Structural Isomer – Definition, Types, Examples

What is Structural isomer?

• Structural isomerism is a fundamental concept in chemistry that describes the relationship between compounds with the same molecular formula but different arrangements of atoms and bonds. It is also known as constitutional isomerism in the IUPAC nomenclature. The distinguishing feature of structural isomers is the logically distinct bonds between atoms within their molecules.
• To illustrate this concept, let’s consider the example provided in the content. Butanol (H3C−(CH2)3−OH), methyl propyl ether (H3C−(CH2)2−O−CH3), and diethyl ether ((H3CCH2−)2O) all have the molecular formula C4H10O. However, due to the differing connectivity of atoms, they are considered three distinct structural isomers.
• This concept extends beyond simple organic compounds and can also be applied to polyatomic ions and ionic compounds. For instance, the cyanate ion (O=C=N−) and the fulminate ion (C−≡N+−O−) have the same total charge but different arrangements of atoms, making them structural isomers. Similarly, ammonium cyanate ([NH4]+[O=C=N]−) and urea ((H2N−)2C=O) are considered structural isomers.
• Structural isomerism stands in contrast to stereoisomerism, where the atoms and bonding scheme remain the same, but the relative spatial arrangement of the atoms differs. Stereoisomers include enantiomers, which are mirror images of each other, and the cis and trans versions of molecules like 2-butene.
• Within the category of structural isomers, there are several subclasses. Skeletal isomers have different carbon backbones, positional isomers (or regioisomers) have different positions of functional groups or substituents, functional isomers have different functional groups, tautomers exhibit isomerism due to the movement of a hydrogen atom and the rearrangement of double bonds, and structural topoisomers have different arrangements of the same atoms in space.
• In summary, structural isomers are compounds with the same molecular formula but distinct arrangements of atoms and bonds. They represent the most radical form of isomerism in chemistry and play a crucial role in understanding the diversity of compounds that can be formed from a given set of atoms.

Types of Structural isomer

Structural isomerism, also known as constitutional isomerism, is a form of isomerism in which the atoms within a molecule are arranged in different orders, resulting in compounds with the same molecular formula but different structural formulae. Structural isomers often exhibit distinct physical and chemical properties. Let’s explore the different types of structural isomerism:

1. Chain Isomerism: Chain isomerism occurs when the carbon skeleton or backbone of the molecule differs between isomers. In other words, the arrangement of carbon atoms varies, leading to different structures. As a result, the physical and chemical properties of chain isomers can vary significantly. For example, butane (CH3-CH2-CH2-CH3) and isobutane (CH3-CH(CH3)-CH3) are chain isomers, both having the molecular formula C4H10 but differing in their branching patterns.
2. Positional Isomerism: Positional isomerism, also known as regioisomerism, arises when functional groups or substituents occupy different positions on the carbon chain. The molecular formula remains the same, but the arrangement of atoms within the molecule differs. This leads to variations in physical and chemical properties. An example of positional isomerism is exhibited by the compounds 1-butanol (CH3-CH2-CH2-CH2-OH) and 2-butanol (CH3-CH2-CH(OH)-CH3), both having the molecular formula C4H10O.
3. Functional Isomerism: Functional isomerism occurs when compounds have the same molecular formula but different functional groups. The functional groups in these isomers are responsible for the variation in chemical properties. For instance, ethanol (CH3-CH2-OH) and dimethyl ether (CH3-O-CH3) are functional isomers, both having the molecular formula C2H6O but differing in the functional group present.
4. Metamerism: Metamerism arises when compounds have the same molecular formula, the same functional group, and the same carbon skeleton, but differ in the alkyl or alkyl group attached to the functional group. This results in distinct physical and chemical properties. An example of metamerism is demonstrated by the compounds ethyl propyl ether (CH3-CH2-O-CH2-CH2-CH3) and methyl butyl ether (CH3-O-CH2-CH2-CH2-CH3), both having the molecular formula C6H14O but differing in the alkyl groups attached to the oxygen atom.
5. Tautomerism: Tautomerism occurs when compounds exist in dynamic equilibrium between two isomeric forms, known as tautomers, which readily interconvert. The interconversion involves the movement of a hydrogen atom and a rearrangement of double bonds. Tautomers exhibit different chemical properties due to the presence of different functional groups or bonding arrangements. An example of tautomerism is observed in the compounds keto-enol tautomerism, where compounds can exist as both keto and enol forms.

These different types of structural isomerism highlight the versatility of organic compounds and the various ways in which their atoms can be arranged. Understanding the different types of structural isomerism is essential for comprehending the diversity of compounds and their unique properties in the field of chemistry.

