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To assign R or S, rank the groups by CIP priority breaking ties atom by atom, view with the lowest priority away, and read 1 → 2 → 3: clockwise R, anticlockwise S; viewed from the wrong side the letter reverses, and swapping two groups flips it.
Paper packet. Every task here also exists on screen, where it is checked automatically; answers written on paper are not assessed by Nydus. When you are back at a device, enter your answers there.
You will rank groups including ties, assign R or S from a stated view, break ties by the first point of difference, give priorities in a real molecule, and follow the letter through swaps.
You can find a stereocentre and rank groups by atomic number. R and S finish the job: which of the two mirror arrangements this stereocentre has.
CIP priority ranks groups 1 (highest) to 4. The first point of difference is the first place two groups' attached atoms differ. R (rectus) and S (sinister) name the two arrangements. A duplicate atom: a double bond counts its partner twice.
1. Rank the four groups. By atomic number of the attached atom. If two tie, list the three atoms attached to each, highest first, and compare: the first difference decides. A double bond counts its partner twice, so –CHO carries (O, O, H).
2. Look with priority 4 pointing away from you.
3. Read 1 → 2 → 3. Clockwise: R. Anticlockwise: S.
If you can only see the molecule with priority 4 pointing towards you, read the direction as usual and then reverse the letter, because you are looking from the wrong side.
Another way: table
Reading the letter.
| Priority 4 points | 1 → 2 → 3 runs | Configuration |
|---|---|---|
| away | clockwise | R |
| away | anticlockwise | S |
| towards you | clockwise | S |
| towards you | anticlockwise | R |
Swapping any two groups on a stereocentre gives its mirror image, so the letter flips; two swaps restore it. That gives a reliable check: if the lowest group is in an awkward place, swap it with whatever is at the back, assign, then flip the letter to undo the swap. And remember what R and S are not: they come from the structure by rule, while the direction a compound rotates light, (+) or (−), must be measured.
R always means a molecule rotates light to the right. R is from the structure; rotation is measured.
Read the direction with priority 4 towards you. Then reverse the letter.
Break ties by the size of the group. Break them atom by atom.
Swapping two groups keeps the configuration. It flips it.
Priorities: Br 1, Cl 2, F 3, H 4.
By atomic number.
Viewed with H away, Br → Cl → F runs anticlockwise.
Look with 4 at the back.
So this arrangement is (S)-bromochlorofluoromethane.
Anticlockwise is S.
What would clockwise mean with H away?
R.
What does viewing from the wrong side do?
It reverses the letter: the centre is S.
Rank the four groups on carbon 2 of butan-2-ol, CH3CH(OH)CH2CH3, by CIP priority, highest first.
Number the steps in order (write the number in the box):
You look at a stereocentre with its lowest-priority group pointing towards you, and the groups ranked 1 → 2 → 3 run clockwise. Is it R or S?
Each pair of groups ties at the first atom. Match each pair to the group that wins.
| –CH2OH wins | –CH2CH3 wins | –CHO wins | –COOH wins | |
|---|---|---|---|---|
| –CH2OH against –CH(CH3)2 | ||||
| –CH2CH3 against –CH3 | ||||
| –CHO against –CH2OH | ||||
| –COOH against –CHO |
Give the CIP priority, 1 (highest) to 4, of each group on the stereocentre of lactic acid, CH3CH(OH)COOH.
| priority | |
|---|---|
| –OH | |
| –COOH | |
| –CH3 | |
| –H |
A model of (R)-butan-2-ol is taken apart and rebuilt with its OH and H groups swapped, all else unchanged. What is the rebuilt model?
Lesson test: one question per skill, one attempt each, no hints. Your answers are checked when you submit.
A stereocentre starts as R. A student swaps pairs of its groups $1$ time(s), one pair at a time. What is its configuration now?
You can assign R or S to a stereocentre. Tell someone what to do when the lowest-priority group points towards them. Next: enantiomers, the mirror-image pairs R and S describe.
8. Your turn: H points towards you and 1 → 2 → 3 runs clockwise, step 3