Indian Journal of ChemistryVol. 27A, December 1988, pp. 1031-1034

Kinetics & Mechanism of Oxidation of Substituted Benzyl Alcohols bySodium Bis[2-ethyl-2-hydroxybutanoato(2- )]oxochromate (V)

G SREELATHA, M PRASADA RAO, B SETHURAM* & T NAVANEETH RAODepartment of Chemistry, Osmania University, Hyderabad 500 007

Received 2 November1987; revised 27 February 1988; accepted 21 March 1988

The kinetics of oxidation of benzyl alcohol and some substituted benzyl alcohols by sodium bis[2-ethyl-2-hydroxy-butanoato(2- )]oxbchromate (V) [Cr(V)) has been investigated in 25% (v/v) aq. acetic acid in the presence of perchloricacid. The order in [oxidant] and [substrate] is unity each. The rate of oxidation increases with increase in [2-ethyl-2-hydroxybutyric acid] (EHBA) and decreases with increase in percentage of HOAc and [H +]. The order of reactivitiesis p-OMe> m-OMe>H>p-Br>p-CI> m-CI> m-N02• The Hammett's plot is linear with o= -0.31 (r=0.974). Amechanism involving transfer of hydride ion from alcohol moiety to Cr{V) has been suggested to explain the kinetic re-sults. Activation parameters have also been calculated and discussed.

It is well established that the most stable oxidationstates of chromium are + 3 and + 6. Interest in theunusual and less stable oxidation states of varioustransition metal ions as oxidants, including chrom-ium is of recent origin'r'. The preparation by Krum-pole and Rocek of several water soluble, air-stablecarboxylato derivatives of chromium(V) has addedyet another metal ion with unusual oxidation state.Sodium bis[2-ethyl- 2-hydroxybutanoato{2- )]oxo-chromate (V) has been found to be the most stableof carboxylato derivatives:', particularly in the pHrange 2-4 and is found to act as one, two and also asa four-electron oxidant in the oxidation of vanadi-um(IV)4, hydrazine" and hydroxylamine" respect-ively.

The present study is aimed at probing further thedifferent aspects of Cr(V) oxidation of aromaticcompounds, particularly those with hydroxyl func-tion such as benzyl alcohols.

Materials and Methods

2-Ethyl-2-hydroxybutyric acid (EHBA, Aldrich)was recrystallized from benzene-hexane. Acetic ac-id was purified by the standard procedure. Benzylalcohol. (Fluka) and HCI04 (BDH) were used assuch. Sodium bis[2-ethyl-2-hydroxybutanoato(2)]oxo-chromate(V) was prepared according to the report-ed method", Stock sctutions of Cr(V) were preparedin 1 x 10-3 mo• ..Jm-3 HCI04 in the presence of1 x 10 - 2 mol dm - 3 EHBA, which formed a stablecomplex with Cr(V). Also EHBA did not undergoperceptible oxidation under the experimental con-ditions. Solutions were standardized iodometrical-ly7 as well as spectrophotometrically using the mo-lar extinction coefficient of Cr(V) complex as

1.68 X 102 dm? mol-I cm "! at 510 nm (ref. 3). Sincebenzyl alcohols (BA) are insoluble in water, 25% (viv) aq. acetic acid was used as the solvent. Acetic aciddid not undergo any perceptible oxidation under theexperimental conditions.

The reaction was initiated by the addition of req-uisite quantities of Cr(V) solution to the mixture ofalcohol, acetic acid and HCIO", after thermo statingthe two solutions separately for 30 min. The reac-tion was monitored by noting the absorbance ofCr(V) complex at 510 nm at regular time intervalsusing Carl-Zeiss VSU-2-P spectrophotometer. ThepH of the solution within the range 2'-4 was main-tained by adding different amounts of HCI04.

Under the conditions of [Cr(V)] ~ [BA]in 25°/0 (viv) acetic acid medium, Cr(V) was allowed to reactwith benzyl alcohol and excess of oxidant was esti-mated. Values of ~[Cr(V W~[BA] under differentconcentration ranges revealed 1:1 stoichiometry(substrate:oxidant). Benzaldehyde was character-ised as the product of oxidation of benzyl alcohol. Itwas also noted that benzaldehyde did not undergooxidation under the experimental conditions em-ployed above.

No polymerisation was observed when acryloni-trile was added to the reaction mixture under nitro-gen atmosphere indicating that free radicals werenot formed during the course of the reaction.

