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Chapter 23: Problem 59

What are the differences between geometric isomers and optical isomers?

Short Answer

Expert verified
Geometric isomers, also known as cis-trans isomers, differ in the arrangement of groups around a carbon-carbon double bond or a ring structure. Optical isomers, on the other hand, are non-superimposable mirror images of each other resulting from an asymmetric carbon atom present in the molecule. The major difference lies in their properties and behavior; geometric isomers differ in physical properties and can be converted into each other, while optical isomers rotate polarized light and cannot be converted without reformation of bonds.

Step by step solution

01

Define Geometric Isomers

Geometric isomerism, also known as cis-trans isomerism or E-Z isomerism, involves the arrangement of atoms or groups around a carbon-carbon double bond or a ring structure where rotation is restricted. The different spatial arrangement gives rise to different compounds.
02

Define Optical Isomers

Optical isomerism occurs when there is an asymmetric carbon atom, known as a chiral center, in the molecule. The presence of this carbon atom leads to molecules that are non-superimposable mirror images of each other, much like left and right hands. They are called enantiomers.
03

Describe Differences

While both geometric and optical isomers are forms of stereoisomerism, they differ in their properties and behavior. Geometric isomers differ in physical properties like melting and boiling points, while optical isomers rotate the plane of polarized light in different directions. Geometric isomers can be converted into one another by simple rotation unless they are part of a ring structure, whereas optical isomers cannot be converted into each other without breaking and reforming bonds.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Stereoisomerism
Stereoisomerism is a form of isomerism where molecules with the same molecular formula have different spatial arrangements of atoms. These variations arise from the fact that certain bonds, especially double bonds and structures involving rings, restrict the ability of the atoms to rotate freely. Stereoisomers are categorized mainly into geometric isomers and optical isomers. Despite having the same types and numbers of atoms, their spatial arrangement dramatically influences their chemical and physical properties, such as how they interact with other molecules and how they are perceived in three-dimensional space.
Chiral Centers
A chiral center, also known as a stereocenter, is a carbon atom that is attached to four different groups or atoms. This special configuration means that the molecule cannot be superimposed on its mirror image, much like how your left hand cannot be perfectly aligned with your right hand when facing each other. Chiral centers are crucial for the existence of optical isomers, leading to compounds with distinct behaviors, especially in how they interact with plane-polarized light and biological systems. The presence of chiral centers usually confers a unique sense of 'handedness' to the molecule, referred to as chirality.
Enantiomers
Enantiomers are a pair of optical isomers that are non-superimposable mirror images of each other. This means that one enantiomer cannot be rotated or flipped to be exactly like its counterpart, just as your left and right hands are mirror images but not the same. They may have identical physical properties like melting points and solubility but interact differently with polarized light and often with biological molecules. This distinction is crucial in fields like pharmaceuticals, where one enantiomer may be significantly more effective or less toxic than its mirror image.
Cis-Trans Isomerism
Cis-trans isomerism, a type of geometric isomerism, occurs in molecules with restricted rotation around a bond, usually a carbon-carbon double bond or within a cyclic structure. If the prominent groups or atoms are on the same side of the bond or plane, it's termed 'cis'. Conversely, if they are on opposite sides, it's called 'trans'. This difference significantly affects the compounds' physical properties and their chemical reactivity due to the distinct spatial arrangement.
E-Z Isomerism
E-Z isomerism is a more comprehensive form of geometric isomerism that considers the relative positions of substituents around a double bond. The terms 'E' (from the German 'entgegen') and 'Z' (from the German 'zusammen') denote whether the higher priority groups are across (E) or together (Z) on the same side. The priority of groups is determined by the Cahn-Ingold-Prelog priority rules. E-Z isomerism is particularly useful for complex molecules where the classical cis-trans designation doesn't adequately describe the isomerism.
Polarized Light Rotation
Polarized light rotation, also known as optical activity, is a phenomenon where a chiral molecule rotates the plane of polarized light passing through it. This rotation can be either to the left (levorotatory) or to the right (dextrorotatory), represented by the symbols (-) or (+), respectively. The degree to which this rotation happens is measurable and known as the optical rotation or specific rotation of a substance. This property is characteristic of optical isomers and is one critical way to distinguish between enantiomers, as they rotate polarized light equally but in opposite directions.

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Most popular questions from this chapter

Give the oxidation numbers of the metals in the following species: (a) \(\mathrm{K}_{3}\left[\mathrm{Fe}(\mathrm{CN})_{6}\right],\) (b) \(\mathrm{K}_{3}\left[\mathrm{Cr}\left(\mathrm{C}_{2} \mathrm{O}_{4}\right)_{3}\right]\) (c) \(\left[\mathrm{Ni}(\mathrm{CN})_{4}\right]^{2-}\)

In a dilute nitric acid solution, \(\mathrm{Fe}^{3+}\) reacts with thiocyanate ion (SCN \(^{-}\) ) to form a dark-red complex: $$ \mathrm{H}_{2} \mathrm{O}+\left[\mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)_{5} \mathrm{NCS}\right]^{2+} $$ The equilibrium concentration of \(\left[\mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)_{5} \mathrm{NCS}\right]^{2+}\) may be determined by how darkly colored the solution is (measured by a spectrometer). In one such experiment, \(1.0 \mathrm{~mL}\) of \(0.20 \mathrm{M} \mathrm{Fe}\left(\mathrm{NO}_{3}\right)_{3}\) was mixed with \(1.0 \mathrm{~mL}\) of \(1.0 \times 10^{-3} \mathrm{M} \mathrm{KSCN}\) and \(8.0 \mathrm{~mL}\) of dilute \(\mathrm{HNO}_{3}\). The color of the solution quantitatively indicated that the \(\left[\mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)_{5} \mathrm{NCS}\right]^{2+}\) concentration was \(7.3 \times 10^{-5} M .\) Calculate the formation constant for \(\left[\mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)_{5} \mathrm{NCS}\right]^{2+}\)

How many geometric isomers are in the following (b) \(\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{3} \mathrm{Cl}_{3}\right] ?\) species: (a) \(\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{2} \mathrm{Cl}_{4}\right]^{-},(\)

The \(K_{\mathrm{f}}\) for the complex ion formation between \(\mathrm{Pb}^{2+}\) and EDTA \(^{4-}\) $$ \mathrm{Pb}^{2+}+\mathrm{EDTA}^{4-} \rightleftharpoons \mathrm{Pb}(\mathrm{EDTA})^{2-} $$ is \(1.0 \times 10^{18}\) at \(25^{\circ} \mathrm{C} .\) Calculate \(\left[\mathrm{Pb}^{2+}\right]\) at equilibrium in a solution containing \(1.0 \times 10^{-3} M \mathrm{~Pb}^{2+}\) and \(2.0 \times 10^{-3} M \mathrm{EDTA}^{4-}\)

Predict the number of unpaired electrons in the following complex ions: (a) \(\left[\mathrm{Cr}(\mathrm{CN})_{6}\right]^{4-},(\mathrm{b})\left[\mathrm{Cr}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{2+}\)
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