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E/Z and R/S come from ranking groups by atomic number: Z puts the higher-priority groups on the same side of a double bond and E on opposite sides; R and S describe the direction of priorities at a stereocentre and say nothing about optical rotation.
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 by CIP priority, assign E or Z from a described alkene, say what each descriptor means, count stereoisomers, and keep R/S apart from optical rotation.
A name so far says which atoms are joined to which. Stereoisomers have the same connections and differ in their arrangement in space, so the name needs one more piece: a letter.
Stereoisomers have the same connectivity and a different 3D arrangement. Cahn–Ingold–Prelog (CIP) priority ranks groups by the atomic number of the atom attached. E and Z describe a double bond; R and S a stereocentre. A stereocentre is a carbon with four different groups.
Rank by atomic number. Look at the atom directly attached: Br (35) > Cl (17) > OH (8) > NH2 (7) > CH3 (6) > H (1). If two attached atoms tie, move out one atom and compare the next set.
E/Z at a double bond. On each carbon of the C=C, pick the higher-priority group. If the two winners are on the same side, the alkene is Z (German zusammen, together); opposite sides, E (entgegen, opposite).
R/S at a stereocentre. Rank the four groups 1 to 4. View the molecule with group 4 pointing away from you. If 1→2→3 runs clockwise, the centre is R (rectus); anticlockwise, S (sinister). The full procedure is lesson 27; here, the point is what the letter tells you.
Another way: steps
To read E/Z:
E and Z replace cis and trans, which only work when each carbon has one hydrogen. In an alkene where carbon 1 carries OH and CH3, the reference group is OH, not a hydrogen, and 'cis' would be ambiguous.
R and S are assigned from the structure. The direction a sample rotates plane-polarized light, written (+) or (−), is measured, and nothing links the two: (S)-alanine is (+), (S)-lactic acid is (+), (S)-glyceraldehyde is (−). None of the four letters is a charge.
Each stereocentre doubles the possible arrangements, so a molecule with n stereocentres can have up to 2 to the power n stereoisomers, each needing its own set of letters.
E/Z labels give a molecule's charge. They give its geometry.
R means it rotates light to the right. Rotation is measured, not named.
Z always means the hydrogens are on the same side. It means the higher-priority groups are.
Priority is by group size. It is by atomic number of the attached atom.
Carbon 1 carries Br and H: Br wins with 35.
Atomic number decides.
Carbon 2 carries CH3 and H: CH3 wins with 6.
One winner per carbon.
Br and CH3 are on the same side, so it is (Z)-1-bromoprop-1-ene.
Same side, Z.
Which group wins on carbon 2?
F (9) beats CH3 (6).
Where are Cl and F?
Opposite sides, since CH3 is on Cl's side: E.
Rank these groups by Cahn–Ingold–Prelog priority, highest first. (Atomic numbers: H 1, C 6, N 7, O 8, Cl 17, Br 35.)
Number the steps in order (write the number in the box):
In an alkene, carbon 1 carries OH and CH3; carbon 2 carries CH2CH3 and H; the CH3 and the CH2CH3 are on the same side. (Atomic numbers: H 1, C 6, N 7, O 8, F 9, Cl 17, Br 35.) Is it E or Z?
Match each descriptor to what it says about a structure.
| higher-priority groups on the same side of a double bond | higher-priority groups on opposite sides of a double bond | priorities run clockwise with the lowest pointing away | priorities run anticlockwise with the lowest pointing away | |
|---|---|---|---|---|
| Z | ||||
| E | ||||
| R | ||||
| S |
A molecule has $4$ stereocentre(s) and no internal mirror symmetry. How many stereoisomers can it have, and so how many different R/S names does it need?
Answer:
A pharmacist reads that a drug is the (S) form and asks whether it must therefore rotate polarized light to the left. What is the right answer?
Lesson test: one question per skill, one attempt each, no hints. Your answers are checked when you submit.
Give E or Z for each alkene. (Atomic numbers: H 1, C 6, O 8, Cl 17, Br 35.)
| E or Z | |
|---|---|
| 1: C1 has Br and H, C2 has CH3 and H; Br and CH3 on the same side | |
| 2: C1 has Cl and H, C2 has CH2CH3 and H; Cl and CH2CH3 on opposite sides | |
| 3: C1 has OH and CH3, C2 has Br and H; OH and Br on the same side |
You can read the geometry a stereochemical letter carries. Tell someone why an (S) drug need not rotate light to the left. Next: using a name to check a structure.
8. Your turn: carbon 1 has Cl and H, carbon 2 has F and CH3; Cl and CH3 on the same side, step 3