Properties of E and Z Isomers

Introduction

E and Z isomers have the same atoms joined in the same order, yet they are genuinely different compounds with different physical constants. Because rotation about the double bond is blocked at ordinary temperatures, each isomer keeps its shape, and that shape controls how polar the molecule is, how molecules pack in a crystal and how much internal strain they carry. This page explains those differences and the data that reveal them.

Core explanation

Dipole moments. A molecule's dipole moment is the vector sum of its individual bond dipoles. In an E isomer with two identical polar groups, such as (E)-1,2-dichloroethene, the two C–Cl bond dipoles point in opposite directions and cancel, so the molecule has zero dipole moment. In the Z isomer they point partly the same way and add, giving a dipole moment of about 1.9 D. Even for hydrocarbons the effect shows up: (Z)-but-2-ene has a small dipole (about 0.3 D) because the weakly electron-donating methyl groups are on one side, while (E)-but-2-ene has none.

Boiling points. Boiling depends on intermolecular forces in the liquid. A Z isomer with a net dipole has permanent dipole–dipole attractions in addition to London forces, so it usually boils higher.

Compound Z isomer bp E isomer bp --- --- --- 1,2-dichloroethene about 60 °C about 48 °C but-2-ene about 4 °C about 1 °C

Melting points. Melting depends on how well molecules pack in the solid, not just on polarity. E isomers are usually more symmetrical and more linear, so they stack neatly into a crystal lattice. Z isomers have a bent "U" shape that packs poorly. As a result the E isomer usually melts higher , even though it often boils lower. (E)-but-2-ene melts at about −106 °C, while (Z)-but-2-ene melts at about −139 °C.

Stability. In a Z alkene, groups on the same side of the double bond are held close together. If they are larger than hydrogen, their electron clouds repel, creating steric strain . The E isomer avoids this, so it is usually more stable. Heats of hydrogenation measure this directly: both isomers of but-2-ene give the same product, butane, so any difference in the enthalpy released must come from the starting alkenes. Hydrogenating (Z)-but-2-ene releases about 120 kJ/mol, and (E)-but-2-ene about 116 kJ/mol. The Z isomer releases more energy because it started higher in energy, by about 4 kJ/mol.

Chemical differences. Geometric isomers often react at different rates, and sometimes differently, because the groups sit in different places relative to each other. A reaction that needs two groups to meet can happen easily in a Z isomer but be impossible in the E isomer.

Step-by-step reasoning

To predict the relative properties of a pair of geometric isomers:

1. Draw both isomers with correct geometry. 2. Add bond dipole arrows and decide whether they cancel or add. 3. Larger net dipole usually means the higher boiling point. 4. Judge symmetry and shape: the more symmetrical, straighter isomer usually melts higher. 5. Judge steric crowding on each side: the less crowded isomer is usually more stable.

Visual explanation

Sketch (Z)- and (E)-1,2-dichloroethene with an arrow along each C–Cl bond pointing towards chlorine. In the Z isomer both arrows lean downwards, and adding them gives a resultant pointing down. In the E isomer one arrow points up-right and the other down-left, so they are equal and opposite, and the resultant is zero.

Real-world analogy

Two people pulling on ropes attached to a boat: if they stand on the same bank and pull in roughly the same direction, the boat moves; if they stand on opposite banks and pull equally in opposite directions, the boat stays still. Bond dipoles in E and Z isomers behave the same way.

Real-world example

Maleic acid and fumaric acid are the Z and E isomers of butenedioic acid. Fumaric acid packs efficiently and forms extensive intermolecular hydrogen bonds; it melts (with decomposition) near 287 °C and is only slightly soluble in water. Maleic acid melts near 135 °C and is far more soluble. Fumaric acid is used as a food acidulant and is an intermediate in the citric acid cycle in living cells.

Why?

Why does the E isomer of butenedioic acid melt so much higher than the Z isomer? In maleic acid the two –COOH groups are close enough to hydrogen bond to each other within one molecule, which uses up hydrogen-bonding capacity that could otherwise link neighbouring molecules. Fumaric acid cannot do this, so all its hydrogen bonds are intermolecular, building a strong network.

Common misconception

"The isomer with the higher boiling point must also have the higher melting point." Boiling reflects attractions in a disordered liquid, while melting also depends on crystal packing. Z isomers are often more polar but pack badly, so they commonly boil higher and melt lower.

Worked example

Question: Heats of hydrogenation of the two but-2-ene isomers are −120 kJ/mol (Z) and −116 kJ/mol (E). Which is more stable, and by how much?

Reasoning: Both reactions give butane, so they end at the same energy level. The Z isomer releases 4 kJ/mol more energy, so it must have started 4 kJ/mol higher in enthalpy.

Answer: (E)-but-2-ene is more stable, by about 4 kJ/mol, reflecting steric strain between the methyl groups in the Z isomer.

Quick check

1. Which isomer of 1,2-dichloroethene has zero dipole moment, and why? Answer: The E isomer, because its two C–Cl bond dipoles point in opposite directions and cancel exactly.

Exam focus

Be ready to explain boiling point differences using dipole moments and melting point differences using packing and symmetry — two separate arguments. For stability questions, use heats of hydrogenation or combustion and remember that a larger energy release means a less stable starting material, provided the products are identical.

Advanced insight

Steric reasoning is not universal. For 1,2-dichloroethene the Z isomer is slightly more stable than the E isomer, an exception called the "cis effect". It is usually explained by favourable orbital interactions between the halogen lone pairs and the C–Cl antibonding orbitals, which outweigh the small repulsion between the chlorine atoms.

Summary

E and Z isomers differ in physical and chemical properties. When polar groups are on the same side, the Z isomer has a net dipole and usually boils higher. The more symmetrical E isomer usually packs better and melts higher. Steric strain usually makes the E isomer more stable, as shown by its smaller heat of hydrogenation, though exceptions exist.

Practice questions

1. Explain why (Z)-but-2-ene boils slightly higher than (E)-but-2-ene. Answer: The Z isomer has a small net dipole moment, adding weak dipole–dipole attractions to London forces; the E isomer's dipoles cancel. 2. Why does (E)-but-2-ene melt at a higher temperature than (Z)-but-2-ene? Answer: Its more symmetrical, straighter shape packs more efficiently in the crystal, so more energy is needed to break up the lattice. 3. Two alkenes give the same alkane on hydrogenation. Alkene A releases 127 kJ/mol and alkene B releases 119 kJ/mol. Which is more stable? Answer: Alkene B, by about 8 kJ/mol, because it releases less energy on forming the same product. 4. Suggest why maleic acid is much more soluble in water than fumaric acid. Answer: Maleic acid's crystal lattice is held together less strongly, partly because of intramolecular hydrogen bonding and poorer packing, so it is easier for water to separate the molecules.