1. Chain isomerism

Chain isomerism is a type of structural isomerism that occurs as a result of different arrangements of carbon atoms, leading to the formation of compounds with linear and branched chains. In chain isomers, the molecular formula remains the same, but the type of chain structure differs.

Chain isomers exhibit similar chemical properties but can have distinct physical properties. One notable difference is seen in their boiling points. Branched chain isomers tend to have lower boiling points compared to their linear counterparts. This difference arises due to the variation in surface area of contact between molecules. Linear chain isomers have a greater surface area of contact, resulting in stronger intermolecular forces of attraction, which in turn leads to higher boiling points.

For instance, let’s consider the chain isomers with the molecular formula C4H12:

1. n-Butane: It is a linear chain isomer of butane. In n-butane, the carbon atoms are arranged in a straight, unbranched chain. The four carbon atoms form a continuous linear structure, with hydrogen atoms attached to each carbon atom. n-Butane is commonly used as a fuel due to its combustible properties.
2. Isobutane (or 2-methylpropane): It is a branched chain isomer of butane. In isobutane, the carbon atoms are arranged in a branched structure. Three carbon atoms form a chain, while the fourth carbon atom branches off, resulting in a methyl group (CH3) attached to it. Isobutane is commonly used as a refrigerant and as a fuel in camping stoves.

Although both n-butane and isobutane have the same molecular formula (C4H12), their different chain structures give rise to different physical properties, such as boiling points. The structural differences impact the intermolecular forces between molecules, affecting their behavior in terms of phase changes.

Chain isomerism is just one example of how variations in the arrangement of carbon atoms can result in distinct isomeric compounds. Understanding these structural differences is crucial in predicting and explaining the properties and behaviors of organic compounds in various chemical and physical processes.

2. Positional isomerism

Positional isomerism is a type of structural isomerism that occurs due to the different positions of side chains, substituents, functional groups, double bonds, or triple bonds on the parent chain or aromatic ring. In positional isomers, the molecular formula remains the same, but the arrangement of atoms within the molecule differs.

Let’s explore a few examples of positional isomerism:

1. Propyl chloride and isopropyl chloride: These compounds have the same molecular formula, C3H7Cl. However, they differ in the position of the chloro group on the main chain. Propyl chloride has the chloro group attached to the first carbon atom, while isopropyl chloride has the chloro group attached to the second carbon atom. This difference in position gives rise to positional isomerism.
2. But-1-ene and But-2-ene: These compounds have the same molecular formula, C4H8. The positional isomerism in this case arises from the different positions of the double bond within the carbon chain. But-1-ene has the double bond at the first carbon-carbon bond, while But-2-ene has the double bond at the second carbon-carbon bond.
3. Propan-1-ol and Propan-2-ol: These compounds have the same molecular formula, C3H7OH. The positional isomerism in this case arises from the different positions of the hydroxyl group (-OH) on the main carbon chain. Propan-1-ol has the hydroxyl group attached to the first carbon atom, while propan-2-ol has the hydroxyl group attached to the second carbon atom.
4. Ortho xylene, Meta xylene, and Para xylene: These compounds are positional isomers that have the same molecular formula, C8H10. The isomerism arises from the different positions of the three methyl groups (-CH3) on the benzene ring. Ortho xylene has the methyl groups attached to adjacent carbon atoms (1,2-positions), meta xylene has the methyl groups attached to carbon atoms with one carbon in between (1,3-positions), and para xylene has the methyl groups attached to carbon atoms with two carbons in between (1,4-positions).

Positional isomerism highlights the significance of the spatial arrangement of atoms and groups within a molecule. Even subtle changes in the position of functional groups or substituents can result in significant differences in physical and chemical properties of the isomers.

3. Functional isomerism

Functional isomerism is a type of structural isomerism that arises due to the presence of different functional groups in compounds. Functional isomers have the same molecular formula but possess distinct functional groups, resulting in differences in their physical and chemical properties.

Let’s explore a few examples of functional isomerism:

1. Ethyl alcohol and dimethyl ether: These compounds have the same molecular formula, C2H6O. However, they are functional isomers due to the presence of different functional groups. Ethyl alcohol contains the hydroxyl group (-OH), making it an alcohol, while dimethyl ether contains an oxygen atom bonded to two methyl groups (-O-CH3), making it an ether. The different functional groups lead to differences in their physical and chemical properties.
2. Acetaldehyde and acetone: These compounds have the same molecular formula, C3H6O. They are functional isomers because they have different functional groups. Acetaldehyde contains an aldehyde functional group (-CHO), whereas acetone contains a ketone functional group (>C=O). The presence of different functional groups imparts distinct chemical reactivity and physical properties to these compounds.
3. Acetic acid and methyl formate: These compounds are functional isomers with the same molecular formula, C2H4O2. Acetic acid is a carboxylic acid, characterized by the carboxyl group (-COOH). On the other hand, methyl formate is an ester, containing the ester functional group (-COOCH3). The different functional groups contribute to variations in their properties, such as acidity, reactivity, and odor.

