Oxidation of Organic CompoundsâII

56 Oxidation of Aromatic Hydrocarbons to Alcohols and Aldehydes

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ELLIS K. FIELDS and SEYMOUR MEYERSON Research and Development Departments, Amoco Chemicals Corp. and American Oil Co., P. O. Box 431, Whiting, Ind. 46394

Aromatic aldehydes and alcohols can be prepared in mod erate yields by oxidizing aromatic hydrocarbons with oxygen at atmospheric pressure, using acetic acid as solvent and cobalt acetate and bromide as catalysts. Low voltage mass spectrometry is useful in following the course of the reaction and predicting, with considerable accuracy, the period for optimum conversions, as well as determining the nature of minor products. Oxidation of toluene-α-ds indicates radical abstraction of hydrogen from the aromatic ring as well as from the side chain.

Alkylated aromatic hydrocarbons have been oxidized to alcohols, aldehydes, and acids by various oxidants such as potassium per manganate, sodium dichromate, nitric acid, and oxygen. A procedure involving the use of oxygen at atmospheric pressure, together with cobalt acetate and hydrobromic acid in refluxing acetic acid, was described by Hay and Blanchard (12); although they showed the formation of some alcohol and aldehyde, they stressed primarily the preparation of carboxylic acids. By using lower concentrations of catalysts, we hoped to convert aromatic hydrocarbons in appreciable yields to aromatic alco hols and aldehydes, as well as to find the products of coupling of hydro carbon radicals. The rate of formation of oxidation intermediates was followed by withdrawing aliquots during oxidation runs and determining intensities of the parent peaks in low voltage mass spectra as measures of the rela tive concentrations of starting hydrocarbon and products. Although sensitivity—i.e., the proportionality factor between parent-peak intensity and concentration—differs from one compound to another, peak heights for any one compound in the spectra of samples of equal size (0.6μ) are 395

In Oxidation of Organic Compounds; Mayo, F.; Advances in Chemistry; American Chemical Society: Washington, DC, 1968.

396

OXIDATION OF

ORGANIC COMPOUNDS

II

directly proportional to concentrations of that compound in the various samples. Closely related compounds have roughly equal sensitivities at the ionizing voltage employed i n our work (10), and, in any case, the use of intensity ratios is perfectly valid for intercomparison of concentration ratios of identical components in separate samples (11, 23) within the limits of reproducibility of the low voltage data.


Experimental Hydrocarbons were purchased from Eastman and Aldrich and, where necessary, purified to 9 9 + % purity. Hemimellitene was furnished by M . C . Hoff, Amoco Chemicals Corp. Acetic acid was J. T. Baker A C S reagent grade; cobalt acetate and cobalt bromide were Fisher certified grade. In a typical oxidation, a solution of 286 ml. ( 5 moles ) of acetic acid, 3.26 grams (0.01 mole) cobalt bromide hexahydrate, 7.47 grams (0.03 mole) cobalt acetate tetrahydrate, and 61.5 ml. (0.5 mole) of p-xylene was refluxed in a Vibra-Mix apparatus ( Chemische Apparat, Zurich ) with oxygen passing in at the rate of 0.6 cu. foot per hour. Aliquots (25 ml.) were withdrawn after 30 minutes and 1 hour and hourly thereafter for 5 more hours. Each aliquot was treated with 100 m l . of water and 150 ml. of ether; the ether layer was washed with three 50-ml. portions of water and dried, and the ether was distilled. The ether-free residue was analyzed by low voltage (approximately 7.5 volts, uncorrected) mass spectrometry on a Consolidated model 21-103c instrument with the inlet system at 250°C. The repellers were maintained at an average potential of 3 volts, the exact values being selected to give maximum sensitivity. Conventional 70-volt spectra were also measured to help establish the identities of components where molecular weight alone was not adequate to remove all ambiguity. Total ion current at 70 volts, measured inde pendently of the spectrum (24), was nearly constant for the samples in each series, confirming that essentially the same amount of sample was admitted in each case. Reproducibility on repeated oxidations was within 7%. Result of a typical analysis are shown in Table I for toluene oxidation. Table I.

Results of Typical Analysis Time, Hours

0.5

1

3

4

5

6

Relative Concentration, Scale Divisions of Peak Height

Compound Toluene Benzaldehyde Benzyl alcohol Benzyl acetate Benzoic acid Bibenzyl

2

490 20 36 6 6 13

340 40 40 10 10 10

210 60 30 14 30 7

80 40 20 16 80 6

40 8 10 10 140 0

20 0 1 0 180 0


16 0 0 0 160 0

56.

