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Geometrical Isomerism: Some molecules have the same molecular formula, but have a different spatial arrangement of the atoms in space within a molecule. Such molecules are called Isomers. Geometrical isomerism is shown by the occurrence of restricted rotation somewhere in a molecule. A compound can exhibit one set of physical properties when arranged in a particular manner and can exhibit another set of entirely different physical properties when arranged in the other.
Geometric isomerism falls under what is known as stereoisomerism. This page explains in detail what is geometrical isomerism and stereoisomers, and how to recognise the possibility of geometric isomers in a given molecule. Read the entire article to get clarity on these important terms and concepts in organic chemistry along with a geometric isomerism example.
The phenomenon by which organic compounds exhibit the same chemical formula, but the different structural formula is called isomerism. The compounds that exhibit Isomerism are termed isomers. Examples of isomers is illustrated below:
There are different types of isomers. The following flowchart (see figure below) depicts the classification of isomers.
The phenomenon during which the atoms making up the isomers are joined up within the same order but manage to possess a unique spacing. Geometric isomerism is one form of stereoisomers. or more specifically, from the above diagram you can understand that compounds that exhibit Geometrical isomerism (cistrans isomers) fall in a category called diastereomers.
The isomerism that arises when atoms or groups having different spatial arrangements are restricted to rotate around a bond or bonds in a molecule is called geometric isomerism. This restricted rotation mainly occurs in a carbon-carbon double bond.
But what happens in a carbon-carbon single bond?
In a carbon-carbon single bond, there is unrestricted rotation about the single bond. Let us consider the example of \(1,\,2\)-dichloroethane. To understand what is geometrical isomerism, study the organic formula as under:
The compound \(1,\,2\)-dichloroethane can be represented as above. It is not an isomer because we can get the above two configurations just by twisting the carbon-carbon single bond.
Now consider a carbon-carbon double bond as in \(1,\,2\)-dichloroethene?
The compound \(1,\,2\)-dichloroethene can be represented as above. The two configurations of the compound \(1,\,2\)-dichloroethane as shown above are not the same. This is because rotation around the carbon-carbon double bond is restricted. The two configurations may look alike but are not the same, hence they are isomers.
1. When the two chlorine atoms are locked on opposite sides of the double bond, the compound forms the trans isomer. (trans: from Latin meaning “across” – as in transatlantic).
2. When the two chlorine atoms are locked on the same side of the double bond. This is known as the cis isomer. (cis: from Latin meaning “on this site”)
Let us take another example – But-\(2\)-ene. The compound But-\(2\)-ene can be represented by the following two configurations.
Note: Both \(1,\,2\)-dichloroethene and but-\(2\)-ene have their respective molecular formulas. It is only the spatial arrangement of atoms that differs which results in the cis and trans isomers.
Organic compounds must satisfy the given conditions to form geometric isomers:
Let us take the example of \(1\)-Chloroethene which is represented as follows-
In Spite of swapping the right-hand groups, the molecule of \(1\)-Chloroethene is still the same. On turning the entire model on the left-hand side about \(180\) degrees, we get the same configuration of the molecule as that on the right-hand side.
Geometric isomers are not possible if we have two similar groups on the same side of the double bond – in this case, the two pink groups on the left-hand end represent two similar groups. So there must be two different groups on the left-hand carbon and two different groups on the right-hand one. The cases we’ve been exploring earlier are like this:
But we can make things even more different and still have geometric isomers, for example in the molecule \(1\)-bromo-\(1\)-chloroethene, which is represented as below:
In the above, the blue and green groups are either on the same side of the bond or the opposite side.
We can also make all the groups around the carbon-carbon double to be entirely different from each other. We will still get geometric isomers. The cis and trans configurations are meaningless in molecules having different substituents around the carbon-carbon double bond. This is where the more sophisticated \({\rm{E – Z}}\) notation comes in.
Now let us look at the various ways in which Geomterical isomersim arise in different types of organic compounds.
We know that the presence of carbon-carbon bonds restricts the bond rotation, which is a necessary condition for a molecule to exhibit geometrical isomerism. The rotation is restricted due to the presence of pi-bond in alkenes. Thus, all of the atoms attached to the alkene dwell in a plane.
For example:
Cis-\(2\)-butene trans-\(\)-butene
Butene has two geometric isomers as shown above and are not interconvertible to each other.
In the cis isomer, the two methyl groups are arranged on the same side of a double bond. Whereas within the trans isomer, they are on the other side.
The bond rotation is not only restricted in alkenes but also cyclic hydrocarbons. In a ring of carbon atoms, there will also be no possibility of rotation about any of the carbon-carbon bonds. In cyclic compounds, substituents attached to a ring system give rise to geometric isomers. The substituents will either be on the same side of the ring or the opposite side of the alkane ring. Thus, cyclic alkanes exhibit cis and trans geometrical isomerism. The letters \({\rm{E}}\) and \({\rm{Z}}\) are not used in cyclic alkanes.