Results and DiscussionUnder the conditions of [alcohol]s- [Cr(V)l the

plots of log (ala - x) versus time were linear, indicat-ing first order dependence in [Cr(V)]. From theslopes of such plots the pseudo-first order rate con-stants (kobs) were evaluated at different [Cr(V)] and.were found to be non-variant. For example. under

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INDIAN J. CHEM., VOL. 27A, DECEMBER 1988

0'75 0·3 O·

:§ ~ 0

•....• • ...... 0

I 0 ..•..• ..••.+

•..... •••0 sIf) -f 1 f

1-050'60'3

0'35 D

!!

e :..•••~+.,

r"6

"4

"2

'·0

:~ <>8

If

0-6

0-4

<>2

0·0

0'85 <>4 <>,

0·65

0·55

0'45

0'25

0,'5

0·50"0 0·4 0·8 0'2_ 2 +'09 [BA] (A)

'·0 '·5 2·0 2'~ 3·0 3·5 4·0 _ ,el/o (B)

0 0·25 0·50 0·75 ';00 1·25 I'~O_ ,02/[EHBA] (e)6·0

-3[H+] (0)0 I· 0 2·0 3'0 4·0 5·0 - 10

Fig. I(A)-Plot of log kobsversus log[BA] ([Cr(V)] = 3.00 x 10-.1 mol dm-.1; [EHBA] = 1.00 x 10-2 mol dm -'; [H+]= 1.00 x 10-.1 moldm-.1;[AcOH]= 25.0% (v/v); temp = 303 K).

(B) Plot of log k " versus liD ([Cr(V)] = 3.00 x 10-.1 mol dm -.1; [BA]= 1.00 x 10-1 mol dm -I; [H+] = 1.00 x lO-.1 mol dm -.1; [EH-BA]= 1.00 x 10-2 mol dm-.1; temp = 303 K).

(C) Plot 01 lIkobs versus I/[EHBA]([Cr(V)]= 3.00 x 10-.1 mol dm ":'; [BA]= 1.00 x 10-1 mol dm-.1; [H+]= 1.00 x 10-.1 mol dm "';[AcOH] = 25.0% (v/v); temp = 303 K).

(D) Plot of IIkobs versus 1([H"']([Cr(V)]=3.00X 10-,' mol dm--'; [BA]= 1.00 x 10-1 mol dm.'; [EHBA]= 1.00 x 10-2 mol dm-.1;[AcOH] = 25.0% (v/v); temp = 303 K).

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SREELATHA et al.: KINETICS OF OXIDATION OF BENZYL ALCOHOLS

the conditions [HClO 4] = l.0 x 10 - 3 mol dm - 3;[EHBA] = l.Ox 10-2 mol dm-3; [BA]= l.Ox 10-1mol dm - 3; temp. = 303 K and solvent = aq. aceticacid (25%, v/v), 104 kot>.remained constant at3.64 ± 0.14 S-I when 103 [Cr(V)] was varied form3.0 to 10.0 mol dm-3• The order in [alcohol] wasnearly unity as obtained from the slope of the linearplot of log k' versus log [alcohol] (Fig. lA). At fixed[Cr(V)], [alcohol] and [H+] increase in [EHBA] in-creased the rate of oxidation (Fig. 1C). The rate ofoxidation decreased with increase in [H +] in the pHrange of 2-4 (Fig. ID). With increase in percentageof HOAc in the solvent medium (decrease in dielec-tric constant) the rate of oxidation increased. Theplot of log k' versus lID was linear with a negativeslope (Fig.lB). The rate of oxidation was unaffectedby the addition of salts like NaCI and NaCI04• Noreaction was observed between Cr(V) and tertiarybutanol.

The Cr(V) complex, [CrV(OXEHBAh]- wasstable in the narrow pH range of 2-4 but below andabove this range decomposition occurred. The pos-sible species of Cr(V) present in the above pH re-gion as suggested by Krumpolc and Rocek" are neu-tral species, i.e., CrV(OHXEHBA}z, monoanionicCrVO(EHBA)i and another species containing onlyone EHBA molecule. The species that absorbs at510 nm is CrVO(EHBA}z- and not any other spe-cies". Alcohols predominantly exist in the neutralform even at very low pH<J. In view of these obser-vations a mechanism involving the reaction betweenCrVO(EHBA)i and alcohol is proposed in the pres-ent case (Scheme I). This receives support from thesolvent and ionic strength effect studies, unit de-pendence in IS]and absence of any kinetic evidencefor complex formation (the pot of 11koosversus 1I[S)is linear with negligible intercept on 11kobs axis).