Table II.

397

Aromatic Hydrocarbons

FIELDS A N D MEYERSON

Products from Reaction of Indium and Benzylic Bromides

Mass

Relative Intensity

168 182 258 272 348 362 438

53 18 100 8 68 13 26

Suggested Identities


Benzyl Bromide Methylbiphenyl and i s o m e r s — C H C H Bibenzyl and isomers— ( C H ) C H (C H ) (C H ) (C H ) C H C H$ (C H ) C H C HgC HgC H C H C HgCgH C HgC H 6

7

6

4

7

7

7

6

4

7

7

7

7

7

7

7

5

7

7

2

2

7

6

7

7

2

7

7

2

7

7

4

7

7

7

7

a-Bromo-o-xylene 92 106 120 134 184-186 196 210 224 288-290 300 314 392-394 404 418 508

54 29 16 61 13 100 39 8 8 95 27 6 45 13 4

Toluene Xylene C H CioHu a-Bromo-o-xylene C H CgH9 (C H ) CgHgCgHn Bromobixylyl C H (C H ) (C H ) (C H ) Bromoterxylyl G H C H (C H )2 C H C H C H C H C H C H C H C H C H 9

1 2

7

7

8

9

7

2

e

8

8

9

8

2

8

9

8

2

7

6

8

8

8

9

8

8

8

8

9

8

9

8

9

8

8

7

6

8

8

8

9

During the workup of the o-xylene oxidation run, a strong lachrymator made its presence felt. This was probably α-bromo-o-xylene, although it was not detected in the low voltage mass spectrum. W e suspected that a strong peak at mass 104, undoubtedly caused chiefly by a fragment ion derived from o-methylbenzyl alcohol by loss of H 0 (1), might also con tain a contribution from benzocyclobutene from the interaction of abromo-o-xylene with the indium tube used to introduce samples into the spectrometer. To test this possibility, benzyl bromide and a-bromo-oxylene were run separately under the same conditions. When the benzyl and xylyl bromides were brought i n contact with indium tubes for sampling, the indium was quickly discolored and pitted. The spectra of the material so taken up were recorded despite the clear evidence of reaction between the indium and the bromide. Relative intensities in the low voltage spectra and suggested identities of the com pounds responsible for the peaks are shown in Table II. The chief result of indium attack on α-bromo-o-xylene was expected to be removal of a bromine atom to produce a xylyl radical. If this were the case, the major stable products should be xylene polymers of molecular weight 210, 314, 2



398

OXIDATION O F

ORGANIC COMPOUNDS

II

etc. This series is present but it is vastly overshadowed by the series of molecular weight 196, 300, etc., which is presumably derived from one benzyl or tolyl group and one or more xylyl group. The component of molecular weight 134 also seems anomalous. The molecular weight and the 134:133:119 intensity distribution (70-volt spectrum) could be ac counted for by tetramethylbenzene, but there seems no obvious reason why such a species should be more abundant than trimethylbenzene, toluene, or even xylene. The explanation may be that indium bromide, formed from indium and the benzylic bromides, acts as a strong Lewis acid. In addition to condensations to diarylmethanes and polyarylpolymethanes, it brings about demethylation and transmethylation. These re actions are being investigated in solution on a larger scale. Despite our failure to find any supporting spectral evidence, the suspected presence of α-bromo-o-xylene and the absence of o-methylbenzyl acetate in the oxidation products from o-xylene suggest a solvolysis rate for this benzylic halide lower than for the isomeric methylbenzyl bromides. Discussion The rates of appearance and disappearance of oxidation intermediates for nine hydrocarbon's are shown in Figures 1 to 10. Ordinate values are peak heights, scaled to an original hydrocarbon concentration of 500; the summation of peak heights may vary considerably among aliquots in a series, reflecting differences in sensitivity among the components. Alde hyde builds up rapidly in toluene and the three xylenes and then is con sumed; In contrast, acetophenone from ethylbenzene oxidation continues to increase in concentration over 6 hours. This behavior agrees with the findings that cobalt is far less effective than manganese i n catalyzing the oxidation of acetophenone (30). The rate of oxidation of p-xylene with the same concentration of cobalt ion with bromide ion (Figure 6) was six times as fast as without bromide ( Figure 7 ). W e have not been able to unscramble the complex kinetics of pxylene oxidation. Ravens studied the second stage of oxidation, that of p-toluic acid in acetic acid with cobalt and manganese acetates and sodium bromide (25), and established the rate equation - d [ 0 ] / d t = fc[Co ] [ N a B r p [ 0 P 2+

2

2

for the autoxidation scheme: Initiation HBr + 0

2

- » Br- + HO.