Let us consider the example of cyclohexane:
When two of the hydrogens in the cyclohexane molecule are replaced by two bromine atoms, results in a molecule which is shown below:
The shape around each carbon atom is tetrahedral, and there are two alternative ways the bromine atoms can arrange themselves.
1. Both the bromine atoms can lie above the cyclohexane ring, resulting in the cis-isomer.
2. One of the bromine atoms can lie above the ring of the cyclohexane and the other below the ring, resulting in the trans- isomer.
When two of the hydrogens in the cyclohexane molecule are replaced by two methyls, results in a molecule which is as shown below:
The cis & trans isomers of \(1,\,4\)–dimethyl cyclohexane is as shown below-
The geometrical isomerism in \(1,\,4\)–dimethyl cyclohexane, the methyl groups are arranged differently about the plane of the cyclohexane ring. These isomers aren’t interconvertible since it’s impossible to rotate the bonds within the cyclohexane ring.
When carbonyl compounds are treated with hydroxylamine, oximes are formed. These are represented as follows:
Where \({\rm{R\& }}{{\rm{R}}^{\rm{1}}}\) are hydrogens; or alkyl or aryl groups.
There are two types of oximes:
The oximes exhibit geometrical isomerism due to the restricted rotation of the \({\rm{C = N}}\) bond. Two geometrical forms are possible for the oximes as shown below.
The descriptors, syn and anti are used to distinguish them.
When both the hydrogen and the hydroxyl \({\rm{ – }}\left( {{\rm{OH}}} \right)\) group are on the same side of the \({\rm{C = N}}\) bond, we get the syn form of aldoxime.
When both the hydrogen and the hydroxyl \({\rm{ – }}\left( {{\rm{OH}}} \right)\) group are on the opposite side of the \({\rm{C = N}}\) bond, we get the anti-form of aldoxime.
For E.g. The syn and anti sorts of acetaldoxime are shown below.
However with ketoximes, the syn and anti descriptors indicate the spatial relationship between the primary group cited within the name and the hydroxyl group. For example, the subsequent ketoxime of butanone is often named as either syn methyl ethyl ketoxime or anti ethyl methyl ketoxime.
1. In disubstituted complexes, the substituted groups may be adjacent or opposite to each other. This gives rise to geometrical isomerism. Thus square planar complexes such as \(\left[ {{\rm{Pt}}\left( {{\rm{N}}{{\rm{H}}{\rm{3}}}} \right){\rm{4C}}{{\rm{l}}{\rm{2}}}} \right]\) can be prepared in two forms, cis and trans. When the chlorine atoms are adjacent to each other it is called cis form. When two chlorine atoms are opposite it is called transform.
1. This type of isomerism is mainly found in coordination compounds with coordination numbers \(4\) and \(6\).
2. In a square planar complex (i.e. coordination compounds with coordination number \(4\) which have \(\left[ {{\rm{M}}{{\rm{X}}{\rm{2}}}{{\rm{L}}{\rm{2}}}} \right]\) type formula \({\rm{X}}\) and \({\rm{L}}\) are unidentate ligands), the two ligands \({\rm{X}}\) may be present adjacent to each other in a cis isomer, or opposite to each other to form a trans isomer.
3. Square planar complex with \({\rm{MABXL}}\) type formula (where \({\rm{A,B,X,L}}\) are unidentate ligands) shows three isomers-two cis and one trans.
4. Cis trans isomerism is not possible for a tetrahedral geometry.
5. But octahedral complexes do show cis-trans isomerism. In complexes with formula \(\left[ {{\rm{M}}{{\rm{X}}{\rm{2}}}{{\rm{L}}{\rm{4}}}} \right]\) type, two ligands \({\rm{X}}\) may be oriented cis or trans to each other.
6. This type of isomerism is also observed when bidentate ligands \({\rm{N}}{{\rm{H}}{\rm{2}}}{\rm{C}}{{\rm{H}}{\rm{2}}}{\rm{C}}{{\rm{H}}{\rm{2}}}{\rm{N}}{{\rm{H}}{\rm{2}}}\left( {{\rm{en}}} \right)\) are present in complexes with \(\left[ {{\rm{M}}{{\rm{X}}{\rm{2}}}{{\left( {{\rm{L – L}}} \right)}{\rm{2}}}} \right]\) type formula.
By just looking at the isomers, we can tell which is the cis and which the transform. All we need to remember is that trans means ‘opposite’ and that cis is the opposite of trans, i.e on the same side. It is an easy and visual way of telling the \(2\) isomers apart. So why do we need another system? To understand this, look at the example given below.
