH}_{3}$ on a hashed wedge, is (2S)-butan-2-ol.

Your turn

1. One like the example

Carbon 2 of 2-bromobutane is drawn as the example is, with the ${CH}_{3}$ straight up on a solid wedge, the ${CH}_{2} {CH}_{3}$ down to the left and the $Br$ down to the right. The hydrogen is not drawn. Is it R or S?

Show the answer and explanation

R. The drawing is (2R)-2-bromobutane.

Bromine, atomic number $35$ , is $1$ . Ethyl beats methyl one atom out, (C, H, H) against (H, H, H), so ethyl is $2$ and methyl $3$ . With the hydrogen behind the page, $Br \to {CH}_{2} {CH}_{3} \to {CH}_{3}$ runs clockwise.

2. A variation

Rank the four groups on the stereocenter of glyceraldehyde, ${HOCH}_{2} - CH (OH) - CHO$ .

Show the answer and explanation

$OH$ , then $CHO$ , then ${CH}_{2} OH$ , then H.

Oxygen outranks carbon, so $OH$ is $1$ and hydrogen is $4$ . $CHO$ and ${CH}_{2} OH$ both start with carbon. The $CHO$ carbon counts its double-bonded oxygen twice, (oxygen, oxygen, H) against (oxygen, H, H), and wins at the second comparison.

3. Somewhere new

A stereocenter is drawn with its hydrogen on a solid wedge, pointing toward you. The path $1 \to 2 \to 3$ around the other three groups runs clockwise on the page. Is the center R or S?

Show the answer and explanation

S.

The rule reads the path with group 4 pointing away. Here it points toward you, so the page shows the view from the wrong side, and the true direction is the reverse of what you see. Clockwise as drawn means counterclockwise, S.

Keep exploring

The Draw (2R)-butan-2-ol button draws this molecule in the Skeletal tool, with a wedge at carbon 2 and its name as the target. Chalk may turn it differently from the description, and turning it never changes the letter. Choose Analyze → Stereochemistry and Chalk lists the ranking and puts an R beside carbon 2. Select the wedged bond and choose ▱ (Hashed wedge), and the R becomes S. Practice $1$ imports the 2-bromobutane drawing without a name, for your own name and letter to be checked.

Return to the concept →

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 (2R)-butan-2-ol Draw L-alanine and read its center See (2R)-butan-2-ol in every representation Practice 1: import the drawing and check your name Open worked example on a board Stereochemistry rules in Chemistry Reference

Your existing work stays on this device. Examples open as editable copies.

Find the stereocenter

Rank by the bonded atom

Break the tie one atom out

Read the path from the far side

Write the name

Result

Your turn

1. One like the example

R. The drawing is (2R)-2-bromobutane.

2. A variation

OHOH, then CHOCHO, then CH2OHCH2​OH, then H.

3. Somewhere new

S.

Keep exploring

Study it, then try it

$OH$ , then $CHO$ , then ${CH}_{2} OH$ , then H.