Slide 1

• Haloakanes and Haloarenes - Molecular Asymmetry

Slide 2

• Definition of Molecular Asymmetry:
  • Molecular asymmetry refers to the property of a molecule having an asymmetric arrangement of atoms or groups of atoms.
  • Asymmetric molecules cannot be superimposed on their mirror images; they exhibit chirality.

Slide 3

• Chirality in Organic Chemistry:
  • Organic compounds can exhibit chirality if they contain asymmetric carbon atoms (chiral centers).
  • Chiral centers are carbon atoms bonded to four different groups or atoms.
• Example:
  • Consider the molecule 2-chlorobutane.
  • It contains a chiral center (C*) because the carbon atom is bonded to a hydrogen atom (H) and three different atoms (Cl, CH₃, and CH₂CH₃).

Slide 4

• Representation of Asymmetric Molecules:
  • Fischer Projection:
    • Fischer projection is a way to represent chiral molecules in a flat two-dimensional form.
    • In Fischer projections, vertical lines represent bonds going behind the plane, and horizontal lines represent bonds coming out of the plane.
• Example:
  • Fischer projection of D-alanine: H | HO-C-COOH | H

Slide 5

• Enantiomers:
  • Enantiomers are pairs of chiral molecules that are non-superimposable mirror images of each other.
  • Enantiomers have identical physical and chemical properties except for their interaction with plane-polarized light.
  • Enantiomers rotate plane-polarized light in equal amounts but in opposite directions (+/-).
• Example:
  • (R)-limonene and (S)-limonene are enantiomers.

Slide 6

• Racemic Mixture:
  • Racemic mixture is a mixture of equal amounts of both enantiomers.
  • Racemic mixtures are optically inactive as the rotation caused by one enantiomer is canceled out by the opposite rotation of the other enantiomer.
• Example:
  • A mixture of (R)-limonene and (S)-limonene in equal amounts forms a racemic mixture.

Slide 7

• Optical Activity:
  • Optical activity refers to the ability of a compound to rotate the plane of polarization of plane-polarized light.
  • Chiral compounds exhibit optical activity, while achiral compounds do not.
• Example:
  • D-glucose is chiral and exhibits optical activity, whereas D-fructose is achiral and does not exhibit optical activity.

Slide 8

• Specific Rotation:
  • Specific rotation is a measure of the extent to which a compound rotates the plane of polarization of plane-polarized light.
  • It depends on the concentration of the compound, the path length of the sample, and the temperature.
  • Specific rotation is denoted by the symbol [α].
• Example:
  • The specific rotation of D-glucose at a concentration of 1 g/mL, a path length of 10 cm, and a temperature of 25°C is +52.7°.

Slide 9

• Optical Purity:
  • Optical purity is a measure of the extent to which a sample of a chiral compound contains only one enantiomer.
  • It is expressed as a percentage and is determined by comparing the observed rotation with the rotation of pure enantiomer.
• Example:
  • A sample containing 80% (R) enantiomer and 20% (S) enantiomer has an optical purity of 80%.

Slide 10

• Importance of Molecular Asymmetry:
  • Pharmaceutical industry:
    • Many drugs exist as enantiomeric pairs, and their enantioselective synthesis and separation are crucial for drug development.
  • Biological systems:
    • Chiral molecules play a significant role in biological processes and their interactions with enzymes, receptors, and other molecules.
  • Material science:
    • Chiral molecules are used in the synthesis of materials with specific optical and electronic properties.

Slide 11

• Haloalkanes:
  • Haloalkanes are organic compounds in which one or more hydrogen atoms in an alkane are replaced by halogen atoms.
  • Common halogens used are chlorine (Cl), bromine (Br), and iodine (I).
  • General formula: RX, where R is an alkyl group and X is a halogen atom.
• Example:
  • Chloromethane (CH3Cl) and bromoethane (CH3CH2Br) are haloalkanes.

Slide 12

• Haloarenes:
  • Haloarenes are organic compounds in which one or more hydrogen atoms in an aromatic ring are replaced by halogen atoms.
  • Similar to haloalkanes, common halogens used are chlorine, bromine, and iodine.
  • General formula: ArX, where Ar is an aromatic ring and X is a halogen atom.
• Example:
  • Chlorobenzene (C6H5Cl) and bromobenzene (C6H5Br) are haloarenes.

Slide 13

• Nomenclature of Haloalkanes:
  • Haloalkanes are named by replacing the -e ending of the corresponding alkane with the appropriate halogen prefix (-fluoro, -chloro, -bromo, -iodo).
  • The position of the halogen atom is indicated by the lowest possible number.
• Example:
  • CH3Cl is named as chloromethane, while CH3CH2Br is named as bromoethane.

Slide 14

• Preparation of Haloalkanes:
  • Free radical halogenation: Alkanes react with halogens in the presence of heat or light to produce haloalkanes.
  • Nucleophilic substitution: Alcohols react with hydrogen halides (HX) or phosphorus halides (PX3, PX5) to produce haloalkanes.
• Examples:
  • Methane reacting with chlorine generates chloromethane.
  • Ethanol reacting with hydrochloric acid produces chloroethane.

Slide 15

• Reactions of Haloalkanes:
  • Substitution reactions: Haloalkanes undergo nucleophilic substitution reactions, where a nucleophile substitutes the halogen atom.
  • Elimination reactions: Haloalkanes can undergo elimination reactions to form alkenes when treated with strong bases.
• Example:
  • CH3Cl reacting with ammonia undergoes nucleophilic substitution to form CH3NH2 and HCl.

Slide 16

• Nomenclature of Haloarenes:
  • Haloarenes are named by indicating the position of the halogen atom using a number and prefix (ortho-, meta-, or para-) for disubstituted haloarenes.
  • The halogen atom is named using the appropriate halogen prefix.
• Example:
  • 1-chlorobenzene is named as ortho-chlorobenzene.

Slide 17

• Preparation of Haloarenes:
  • Electrophilic aromatic substitution: Aromatic compounds react with halogens in the presence of a Lewis acid catalyst to give haloarenes.
• Example:
  • Benzene reacting with bromine in the presence of FeBr3 catalyst produces bromobenzene.

Slide 18

• Reactions of Haloarenes:
  • Nucleophilic substitution: Similar to haloalkanes, haloarenes can undergo nucleophilic substitution reactions with certain nucleophiles.
  • Other reactions: Haloarenes can be further transformed through various reactions like diazotization and Sandmeyer reactions.
• Example:
  • Chlorobenzene reacting with sodium hydroxide undergoes nucleophilic substitution to form phenol.

Slide 19

• Biodegradation of Haloalkanes and Haloarenes:
  • Haloalkanes and haloarenes are persistent pollutants and can have harmful effects on the environment and human health.
  • Some microbes, called degraders, have the ability to metabolize and break down these compounds through microbial degradation.
• Example:
  • Bacterial strains like Pseudomonas putida and Dehalococcoides ethenogenes can degrade chlorinated compounds like trichloroethylene (TCE).

Slide 20

• Summary:
  • Haloalkanes and haloarenes exhibit molecular asymmetry due to the presence of a chiral carbon or asymmetric arrangement of atoms.
  • Enantiomers are non-superimposable mirror images, whereas racemic mixtures contain equal amounts of both enantiomers.
  • Chiral compounds exhibit optical activity and have specific rotations.
  • Haloalkanes and haloarenes have various methods of preparation, reactions, and environmental impacts.

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