This video gives a good overview of this topic. Before you begin reading this section, review section 1.5, which contains an introduction to isomerism.
IUPAC defines constitutional isomerism as “isomerism between structures differing in constitution and described by different line formulae e.g. CH3OCH3 and CH3CH2OH.” Recall that there are three types of constitutional isomer commonly seen: Chain, positional and functional.
Isomers are molecules that have the same molecular formula, but have a different arrangement of the atoms in space. Consider butane:
There are also endless other possible ways that this molecule could twist itself. There is almost completely free rotation around all the carbon-carbon single bonds. If you had a model of a molecule in front of you, you would have to take it to pieces and rebuild it if you wanted to make an isomer of that molecule. If you can make an apparently different molecule just by rotating single bonds, it’s not a constitutional isomer – it’s still the same molecule, just in a different “conformation” (see section 3.5. and 3.6.).
In constitutional isomerism, the atoms are arranged in a completely different order. This is easier to see with specific examples. What follows looks at some of the ways that structural isomers can arise. The names of the various forms of constitutional isomerism probably don’t matter all that much, but you must be aware of the different possibilities when you come to draw isomers.
Chain isomerism
These isomers arise because of the possibility of branching in carbon chains. For example, there are two isomers of butane, [latex] C_4H_{10} [/latex]. In one of them, the carbon atoms lie in a “straight chain” whereas in the other the chain is branched.
Be careful not to draw “false” isomers which are just twisted versions of the original molecule. For example, this structure is just the straight chain version of butane rotated about the central carbon-carbon bond.
You could easily see this with a model. This is the example we’ve already used at the top of this page.
Example: Chain Isomers in Pentane
Pentane, C5H12, has three chain isomers. If you think you can find any others, they are simply twisted versions of the ones below. If in doubt make some models.
Position isomerism
In position isomerism, the basic carbon skeleton remains unchanged, but important groups are moved around on that skeleton.
Example: Positional Isomers in C5H12
For example, there are two structural isomers with the molecular formula C3H7Br. In one of them the bromine atom is on the end of the chain, whereas in the other it’s attached in the middle.
If you made a model, there is no way that you could twist one molecule to turn it into the other one. You would have to break the bromine off the end and re-attach it in the middle. At the same time, you would have to move a hydrogen from the middle to the end.
Another similar example occurs in alcohols such as [latex] C_4H_9OH [/latex]
These are the only two possibilities provided you keep to a four carbon chain, but there is no reason why you should do that. You can easily have a mixture of chain isomerism and position isomerism – you aren’t restricted to one or the other.
So two other isomers of butanol are:
You can also get position isomers on benzene rings. Consider the molecular formula [latex] C_7H_7Cl [/latex]. There are four different isomers you could make depending on the position of the chlorine atom. In one case it is attached to the side-group carbon atom, and then there are three other possible positions it could have around the ring – next to the [latex] CH_3 [/latex] group, next-but-one to the [latex] CH_3 [/latex] group, or opposite the [latex] CH_3 [/latex] group.
Functional group isomerism
In this variety of constitutional isomerism, the isomers contain different functional groups – that is, they belong to different families of compounds (different homologous series).
Example: Isomers in C3H6O
A molecular formula [latex] C_3H_6O [/latex] could be either propanal (an aldehyde) or propanone (a ketone).
There are other possibilities as well for this same molecular formula – for example, you could have a carbon-carbon double bond (an alkene) and an -OH group (an alcohol) in the same molecule.
Another common example is illustrated by the molecular formula [latex] C_3H_6O_2 [/latex]. Amongst the several constitutional isomers of this are propanoic acid (a carboxylic acid) and methyl ethanoate (an ester).
Contributors to the above section
Jim Clark (Chemguide.co.uk)
Stereoisomers
Stereoisomers are isomers that differ in the spatial arrangement of atoms, rather than the order of atomic connectivity. We gave a basic description in section 1.5, and stereoisomerism will be the main focus of chapter 4. However, one specific type of stereoisomer – geometric isomers – warrants further discussion as we begin looking in detail at hydrocarbon structures.
Geometric or cis-trans isomerism
We have defined isomers in a very general way as nonidentical molecules that possess the same number and kind of atoms. However, there are several ways in which isomers can be nonidentical. Among the alkenes, 1- and 2-butene are position isomers, because in these compounds the double bond has a different position in the carbon chain
Most, but not all alkenes, have stereoisomers that are not identical because of differentspatialarrangements of the component atoms. Thus there are two stereoisomers of 2-butene that differ in the geometric arrangement of the groups attached to the double bond. In one isomer, both methyl groups are on thesameside of the double bond (cis-2-butene) and in the other, the methyl groups are onoppositesides of the double bond (trans-2-butene):
The two isomers clearly have the same structural framework but they differ in the arrangement of this framework in space – hence the designationstereoisomers. They owe their separate existence to the fact that the double bond is rigid and the parts of the molecule are not free to rotate with respect to each other about this bond. Therefore the isomers do not interconvert without breaking the double bond, and they exist as different compounds, each with its own chemical and physical properties. Ball-and-stick models ofcis-andtrans-2-butene are shown below, and the rigidity of the double bond is simulated in the model by a pair of stiff springs or bent sticks connecting the two carbons of the double bond.
