What is a geometric isomer?

Geometric isomers are cis-trans isomerism, or EZ isomerism. Their actions are based on limited rotation of double or single carbon bonds in cyclic compounds. In a cyclic connection, the rotation between the carbon single bond is limited and two different groups are attached to each carbon isomerism in a similar way. Such geometric isomers often differ in their physical properties. This is due to the shape of the isomers and the total dipole moment. If the two atoms with the highest priority remain on the same side of the isomer, they are denoted as Z, and if they are in the opposite side, E.

Brief History of Isomerism

The concept of constitutional isomerism is a significant step forward in the history of modern chemistry, and especially in the development of organic chemistry. By the end of the 1700s, several pure substances had been isolated by studying "animal" and "plant" chemistry. Many were received by Karl Wilhelm Scheele (1742-1786).Because of the great diversity of organic compounds, each new substance represented a different elemental composition, which corresponded to a generalized observation of the "mineral" chemistry. The number of isolated organic compounds increased in the early 1800s, through the identification of various substances.

In his history book of chemistry, Thomas Thomson wrote in 1830 that:

Berzelius applied the theory of the atom also to the plant kingdom, after analyzing several plant acids, and showing his atomic constitution, but here a difficulty arises, which we cannot overcome in the present state of our knowledge. There are two acids that consist of exactly the same atoms. Now, how should we account for this striking difference in properties? Undoubtedly, in various ways in which the atoms are located in each of them.

Thomson then used various atomic symbol schemes used at that time to explain why two acids with the same elemental composition, related to geometric isomers, have different physical and chemical properties.

Until the beginning of the XIX century, it was believed that these chemicals found in living organisms have a special vitality,associated with living things, and that they are needed in living systems for reproduction. In 1828, Wohler synthesized a urea sample, (NH2) 2CO (also CH4N2O), which was indistinguishable from urea isolated from biological urine.

He prepared this “animal” substance from a clearly inorganic (mineralogical) starting material of ammonium cyanate, (NH4) NCO (also CH4N2O), which is the result of combining ammonium chloride and silver cyanate. Thus, the barrier between “living” and “inanimate” isomerism collapsed.

Designation of isomers

In the cis-isomer, two such groups remain on the same side of the double bond, while in the trans-isomer they remain in the opposite direction. For example, 2-butene has two cis and trans isomers.

Designation of isomers

In the cis-isomer, two methyl groups and two hydrogen groups remain in the same side of the double bond, while in the trans-isomer they remain in the opposite direction.

When one or more of the groups linked to a double bond are not the same, the isomers are called E or Z. To notify this type, the user will need to specify the formulas of hydrocarbons that have geometric isomers and determine the atom with the highest priority (highest atomic number) attached to each double bond C.If the two atoms with the highest priority remain on the same side of the isomer, which is designated as Z, and, if they are in the opposite side, are designated as E.

Formulas of hydrocarbons

For example, 1 - bromo - 1 - fluoropropane has two isomers. In Z-1 - bromine - 1 - fluoropropane, it can be seen that bromine has a higher priority or a higher atomic number (35) than fluorine (9), which are attached to C-1. Carbon has a higher atomic number (6) than hydrogen (1), which is attached to C-2 of this compound. Since the carbon atoms with the highest priority (from the -CH3 group) and bromine attached to these two carbon atoms are on the same side, this compound is defined as Z. On the other hand, in E-1 - bromine - 1 - fluoropropane with the highest priority C and bromine are in the opposite direction, so it is called the E-isomer.

Carbon-carbon double bond

Isomers are two molecules that have the same atomic composition, but are not identical. Atoms in two isomers may be linked in a different order (structural isomerism), or they may be linked in the same way, but have a different orientation — spatial stereoisomerism.

The structural and geometric isomer in special cases, the stereoisomer, must meet two requirements:

  1. There is a limited rotation in the molecule.
  2. Both atoms involved in a restrictive bond have two different functional groups for them.

A common example of limited rotation is a carbon-carbon double bond. These connections include the pi connection, in most conditions it is not profitable to break them.

Link pi

Geometric isomers have a structure that affects the physical properties of a compound.

Cis / Trans system

Naming Cis / Trans is the easiest connection system to use. First, the longest carbon chain in the molecule is identified, and then the functional groups of interest are identified. In the cis isomer, the two groups under consideration are on the same side of the double bond (cis means “on the same side” in Latin). In the trans-isomer, the two groups under consideration are on opposite sides of the double bond (trans means across the Latin language). For example, two different geometric isomers of butene-2.

Cis-trans isomerism

Both atoms with a double bond have the same two groups as these two groups, but they differ from one another on one of the double carbons. The task is complicated as the side chains and functional groups become more complex.

The official IUPAC naming convention is E / Z.There is no specific relationship between cis / trans and E / Z, and the two systems are not interchangeable. The E / Z designation uses the Cahn-Ingold-Prelog priority rules, and is considered more reliable. The fumaric acid IUPAC name is the trans-isomer with the formula HO2CCH = CHCO2H, and maleic acid is the cis-butandiic acid.

