Indian Journal of ChemistryVol. 27 A. November 1988. pp. 956-958

Regeneration of Carbonyl Compounds from Semicarbazones byManganese(III) Acetate

K R SAN KARAN. KALYANI RAMAKRISHNANf & VANGALUR S SRINIVASAW

Department of Chemistry. Ramakrishna Mission Vivekananda College. Madras 600 004

Received 14 September 1987: revised 27 January 1988; accepted 1\ February 1988

The kinetics of manganese( 11\) triacetare dihydrate oxidation of sernicarbazones have been studied in 76% (v/v) aqacetic acid at 28 ± O.2°C. The reaction exhibits total second order kinetics-first order in each reactant. The rates of thesereactions are susceptible to electronic influence of substituents on the phenyl ring. To account for the negative Hammettreaction constant (p = - I.S7) in the case of benzaldehyde and substituted benzaldehydes, a rate-determining cleavage ofN-Mnlll bond has been proposed. As the Hammett reaction constant obtained in the case of acerophenone/benzophenonesemicarhazones is positive. an alternate scheme involving rate-determining addition of Mnl OAc), across of C = N is pro-posed. At ambient temperature. manganese( 111)acetate regenerates the corresponding carbonyl compound in about SO'\,;,yield.

Thallium triacetate regenerates I benzaldehyde fromits semicarbazone in about 90% yield at ambient tem-perature. However, the same reagent regeneratescarbonyl compounds- from semicarbazones of ace-tophenone and substituted acetophenones, after longreflux times in 65-90% yield along with an acylatedproduct. Potassium bromate has also been shown toregenerate carbonyl compounds quantitatively atambient temperature. Presently we have shown thatmanganese(Ill) triacetate regenerates carbonyl com-pounds quantitatively from semicarbazones of alde-hydes and ketones at ambient temperature. The kin-etics of the oxidation reaction have been studied in76% (v/v) aq acetic acid.

Materials and MethodsThe semicarbazones of benzaldehyde, substituted

benzaldehydes, acetophenones, substituted ace-tophenones, benzophenone and substituted ben-zophenones were prepared from extra pure com-pounds by literature procedures and their puritieschecked. Manganese(III) acetate dihydrate, preparedin the laboratory was of99% purity. As the decompo-sition of manganese(Ill) acetate at room temperaturewas considerable, the solution was prepared as andwhen necessary in 100% acetic acid, left overnightand filtering off the resultant solution. The other rea-gents used were of reagent grade (BDH).

The rate of reaction was measured following dcc-rcasc in absorbanceofMn(lII)at 350 nm using a Carl-Zeiss VSU 2-P spectrophotometer. When Mn(lII)was in excess, the absorbance value was corrected us-

t Department of Chemistry. Mccnakshi College for Women.Madras 60() 024. India.

<)56

ing about 70% of that concentration of Mn(Ill) in thereference compartment, so that the change in absorb-ance could be measured precisely. The specific ratescalculated using integrated rate equations from du-plicate runs agreed within ± 7% and these values alsoagreed with those obtained from the slopes of linearplots of log of change in absorbance versus time.

Product analysis and stoichiometryReaction mixture containing semicarbazone (1.0

to 2.0 mmol) and manganese(lll) acetate (10 to 20mmol) in 76% (v/v) aq acetic acid at 28°C, after ninehalf-lives, was diluted with equal volumes of ice-waterand extracted with CHClJ (3 x 5 rnl), The combinedchloroform extract was dried (MgSO~), filtered andacetic acid was neutralised by adding saturated NaH-C01 solution and extracted with ether. The organiclayer gave the product. From UV and IR spectra, theproduct was identified as the corresponding aldehydeor ketone. The amount of benzaldehyde, benzophe-none or acetophenone formed was determined bymeasuring the absorbance of the organic layer at 250nrn+e = 1l,400dmJmol-1cm-I),257nm(E= 18,500dm1mol-1cm-l) or 246 nm (E=12,600drrr'mol" 'ern - I )~.:;respectively. The amount of ben-zaldehyde or acetophenone or henzophenoneformed from the respective semicarbazones was he-tween 73°/c,to ~5%, when [scmicarbazonc] = 1.04 to3.5xlO-Jmol dm-1 and [Mn(1II)=1O.0 to35 x 10- Jmol dm -.\ The inorganic products werenot identified.