It should be clear to you that there will be no cis-trans isomers of alkenes in which one end of the double bond carries identical groups. Thus we don not expect there to be cis-trans isomers of 1-butene or 2-methylpropene, and
Ball-and-stick models of cis- and trans-2-butene
indeed none are known:
You may wish to verify this by making your own models of these substances.
Ring formation also confers rigidity on molecular structure such that rotation about the ring bonds is prevented. As a result, stereoisomerism of the cis-trans type is possible. For example, 1,2-dimethylcyclopropane exists in two forms that differ in the arrangement of the two methyl groups with respect to the ring.
Ball-and-stick models of cis and trans isomers of 1,2-dimethylcyclopropane
In the cis isomer, the methyl groups both are situated above (or below) the plane of the ring and in the trans isomer they are situated one above and one below, as shown in the figure. Interconversion of these isomers does not occur without breaking one or more chemical bonds.
Stereoisomers that do not interconvert rapidly under normal conditions, and therefore are stable enough to be separated, specifically are calledconfigurational isomers. Thuscis– andtrans-2-butene are configurational isomers, as arecis– andtrans-1,2-dimethylcyclopropane. The termscis-trans isomerismorgeometric isomerismcommonly are used to describeconfigurational isomerismin compounds with double bonds and rings. When referring to theconfigurationof a particular isomer, we mean to specify its geometry. For instance, the isomer of 1,2-dichloroethene shown below has the trans configuration; the isomer of 1,3-dichlorocyclobutane has the cis configuration:
Cis-trans isomerism is encountered very frequently. By one convention,the configuration of a complex alkene is taken to correspond to the configuration of the longest continuous chain as it passes through the double bond. Thus the following compound istrans-4-ethyl-3-methyl-3-heptene, despite the fact that two identical groups are ciswith respect to each other, because the longest continuous chain is trans as it passes through the double bond:
Notice that cis-trans isomerism is not possible at a carbon-carbon triple bond, as for 2-butyne, because the bonding arrangement at the triply bonded carbons is linear:
Many compounds have more than one double bond and each may have the potential for the cis or trans arrangement. For example, 2,4-hexadiene hasthreedifferent configurations, which are designated as trans-trans, cis-cis, and trans-cis. Because the two ends of this molecule are identically substituted, the trans-cis becomes identical with cis-trans:
The importance of drawing geometric isomers properly
It’s very easy to miss geometric isomers in exams if you take short-cuts in drawing the structural formulae. For example, it is very tempting to draw but-2-ene as
CH3CH=CHCH3
If you write it like this, you will almost certainly miss the fact that there are geometric isomers. If there is even the slightest hint in a question that isomers might be involved, always draw compounds containing carbon-carbon double bonds showing the correct bond angles (120°) around the carbon atoms at the ends of the bond. In other words, use the format shown in the last diagrams above.
How to recognize the possibility of geometric isomerism
You obviously need to have restricted rotation somewhere in the molecule. Compounds containing a carbon-carbon double bond have this restricted rotation as do compounds with multiple groups attached to a ring, so you need to consider the possibility of geometric isomers. Think about this case with a C=C double bond:
Although we’ve swapped the right-hand groups around, these are still the same molecule. To get from one to the other, all you would have to do is to turn the whole model over. You won’t have geometric isomers if there are two groups the same on one end of the bond – in this case, the two pink groups on the left-hand end. 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 you could make things even more different and still have geometric isomers:
Here, the blue and green groups are either on the same side of the bond or the opposite side. Or you could make everything different. You still get geometric isomers, but by now the words cis and trans are meaningless. This is where the more sophisticated E-Z notation comes in, and this will be covered soon in section 4.3..
Summary
To get geometric isomers you must have:
- restricted rotation (often involving a carbon-carbon double bond for introductory purposes);
- two different groups on the left-hand end of the bond and two different groups on the right-hand end. It doesn’t matter whether the left-hand groups are the same as the right-hand ones or not.
A conformation (or conformational isomer) of an acyclic molecule is a specific disposition of atoms in the molecule in space within the molecule due to free rotation around bonds.
Example: Ethane (CH3CH3)
Due to free rotation around the carbon-carbon bond, the ethane molecule could assume an infinite number of conformations, two of which are shown below as “saw-horse formulae”:
Further Reading
Khan Academy
FAQs
What is an isomer PDF? ›
ISOMERISM. Isomerism is the phenomenon in which more than one compounds have the same chemical formula but different chemical structures. Chemical compounds that have identical chemical formula but differ in properties and the arrangement of atoms in the molecule are called isomers.
