Question

Organic compounds having the same molecular formula, but different structural formulae are called

Short answer

Answer: The term for organic compounds with the same molecular formula but different structural formulae is isomers. Examples of isomers include: 1. Structural isomers: (a) Chain isomers: n-pentane, 2-methylbutane, and 2,2-dimethylpropane (all have the molecular formula C5H12). (b) Positional isomers: 1-butanol and 2-butanol (both have the molecular formula C4H8O). (c) Functional group isomers: ethanol (ethyl alcohol) and dimethyl ether (both have the molecular formula C2H6O). 2. Stereoisomers: (a) Geometrical isomers: cis-2-butene and trans-2-butene (both have the molecular formula C4H8). (b) Optical isomers: L-alanine and D-alanine (both have the molecular formula C3H7NO2).

Step-by-step solution

Define Isomers

Isomers are organic compounds with the same molecular formula, but they have different structural formulae. This means that they have the same number and types of atoms, but these atoms are arranged differently, leading to different chemical and physical properties.

Types of Isomers

There are two main types of isomers: 1. Structural (constitutional) isomers: In these isomers, atoms are connected in different orders. They can be further classified into chain, positional, and functional group isomers. 2. Stereoisomers: In these isomers, atoms are connected in the same order but have different spatial arrangements. They can be further classified into geometrical (cis-trans) isomers and optical isomers (enantiomers and diastereomers).

Examples of Isomers

Let's take a look at some examples of isomers: 1. Structural isomers: (a) Chain isomers: C5H12 can be expressed as \(\text{n-pentane}\), \(\text{2-methylbutane}\), and \(\text{2,2-dimethylpropane}\). (b) Positional isomers: C4H8O can be expressed as \(\text{1-butanol}\) and \(\text{2-butanol}\). (c) Functional group isomers: C2H6O can be expressed as \(\text{ethanol}\) \((\)ethyl alcohol\()\) and \(\text{dimethyl ether}\). 2. Stereoisomers: (a) Geometrical isomers: \(\text{cis-2-butene}\) and \(\text{trans-2-butene}\) are geometrical isomers with different spatial arrangements of the double bond. (b) Optical isomers: L-alanine and D-alanine are enantiomers that are non-superimposable mirror images of each other in terms of their three-dimensional structures. In conclusion, isomers are organic compounds that exhibit the same molecular formula but have different structural formulae, resulting in different properties. They can be categorized into structural isomers and stereoisomers.

Key concepts

Structural Isomers

When two or more compounds share the same molecular formula but have distinct connectivity of their atoms, they are known as structural isomers. This unique characteristic means that the molecules possess the same number and types of atoms but differ in how those atoms are linked together.

For instance, the molecular formula C₄H₁₀ could represent butane or isobutane. The difference lies in the arrangement of the carbon atoms; butane has a straight-chain structure, while isobutane has a branched form. These differences in structure can give structural isomers very diverse chemical and physical properties even though they have the same molecular composition.

Structural isomerism itself falls into several categories, including chain isomerism, positional isomerism, and functional group isomerism, each with unique variations in atom connectivity.

Stereoisomers

Stereoisomers are fascinating in that they have the same molecular formula and sequence of bonded atoms as another molecule, but their atoms are oriented differently in three-dimensional space. It's like having the same ingredients and recipe for a dish, but the plating can vary from chef to chef.

This concept becomes especially interesting when we understand that such isomers can exhibit markedly different biological activities despite their common structures, highlighting the importance of molecular orientation in chemical reactions and interactions.

Molecular Formula

The molecular formula is a representation that provides the actual number of each type of atom in a molecule. It's like the ingredient list for a molecule, detailing what's inside without explaining how everything is put together.

For example, glucose and fructose both have a molecular formula of C₆H₁₂O₆, although they are distinctly different in structure. The molecular formula is a starting point in the identification of a compound but doesn't reveal the arrangement of atoms, which can significantly impact the properties and reactions of the substance.

Geometrical Isomers

Geometrical isomers, also known as cis-trans isomers, are a type of stereoisomer where molecules differ in the spatial positioning around a double bond or a ring structure.

This type of isomerism arises because the pi bond present in double bonds restricts the rotation of the bonded atoms, locking them in place. Geometrical isomers often possess different physical properties such as melting points, boiling points, and solubility. An example includes cis-2-butene and trans-2-butene, where 'cis' indicates that the higher priority groups are on the same side of the double bond, while 'trans' stands for them being on opposite sides.

Optical Isomers

Optical isomers, also called enantiomers, are non-superimposable mirror images of each other, much like your left hand is to your right hand. This type of isomerism is found in molecules that have a chiral center, typically a carbon atom with four different atoms or groups attached to it.

Enantiomers have identical physical properties but differ in one unique way — how they interact with polarized light. One will rotate the light clockwise (dextrorotatory) and the other counterclockwise (levorotatory). Additionally, their interactions with biological systems can be drastically different, making the study of optical isomers critical in fields like pharmaceuticals where one enantiomer can be therapeutic while the other could be harmful.