(1)

56.

399



R C H + Br- - » R C H - + HBr

(2)

RCH - + 0

(3)

3

2

2

2

-> R C H 0 0 2

Propagation RCH 00- + RCH -» RCH OOH + RCH 2

3

R C H O O H + C o ' -> R C H 0 - + C o 2

2

RCH OOH + Co Downloaded by NORTH CAROLINA STATE UNIV on December 8, 2012 | http://pubs.acs.org Publication Date: January 1, 1968 | doi: 10.1021/ba-1968-0076.ch056

2

Termination 2 RCH 002 RCH '

s t

2

(4)

+ OH"

(5)

2

2

3+

-> R C H 0 0 - + C o - + H a

2

+

- n —> > nonradical products

2

2

RCH 00- + RCH - -») 2

2

(6) (7) (8) (9)

Sodium bromide with acetic acid furnished hydrogen bromide: NaBr + HOAc ^ HBr + NaOAc Hydrogen bromide is a weak acid in acetic acid solution and exists mainly in the unionized form (17). Reaction 1 has been postulated both in oxidations of alkanes i n the vapor phase (29) and in the anti-Markovnikov addition of hydrogen bro mide to olefins i n the liquid phase (14). Reaction 2 involves the estab lished mechanism for free-radical bromination of aromatic side chains (2). Reaction 4 as part of the propagation step, established i n earlier work without bromine radicals (26), was not invoked by Ravens, because of the absence of [ R C H ] i n the rate equation. Equations 4 to 6, i n which Reaction 6 was rate-determining, were replaced by Ravens by the reaction of peroxy radical with C o : 3

2 +

RCH 00- + Co 2

RCH 00" 2

and by reduction of C o ion and water (5,6): Co

3 +

3+

2+

-> R C H 0 0 " + C o 2

3+

RCHO + OH"

(4a) (5a)

by H B r , analogous to the reaction of cobaltic + H B r -> C o

2+

+ Br- + H

+

(6a)

In our study, bromide ion was present i n half the concentration of cobalt ion. If bromide ion were active only i n a propagation step such as Reaction 6a proposed by Ravens (25) or, alternatively, Reactions 7a and Sa: Br" + R O O H - » Br- + O H " + R O

(7a)

Br- + R O O H -> Br" + H + R 0 -

(8a)

+

2

the rate enhancement should have been approximately 50% rather than 600% over the oxidation without bromine. The large rate increase by bromide ion strongly suggests that its major role is to initiate rather



400

OXIDATION

OF

ORGANIC COMPOUNDS

II

than to propagate many radical chains; its behavior in the liquid phase thus parallels its behavior in the vapor phase (29). Although an appreciable amount of p-tolualdehyde was formed in oxidations with and without bromide, relatively little p-methylbenzyl alcohol and acetate resulted in the absence of bromide (1/10 to 1/25 as much as with bromide ion). The contrasting results suggest that the benzyl alcohol is a minor product in the autoxidation chain, as previ ously postulated by Boland, Bateman, and others (4, 7, 8, 15, 27), and is formed in the presence of bromide by solvolysis: R C H + Br- -> R C H - + HBr or ROOor R 0 H 3

2

2

R C H - + Br- —> R C H B r 2

or

2

R C H - + CoOAcBr - » R C H B r + CoOAcBr 2

2

2

R C H B r + Ν" - » R C H N + Br" 2

2

( N " = O H - or Ο A c ) Such rapid solvolysis has been demonstrated for benzyl bromide i n acetic acid containing cobalt acetate (10). Under conditions of high partial pressures of oxygen, the concentra tion of Br- is much lower and that of 0 higher, so the benzyl radicals give peroxy radicals and hydroperoxides rather than benzyl alcohol and acetate. Evidence for this is the appreciable amount of benzyl dimer (dimethylbibenzyl) resulting in the presence of bromide i n our oxida tions at relatively low partial pressure of 0 . Benzyl radicals are formed so rapidly that they cannot be scavenged by oxygen and dimerize instead. In the much slower oxidation without bromide all benzyl radicals can find oxygen with which to react, and dimethylbibenzyl is totally absent. Mass spectrometer analyses of the fractions taken at regular intervals indicate the optimum conversions of aromatic hydrocarbons directly to aldehydes and alcohols by oxidation, as shown in Table III. Semiquanti tative yields derived from low voltage mass spectral intensities and values found by gas chromatography were generally in good agreement—for ex ample, oxidations of toluene and p-xylene, worked up after 2 hours, gave the results shown i n Table IV. The ratio of alcohol to acetate depends upon workup procedure, which differed in the two cases. In addition, acetates of benzylic alcohols typically break down under electron impact by a low energy process to produce ketene and the corresponding alcohol ion. The peak at the parent mass of the alcohol, throughout this work, most likely contains a contribution so derived from the acetate. A careful distinction between the alcohol and acetate was not deemed important for our purposes, so 2

2


56.

