3.1.5Organic Chemistry
Stereoisomerism — Edexcel AS Chemistry Revision Notes & Topical Questions
5 min read 5 parts Lesson 18 of 42
Curriculum Equation Key معادلة تفاعل تكوين الإستر (الأسترة) - Esterification Reaction EquationFast Exam Solution
R-COOH + R2-OH ⇌ (conc. H₂SO₄) R-COO-R2 + H₂O
Word Equation & Conditions: حمض كربوكسيلي + كحول (في وجود حمض الكبريتيك المركز كعامل حفاز ونازع للماء) ⇌ إستر + ماء
Location & Specifications: الكيمياء العضوية للثانوية العامة والتحصيلي و Edexcel Chemistry.
Part 1 of 5Stereoisomers and the C=C Bond
Definition
- Definition Stereoisomers have the same structural formula but a different arrangement of the atoms in space.
Restricted rotation
- Restricted rotation The π bond in a C=C bond stops the two carbon atoms from rotating freely about the double bond.
Fixed positions
- Fixed positions Groups on each carbon are therefore locked above or below the plane of the double bond.
Condition
- Condition E/Z isomerism needs two different groups on each carbon of the double bond.
No isomerism
- No isomerism But-1-ene has two hydrogens on one carbon of the double bond, so it has no E/Z forms.
Part 2 of 5Assigning E and Z
1
Look at the two groups on one carbon of the double bond and rank them by the atomic number of the atom directly attached.
2
If the atoms attached are the same, look at the atoms attached to them in turn, going outward until a difference appears.
3
Repeat for the other carbon of the double bond to find its higher-priority group.
4
If the two higher-priority groups are on the same side of the double bond, the isomer is Z (from the German zusammen).
5
If the two higher-priority groups are on opposite sides, the isomer is E (from the German entgegen).
Part 3 of 5Examples
Examples
Z-but-2-ene
- Z-but-2-ene Both CH₃ groups are on the same side of the double bond, so this isomer is Z.
E-but-2-ene
- E-but-2-ene The CH₃ groups are on opposite sides of the double bond, so this isomer is E.
Bromochloroethene
- Bromochloroethene In CHBr=CHCl, bromine outranks hydrogen and chlorine outranks hydrogen, so Br and Cl decide the label.
Cis and trans
- Cis and trans Cis-trans labels can be used when each carbon carries the same group as the other, but E/Z works for every case.
Properties
- Properties E and Z isomers differ in shape, so they have different melting points, boiling points and sometimes polarities.
| Higher-priority groups | Label |
|---|---|
| Same side | Z |
| Opposite sides | E |
Part 4 of 5Worked Example
Question Name the geometrical isomer of 1-bromo-2-chloroethene in which the bromine and chlorine are on the same side of the C=C bond.
- On carbon 1 the groups are Br and H;
- Br has the higher atomic number (35 compared with 1), so Br has priority.
- On carbon 2 the groups are Cl and H; Cl (17) has priority over H (1).
- Br and Cl are on the same side of the double bond.
- Higher-priority groups on the same side means Z.
Answer Z-1-bromo-2-chloroethene
Part 5 of 5Exam Checklist & Tips
3.1.5 – Key Points for the Exam
- Stereoisomers Isomers with the same structural formula but different arrangements of atoms in space.
- E/Z isomerism Stereoisomerism caused by restricted rotation about a C=C bond with two different groups on each carbon.
- Priority Rank of a group set by the atomic number of the atoms attached to the double-bond carbon.
- Restricted rotation Inability to rotate freely about a double bond because of the π bond.
- Z isomer The isomer with higher-priority groups on the same side of the double bond.
Common Mistakes
- Assuming that every alkene shows E/Z isomerism, when each carbon must carry two different groups.
- Ranking by the size of a group instead of the atomic number of the atom attached to the double bond.
- Confusing Z with the opposite sidesZ means zusammen (same side).
- Using cis and trans for compounds where the two atoms on each carbon are not the same.
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