E/Z Isomerism Lecture

13 question-and-answer flashcards covering the definition, causes, rules, determination, examples, and significance of E/Z isomerism.

What is E/Z isomerism?

A type of stereoisomerism in alkenes caused by restricted rotation about a C=C double bond, giving two different spatial arrangements of the same atoms.

Why does E/Z isomerism occur in alkenes?

Because the π bond in a carbon–carbon double bond prevents rotation, locking attached groups into fixed positions and allowing different spatial arrangements.

What two conditions must be met for a molecule to exhibit E/Z isomerism?

1) It must contain a C=C double bond. 2) Each carbon of that double bond must be attached to two different groups.

What do the letters “E” and “Z” stand for, and what do they indicate?

E (Entgegen, “opposite”) means the highest-priority groups are on opposite sides of the double bond; Z (Zusammen, “together”) means the highest-priority groups are on the same side.

What is the Cahn–Ingold–Prelog (CIP) priority rule?

A set of rules for ranking the two groups on each carbon of a C=C bond: the atom with the higher atomic number gets higher priority; if the directly attached atoms are identical, look outward until a difference is found; multiple bonds are treated as if each bond is to a separate atom.

How are multiple bonds treated when applying the CIP rules?

A multiple bond is counted as if the atom is bonded to the same atom multiple times (e.g., a C=O is treated as C–O and C–O when assigning priorities).

What is the step-by-step procedure for deciding whether an alkene is E or Z?

1) Assign priorities to the two substituents on each carbon of the double bond using CIP rules. 2) Compare the positions of the two highest-priority groups: if they are on the same side, the alkene is Z; if on opposite sides, it is E.

Give an example of a compound that shows E/Z isomerism and describe its two forms.

But-2-ene (CH₃CH=CHCH₃) has Z-but-2-ene with both CH₃ groups on the same side and E-but-2-ene with the CH₃ groups on opposite sides.

How does E/Z isomerism differ from cis/trans isomerism?

Cis/trans naming applies only when each carbon of the double bond has one hydrogen and the other two groups are identical; E/Z is a general system that works when all four groups are different.

Can a molecule with identical groups across a double bond still show stereoisomerism?

Yes; cis/trans notation can be used when two identical groups are present, while E/Z notation is required when all four groups are different.

Do E and Z isomers have different physical properties?

Often yes. They can differ in boiling point, melting point, solubility, and dipole moment because of their different spatial arrangements.

Do E and Z isomers always react in the same way chemically?

Frequently they show similar reactivity, but in certain biological or stereospecific reactions one isomer can be more reactive or preferred.

Why is understanding E/Z isomerism important in organic chemistry?

It explains how compounds with the same molecular formula can differ in shape, physical properties, and biological activity—crucial considerations in drug design, materials science, and biochemistry.