401




we have in many cases taken the sum of the two presumed parent peaks as a measure of the total concentration of the two components. Although total conversion of hydrocarbon to aldehyde plus alcohol and acetate is not always high, the yields are sufficient to be attractive for the less common aromatic aldehydes and alcohols.

0

1

2

Figure 1.

3

4

TIME,

HOURS

5

6

7

Oxidation of toluene

Comparing results for the three xylene isomers (Figures 4 to 6), we find m- and p-xylene are similar, whereas o-xylene differs sharply i n one important respect from the other two. The concentration of 2,2'-dimethylbibenzyl exceeds or equals that of tolualdehyde for the first half of the


402

OXIDATION

O F ORGANIC COMPOUNDS

Π

oxidation period, whereas tolualdehyde i n the other two cases is greater by factors of 1.5 to 5 than the dimethylbibenzyl. The o-methylbenzyl radical thus appears to be substantially more stable than the m- and p-isomers, and prefers to dimerize rather than react with oxygen. The reaction: ArCH - + 0 -» ArCH 0 Downloaded by NORTH CAROLINA STATE UNIV on December 8, 2012 | http://pubs.acs.org Publication Date: January 1, 1968 | doi: 10.1021/ba-1968-0076.ch056

2

2

2

2

supposedly has a zero energy of activation; this may not hold true for A r = o-methylbenzyl. Minor oxidation products are shown i n Table V ; the numbers are relative intensities in the low-voltage mass spectra. The trimeric products

0

1

2

3 TIME,

Figure 2.

4

5

6

HOURS

Oxidation of ethylbenzene


7

56.

FIELDS


C24H24

and

A N D MEYERSON

C24H26


403

from the xylenes, presumably

are formed in greatest amount from o-xylene, providing further evidence for the stability of the o-methylbenzyl radical. The rates of oxidation of aromatic hydrocarbons are shown i n Table V I . The rates were determined by following disappearance of the original hydrocarbon rather than oxygen absorption—the technique used by most investigators. p-Xylene oxidized most rapidly, followed by o- and m-xylene. This is the reverse of the basicities of the three xylenes (20) and would suggest a τΓ-complex between the aromatic hydrocarbon and peroxy or other free radical, such as found by Russell for chlorine atoms (28). However, this order of base strength cannot be correlated with the rates of oxidation of other hydrocarbons; ethylbenzene and cumene are only slightly more basic than toluene, and the methylnaphthalenes are about as basic as m-xylene (19). p-Xylene has twice as many benzylic hydrogens as toluene, and oxi dizes about twice as fast. o-Xylene, also with six benzylic hydrogens, oxidizes only slightly faster, and m-xylene more slowly, than toluene. Further, on a purely statistical basis, hemimellitene has 1.5 times as many benzylic hydrogens as p-xylene and should therefore oxidize 1.5 times as rapidly. Its rate is less than 1/10 that of p-xylene; evidently, à steric, or perhaps polar, effect is far more important than the statistical factor. C o oxidations are under way to confirm these differences.



404

OXIDATION

OF

ORGANIC COMPOUNDS

Π

Cumene oxidized relatively slowly, at about 1/13 the rate of p-xylene. This was not caused by the formation of phenol, as might be expected by an acid-catalyzed rearrangement of cumene hydroperoxide. N o phenol or product clearly derived from phenol, as by radical attack or by oxidation to a quinone, was detected at any time in the reaction mixture. The two major products were α-methylstyrene and 2-phenylpropylene oxide; their concentrations increased with time. The group at Shell also observed the formation of α-methylstyrene and 2-phenylpropylene oxide among the products of cumene oxidation in butyric acid at 140 °C. with cobalt and manganese catalysts (30). a-MethyJstyrene rather than phenol might function as the retarder, consuming hydroperoxide by the following reactions.


56.

FIELDS A N D MEYERSON CH

I

CH

3

C H COOH e

5

I CH


ÇH

6

I

-Η,Ο

3

C H —C=CH

5

e

5

2

i CH3 ;

CH

3

I 5

CH

3

C H —C—OH

3

2

5

I CH

3

Figure 4.