In the example of \(1,\,2\)-dichloroethene, there are two similar groups present around the carbon-carbon double bond. But, what if all the substituents around the carbon-carbon double bond are different? For example, could we name the following isomers using cis and trans?
The cis- and trans- isomers cannot be judged by just looking at the isomers. This is because all the substituents attached to the carbon-carbon double bond are different, there aren’t any obvious things that we can think of as being “cis” or “trans” to each other. The presence of more than two different substituents on a double bond makes it difficult for the geometric isomers to be named using cis- and trans- descriptors. Hence, another system of naming compounds displaying geometrical isomerism known as the \({\rm{E – Z}}\) system has been introduced.
In the \({\rm{E – Z}}\) system of nomenclature, the geometry of the isomers is determined by prioritizing the substituents on the two carbons of the double bond. The priorities of the substituents are determined by the atomic number with atoms of a higher atomic number having higher priority.
According to this system:
1. Isomer \({\rm{“E”}}\) – The isomer is denoted by \({\rm{E}}\) if the higher priority group is present on the opposite sides of the double bond.
Where \({\rm{E = }}\) Entgegen ( the German word for ‘opposite’)
However,
2. Isomer \({\rm{“Z”}}\) – The isomer is denoted by \({\rm{Z}}\) if the higher priority group is present on the same side of the double bond.
Where \({\rm{Z = }}\) Zusammen (the German word for ‘together)
The letters \({\rm{E}}\) and \({\rm{Z}}\) are represented within parentheses and are separated from the remainder of the name with a hyphen.
The following procedure is to be adopted to denote the geometrical isomerism using \({\rm{E\& Z}}\) descriptors:
The priorities of the substituents are assigned by following the Cahn-Ingold-Prelog sequence rules (CIP rules) described below.
i. Rank the atoms directly attached to the olefinic carbon according to their atomic number. High priority is given to the atom with a higher atomic number.
ii. If isotopes of the same element are present, the upper priority is given to the isotope with a higher mass.
For E.g. Deuterium is an isotope of hydrogen having a relative atomic mass of \(2\). It has just one proton, but still has an atomic mass of \(1\). However, it is not equivalent to an atom of “ordinary” hydrogen, then these two compounds exhibit geometrical isomerism:
The hydrogen and deuterium have the same atomic number, so they would have the same priority on that basis. In a case like that, the one with the higher relative atomic mass has the higher priority. So in these isomers, the deuterium and chlorine are the higher priority groups on each end of the double bond.
That means that the left-hand isomer within the last diagram is that the \(\left( {\rm{E}} \right)\)- form, and therefore the right-hand one the \(\left( {\rm{Z}} \right)\)-
The \({{\rm{C}}^{{\rm{13}}}}\) isotope has more priority than \({{\rm{C}}^{{\rm{12}}}}{\rm{.}}\)
3. If the atoms directly attached to the olefinic carbon are still identical, the next set of atoms are taken into consideration and examined along the chain until a “first point of difference” is found. This is done by making an inventory of atoms linked to the primary atom, i.e the carbon atom having a double bond. Each list is arranged in the decreasing atomic number. Then the higher priority is given to the list which contains atoms with the higher atomic number at the first point of difference.
E.g. Examine the lists of atoms directly linked to the highlighted carbons (depicted in *) in the following compound.
4. The descriptor, \({\rm{Z,}}\) is employed to represent the isomer of the above compound. This is because on the left-hand side of the double bond the \(\left( {{\rm{C,H,H}}} \right)\) list has more priority over \(\left( {{\rm{H,H,H}}} \right)\) whereas, the \(\left( {{\rm{O,H,H}}} \right)\) has more priority over \(\left( {{\rm{C,C,C}}} \right)\) on the right-hand side of the highlighted carbon double bond.
5. As the groups with the highest priorities are present on the same side of the double bond, the descriptor, \({\rm{Z,}}\) represents the compound’s stereochemistry.
6. Hence, the name of the compound is \(\left( {{\rm{2Z}}} \right){\rm{ – 2}}\)-tert-Butyl-\(3\)-methylpent-\(2\)-en-\(1\)-ol.
Since the double bond is present at \({\rm{C2}}\) of the pentene chain, the number \(2\) is used before the descriptors i.e., (\{\rm{‘2Z’}}\) or \({\rm{‘2E’}}.\)
The multiple bonds are counted as multiples of that same atom i.e., each \({\rm{\pi }}\) bond is treated as if it were another \({\rm{\sigma }}\) bond to that type of atom.
The geometrical isomerism in oximes is better differentiated by using \({\rm{E – Z}}\) notations.
