Order of attraction: H-bond >> dipole-dipole > London Boiling point (bp) : the greater the molecular weight, the greater the number of electrons, and the greater the van der Waals forces. The bp of alkanes increase as their size increases. Branching lowers the bp because it reduces the area of contact.
Chemistry 2461– Fall 2008 Dr. G. Vicente 19 The bp of alcohols is higher than alkanes and ethers because alcohols can form hydrogen bonds . Ethers do not form hydrogen bonds because they lack a hydrogen attached to a very electronegative atom. Primary and secondary amines can also form hydrogen bonds. Melting point (mp) : in general increases as the molecular weight increases (as bp). Solubility : polar compounds dissolve in polar solvents and nonpolar compounds dissolve in nonpolar solvents. • Conformations of alkanes (2.10) Conformations : different arrangement of atoms that result from rotation around a C-C single bond. Different conformations have different energies and stability. Newman projections : viewing the molecule along the C- C bond, so the back C is hidden by the front C. The circle aids in distinguishing the bonds on the front C, which touch at the center of the circle from the bonds on the back C, which touch at the circumference of the circle. staggered conformation (lowest in energy, most stable) eclipsed conformation (highest in energy, least stable) Torsional strain : repulsion between the bonding electrons of one substituent as they pass close to the bonding electrons of another substituent.
Chemistry 2461– Fall 2008 Dr. G. Vicente 20 Steric strain : results when atoms or groups are too close to each other. For C2-C3 rotation in butane: CH 3 CH 3 H H H H H 3 C CH 3 H H H H 3 C H CH 3 H H H H CH 3 anti conformation gauche conformations CH 3 least stable eclipsed conformation In the anti conformation the largest substituents are opposite each other most stable conformation. In the gauche conformations the largest substituents are adjacent to each other All eclipsed conformers have torsional and steric strain. • Ring strain in cycloalkanes (2.11) Cyclolkanes may twist and bend to minimize 3 different kinds of potential strain: 1. Angle Strain (deviation from 109.5° bond angle) 2. Torsional Strain (repulsion of the bonding electrons between nearby substituents) 3. Steric Strain (caused by atoms or groups of atoms approaching each other too closely) = Axial (perpendicular) Equatorial (same plane) 60 o 88 o 108 o 109.5 o • Conformations in cyclohexane (2.12) Two main conformations: CHAIR and BOAT
Chemistry 2461– Fall 2008 Dr. G. Vicente 21 Both conformations are free of angle strain; however, the boat conformer has more torsional and steric strain than the chair. CHAIR BOAT • Monosubstituted cyclohexanes (2.13) Ring-flip : cyclohexane interconverts between two chair conformations (interconversion of axial and equatorial positions): CH 3 H H CH 3 1,3-diaxial interactions more stable conformer H H less stable ring-flip Substituents in axial positions have 1,3-diaxial interactions (unfavorable steric interactions).
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