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Alkanes and Cycloalkanes Help (page 2)

By — McGraw-Hill Professional
Updated on Aug 16, 2011

Geometric Isomerism

Atoms in rings cannot rotate completely about their σ bonds, and this leads to cis-trans isomers in cycloalkanes. For example, there are 2 isomers of 1,2-dimethylcyclopropane. The isomer in which the 2 CH3 groups are on the same side of the ring is called the cis isomer, and the isomer in which the 2 CH3 groups are on different sides is called the trans isomer.

Geometric Isomerism

There are several ways to draw these structures. In some diagrams, the ring is drawn flat in the plane of the paper, with "wedges" projecting toward the viewer and "dashes" (hashed lines) away from the viewer. Drawings such are the ones below assume that there is a carbon atom at each vertex, along with the appropriate number of hydrogens to make each carbon have 4 bonds.

Geometric Isomerism

Conformations of Cycloalkanes

There are two common conformations of cyclohexane, the chair form and the boat form. The chair conformation is more stable than the boat form, since there is less of a tendency for two substituents (in this case hydrogens) to "bump" into each other. Two groups trying to occupy the same space results in unfavorable steric strain. Chair conformations and boat conformations interchange through simple bond rotation.

Conformations of Cycloalkanes

Conformations of Cycloalkanes

Axial and Equatorial Bonds in Chair Cyclohexane

Six of the twelve H's of cyclohexane are equatorial (He); they project outwards from the ring, forming a belt around the ring perimeter. The other 6 H's are axial (Ha); they are perpendicular to the plane of the ring and parallel to each other. Three of these axial H's point up and the other 3 point down. Converting one chair conformer to the other changes all of the equatorial bonds in the first conformer, shown as heavy lines, to axial bonds in the second conformer. All of the axial bonds similarly become equatorial bonds. Note how the circled atom below changes from being axial to being equatorial as the first chair converts to a new chair.

Conformations of Cycloalkanes

Substituted Cyclohexanes

There are two chair conformations for substituted cyclohexanes such as methylcyclohexane, one in which the CH3 is axial and one in which the CH3 is equatorial. The conformer with the axial CH3 is less stable due to 1,3-diaxial interactions. The axial CH3 is closer to the two axial H's (located on the 3rd carbon over) than is the equatorial CH3 to the adjacent equatorial H's, even those on the adjacent carbons. In general, substituents prefer the less crowded equatorial position to the more crowded axial position.

Substituted Cyclohexanes

Chemical Properties of Alkanes and Cycloalkanes

Alkanes are unreactive except under vigorous conditions.

  1. Pyrolytic Cracking [heat (Δ) in absence of O2; used in making gasoline]:
      Alkane → mixture of smaller hydrocarbons
  2. Combustion [burning of methane or other alkanes]
      CH4 + 2O2 → CO2 + 2H2O
      ΔH of combustion = –809.2 kJ/mol
  3. Halogenation
      RH + X2 → RX + HX
      (reactivity of X2: Cl2 > Br2. I2 does not react; F2 destroys the molecule)

Halogenation of alkanes such as methane, CH4, proceeds by a radicalchain mechanism, as follows:

Initiation Step: Cl–Cl → 2Cl•

The energy required to initiate the reaction comes from ultraviolet (UV) light or heat.

Propagation Steps:

  1. H3C–H + Cl• → H3C• + H:Cl       ΔH= –4 kJ/mol
  2. H3C• +Cl:Cl → H3C–Cl + Cl•       ΔH= –96 kJ/mol

The sum of the two propagation steps is the overall reaction,

    CH4 + Cl2 → CH3Cl + HCl       = –100 kJ/mol

In propagation steps, free-radical intermediates, here Cl• and H3C•, are being formed and consumed. Chains terminate when two free-radical intermediates form a covalent bond:

    Cl• + Cl• → Cl2, H3C• + Cl• → H3CCl, H3C• + •CH3 → H3C–CH3

Chemical Properties of Alkanes and Cycloalkanes

In more complex alkanes, the abstraction of each different kind of H atom gives a different isomeric product. Three factors determine the relative yields of the isomeric product.

  1. Probability factor. This factor is based on the number of each kind of H atom in the molecule. For example, in CH3CH2CH2CH3 there are six equivalent 1° H's and four equivalent 2° H's. The statistical probability of reaction is 6 to 4 (primary to secondary).
  2. Reactivity of H. The order of reactivity for different hydrogens in the same molecule is 3° > 2° > 1°.
  3. Reactivity of X. The more reactive Cl is less selective and more influenced by the probability factor. The less reactive Br is more selective and less influenced by the probability factor. As summarized by the reactivity-selectivity principle: If the attacking species is more reactive, it will be less selective, and the yields will be closer to those expected from the probability factor. An example of this is shown below, in the distribution of 1˚ and 3˚ products of halogenation of 2-methylbutane. Bromination, involving the less reactive Br• radical, is much more selective than chlorination.

Chemical Properties of Alkanes and Cycloalkanes

Practice problems for these concepts can be found at:

Alkanes and Cycloalkanes Practice Problems

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