CH

3

I

Γη

+ C H C—OOH e

CH

3

I

C H C=CH e

l

-[Ο] —1—U-

405


,

I

C H C—CH e

3

5

2

+ C H C—OH e

5

V

I

Ο

CH

Oxidation of o-xylene


3

406

OXIDATION OF


• • • • Ο Δ

ORGANIC COMPOUNDS

Π

m-XYLENE m-TOLUALDEHYDE \ Ο m-METHYLBENZYLALCOHOL m-METHYLBENZYL ACETATE m TOLUIC ACID 3,3· D I M E T H Y L ΒI B E N Z Y L

Figure 5.

Oxidation of m-xylene

This is similar to the process in which ethylbenzene is oxidized by air to the hydroperoxide, then treated with propylene i n the presence of transition metal oxides or salts to yield α-phenylethanol and propylene oxide (18), although, as Mayo points out (21), C o is a poor catalyst in this reaction. Phenylethylene oxide might be expected among the oxidation prod ucts from ethylbenzene by similar reactions. Its absence may be caused by the relative reluctance of styrene to react with peroxy radicals. α-Methylstyrene, which can yield the more stable ^-substituted a-methylα-phenethyl radical, is 2 to 3 times as reactive towards peroxy radicals as is styrene (22).


56.

407



The methylnaphthalenes were slowest to oxidize of the hydrocarbons studied; after the standard 6 hours only a few per cent had been con verted to the products shown: Intensity

a


Product

1-Methylnaphthalene

2-Methylnaphthalene

4 5 13 4

2 2 6 3

Aldehyde Carbinol Carbinyl acetate Dinaphthylethane β

Relative to methylnaphthalene = 100.

0

1

2

3

4

5

6

7

TIME, HOURS

Figure 6.

Oxidation of p-xylene


408

OXIDATION OF ORGANIC COMPOUNDS

II


Neither the relative number of benzylic hydrogens nor the base strength accounts for the slow oxidation rate of the methylnaphthalenes. Formation of radicals i n the presence of aromatic hydrocarbons can lead to radical attack on the aromatic ring. Addition of phenyl or methyl radical to the ring gives a cyclohexadienyl radical that may dispropor tionate or dimerize, or undergo hydrogen abstraction by another radical (3, 9,13).

TIME. HOURS

Figure 7.

Oxidation of p-xylene (no CoBr ) 2


56.

FIELDS


AND MEYERSON

409


Abstraction of ring hydrogen may compete with that of benzylic hydrogen. The decomposition of acetyl peroxide in ring-deuterated toluene, ethylbenzene, and cumene gives rise to methane-d (31). Apparent abstraction of ring hydrogen in competition with benzylic hydrogen probably occurs by addition of methyl radical to the benzene ring, fol lowed by abstraction of the cyclohexadienyl hydrogen by another methyl radical. CH

CH,

3

This process competes favorably with benzylic hydrogen abstraction in toluene, less in ethylbenzene, and least in cumene (31). Such reactions do not seem significant in the oxidation of benzene derivatives. However, naphthalene reacts about 20 times as rapidly with phenyl radical as does benzene (16), and radical addition to the naphthalene nucleus may at least partly account for the slow oxidation rate in the methylnapthalenes. Among the minor products from both methylnaphthalene oxidations were compounds of molecular weight 296: C H C—C H CH 1 0

7

1 0

6

3

II

ο

probably derived by oxidation of a methyl dinaphthylmethane, which i n turn contributes to the intensity at mass 282, along with dinaphthylethane and perhaps dimethyldinaphthyls. Further evidence of nuclear attack was furnished by the oxidation of toluene-a-d under conditions identical to those i n Table IV. After 90 minutes the product contained an estimated 6% benzaldehyde-di and 11% benzyl alcohol-d plus benzyl acetate-d . In addition, samples taken at three intervals gave these isotopic analyses for the recovered toluene: 3

2

2


410

OXIDATION

Time, Minutes

0 x

2


3

5

6

60

20

0

COMPOUNDS

90

Toluene Isotopic Composition

No. of Deuterium Atoms d d d d d, d d

O F ORGANIC

0.1 1.2 6.6 88.0 3.1 0.7 0.2

0.1 0.7 6.1 89.1 3.1 0.7 0.2

0 0.2 4.7 91.1 3.1 0.6 0.3

0.4 0.8 12.1 82.6 3.3 0.7 0.1

500

400

f—

300

h"

z ο

Oxidation of Organic CompoundsâII

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