Functional isomerism demonstrates how the presence of different functional groups can greatly influence the chemical behavior and properties of organic compounds. Even with the same number and types of atoms, the arrangement and connectivity of functional groups result in unique compounds with diverse characteristics. Understanding functional isomerism is crucial in areas such as organic synthesis, drug design, and the study of chemical reactions involving different functional groups.

4. Metamerism

Metamerism is a type of structural isomerism that occurs when different alkyl groups are attached to the same functional group within a molecule.

Let’s explore a few examples of metamerism:

1. Diethyl ether, methyl propyl ether, and isopropyl methyl ether: These compounds exhibit metamerism because they all contain the ether functional group (-O-) but differ in the nature of the alkyl groups attached to the oxygen atom. Diethyl ether has two ethyl groups (-CH2CH3) attached to the oxygen atom, methyl propyl ether has a methyl group (-CH3) and a propyl group (-CH2CH2CH3) attached, and isopropyl methyl ether has an isopropyl group (-CH(CH3)2) and a methyl group attached. The different alkyl groups give rise to metamerism, resulting in variations in their physical and chemical properties.
2. Metamerism in amines: Metamerism can also occur in amines, which are organic compounds containing the amino functional group (-NH2). In the case of amines, metamerism arises when different alkyl groups are attached to the nitrogen atom. The nature of the alkyl groups attached to the amino group affects the overall chemical behavior and properties of the compound, leading to metamerism.

Metamerism highlights the importance of the arrangement and nature of alkyl groups in organic compounds. Even though the functional group remains the same, the variation in the alkyl groups can result in distinct isomers with different chemical and physical characteristics. Understanding metamerism is crucial in organic chemistry as it helps explain the diversity of compounds that possess the same functional group but exhibit variations in reactivity, solubility, and other properties.

5. Tautomerism

Tautomerism refers to the dynamic equilibrium between two compounds with the same molecular formula but different functional groups. It is a special case of functional group isomerism where tautomers exist in a rapid interconversion between each other.

There are different types of tautomerism, and two notable examples are keto-enol tautomerism and nitro-aci tautomerism.

a. Keto-enol tautomerism

This type of tautomerism is observed in carbonyl compounds that contain at least one α-hydrogen atom. The carbonyl group can undergo a tautomeric transformation, converting into an enol form through the transfer of a hydrogen atom to the oxygen atom. The keto form represents the carbonyl compound, while the enol form contains an alkene alcohol group.

The composition of the tautomeric mixture between the keto and enol forms depends on the relative stabilities of the two forms. In general, the keto form is more stable and exists in a higher percentage. However, in certain cases, the enol form can be stabilized by other factors, leading to its appreciable stability.

For example, acetone exhibits keto-enol tautomerism, where the keto form is the dominant species. On the other hand, in 1,3-dicarbonyl compounds like acetoacetic ester (ethyl acetoacetate), the enol form is relatively more stable.

In the case of phenols, the enol form can be more stable than the keto form due to resonance stabilization of the aromatic ring.

b. Nitro-aci tautomerism

Nitro compounds that contain at least one α-hydrogen atom can exhibit nitro-aci tautomerism. During this tautomeric conversion, the hydrogen atom is transferred completely from one atom to another, resulting in the interconversion between the nitro and aci forms.

An example of nitro-aci tautomerism is observed in nitromethane, where the nitro form and aci form can exist in dynamic equilibrium.

It is important to note that tautomers are not simply resonance structures of the same compound. Instead, they are distinct compounds that undergo rapid interconversion. Tautomerism plays a significant role in the reactivity, stability, and properties of compounds, and understanding it is crucial in various areas of chemistry, including organic synthesis, biochemistry, and drug design.

Stereoisomers vs Structural isomers

Structural isomers and stereoisomers are two distinct types of isomerism that arise in chemistry. While both involve differences in the arrangement of atoms, they differ in the specific aspects of molecular structure that are responsible for the isomeric variations.

Structural isomers, also known as constitutional isomers, exhibit differences in the connectivity of atoms within a molecule. These isomers have the same molecular formula but differ in the way atoms are bonded to each other. The example of butanol illustrates structural isomerism, where the position of the alcohol group differs while the atoms involved remain the same. In structural isomers, the spatial arrangement of atoms is not considered, only the connectivity between them.

