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When we study the isomers in chemistry, we come across different forms of isomers. Although isomers are divided into two different groups – stereoisomers and constitutional isomers, there are other sub-divisions of isomers as well, such as cis and trans isomers. However, there is a difference between cis and trans isomers.
Generally, the cis and trans isomerism is found in both organic and inorganic compounds. The ‘cis’ and ‘trans’ are Latin words that refer to this side of and another side of respectively. Cis isomers are the isomers that have two similar atoms lying on the same side of the double bond in a molecule. The trans isomers have molecules with two similar atoms that are placed on opposite sides of the double bond. Both the cis and trans isomers possess the same molecular formula but differ in a few aspects. In this article, we will learn about the difference between cis vs trans-isomers.
What is Cis and Trans Form of Isomers?
The atoms that make up isomers in stereoisomerism join up in the same order, but have a different spatial arrangement. Geometric isomerism is a type of stereoisomerism. These kinds of isomers tend to occur when the rotation in a molecule is restricted. The examples of stereoisomers generally consist of the carbon-carbon double bond.
Difference between Cis and Trans Isomers
Talking about the difference between the cis and trans stereoisomers, the main difference between the two lies in the arrangement of the atoms in the double bonds. However, some of the major difference between the isomers cis and trans are tabulated below. Let us take a look at them.
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CIS |
TRANS |
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Cis isomers are referred to as the molecules having the same connectivity of the atoms. They constitute similar side groups that are placed on the same side of the double bond. |
Trans isomers, on the other hand, consist of molecules having similar side groups that are placed on opposite sides of the double bond. |
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Cis isomers are generally alomost polar. |
Trans isomers are not much polar. Many of the known trans isomers are non-polar molecules. |
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Because of the loosely packed molecules, the cis isomers possess relatively lower melting points when compared to the trans- isomers. |
Because of the tightly packed molecules, the melting points of the trans isomers are genereally higher than that of the cis isomers. |
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Because of the presence of the strong attractives forces between the two atoms, the cis isomers tend to possess a higher boiling point. |
The boiling point of the trans isomers is comparatively low since there are no such strong attractive forces. |
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In the inert solvents, the cis isomers tend to possess a strongly soluble content. |
The solubility of the trans isomers in neutral solvents is relatively smaller. |
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Both the forms of the cis isomers possess a higher acid power. |
The protons in the trans isomers are not readily emitted. Hence, it has a lesser acidic strength. |
Boiling Points and Melting Points of Cis and Trans Isomers
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Boiling Point
The boiling point of a cis- isomer is higher than the boiling point of a trans- isomer of the same molecule. For example, boiling points of cis- isomer and trans- isomer of 1,2-dichloroethene are 60 degrees Celsius and 48 degrees Celsius respectively. This is because, in cis- isomer, the heavy groups are on the same side of the bond which makes the cis-isomer polar and results in dipole attraction between the molecules.
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Melting Point
The melting point of the cis-isomer is lower than the melting point of the trans- isomer of the same molecule. For example, melting points of cis- isomer of 1,2- dichloroethene are -80 degrees celsius and -50 degrees celsius respectively. This is because the molecules of the trans- isomers(straight-chain) well- packed than the molecules of the cis-isomers (U-chained). Therefore, the intermolecular forces are stronger in trans-isomers as compared to the cis-isomers.
The Polarity of Cis – Isomers and Trans – Isomers
Both the cis-isomers and trans-isomers have an equal number of the same type of atoms joined together. However, the cis-isomers are polar and trans-isomers are non-polar because, in trans-isomers, the groups are attached in opposite directions. Due to this, the dipole moment of the bond cancels out making the trans- isomer non-polar. Since the cis-isomers are polar, they are also unstable in nature whereas the trans-isomers, being non-polar are stable in nature.
Acidic Strength of Cis and Trans – Isomers
The cis-isomers have greater acidic strength than the trans-isomers. The cis-isomer also called the maleic acid has greater acidic strength than the trans-isomer also called the fumaric acid because the maleic acid is able to form an intra-hydrogen bond. This bond forms because the maleic acid is able to lose its hydrogen ion. No such bond is formed in fumaric acid because in trans- isomers, strong interaction with the oxygen atoms of the carboxylic group exists.
Examples of Cis and Trans Isomers in Alkenes
1. But-2-ene is one of the most common examples of geometric isomerism in Alkenes.
But-2-ene can show both cis and trans isomerism because of the restricted rotation around the double bond. The double-bonded carbons in cis- isomer and trans- isomer have the same two groups attached to them that are hydrogen and methyl groups. The only difference is in the placement of these groups. In the cis-isomer, the two groups are present on the same side of the double bond whereas, in the trans- isomer, the two groups are present on the opposite sides of the double bond. Both the compounds have different geometrical structures, physical properties and chemical properties.
Examples of Cis and Trans Isomers in Cyclic Compounds
1. 1,3-dimethylcyclobutane is one of the most common examples of cis and trans isomers in cyclic compounds.
1,3-dimethylcyclobutane can show both cis and trans isomerism because of the restricted rotation around the double bond. This is because if the rotation occurs, then the cyclic ring will break. The groups attached to the ring system will vary in their positions. The cis-isomer will have the groups attached to the same side of the ring whereas the trans- isomer will have the groups attached to the opposite sides of the ring.