Organic Chemistry: Alkanes and Reactions

Pharmaceutical Organic Chemistry-1 (PC102) Lecture 5

• Conformational Isomerism: Produced by rotation around single bonds, often rapidly interconverting at room temperature. Also known as conformers, rotamers, or rotational isomers.
• Conformational Analysis: The study of energy changes due to this free rotation.
• Newman Projection: The best way to represent conformations.

Conformational Analysis of Ethane

• Staggered Conformation: Each C-H bond of one carbon bisects the H-C-H angle of the other carbon.
• Eclipsed Conformation: Each C-H bond of one carbon is aligned with a C-H bond of the other carbon.

Conformational Analysis of Butane

• Staggered Conformers (Butane): Butane has two different staggered conformations: Gauche and anti.
• Types of Strain:
  • Eclipsed: Torsional strain and steric strain (Van der Waals strain).
  • Gauche: Steric strain.
  • Anti: No strain—most stable.

Conformation of Cycloalkanes with Small Rings

• Cyclopropane: Contains three carbon atoms; planar; C-C-C bond angle is only 60°, much less than the typical tetrahedral angle of 109.5°; hydrogen atoms lie above and below the carbon plane, and hydrogens on adjacent carbons are eclipsed. Exhibits angle strain and torsional strain.
• Cyclobutane: Can reduce torsional strain by adopting a nonplanar "puckered" conformation, decreasing eclipsing interactions. It adopts an envelope shape.
• Cyclopentane: Acquires an envelope shape.

Conformation of Cyclohexane

• Chair Conformation: The most stable conformation for cyclohexane; nonplanar; C-C-C bond angle is 111° with no torsional and no angle strain.
• Boat Conformation: Less stable than the chair; no angle strain, but it has eclipsing strain from pairs of hydrogen atoms on carbons 1-4, 2-3, and 5-6.
• Twist-Boat Conformation: More stable than the boat.
• Half-chair Conformation: The least stable; exhibits both angle and eclipsing strain.

Axial and Equatorial Bonds of Cyclohexane

• Axial Bonds: Bonds parallel to the vertical axis (up and down on adjacent carbons).
• Equatorial Bonds: Bonds arranged along the equator of the molecule.

Synthesis of Alkanes

• Clemmenson Reduction: Reduces carbonyl compounds to alkanes using Zinc amalgam (Zn/Hg) and HCl.
• Catalytic Hydrogenation: Adds hydrogen to alkenes and alkynes, often producing a syn addition. Requires a transition metal catalyst (e.g., platinum, palladium, or nickel) under specific temperature and pressure conditions.

Reactions of Alkanes and Cycloalkanes

• Combustion: Alkanes react with oxygen to form carbon dioxide and water.
• Halogenation: Substitution of hydrogen atoms in alkanes with halogen atoms (e.g., chlorine, bromine), often a free radical substitution reaction initiated by ultraviolet light or high temperatures. Bromination is selective, favoring tertiary > secondary > primary hydrogens.
• Reaction between methane and chlorine: A photochemical chain reaction.

Multiple Substitutions in Methane/ Chlorine Reaction

• Multiple substitution of chlorine in methane can occur to form chloromethane, dichloromethane, trichloromethane, or tetrachloromethane .

Additional Information

• Regioselectivity: Bromination of alkanes occurs preferentially at tertiary then secondary then primary hydrogen.

Test your knowledge of organic chemistry focusing on alkanes and their reactions. This quiz covers various concepts such as coupling reactions, catalytic hydrogenation, combustion products, and the role of ultraviolet light in halogenation. Challenge yourself and see how well you understand these essential topics!