Slide 1: Haloalkanes and Haloarenes - Optically Active Compounds

  • In organic chemistry, some compounds exhibit optical activity
  • Optical activity is the ability of a compound to rotate the plane of polarized light
  • Haloalkanes and haloarenes are two classes of compounds that can show optical activity
  • We will explore these compounds and their optical properties in this lecture

Slide 2: Haloalkanes and Haloarenes

  • Haloalkanes are compounds that contain a halogen atom (fluorine, chlorine, bromine, or iodine) bonded to a carbon atom
  • Haloarenes are compounds that contain a halogen atom bonded to an aromatic ring
  • Both haloalkanes and haloarenes can exhibit optical activity when they have an asymmetric carbon or an asymmetric carbon within the ring, respectively

Slide 3: Asymmetric Carbon in Haloalkanes

  • An asymmetric carbon is a carbon atom that is bonded to four different groups
  • In haloalkanes, an asymmetric carbon leads to optical activity
  • Optical activity arises due to the spatial arrangement of the groups around the asymmetric carbon

Slide 4: Chirality in Haloalkanes

  • Chirality refers to the property of having a nonsuperimposable mirror image
  • Haloalkanes with an asymmetric carbon are chiral compounds
  • Chiral compounds exist as a pair of enantiomers, which are mirror images of each other

Slide 5: Enantiomers

  • Enantiomers have the same physical and chemical properties except for their interactions with plane-polarized light
  • Enantiomers rotate plane-polarized light either to the left (Levorotatory - symbol “(-)”) or right (Dextrorotatory - symbol “(+)”)
  • The amount of rotation is given by the specific rotation (α)

Slide 6: Specific Rotation

  • Specific rotation (α) is a measure of the amount and direction of optical rotation
  • It is defined as the observed rotation (observed with a polarimeter) divided by the concentration of the compound and the path length of the solution
  • Specific rotation is expressed in units of degrees per unit concentration and path length (°/(g·cm))

Slide 7: Calculation of Specific Rotation

  • To calculate specific rotation, we use the formula: α = α_observed / (c * l)
  • α_observed is the observed rotation in degrees
  • c is the concentration of the compound in g/mL
  • l is the path length of the polarimeter tube in cm
  • The sign of α indicates whether the compound is levorotatory or dextrorotatory

Slide 8: Optical Activity in Haloarenes

  • Haloarenes can also show optical activity if they have an asymmetric carbon within the ring
  • In haloarenes, optical activity arises due to the spatial arrangement of the groups around the asymmetric carbon in the aromatic ring

Slide 9: Optically Active Haloarenes

  • Just like in haloalkanes, an asymmetric carbon in haloarenes leads to optical activity
  • The optical rotation of haloarenes is also measured using a polarimeter and expressed in terms of specific rotation
  • The specific rotation of optically active haloarenes can be calculated using the same formula as for haloalkanes

Slide 10: Examples of Optically Active Compounds

  • Let’s explore some examples of optically active haloalkanes and haloarenes:
    • (S)-2-Chlorobutane: An optically active haloalkane, with the chlorine atom bonded to an asymmetric carbon
    • (R)-1-Bromo-3-chlorocyclohexane: An optically active haloarene, with the bromine and chlorine atoms bonded to an asymmetric carbon in the aromatic ring

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  1. Optical Activity in Haloalkanes:
  • Haloalkanes with an asymmetric carbon exhibit optical activity
  • The presence of different substituents around the asymmetric carbon results in nonsuperimposable mirror images
  • These mirror images are called enantiomers
  • Enantiomers have identical physical and chemical properties except for their interaction with plane-polarized light
  • The specific rotation of enantiomers can be determined experimentally
  1. Calculation of Specific Rotation in Haloalkanes:
  • Specific rotation (α) is calculated using the formula: α = α_observed / (c * l)
  • α_observed is the observed rotation in degrees
  • c is the concentration of the compound in g/mL
  • l is the path length of the polarimeter tube in cm
  • The sign of α indicates the direction of rotation (levorotatory or dextrorotatory)
  1. Optical Activity in Haloarenes:
  • Haloarenes can also exhibit optical activity if they possess an asymmetric carbon within the aromatic ring
  • The spatial arrangement of substituents around the asymmetric carbon determines the optical activity
  • Just like in haloalkanes, haloarenes exist as enantiomeric pairs
  1. Racemic Mixture:
  • A racemic mixture is a 50:50 mixture of two enantiomers
  • It does not exhibit optical activity because the rotations of the two enantiomers cancel out
  • Racemic mixtures are indicated by a (+/-) prefix in their names or by a racemic symbol (dotted line) in their structural formulas
  1. Resolving Agents:
  • Resolving agents can be used to separate enantiomers from a racemic mixture
  • They selectively react with one enantiomer, converting it into a diastereomer or a salt that can be separated from the other enantiomer
  • Common resolving agents include tartaric acid, cinchona alkaloids, and chiral stationary phase columns in chromatography
  1. Chiral Drugs:
  • Optical activity is relevant in pharmaceuticals because some enantiomers exhibit different biological activities
  • One enantiomer may have the desired therapeutic effect, while the other may be inactive or even have adverse effects
  • Chiral drugs are typically formulated as single enantiomers to ensure maximum efficacy and safety
  1. Examples of Optically Active Compounds:
  • (S)-2-Chlorobutane: An optically active haloalkane with an asymmetric carbon
  • (R)-1-Bromo-3-chlorocyclohexane: An optically active haloarene with an asymmetric carbon in the aromatic ring
  • Both compounds exist as pairs of enantiomers and show optical rotation
  1. Importance of Optical Activity in Chemical Industries:
  • Optical activity is relevant in the production of pharmaceuticals, flavors, fragrances, and agrochemicals
  • Enantiomerically pure compounds are required to ensure desired therapeutic effects and minimize side effects
  • Optical activity also plays a role in the synthesis of chiral ligands catalysts for asymmetric synthesis
  1. Stereochemistry and Molecular Recognition:
  • Optical activity is linked to stereochemistry, which focuses on the three-dimensional arrangement of atoms in molecules
  • Enantiomers can have different interactions with other chiral molecules or biomolecules due to their unique stereochemical properties
  • This can affect molecular recognition, receptor binding, and biological activity
  1. Conclusion:
  • Haloalkanes and haloarenes with an asymmetric carbon can exhibit optical activity
  • Enantiomers are mirror images but not superimposable
  • Specific rotation quantifies optical activity, with α indicating the direction of rotation
  • Optical activity is relevant in pharmaceuticals, chemical industries, and molecular recognition processes
  • Resolving agents and chiral drug formulations are used to isolate and utilize specific enantiomers for desired effects.