Cis-butendeic acid

IUPAC is the International Union of Pure and Applied Chemistry, which sets international rules and standards for labeling chemicals in all languages.

Cyclic compounds

In a cyclic connection, the rotation between the carbon single bond is limited. Thus, isomerism is also possible for this type of compound if two different groups are attached to each carbon. There are two isomers of 1,2-dimethylcyclopropane.

Cyclic compounds

One of them is the cis isomer, where the two methyl groups are on one side, and the other is the trans isomer, where the two methyl groups are on the other side.

Geometric isomers differ in their physical properties. This is due to the shape of the isomers and the total dipole moment. For example, they differ in boiling point.The boiling point of cis- and trans-isomers of 1,2-dichloroethylene is 60.3 ° C and 47.5 ° C, respectively.

In the cis-isomer, the presence of two dipole bonds (C-Cl) gives the total molecular dipole. This leads to intermolecular dipole-dipole forces. For this force, the cis-isomer has a higher boiling point than the trans-isomer, where two dipole bonds (C-Cl) are canceled due to their position in the opposite direction.

The reason why it is impossible to rotate a carbon-carbon double bond is that there are two bonds connecting the carbon atoms together and you have to break the pi bond. Pi connections are formed during lateral overlap between p-orbitals. If it tries to turn a carbon-carbon double bond, the p-orbitals will no longer line up, and therefore the pi link will be broken. Energy is expended on this, and this only happens if the compound is very hot.

Construction of structural formulas

It is very easy to omit important elements of geometric isomers during the construction of structural formulas during contraction. For example, it is very tempting to draw but-2-ene. If the user writes this incorrectly, then the compound will no longer be an isomer.If there is even the slightest hint that an isomer can be used, it is always necessary to use compounds containing carbon-carbon double bonds showing the correct angles (120 °) around the carbon atoms at the ends of the bond. In other words, you must use the format shown in the figure.

Construction of structural formulas

Compounds containing a carbon-carbon double bond have a limited rotation. To obtain isomers, the following conditions must be met:

  • limited rotation, usually involving a carbon-carbon double bond;
  • two different groups on the left end of the link and two different groups on the right end.

It does not matter whether the left groups coincide with the right or not.

Alkenes: Molecular and Structural Formulas

Geometric isomers of alkenes include a number of compounds that consist of C and H atoms in a carbon chain. This group includes a homologous series with the formula CnH2n. The simplest alkene is eten, it has two C atoms and the formula C2H4.

Alkenes: Molecular and Structural Formulas

The structural formula for ethene is shown in the figure above. In longer alkene chains, additional carbon atoms are attached to each other using some covalent bonds.Each carbon atom is also attached to sufficient hydrogen atoms to produce a total of four single covalent bonds.

In chains with four or more C atoms, the double bond can be located in different positions, which leads to the formation of structural isomers. In addition to structural isomers, alkenes also form stereoisomers. Since the rotation around the multiple bond is limited, the groups attached to atoms with a double bond always remain in the same relative positions.

These “blocked” positions allow chemists to identify various isomers from substituents in order to determine which substance has geometric isomers. For example, one structural isomer C5H10 has the following stereoisomers.

Structural isomer C5H10

The isomer on the left, in which two substituents (methyl and ethyl groups) are on the same side of the double bond, is called the cis isomer, while the isomer on the right with two non-hydrogen substituents on opposite sides is the trans isomer.

For example, chlorine takes precedence because it is heavier. On the right side, bromine is superior to carbon.Third, the positions of two atoms of higher rank are determined. If two atoms are in the cis position, the location is Z (from German zusammen, which means "together"). If the atoms, or groups, are in a trans position, the location is E (from the German entgegen, which means "the opposite").

Hard double bond compounds

Geometric isomers butene is an alkene with a hard double bond. This means that there are actually four isomers, not three, in the position of a double bond. There are fifth and sixth hydrocarbons of the same composition, but they are not alkenes, despite the same formula.

Hard double bond compounds

The ring formation of cyclobutane or methylcyclopropane occupies the space of two hydrogen atoms as a double bond, which leads to the fact that they have formulas that are identical to different butenes.

Geometric isomers examples:

  • 1-butylene (1-butene);
  • iso-butylene (2-methyl-propene);
  • cis-2-butylene (cis-2-butene);
  • trans-2-butylene (trans-2-butene).

And bonuses: cyclobutane and methylcyclopropane, both of which have the same empirical formula as butene isomers, but are not alkenes. The first name is “common” or “trivial” name, and the name in parentheses is the name of IUPAC.

Butene isomers

Butene has many uses, from fuel in a car to grocery bags that are carried daily by hundreds of millions of people around the world.The chemical formula for butene: C4H8, which means that it consists of four C atoms and eight H atoms, the compound refers to an alkene.

There are several different isomers or molecular structures that can form this compound (the IUPAC names are shown in parentheses):

  • alpha-butylene (but-1-ene);
  • cis-beta-butylene - ((2Z) -but-2-ene);
  • trans-beta-butylene - ((2E) -but-2-ene);
  • isobutylene (2-methylprop-1-ene).
Butene isomers

Although they all have the same formula, their structures are different. The ratios between each of these geometric isomers are basically constitutional, which means that they have the same molecular formula, but different bonds. The exception is cis-beta-butylene and trans-beta-butylene.

