Oxidation 4-chlorobenzyl alcohol

Scheme 13 Photo-oxidation of -chlorobenzyl alcohol by Fl-2Sc via photoinduced electton transfer. (From Ref. 114.)...

All of these control experiments are consistent with a mechanism in which the stilbenes 5 are in dynamic equilibrium with the equivalent benzylic alcohols 4 and the oxidative-cleavage reaction occurs on the latter. This equilibrium explains 1) the observed requirement for water for the stilbene oxidations 2) the unsymmetrical product distribution from both the alcohols and stilbenes and 3) the enhanced reaction rate of the chlorobenzyl alcohol 4b as compared to the stilbene analogue 5b at 120 °C. 

J. Chem., Sect. B, 1986,25, 228.) It appears to be a selective oxidant for benzyUc alcohols, e.g. treatment of 2-chlorobenzyl alcohol with PBC for 3 h in chloroform at reflux gives the corresponding aldehyde in 95% yield. However, it has not been widely adopted. 

M. N. Bhattacharjee, M. K. Chaudhuri, H. S. Dasgupta, N. Roy and D. T. Khathing, Synthesis, 1982, 588. Pyridinium Bromochiomate (PBC) has also been prepared. (N. Narayanan and T. R. Balasubramanian, Indian J. Chem., Sect. B, 1986, 25, 228.) It appears to be a selective oxidant for benzylic alcohols, e.g. treatment of 2-chlorobenzyl alcohol with PBC for 3 h in chloroform at reflux gives the corresponding aldehyde in 95% yield. However, it has not been widely adopted. 

In mammalian species, CS rapidly hydrolyzes to form 2-chlorobenzaldehyde and malononitrile (Leadbeater, 1973 Paradowski, 1979 Rietveld et a/., 1986). The malononitrile intermediate is further metabolized from two cyanide moieties, which are converted to thiocyanate (Cucinell et al, 1971). The aldehyde intermediate undergoes oxidation to 2-chlorobenzoic acid or reduction to 2-chlorobenzyl alcohol. These metabolites are conjugated and excreted in the urine. 

Benzyl alcohols as aldehyde precursors were also added to similar reactions utilizing the oxidizing properties of sulphur (87KG639). Diamine 109 and benzyl alcohol were heated with sulphur at 180-200 °C to obtain 2-phenyl-IP 54 in a poor yield (20%). A similar reaction occurred easier with -chlorobenzyl alcohol here the yield of 2-/ -chlorophenyl-IcP 67 reached 60% (87KG639). 

In the experiments that follow, several representative oxidations of alcohols using hypochlorous acid will be performed. In the first experiment, cyclodode-canol (1), a secondary alcohol, is oxidized to cyclododecanone (2) using sodium hypochlorite or commercial household bleach (Eq. 16.20). In the second experiment, 4-chlorobenzyl alcohol (3) is oxidized directly to 4-chlorobenzoic acid (4) using calcium hypochlorite (Eq. 16.21). 

Consider the oxidation of 4-chlorobenzyl alcohol (3) to 4-chlorobenzoic acid (4) using hypochlorous acid to answer the following questions. 

One of the side reactions that may occur when hypohalite is used as an oxidizing agent for benzylic alcohols is halogenation of the aromatic ring. Although this side reaction is a problem when benzyl alcohol is oxidized with calcium hypochlorite, it is not observed in the oxidation of 4-chlorobenzyl alcohol. Explain this observation. 

Discuss the differences observed in the IR and NMR spectra of 4-chlorobenzyl alcohol and 4-chlorobenzoic acid that are consistent with the oxidation of an alcohol to a carboxylic acid rather than to an aldehyde in this experiment. 

To a mixture of 100 ml of liquid ammonia and about 80 mg of black iron oxide was added 0.78 g (0.02 atom) of potassium. When all of the potassium had reacted, 3.3 g of N,N-dimethyl-N -(2-pyridyl)ethylenediamine was added. After the addition of 75 ml of dry toluene the ammonia was removed on the steam bath. To the cooled and stirred mixture was added 4.26 g of p-chlorobenzyl chloride, and the reaction mixture was stirred on the steam bath for 11 hours. It was then filtered and concentrated to an oil. This concentrate was taken up in ether, and the ethereal solution was washed with water, dried over sodium sulfate, and concentrated. Distillation gave 2.96 g of yellow liquid. Treatment of this distillate with an equivalent quantity of hydrogen chloride in absolute alcohol and precipitation by the addition of anhydrous ether gave 2.33 g of the N,N-dimethyl-N -(4-chlorobenzyl)-N -(2-pyridyl)ethylenediamine hydrochloride. 

Control experiments with 19a showed that in the absence of olefins hydrodimers were also formed but the reaction rate was reduced by about 90 %. Either the CdS itself (under photocorrosion) or the solvent must be involved in the oxidative step of this reaction. Accordingly, irradiation of CdS in a solution of 19a in different alcohols transforms the imine at different rates and the corresponding addition products 22a-e and the hydrodimer 23 are isolated (Figure 31). Except for methanol and 2-propanol, the products are racemic diastereomeric mixtures which are isolated in low yields (5-20 %) they are often mixed with the two-electron reduction product A-4-chlorobenzyl-4-chloroaniline. The major product in all reactions is the hydrodimer 23, obtained in yields of 10 % (MeOH), 28 % (BuOH), 29 % (PrOH), 42 %... 

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