K,CrV(OHXEHBAh ~ CrVO(EHBA)i +H+ ... (1)

K,CrV(OXEHBA)i ;=! CrVO(EHBAt + EHBA ... (2)

kCrVO(EHBA); + Ph.CHzOH -

- slow

Ph - C- H + Cr(III) + H +Ho (Scheme I)

... (3)

According to Eq. (1) if the monoanionic species isassumed to be the active species, the rate should de-crease with increase in [H +] which in fact was ob-served in the present study. The rate of oxidation in-creased with increase in [EHBA] further confirmingthat probably the monoanionic species, i.e.

CrVO(EHBA)i is the most likely reactive species(Eq. 2). Scheme 1 leads to the rate law (4 ).

-~Cr(V)]tdt

kK,[EHBA][SllCr(V)],[EHBA]{[H+j+ Kd+ K,Kz

... (4)

- din [Cr(V))tdt kohs

kK,[EHBAj[S] 5[EHBA]{[H+J+ K,l+ K,K2 ••• ( )

or

1 [EHBA]{[H+]+ K,l+ K,K2kot>. kKI[EHBA][S]

As fixed [S]and (EHBA] Eq. (6) can be written as

... (6)

_1_=K[H+]+Kkoo.' y

where

... (7)

1 [EHBA]+K2K. = K, k(S];Ky = k(EHBA][S]

According to Eq. (7) a plot 11kot>. versus [H +]should be linear with intercept which has been ob-tained in the present case (Fig. 1D).

As fixed IS]and [H+] Eq. (6) reduces to Eq. (8)

_1_= K. +Kkoo. [EHBA] Y

... (8)

where

According to Eq. (8) a plot of 1I""t>.versus 1I[EH-BA] should be linear with an intercept. This hasbeen realised in the present study (Fig. 1C).

In order to calculate k, KI and K2, the kobs valueswere determined by varying [H +] under differentconcentrations of EHBA and also by varying [EH-BA] under different [H+] by keeping [S] fixed. Theplots of Ky versus 1I[EHBA] and K; versus [H +]were linear with an intercept. From the values ofslopes and intercepts of these plots k, K, and K2were evaluated to be 38.5 x 10 - 3 dm ' mol- I s - "27.8 X 10-3 mol dm ? and 2.10 x 10'-3 mol dm? re-spectively confirming the proposed mechanism (seeScheme 1).

The effect of substituents on the reaction rate wasstudied using different substituted benzyl alcohols

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INDIAN J. CHEM., VOL. 27A, DECEMBER 1988

Table I-Effect of Substituent on Second Order RateConstants and Activation Parameters

(Cr(V)] = 3.00 x .0-3 mol dm-3; (HOAc]= 25% (v/v)[EH-BAj= 1.00 x 10-2 mol dm-3; (HCIO.]= 1.00 x 10-3 mol dm-3;

Temp=308K

Substi- 103 k' Ecxp 6.Ht 6. G ttuent dm! mol- I s - I)

~OMe 7.76 56.8 54.3m-OMe 5.85 52.7 50.2H 5.80 51.0 48.4~Br 5.75 50.0 47.4~CI 5.08 46.6 44.0m-CI 4.60 43.9 41.3m-N01 3.33 38.2 33.6Ecxp.6.H· and 6.G· values in k J mol" I

87.988.688.688.789.089.390.1

-6.st(J K-I mol-I)

109124130134146155183

and the reactivities follow the order: ]rOCH, > m-OCH} > H > ]rBr > ]rCI > m-N02• The Hammett'splot is linear with p = - 0.31 (r= 0.974). The nega-tive o-value increases with increase in temperature.The o-values at temperatures 293,298,303,308 Kare - 0.19, - 0.23, - 0.28 and - 0.31 respectively.

In view of the negative test obtained for free radi-cals, negative slope for the Hammett's plot, and noreaction with r-butanol, a hydride ion transfer me-chanism has been suggested despite the low valuefor p obtained. Such low values are not uncommonin hydride ion transfer mechanism 10.

The second order rate constants (k 0) and the acti-vation parameters, viz. I:.~,~Hf, ~Sf and ~Gfhave been calculated and the values are listed inTable 1.The constancy in ~G f values suggest that a

1034

similar mechanism is operative with all the sub-strates used. The isokinetic temperature (~) ob-tained from the plot of ~H t versus ~S t is 285 Kwhich is well below the experimental temperaturerange (293-308 K), suggesting that the reactions areentropy-controlled. The plots of log k versus 1/T al-so converge at 286 K which is nearly equal to theabove value obtained by the above method.

AcknowledgementOne of the authors (GS) is thankful to the CSIR,

New Delhi, for the award of a research fellowship.

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