1. When the oxime has a hydroxyl group and the group with higher priority on the same side of \({\rm{C = Z}}{\rm{.}}\) It is designated as a \({\rm{‘Z’}}\) isomer.
2. When the oxime has a hydroxyl group and the group with higher priority on the opposite side of \({\rm{C = Z}}{\rm{.}}\) It is designated as an \({\rm{‘E’}}\) isomer.
E.g. The syn acetaldoxime is named as \(\left( {\rm{E}} \right)\)-acetaldoxime since the hydroxy group and the group with higher priority i.e., methyl group are on the different sides of the \({\rm{C = Z}}{\rm{.}}\) Whereas the anti-form is named as \(\left( {\rm{Z}} \right)\)-acetaldoxime.
The syn and anti forms of acetaldoxime are named as follows:
The effect that geometrical isomerism has on various physical properties of the molecules are discussed below:
The table below shows the melting point and boiling point of the cis and trans isomers of \(1,\,2\)-dichloroethene:
| Isomer | Melting Point (\left( {{}^{\rm{o}}{\rm{C}}} \right)) | Boiling Point (\left( {{}^{\rm{o}}{\rm{C}}} \right)) |
| cis | \( – 80\) | \(60\) |
| trans | \( – 50\) | \(48\) |
Following observations are made from the table given above:
1. the trans isomer has the higher melting point;
2. the cis isomer has a higher boiling point.
The same effect is observed in the cis and trans isomers of but-\(2\)-ene.
In Geometrical Isomerism, the polarity of the molecules best explains the reason behind the high boiling point of the cis isomer. The polarity of the molecules strongly influences the strength of the intermolecular forces. Taking \(1,\,2\)-dichloroethene as an example:
The cis- and trans- isomer of \(1,\,2\)-dichloroethene have-
a. the same atoms joined up in the same order, which means that the van der Waals dispersion forces between the molecules will be identical in both cases.
b. polar chlorine-carbon bonds.
The difference lies in the polarity of the two molecules. In the cis isomer, the polar chlorine carbon bonds are both on the same side of the molecule. That means that one side of the molecule will have a slight negative charge while the other is slightly positive. The molecule is therefore polar.
In the trans isomer, the polar chlorine carbon bonds are on the opposite side of the molecule. That means the charges will get cancelled and the molecule will have zero dipole moment. This makes the trans isomer non-polar.
The cis isomer has a net dipole moment and dipole-dipole attractions between its molecules in addition to the van der Waal’s forces, whereas trans \(1,\,2\) dichloroethene which is non-polar has only van der Waal’s forces. An extra amount of energy is required to overcome the dipole-dipole interaction in a cis isomer than that of the trans isomer. The boiling point of the cis-isomers is therefore higher.
The symmetry affects the packing of the molecules in the solid-state. The trans isomers pack together better than that of the cis isomer. This is because the trans isomers have greater symmetry compared to the cis isomer. The poorer packing in the cis isomers means that the intermolecular forces aren’t as effective as they should be and so less energy is needed to melt the molecule – cis isomers have a lower melting point
In this article, we have learned about the organic compounds that exhibit geometrical isomerism and the necessary conditions required to do so. We have also discussed the nomenclature of the geometrical isomers and their effect on the physical properties of the organic compounds.
Here are the answers to some of the frequently asked questions on geometrical isomerism.
Q.1: Do cis and trans have the same properties?
Ans: The cis and trans isomers have different physical properties. This is because the trans isomer is non-polar as is a symmetrical molecule compared to the cis-isomer. In cis isomer, there is an additional dipole interaction in addition to the van der Waals forces which makes it an asymmetrical molecule.
Q.2: Why trans isomer has more melting point than the Cis-isomer?
Ans: The trans isomer is an asymmetrical non-polar molecule. It packs better into the crystal lattice. This allows a closer approach and larger attractive forces, resulting in higher melting points.
Q.3: How do you identify geometrical isomerism?
Ans: A compound will exhibit geometrical isomerism if it satisfies the following conditions:
a. restricted rotation around a carbon-carbon double bond.
b. Presence of two different groups on the left-hand end as well as on the right-hand end of the double bond.
Q.4: What is the reason for Geometrical isomerism?
Ans: Cis-trans isomerism or geometrical isomerism exists when there is restricted rotation around an olefinic carbon atom and when there are non-identical groups on both ends of the carbon atom.
Q.5: What is the geometrical isomerism definition?
Ans: The geometrical isomerism definition is – The isomerism that arises when atoms or groups having different spatial arrangements are restricted to rotate around a bond or bonds in a molecule is called geometric isomerism.
We hope this detailed article on Geometrical Isomerism helps you in your preparation. If you get stuck do let us know in the comments section below and we will get back to you at the earliest.
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