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Structural Isomerism

Last updated date: 27th Nov 2023
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This article is about the chemical concept of isomers. In chemistry, isomers are polyatomic or molecules or ions with identical chemical formulas but distinct arrangements of atoms in a particular space. Necessarily Isomers do not share similar properties. Two main forms of isomerism is stereoisomerism (or spatial isomerism, in which the orientations of atoms differ).

Structural isomerism or constitutional isomerism, these are in which bonds differ. Structural isomers differ in terms of the connectivity of some of the atoms or all constituent atoms.

 For example: methoxyethane and propanols.

Isomers simple example is given by propanol that It has the formula C3H8O (or C3H7OH) and it occurs as two isomers: propan-2-ol (isopropyl alcohol; 

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These two molecules are known as the position isomers of each other, it is called so because the position of the hydroxyl group differs between the two: in the first isomer It is attached to an end carbon in the, and to the center carbon in the second.

Other isomer of C3H8O is methoxyethane (ethyl-methyl-ether; III). Unlike the isomers of propanol, methoxyethane has an oxygen which is connected to two carbons rather than  one hydrogen and one carbon. Methoxyethane is an ether, because it lacks a hydroxyl group, and it has chemical properties more similar to other ethers than to either of the above alcohol isomers.

For example: propyne and propadiene

Propyne (or methylacetylene), Propadiene (or allene)  are examples of isomers. Two double bonds are contained by Propadiene, whereas propyne contains one triple bond.

Structural Isomers

Constitutional isomer or a structural isomer, (per IUPAC), is a type of isomer in which molecules with the same molecular formula have different bonding patterns and organisation of atoms, as opposed to stereoisomers, in which molecular bonds are in the same order and only spatial arrangement differs. For structural isomers there are multiple synonyms.

Structural isomers three categories are skeletal, positional, and functional isomers. Positional isomers are also known as regioisomers.

Structural isomers are those isomers in which the molecular formula is the same and atoms are completely arranged in a different order.

These are the molecules having the same kind of molecular formula with different connectivities which is depending upon the order they are put together with. One of the simplest examples is the structure of Alkane (C₄H₁₀) representing a structural isomer with different isomers. The structural isomers increase With the increase in the number of Carbon atoms in the alkane molecule.


Functional Group Isomers

Structural isomers are functional isomers that have the same molecular formula (that is, the same atoms number of the same elements), but the atoms are connected in different ways so that the groupings are not very similar. These groups of atoms are called functional groups or sometimes as functionalities.

For example, 1-hexene and cyclohexane both have the formula C6H12. These two are considered as the functional group isomers because 1-hexene is an alkene and cyclohexane is a cycloalkane.

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To be functional isomers for two molecules, they must contain key groups of atoms which are arranged in particular ways. Some of the best examples can be given from organic chemistry. C2H6O is the molecular formula of this group. Depending on how the atoms are arranged, it can represent in two different compounds ethanol CH3CH2-O-H or dimethyl ether CH3-O-CH3. Dimethyl ethanol and ether are functional isomers. The first one is known as ether. The carbon chain and -oxygen-carbon chain functionality is called an ether group. The second is the alcohol group. The carbon chain-oxygen-hydrogen functionality is known as an alcohol.

The location changes but If the functionalities stay the same, then the structural isomers are not functional isomers.  2-propanol and 1-Propanol are structural isomers, but they are not known as functional isomers. Both of them are called alcohols and are alcohols too. The functional group (carbon chain-O-H) is present in both of these compounds, but they are not the same compounds.

While some chemists use the terms functional isomer and structural isomer interchangeably, not all structural isomers are functional isomers.

In chemistry the functional isomers are most often identified with infrared spectroscopy. Infrared radiation corresponds to the energies associated with molecular vibration primarily. Due to hydrogen bonding the alcohol functionality has a very distinct vibration called OH-stretch. All alcohols in solid and liquid form absorb infrared radiation at certain wavelengths.

With the same functional groups the compounds will all absorb certain wavelengths of infrared light because of the vibrations associated with those groups. In fact, the infrared spectrum is bifurcated into two regions. The first part is known as the functional group region. Ethanol Dimethyl ether would have dissimilar infrared spectra in the functional group region.

Infrared spectrum in the second part of the is called the fingerprint region; it is associated with types of motion allowed by the structure of the molecule and influenced by the bond energies. More specific is the fingerprint region is to an individual compound. Even though 2-propanol and 1-propanol have similar infrared spectra in the functional group region, they differ in the fingerprint region.

If we say in simple terms, the functional isomers are structural isomers that have different functional groups like ether and alcohol.

FAQs on Structural Isomerism

Q1. Explain Different Types of Structural Isomerism.

Ans: The structural isomers have the same structural formulas but they have different arrangements of atoms. There are mainly three types of isomers: positional isomers, functional group isomers, chain isomers. The same molecular formula is of chain isomers, but they have different arrangement branches.

Q2. How Can We Identify a Structural Isomer?

Ans: We can identify the structural isomers by their bonding patterns. The compounds atoms are the same but they are connected in such a way as to make different functional groups. An example is n-butane is a straight hydrocarbon chain with four carbons while isobutane is branched. 

Q3. Give an Example and Explain What Structural Isomers?

Ans: Structural isomers are molecules with the same molecular formulas but their arrangement of atoms is different or the bonds are different. For example: isobutane and butane have the same number of hydrogen and carbon atoms so their molecular formula is also the same.

Q4. List a Few Characteristics of Isomerism.

Ans: They have different physical properties, they possess different chemical properties as well, they show the same properties when they are in the same functional groups, they have different structural formulas but same molecular formula, the isomers have different boiling points, they have the same number of atoms and have the same atomic weight.