Organic Mechanisms. Xiaoping Sun

the hydrogen bond formed on the transition state is strong, greatly stabilizing the transition state (Fig. 1.24b). [TS]_O^ǂ and [TS]_W^ǂ represent the transition states for the reactions taking place in organic media (homogeneous) and in hydrophobic water/organic interface (heterogeneous), respectively. E_a,O and E_a,W are the corresponding activation energies for the homogeneous and heterogeneous reactions (E_a,W < E_a,O), respectively. Both aggregation of the organic reactants and especially, the hydrogen‐bond stabilization for the transition state result in decrease in the activation energies of the organic reactions on the hydrophobic interface and greatly speed up the reactions relative to those performed in organic media.

FIGURE 1.24 Hydrophobic effects on organic reactions. (a) The intermolecular hydrogen bond between water and transition state of reactions taking place in the hydrophobic interface, and (b) comparison of the energetics for the homogeneous reactions (E_a,O and [TS]_O^ǂ) in organic media and heterogeneous reactions (E_a,W and [TS]_W^ǂ) in the water–organic interface.

      Many types of organic reactions, such as Diels–Alder cycloadditions and Claisen rearrangements, have been carried out in water with fast speeds and good yields [9–12]:

(1.74)

For example, it only takes 10 min for Reaction 1.74 to complete in water as the reaction mixture is vigorously agitated at 23 °C. When the reaction is performed by neat reactants with no solvent, the completion time is 48 h. It takes more than 120 h for the same reaction to complete in toluene homogeneously [12].

(1.75)

The rate constant for Reaction 1.75 performed in water (k_water) is many times greater than the rate constants for the reaction performed in organic solvents (k_solvent). Research has shown that the k_water/k_solvent ratios at room temperature for isooctane, acetonitrile, and methanol are 740, 290, and 58, respectively [9].

      More examples and mechanistic details of organic reactions carried out in water will be discussed in individual chapters (Chapters 4, 8, and 10) in the sections of green chemistry methods and applications.

      PROBLEMS

      1 1.1 Dibenzylmercury (PhCH2)2Hg contains linear C─Hg─C bonds. At 140 °C the compound undergoes thermal decomposition to give 1,2‐diphenylethane PhCH2CH2Ph and mercury. Suggest a concerted mechanism and a stepwise mechanism for this reaction.

      2 1.2 The following are two alternative mechanisms for the AlCl3 catalyzed Friedel–Crafts reaction of benzene with chloroethane. Show qualitative reaction profiles for both mechanisms. Which mechanism is more plausible? Explain.

      3 1.3 The decomposition of bromoethane in gaseous phase to ethene and hydrogen bromide follows a first‐order rate law. Suggest a possible concerted and a stepwise mechanisms which are consistent with the rate law.

      4 1.4 The rate of nitration of benzene in a mixture of nitric acid and sulfuric acid does not depend on the concentration of benzene. What conclusion can be drawn from this? Show qualitative energy profile for the stepwise reaction.

      5 1.5 The mechanism for the autoxidation of hydrocarbons by oxygen is shown below:Apply the steady‐state approximation to work out the expected kinetics for the reaction.

      6 1.6 For the SN1 solvolysis of the tertiary chloroalkanes R–Cl in ethanol at 25°C, the relative rates for three different chloroalkanes are 1, 1.2, and 18.4 for R = Me3C, tBuMe2C, and tBu2MeC, respectively. Is the difference due to electronic or steric effects? Explain. Draw qualitative energy profiles for the SN1 solvolysis of the three chloroalkanes.

      7 1.7 For the following each pair of anions, compare their basicity and nucleophilicity. Account for the difference.(1) OH− and SH− (2) CH3COO− and C6H5COO−

      8 1.8 Which of the following SN2 reactions goes faster? Explain by kinetic isotope effect.

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