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24.4 Common Classes of Organic Reactions

  • Page ID
    6529
  • . The overall reaction is as follows:

    \[\mathrm{CH_3CH_3+Br_2 \overset{400^\circ C}{\underset{or\;h u}{\rightleftharpoons}}CH_3CH_2Br + HBr} \tag{24.4}\]

    Radical chain reactions occur in three stages: initiation, propagation, and termination. At high temperature or in the presence of light, the relatively weak Br–Br bond is broken in an initiation step that produces an appreciable number of Br atoms (Br·). During propagation, a bromine atom attacks ethane, producing a radical, which then reacts with another bromine molecule to produce ethyl bromide:

    \(\begin{align}
    \mathrm{Br\cdot+ CH_3CH_3} & \rightarrow \mathrm{CH_3CH_2\cdot+HBr} \\
    \underline{\mathrm{CH_3CH_2\cdot+ Br_2^-}} & \rightarrow \underline{\mathrm{CH_3CH_2Br +Br\cdot}} \\
    \mathrm{Br_2+CH_3CH_3} & \rightarrow \mathrm{CH_3CH_2Br + HBr}
    \end{align} \tag{24.4.5}\)

    The sum of the two propagation steps corresponds to the balanced chemical equation for the overall reaction. There are three possible termination steps: the combination of (1) two bromine atoms, (2) two ethyl radicals, or (3) an ethyl and a bromine radical:

    \(\begin{align}
    & \mathrm{Br\cdot+ Br\cdot} && \rightarrow \mathrm{Br_2} \\
    & \mathrm{CH_3CH_2\cdot+\cdot CH_2CH_3} && \rightarrow \mathrm{CH_3CH_2CH_2CH_3} \\
    & \mathrm{CH_3CH_2\cdot+ Br\cdot} && \rightarrow \mathrm{CH_3CH_2Br}
    \end{align} \tag{24.4.6}\)

    Because radicals are powerful nucleophiles and hence highly reactive, such reactions are not very selective. For example, the chlorination of n-butane gives a roughly 70:30 mixture of 2-chlorobutane, formed from the more stable radical by reacting a secondary carbon and 1-chlorobutane.

    There are common patterns to how organic reactions occur. In a substitution reaction, one atom or a group of atoms in a substance is replaced by another atom or a group of atoms from another substance. Bulky groups that prevent attack cause the reaction to be sterically hindered. In an elimination reaction, adjacent atoms are removed with subsequent formation of a multiple bond and a small molecule. An addition reaction is the reverse of an elimination reaction. Radical reactions are not very selective and occur in three stages: initiation, propagation, and termination. Oxidation–reduction reactions in organic chemistry are identified by the change in the number of oxygens in the hydrocarbon skeleton or the number of bonds between carbon and oxygen or carbon and nitrogen.

    Conceptual Problems

    1. Identify the nucleophile and the electrophile in the nucleophilic substitution reaction of 2-bromobutane with KCN.
    2. Identify the nucleophile and the electrophile in the nucleophilic substitution reaction of 1-chloropentane with sodium methoxide.
    3. Do you expect an elimination reaction to be favored by a strong or a weak base? Why?
    4. Why do molecules with π bonds behave as nucleophiles when mixed with strong electrophiles?

    Answer

    1. CN is the nucleophile, and C2H5Cδ+HBrCH3 is the electrophile.

    Structure and Reactivity

    1. Sketch the mechanism for the nucleophilic substitution reaction of potassium cyanide with iodoethane.
    2. Sketch the mechanism for the nucleophilic substitution reaction of NaSH with 1-bromopropane.
    3. Sketch the mechanism for the elimination reaction of cyclohexylchloride with potassium ethoxide. Identify the electrophile and the nucleophile in this reaction.
    4. What is the product of the elimination reaction of 1-bromo-2-methylpropane with sodium ethoxide?
    5. Write the structure of the product expected from the electrophilic addition of HBr to cis-3-hexene.
    6. Write the structure of the product expected from the electrophilic addition of 1-methylcyclopentene to HBr. Identify the electrophile and the nucleophile, and then write a mechanism for this reaction.
    7. Write a synthetic scheme for making propene from propane. After synthesizing propene, how would you make 2-bromopropane?
    8. Write a synthetic scheme for making ethylene from ethane. After synthesizing ethylene, how would you make iodoethane?
    9. From the high-temperature reaction of Br2 with 3-methylpentane, how many monobrominated isomers would you expect to be produced? Which isomer is produced from the most stable radical?
    10. For the photochemical reaction of Cl2 with 2,4-dimethylpentane, how many different monochlorinated isomers would you expect to be produced? Which isomer is produced from the most stable precursor radical?
    11. How many different radicals can be formed from the photochemical reaction of Cl2 with 3,3,4-trimethylhexane?
    12. How many monobrominated isomers would you expect from the photochemical reaction of Br2 with
    1. isobutene?
    2. 2,2,3-trimethylpentane?
    1. Arrange acetone, ethane, carbon dioxide, acetaldehyde, and ethanol in order of increasing oxidation state of carbon.
    2. What product(s) do you expect from the reduction of a ketone? the oxidation of an aldehyde?
    3. What product(s) do you expect from the reduction of formaldehyde? the oxidation of ethanol?

    Answers

    1. four; 3-bromo-3-methylpentane
    1. seven
    1. methanol; acetaldehyde, followed by acetic acid and finally CO2