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An isomer is one of several different compounds that have the same molecular formula . That is, isomers are chemical compounds formed by the same atoms, but which have some difference, either in their structure or in the spatial orientation of their atoms, which causes them to have different properties. In some cases the differences in properties are very subtle and even difficult to detect, while in other cases the isomers are radically different compounds.
Isomeric compounds can be both molecular and ionic compounds, although in the latter case it is generally required that at least one of the ions be a covalent or molecular ion.
On the other hand, isomers can also be both organic and inorganic or organometallic chemical compounds. However, despite the fact that isomerism is studied in all fields of chemistry, it finds its greatest potential in organic chemistry, thanks to the chemical richness of carbon, which can form thousands of different compounds when combined with other elements such as hydrogen, nitrogen, oxygen, phosphorus and the halogens.
Etymology and origin of the term isomer
The term isomer was coined by the Swedish chemist Jöns Jacob Berzelius (1779-1848) in the 19th century. The word is formed by the combination of the Greek terms iso- , which means equal, and -meros , which means part or portion. Thus, the literal meaning of isomer is that which is made up of equal parts or portions. In chemistry, parts or portions refer to the constituent atoms of chemical compounds.
Classification of isomers or types of isomerism
Isomers can have different degrees of relationship with each other. That is, there are isomers that are chemically and structurally very similar and, therefore, are part of the same family of chemical compounds, while, in other cases, the isomers are totally different compounds, with opposite or very different chemical properties. and with different structures (and even types of links). This gives rise to different types of isomers or isomerism.
The following figure shows a scheme of the general classification of isomers. As can be seen, isomers are classified into two large groups, which are structural isomers and stereoisomers. Then each of them is divided into other subclasses.
Below is a short definition of each of these types of isomers.
structural isomers
Structural isomers are those isomers that differ in the connectivity between their atoms. That is, they are compounds formed by the same atoms, but in which they are linked together in a different order, thus giving rise to compounds with different structures.
In organic chemistry, structural isomers are often divided into different types, depending on how the difference in the connectivity of the atoms manifests itself. These are:
Chain isomers: are those that differ in the fundamental structure of the carbon chain. That is, they are compounds that have different main chains, that have different branches, or both. For example, n-butane and isobutane are chain isomers, since the former has a 4-carbon backbone with no branches, while the latter has a 3-carbon backbone and a 1-carbon branch.
Positional Isomers: These isomers have the same main chain, but differ in the position of branches, functional groups, or other structural elements. For example, 2-methylhexane and 3-methylhexane have the same 6-carbon backbone and 1-carbon branch, but in the former the methyl is at position 2, while in the latter it is found in position 3.
Functional isomers: These are compounds with the same molecular formula but which have different functional groups, such as ethers and alcohols, or cycloalkanes and alkenes.
spatial isomers or stereoisomers
Spatial isomers are isomers in which all the atoms are linked in the same order and by the same type of bonds but have a different spatial orientation. That is, they are compounds in which there is the same connectivity between all the atoms, but in which the atoms do not all point in the same direction.
There are two main classes of stereoisomers or spatial isomers which are:
Enantiomers: are stereoisomers that are related to each other because they are non-superimposable mirror images. Due to the fact that you can only have a mirror image, enantiomers only come in pairs, they share most of the physical (they have exactly the same melting and boiling points, the same solubility, etc.) and chemical (they have the same same enthalpies of formation, of combustion, the same chemical reactivity towards reagents that are not themselves enantiomers, etc.).
However, enantiomers have a unique property, which is their ability to rotate plane-polarized light, a property called optical activity. An enantiomer differs from its mirror image in that the two rotate the plane of polarized light in opposite directions. Furthermore, they also differ in their reactivity to other optically active compounds.
The vast majority of compounds of biological importance are enantiomers and are usually identified by a combination of letters R and S indicating the absolute configuration of the chiral centers or asymmetric carbon atoms. There are also other conventions such as the letters lod that indicate whether an isomer rotates light to the left (l for levorotatory) or to the right (d for dextrorotatory), or by the letters L or D which is a convention used by biochemists, pharmacists and medical science specialists to identify one of the two possible enantiomers. For example, any drug that is preceded by the letters L or D in its generic name is an enantiomer.