Many people know that trans fats are harmful to humans, and unsaturated fats are good for them. The only difference between these two fats is that one has a trans link, and the other has a cis link, however, this small differentiation can significantly affect the function of the molecule.

With cis-beta-butylene and trans-beta-butylene, the atoms are in the same order, but the polarities are different. The cis isomer is polar, with both CH3 groups on the same side. This makes it really cumbersome and complex. The trans-isomer is non-polar, the volumetric groups of CH3 alternate, which gives more space in the molecule.This relationship is called cis-trans-isomerism. The cis isomers are polar, while the trans isomers are not.

Despite the fact that each of these isomers of butene consists of the same materials, each of them has different physical properties. For example, boiling point:

  1. Cis-beta-butylene: 3.7 ° C.
  2. Trans Beta Butylene: 0.8 ° C.
  3. Isobutylene: -6.9 ° C.
  4. Alpha Butylene: -6.3 ° C.

Material for the production of plastics

Butenes are alkenes with four carbon atoms, C4H8. There are several different structural or configurational isomers of butene, including geometric and optical isomers. All four butenes have similar physical properties, being colorless gases, heavy in water, readily soluble in ether and alkanes. Differences in physical properties are explained by the structure of molecules. For example, cis-But-2-en has a higher boiling point than trans-But-2-en, because it is a stronger dipole.

Butene isomer

The two alkyl groups of the cis-isomer work with their + I effect in one direction and thereby enhance, while the two alkyl groups of the trans-isomer act in opposite directions and, thus, weaken each other.Formulas for hydrocarbons with geometric isomers are indicated by IUPAC standards. But-1-en has such a low melting point, since CC is a single bond between the second and third carbon atoms, free rotation and the ethyl group can rotate around the axis of rotation in all directions.

This makes it difficult to classify a molecule into a solid crystalline structure. The remaining three butenes with a double bond between the 2nd and 3rd C atoms are very rigid and can be easily classified into a crystal structure. Therefore, they have relatively high melting points. These arguments are not always valid, as the example shows 2-Methyl-but-2-en (or isobutene). The two methyl groups with their + I-effects act in the same direction as cis-But-2-en, and in fact need to be improved. However, isobutene has a very low boiling point of only -7 ° C.

But-1-en and But-2-ene are used to produce butadiene and butan-2-ol. In addition, alkenes are used as alkylating agents. Thus, an important fuel 2,2,4-Trimethyl-pentan, better known as isooctane, is obtained from isobutene and isobutane. Finally, butenes are the starting materials for the production of some plastics, as they are easy to polymerize. Known plastic on But-1-en basis, is polybutene-1, from which pipes are made.

Pentane, n-pentane, isopentane

Pentane, or n-pentane, is one of the saturated alkane hydrocarbons. Almost odorless, n-pentane is a liquid under environmental conditions and is a isomer of herakethite of 3 isomers. Branched C5 - C16 liquid isoalkanes are increasingly used as fuel (Otto, Diesel). In addition, these alkanes are present in heating oils and lubricating oils. They provide complete burning. Before you know the characteristics of such compounds, you must specify the formulas of hydrocarbons with geometric isomers:

  1. Physical state - fluid.
  2. Color - colorless.
  3. Smell - almost not.
  4. Easily flammable.
  5. Vapors may form explosive mixtures when exposed to air.
  6. Water solubility is very low (practically insoluble).
  7. Very unstable connection.



Chemical sign


Molar mass

72.1488 kg / kmol

Flash point

224 K


533 bar

Boiling point - 1,013 bar

309.2 K

Critical temperature

469.8 K

Critical pressure

33.6 bar

Heat of evaporation at boiling point

357.46 kJ / kg (25.79 kJ / mol)


1.4 vol.% (41 g / m3)


7.8% by volume (235 g / m3)


9.5 bar

The most important sources of pentane are petroleum oils, which differ significantly in their composition depending on their origin.Separation occurs by fractional distillation. The following fractions are obtained here:

  1. Paraffin oil (boiling point> 320 ° C).
  2. Oil (boiling point 180 to 250 ° C).
  3. Heating / diesel fuel (boiling point 250 to 320 ° C).
  4. Crude gasoline (boiling point up to about 180 ° C).
  5. In naphtha there are branched hydrocarbons (alkanes) from C5 to C10.
  6. Combustion of pentane with oxygen (stoichiometric).
  7. The end products are carbon dioxide and water. Formulas of geometric isomers: C5H12 + 8O2 ⟹ 5CO2 + 6H2O.



Calorific value HU [MJ / kg]

Calorific value HU [kWh / kg]


































C 9 H 20







The geometric isomer pentene-2 ​​is a universal solvent. It is used for foaming phenolic resin and polystyrene. It is also required as a reference substance in gas chromatography, and as a propellant in spray cylinders.

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