Taking manganese( III)acetate in 10-30 fold excess.stoichiometric runs for the Mn(lll) oxidation of semi-carbazoncs were carried out. After nine half-lives. the

SANKARAN et al. : REGENERATION OF CARBONYL COMPOUNDS FROM SEMICARBAZONES

Table 1-Kinetic Data for Mn(lII) Acetate Oxidation ofSemicarbazones+"

[C,Hs-C(RI) = N-NHCO NH~ I

ltr'[semicarbazone] 103[Mn(II1lJ lO'kl(rnol drn r ') (moldm-') (S-I)

RI=H2.0 1.084.0 1.082.0 2.03.0 2.0

k;(dm.1moi-IS - I)

0.730.070.730.70

RI=CH,2.0 1.2 3.03.2 1.2 3.H4.0 1.2 3.26.0 1.2 3.0

RI =C,H,20 0.20 0.53 0.2740 0.20 1.01 0.2560 0.20 1.62 0.2720 0.10 0.53 0.2720 0.30e 0.42'

(a) Reactions were carried out at 28 ± 0.2°C in 76% aq acetic acidat constant ionic strength.(b) Decomposition/disproportionation of Mn(II1) is minimum(less than 5%) only in 76% (v/v) aq acetic acid.(c) When [Mn(II1)j was more than 3.0 x 1Q-4mol dm -3, the spe-cific rate, kl(s - I) was lower than 0.53, probably due to nature ofoxidant which is known to exist in different forms such as [Mn30(OAC)6(OAc)HOAcj.5Hp (ct. Fristad, W.E. Peterson, J.R. &Ernst A, J org Chern, 50 (1985) 3143.

unreacted Mn(III) was determined spectrophotomet-rically. It was found that one mol of semicarbazoneconsumed 2 mol of Mn(III).

Results and DiscussionTable 1 summarises the kinetic data for Mn(III) oxi-

dation of semicarbzones. As manganese(IlI} reac-tions with semicarbazones of benzaldehyde and ace-tophenone proceed faster, these reactions have beenfollowed with oxidant in excess. The reaction exhibitstotal second order kinetics, one each in reactant(Table 1).

Substituent effectsThe rate of Mn(II1) acetate oxidation of sernicarba-

zones of benzaldehydes and substituted benzalde-hydes are susceptible toclectronic influence of sub-stituents in the phenyl ring. Electron donating groupsenhance the rate while electron withdrawing groupsretard the rate (Table 2). The Hammett reaction con-stant, o, calculated using the substituent constants (0)is - 1.57 indicating an electron deficient transitionstate. The ortho substituent, o-CI seems to retard therate due to its - I andstcric effects.

Table 2-Substituent Effect on Rate of Manganese(lII)Acetate Oxidation of Semicarbazones+"

[R~C6H4-C(RI) = N-NH-CO-NH~I

RI R~ k~(drn'mol : IS -I)

H H 0.73H p-CH, 1.00H p-CI 0.30H IIl-NO~ 0.40H o-CI 0.21CH, H 3.6CH, p-CI 4.3CH, p~H, 0.70CH, o~1 5.0C,H, H O.26b

C,H, p-NO~ 0.66b

C"H, p-CI O.38h

C,H, p-CH, O.50h

(a) Reactions of benzaldehyde and acetophenone semicarba-zones were carried out with the oxidant in 10-fold excess, viz,[Mn(lIl)] = 2.0 x IW'mol dm -.1 aad [semicarba-zone I= 2.0 x 10 4mol dm' at 2H± O.2°C in 76°;', (vIv) aq aceticacid,(b) Reactions of benzophenone semicarbazones were carried outas above with [semicarbazone] in IO-fold excess, viz. [semicarb-zone] = 2.0 x 10 -3moldm -3and[Mn(I1I)]= 2.0 x 10 -4moldm -.1.

(c) Specific rate for the maganese(II1) acetate oxidation of semi-carbazide in concentration range, [semicarba-zide] = 1.92 x 1O-'mol dm - 3,[Mn(Ill) = 2.1 x lO-4 mol dm -, at28±0.2°Cis5.1 x 1O-3s-l.

In contrast, the rates of oxidation of semicarba-zones of acetophenone and substituted acetophe-nones, are retarded by electron donating groups (p-CH3) and enhanced by substituent like p-Cl (Table 2).The plot of log [specific rate] versus Hammett substi-tuent constant (o), which is a fairly linear, gives Ham-mett reaction constant (p) = + 0.41 (correlation coef-ficient y = 0.98). This p-value indicates an electronrich transition state. Similar electronic influence hasbeen observed in Mn(III} oxidation of semicarba-zones of benzophenone and substituted benzophe-nones (Table I), with p = + 0.44. As the magnitude ofreaction constant, p, is low, probably these reactionsare less susceptible to electronic influence than thesemicarbazones of benzaldehydes.