How many C 3h 8o constitutional isomers are possible? ›Therefore, C 3 H 8 O exhibits three constitutional isomers.
What is an isomer for dummies? ›Molecules can differ in the way the atoms are arranged - the same combination of atoms can be assembled in more than one way. These compounds are known as isomers. Isomers are molecules with the same molecular formulas, but different arrangements of atoms.
How do you read isomers? ›If the highest priority groups for each carbon are on the same side of the molecule, that molecule is denoted as the 'cis' or 'Z' isomer. If they're on opposite sites, it's denoted as the 'trans' or 'E' isomer.
How many isomers are there? ›Hint: Isomers are defined as the molecules which have the same molecular formula but different molecular geometries. There are two types of isomers such as conformational isomers and constitutional isomers.
What are isomers A level? ›Isomers are molecules that have the same molecular formula, but have a different arrangement of the atoms in space.
What are the three main isomers? ›The three types of structural isomerism are functional groups, chains, and positional isomers. Functional group isomers have the same chemical formula but different functional groups.
How do you write isomers? ›1. Draw the main chain (i.e. the straight chain containing all the C atoms). 2. Draw the main chain minus 1 carbon, and add a methyl group to as many positions as possible; in other words, chop a C from one of the ends and attach it in as many places as you possibly can.
What is the summary of isomers? ›Isomers are compounds that have the same molecular formula but possess a different chemical structure, potentially leading to vastly different physical, chemical, and biological properties. Constitutional isomers possess a different connectivity from one another.
How many isomers of c4 h9 are possible? ›Hence, a total of 8 isomers are possible.
How many isomers of C 6h 14 are possible? ›
Therefore, C6H14 has 5 structural isomers.
How many isomers are formed by c3 h9 in? ›The number of structural isomers possible from the molecular formula C3H9N is 4.
How many isomers can be possible? ›...
Number of Isomers of Alkanes.
| Number of C Atoms | Possible Isomers |
|---|---|
| 1-3 | 1 |
| 4 | 2 |
| 5 | 3 |
| 6 | 5 |
Total no. of stereoisomers = [ 2(n-1)-2(n/2-1/2)] + 2(n/2-1/2)]
How do I know how many isomers there are? ›Formula for number of isomers for a compund = 2^n, where n = number of chiral carbons in said compound.
What is a real life example of an isomer? ›The substituted xanthines are a good example of an isomer found in food and drugs. Theobromine, caffeine, and theophylline are isomers, differing in the placement of methyl groups. Another example of isomerism occurs in phenethylamine drugs.
What is the difference between an isomer and an isotope? ›Isotopes and isomers are two different concepts in chemistry. Isotopes are atoms of the same element with the same number of protons but a different number of neutrons, while isomers are molecules with the same molecular formula but different arrangements of atoms.
What is a good example of an isomer? ›Examples of Isomers-
Ethyl alcohol and dimethyl ether are isomers of each other as both the compounds have the same molecular formula – C2H6O while different structural formulae. 2. Compounds such as pentane, iso-pentane and neopentane are isomers of each other.
Isomers refer to compounds that have the same molecular formula but are structurally different. When isomers only differ in the spatial arrangement of their atoms, they constitute spatial isomers, or stereoisomers, a group that also includes the optical isomers.
What is E and Z in organic chemistry? ›If both substituents ranked 1 are on the same side of the pi bond, the bond is given the descriptor Z (short for German Zusammen, which means “together”). If both substituents ranked 1 are on the opposite side of the pi bond, the bond is given the descriptor E (short for German Entgegen, which means “opposite”).
Do isomers have different names? ›
As is true for all constitutional isomers, each different compound has a different IUPAC name. Furthermore, the molecular formula provides information about some of the structural features that must be present in the isomers.
Which has largest isomers? ›[Co(en)2C12]+ has largest number of isomers which includes geometrical isomers (cis and trans isomers) as well as stereoisomers (d form, l form and optically inactive form).
Are isomers important? ›Because isomers have different pharmacokinetic and pharmacodynamic features, isomerism is important in clinical pharmacology and pharmacotherapeutics. Currently, understanding isomerism has aided in the development of safer and more effective pharmacological alternatives for both new and existing pharmaceuticals.
Do all organic compounds have isomers? ›All organic compounds have isomers. Isomer means that it is the same molecular formula but structured differently. The connectivity of the compound can ultimately determine the functionality of the compound. For example, the same molecular formula could be used to identify an alcohol or an ether.
Which compound has an isomer? ›Hydrocarbons with four or more carbon atoms have isomers. The more carbon atoms a hydrocarbon has, the greater the number of isomers. Isomers are different compounds with different properties, such as different boiling and melting points.