Diastereomers: are those stereoisomers that are not mirror images (and are not superimposable with each other). Diastereomers can, in turn, be cis-trans isomers, in which the same or main groups point in the same or opposite directions, and conformers that are the same compound in different conformations which can be interconverted by of single bond rotation.
Examples of isomers
Here are some examples of the different types of isomers along with their respective structures:
Examples of Structural Chain and Functional Isomers
The following are all examples of isomers with the molecular formula C 6 H 6 :
The names of these isomers are:
I.- Cyclohexane
II.- Hex-1-ene
III.- 2-methylpent-1-ene
These three compounds are examples of structural isomers. Isomers II and III are chain isomers since they are compounds of the same type (alkenes) whose difference is the connectivity between the carbon atoms of the main chain and of the branches. In fact, the main chain of isomer II is 6 carbons while the main chain of III is 5.
On the other hand, isomer I, cyclohexane, is a functional isomer of compounds II and III, since it is a compound with the same molecular formula but with different functional groups. I is a cycloalkane, while II and III are alkenes that have a double bond as a functional group.
Examples of Structural Positional Isomers
The names of these isomers are:
IV.- 1,1-dimethylcyclobutane
V.- 1,2-dimethylcyclobutane
VI.- 1,3-dimethylcyclobutane
As we can see, these compounds all have the same functional groups (all are substituted cycloalkanes), the same main chain (cyclobutane) and the same branches (two methyl groups). However, the substituents are found in different positions on each of them, making them positional isomers.
It should be noted that compounds V and VI can also present another type of isomerism, since, being cycloalkanes, the main chain does not have rotational freedom, which means that the relative positions of the substituents can give rise to different compounds. Depending on whether the two methyls in V or VI are on the same side of the ring or on opposite sides, each of these compounds can occur as a cis or trans isomer.
Thus, there is the isomer cis-1,2-dimethylcyclobutane and trans-1,2-dimethylcyclobutane, which only differ in the spatial orientation of the methyls and are therefore diastereomeric spatial isomers. There are also two diastereomers of compound VI, respectively cis-1,3-dimethylcyclobutane and trans-1,3-dimethylcyclobutane.
Examples of enantiomers
Both compound VII and VIII correspond to 2-hydroxypropanal. However, this compound has a chiral center (carbon 2) that makes the molecule non-superimposable with its mirror image. In fact, compound VII is the mirror image of VIII and, as can be easily verified, it is not possible to rotate or flip either of the two molecules in such a way that all the atoms that make them coincide in space.
The difference in the spatial orientation of the atoms makes them stereoisomers, while the fact that they are mirror images makes them a pair of enantiomers. Compound VII corresponds to the S isomer while VIII corresponds to the R isomer.
References
Carey, F., & Giuliano, R. (2021). Organic Chemistry (11th ed .). McGraw-Hill Interamericana de España SL
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Guerra Medrano, L. (2019, June). stoichiometry. Introduction and basic concepts . UAEH. https://www.uaeh.edu.mx/docencia/P_Presentaciones/b_sahagun/2019/lgm-quimorg.pdf
ISOMER . (nd). Chilean etymologies. http://etimologias.dechile.net/?iso.mero
Martínez, FJ, Mendoza, FA, Parra, BU, & Ramírez , BG (2020, October 12). Isomeria – Chemistry – UACJ . StuDocu. https://www.studocu.com/es-mx/document/universidad-autonoma-de-ciudad-juarez/quimica/isomeria/10428774
Roberts, JD, & Caserio, MC (2021, July 30). 5.5: The D,L Convention for Designating Stereochemical Configurations . Chemistry LibreTexts. https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Book%3A_Basic_Principles_of_Organic_Chemistry_(Roberts_and_Caserio)/05%3A_Stereoisomerism_of_Organic_Molecules/5.05%3A_The_D_L_Convention_for_Designating_Stereochemical_ Settings