On the other hand, stereoisomers involve the same connectivity of atoms but differ in the spatial arrangement or conformation of the molecule. Stereoisomers can be further categorized into various types, such as geometric isomers (cis-trans isomers) and optical isomers (enantiomers). Geometric isomers, like cis-2-butene and trans-2-butene, have the same bonds between the same atoms, but differ in the spatial orientation around a double bond. The cis-isomer has substituents on the same side of the double bond, while the trans-isomer has them on opposite sides. The key point is that the connectivity of atoms remains unchanged between these stereoisomers.

Unlike structural isomers, stereoisomers possess identical connectivity but exhibit differences in the arrangement of atoms in three-dimensional space. This difference in spatial arrangement can lead to distinct physical and chemical properties. Stereoisomers are often characterized by their ability to exhibit optical activity, where enantiomers exist as non-superimposable mirror images of each other.

In summary, the main distinction between structural isomers and stereoisomers lies in the aspect of molecular structure that is responsible for the isomeric differences. Structural isomers differ in the connectivity of atoms, whereas stereoisomers have the same connectivity but differ in the spatial arrangement of atoms. Understanding these different types of isomerism is essential for comprehending the diversity of molecular structures and their properties in the field of chemistry.

Aspect	Structural Isomers	Stereoisomers
Definition	Differ in the connectivity of atoms within the molecule	Have the same connectivity of atoms but differ in spatial arrangement
Molecular Formula	Same	Same
Bonds	Different bonds between atoms	Same bonds between atoms
Arrangement	Connectivity of atoms is different	Spatial arrangement is different
Example	Butanol: H3C−(CH2)3−OH, Methyl propyl ether: H3C−(CH2)2−O−CH3, Diethyl ether: (H3CCH2−)2O	Trans-2-butene and cis-2-butene
Physical/Chemical	Different physical and chemical properties	Can exhibit different physical and chemical properties
Types	Chain isomerism, positional isomerism, functional isomerism, metamerism, tautomers, structural topoisomers	Geometric isomerism (cis-trans), optical isomerism (enantiomers)

Structural vs geometric isomers

Aspect	Structural Isomers	Geometric Isomers
Definition	Differ in the connectivity of atoms within the molecule	Have the same connectivity but differ in spatial arrangement
Molecular Formula	Same	Same
Bonds	Different bonds between atoms	Same bonds between atoms, but differ in spatial orientation
Arrangement	Connectivity of atoms is different	Spatial arrangement around a bond is different
Example	Butanol: H3C−(CH2)3−OH, Ethanol: H3C−CH2−OH	Trans-2-butene and cis-2-butene
Physical/Chemical	Different physical and chemical properties	Can exhibit different physical and chemical properties
Types	Chain isomerism, positional isomerism, functional isomerism, etc.	Geometric isomerism (cis-trans)

Draw all the structural isomers for the molecular formula c4h9br – structural isomers of c4h9br

1. n-Butyl bromide: CH3CH2CH2CH2Br
2. sec-Butyl bromide: CH3CH(Br)CH2CH3
3. iso-Butyl bromide: CH3CH(CH3)CH2Br
4. tert-Butyl bromide: (CH3)3CBr

These four isomers have the same molecular formula (C4H9Br), but they differ in the arrangement or position of the bromine atom in the carbon chain.

c5h12 structural isomers

Here are the structural isomers for the molecular formula C5H12:

1. Pentane: CH3CH2CH2CH2CH3
2. Isopentane (or 2-methylbutane): CH3CH(CH3)CH2CH3
3. Neopentane (or 2,2-dimethylpropane): (CH3)3CCH3
4. Cyclopentane: (CH2)4
5. 2-Methylpentane: CH3CH2CH(CH3)CH3
6. 3-Methylpentane: CH3CH2CH2CH(CH3)2
7. 2,2-Dimethylbutane: (CH3)3CCH2CH3

These are the seven structural isomers for the molecular formula C5H12. They have the same number of carbon and hydrogen atoms, but their arrangement differs, resulting in different chemical properties.

Structural isomer example

Here are some examples of structural isomers:

1. Butane and Isobutane:
   • Butane: CH3CH2CH2CH3
   • Isobutane: (CH3)3CH
2. Pentane and Neopentane:
   • Pentane: CH3CH2CH2CH2CH3
   • Neopentane: (CH3)4C
3. Ethanol and Dimethyl Ether:
   • Ethanol: CH3CH2OH
   • Dimethyl Ether: CH3OCH3
4. Butanone and Butanol:
   • Butanone (Methyl ethyl ketone): CH3COCH2CH3
   • Butanol: CH3CH2CH2CH2OH
5. Acetone and Acetaldehyde:
   • Acetone: CH3COCH3
   • Acetaldehyde: CH3CHO

These examples illustrate different arrangements of atoms and bonds within molecules, resulting in distinct structural isomers.

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