How many isomers are there in organic chemistry? ›There are two primary types of isomerism, which can be further categorized into different subtypes. These primary types are Structural Isomerism and Stereoisomerism. The classification of different types of isomers is illustrated below.
What is a functional isomer? ›“Functional isomers are structural isomers that have the same molecular formula (that is, the same number of atoms of the same elements), but the atoms are connected in different ways so that the groupings are dissimilar.
What is an isomer structure? ›In chemistry, a structural isomer (or constitutional isomer in the IUPAC nomenclature) of a compound is another compound whose molecule has the same number of atoms of each element, but with logically distinct bonds between them. The term metamer was formerly used for the same concept.
What are important facts on isomers? ›isomers. isomers Chemical compounds having the same molecular formula but different properties due to the different arrangement of atoms within the molecules. Structural isomers have atoms connected in different ways. Geometric isomers, also called cis-trans isomers, differ in their symmetry about a double bond.
What causes isomers? ›Cis-trans isomers can occur when atoms or functional groups are situated on either end of a rigid carbon-carbon bond, such as a double bond. In this case, restricted rotation about the double bond means that the atoms or groups attached to either end can exist in one of two possible configurations.
How many total isomers are formed by C5 h10? ›
The structures of these compounds are given below: Hence, the compound having molecular formula C5H10 has four chain isomers.
How many isomers are there in c4 h8 o2? ›The three isomers with the molecular formula C4H8O2 contain different functional groups and carbon skeletons.
What are the isomers of c4 h10 O? ›There are seven isomers in C4H10O. Out of these seven isomers, four are alcohol and three are ether.
How many isomers are there of c4 h10 of total? ›C 4 H 10 has two isomers. Among these, “n-Butane” is a straight-chained compound while “2-Methylpropane” (isobutane) is the branched-chain isomer.
How many isomers of c4 h10 O are possible? ›There are seven isomers in C4H10O. Out of these seven isomers, four are alcohol and three are ether.
What is the total number of isomers for c4 h10? ›How Many Isomers Does Butane have? Butane is an alkane with four carbon atoms so molecular formula is C4H10. It has two isomers; n-butane and isobutane.
How many isomers of c4 h8 are possible? ›There are five possible isomers with the given formula, C 4 H 8 . These include cyclobutane, but - 1 - ene, but - 2 - ene, 2 - methylpropane, and methyl cyclopropane.
How many isomers does c3 h4 have? ›There are three structural isomers of C3H4, propadiene, propyne, and cyclopropene.
How many isomers does c3 h6 o2 have? ›Correct answer is '4'.
What is an isomer? ›(I-soh-mer) One of two or more compounds that have the same chemical formula but different arrangements of the atoms within the molecules and that may have different physical/chemical properties.
What is isomers best definition? ›
Chemical compounds that have identical chemical formulae but differ in properties and the arrangement of atoms in the molecule are called isomers. Therefore, the compounds that exhibit isomerism are known as isomers. The word “isomer” is derived from the Greek words “isos” and “meros”, which mean “equal parts”.
What is the isomer in chemistry? ›Isomers refer to compounds that have the same molecular formula but are structurally different. When isomers only differ in the spatial arrangement of their atoms, they constitute spatial isomers, or stereoisomers, a group that also includes the optical isomers.
What is an isomer and why is it important? ›Isomers are different compounds with the same molecular formula but that are structurally different in some way. It is important to be able to recognise isomers because they can have different chemical, physical properties and biological properties.
What are the 3 types of isomer? ›Types of Isomers: Constitutional, Stereoisomers, Enantiomers, and Diastereomers.
What is an example of an isomer? ›Examples of Isomers-
1. Ethyl alcohol and dimethyl ether are isomers of each other as both the compounds have the same molecular formula – C2H6O while different structural formulae. 2. Compounds such as pentane, iso-pentane and neopentane are isomers of each other.
- The number of optical isomers of a compound is determined by calculating the number of chiral centres in it. - The maximum number of optical isomers is given by the formula ${{2}^{n- 1}}$, where n is the number of chiral centres.
How are isomers used in real life? ›Isomers are especially important in nutrition and medicine because enzymes tend to work on one isomer over another. The substituted xanthines are a good example of an isomer found in food and drugs. Theobromine, caffeine, and theophylline are isomers, differing in the placement of methyl groups.
What is true for all isomers? ›Answer and Explanation: True: Having the same molecular mass in the isomers is an important part of the definition of isomerism. Isomers have the same chemical formulas but different chemical structures.
What makes isomers different? ›Isomers are molecules that have the same molecular formula, but have a different arrangement of the atoms in space. That excludes any different arrangements which are simply due to the molecule rotating as a whole, or rotating about particular bonds.