As acetopheone semicarbazone gets oxidised byMn(lII) faster than that of benzaldehyde, possiblyelectron donation to a-carbon by - CHJ group facili-tates the reaction; benzophenone semicarbazonereacts at the slowest rate, possibly, because of stericinfluence of phenyl group at a-carbon in benzophc-none (Table 2).

MechanismTo account for the high yield ( ~ H()'Yc,) ofbcnzalde-

hydc at all [Mn(III)]and nearly samc amount ( ::::H()'X,)

1)57

INDIAN J. CHEM .. VOL. '27A. NOVEMBER 19KK

R II 0I I II

Nn(o",~ + Ph-C"'N-N -C -1'1112

R"".C~.,&,\~ ~n(O-"~

I'll"' =1'1- 1'1 -~ -N~+ HOAe.

~l"O~RI .

I'II-C=N -N-~-N~+ "'nUll

olost ."'nIOAc~! lip

R,I'll -7.-1'1=1'1 -fr -N~. Nnllll

II /0, II 0

1-11'

Inor90nic products(not identified)

SCHEME 1

of ketones at lower] Mn(IlI)] from the respective semi-carbazones and the stoichiometry, [1 mol of semi-carbazone requiring nearly 2 mol of Mn(lll)], thereaction Scheme 1 is proposed. Scheme 1 envisagesthe formation of N-manganese(III) intermediatewhich then decomposes in a slow step with electrontransfer to Mn(IIl). Such an electron transfer toMn(lll) will be facilitated by electron donating groupsand retarded by electron withdrawing groups. Thistrend is observed in the Mn(lII)-benzaldehyde semi-carbazones only. Hence an alternate mechanism in-volving the addition of Mn( OAc h to C = N has beenproposed (Scheme 2).

The presence of electron withdrawing group at thepara position of phenyl ring (p- CHi) may make thenitrogen of C = N electron rich, hindering the addi-tion of Mn(OAc), to C = N, while an electron with-drawing p-N02 group will favour the electron flow inthe opposite direction towards the carbon of C = Nfacilitating addition of Mn(OAc l". This trend in reac-tivity is observed in the case of Mn(lII) oxidation ofsemicarbazones of acetophcnoncs and benzophe-

RHOI I II

Mn(OAc~ + Ph-C=N-N -C -NHz

RI

Ph-CIIo

R II 0I I II

Ph- C- N- N- C- NilI I 2

AcO MJII/ ,

AcO 0Ac:

lost ~ MnIOAc~

R 0I II

Ph-~ -N =1'1 -C- I'll)' 2Mn(ll)

AcO

lost !¥R 0I II

Ph -C -1'1-=1'1 -C -NH'. 2,0,

II ". I lost-II V-"OAc

R 0I II

Ph-~ -N=N-C-NI)

0,lost 1 II

o• II

• 1'1EN.-C - HII2

+Inorgonic products (not Iclentai.d)

SCHEME 2

nones. Hence it can be said that Scheme 1 is a prob-able mode of conversion of benzaldehyde semicarba-zone to benzaldehyde while the reaction of semicarb-azones of acetophenonc/benzophenone with Mn( III)seems to prefer alternate reaction Scheme 2.

AcknowledgementOne of the authors (KRS) is thankful to UGC, New

Delhi, for the award of a junior research fellowship.

ReferencesI Balakrishnan R & Srinivasan V S. Proc Indian Acud Sci. 93

(1984) 171.2 Butler R N. Morris G J & O'Donohue A M. J chem Rei.,S).

(19KI) 61.3 Narayanan S & Srinivasan V S. J chem SOl' Perkin II. ( 19K6)

1557.4 Dyer J R. Applications of absorption spectroscopy of orgaIIic

compounds+ Print ice Hall. Englewood Cliffs) 1965. p.IK.5 Streitwieser A Jr & Heathock C H. Introduction to organic

chemistry(Macmillan. New York) 1973 p. 596.6 (a) Hine J. Physical organic chemistry (McGraw Hill. New

York)( 1%2). (0) Brown H C & Okornoto JAm chem Soc.80 (195Kj4971.J.