:Indian Journal of Chemistry-Vol. 17A, January 1978, pp. 63-69

Kinetics & Mechanism of Halogenation of Substituted Benzaldehydesby N-Bromosuccinimide in Aqueous Acetic Acid &

Sodium Acetate Buffer MediumP. s. RADHAKRISHNAMURTI* & B. SAHU

Department of Chemistry. Berhampur University. Berhampur 760007

Received 28 February 1978; accepted 3 July 1978

Kinetics of halogenation of substituted benzaldehydes bY N-bromosuccinimide is reportedin aqueous acetic acid and sodium acetate buffer medium. The complete disappearance of thesubstituent effect in the case of benzaldehydes, except the hydroxybenzaldehydes is a novelfeature of these reactions. Exocyclic C-halogenation in the case of substituted benzaldehydesis postulated, while nuclear C-halogenation has been postulated for the hydroxybenzaldehydes.The reaction between benzaldehydes and N-bromosuccinimide has been shown to be of a dipole-dipole type.

EARLIER we have reported Os04-catalysedoxidation of benzaldehydes by chloramine-T(CAT) in aqueous alkaline medium", and

the halogenation of benzaldehydes by CAT in acidmedium>. In view of the lack of kinetic data onthe halogenation of benzaldehydes, the kinetics ofreactions of N-bromosuccinimide (NBS) with a series-of benzaldehydes in the presence of mercuric acetatein aqueous acetic acid-sodium acetate buffers hasbeen undertaken and the results reported in thispaper. NBS is known to function either throughmolecular bromine or through the positive halogen-end. The functioning of NBS through molecularbromine has been totally suppressed by the use ofmercuric acetate which also fixes the Be that maybe produced as un-ionized HgBr2 or more likelyHgBr~- (refs. 3 and 4).

Materials and MethodsAll the substrates (see Table 1) were of AR grade

.and Were redistilled or recrystallized before use.Analar acetic acid was purified by standard proce--dure before use. All the other chemicals usedwere of AR grade. The disappearance of NBSwas estimated by standard .iodornetric method.The kinetic runs were carried out in aq_ acetic acid-sodium acetate buffer in the presence of mercuricacetate.

Results and DiscussionThe pseudo first-order rate constant (k1) remains

almost same When different concentrations of thebenzaldehydes are employed. This proves the zero--order nature of the reaction with respect to thebenzaldehydes (Table 1).

The hydroxybenzaldehydes show a one-half order.dependence with respect to the substrate. Theplot of log kl VO; logeS] is linear having a dope of-0·5. Even in pure acetic acid medium also the-one-half dependence on substrate still persists forhydroxybenzaldehydes. The constancy of the rate<constants at different [NBS] indicates the first order

TABLE 1 - DEPENDENCE OF RATE CONSTANTON[SUBSTRATEJAND [NBSJ IN AQUEOUSACETIC ACID AND

PURE ACETIC ACID

[Hg(OAc), = 0-005M; NaOAc = 0-02M; temp. = 600J

Compound 10'[SJ. M 102[NBSJ, M 102k!minot

HOAc-WATER (75: 25). % v]v

Benzaldehyde 5'01 0,1 2-437·19 0-1 2-709·58 0·1 2-56

11-86 0-1 2-7514·35 0·1 2-7422-88 0-1 3-075-01 0'075 2-595-01 0-094 2,435·01 0'140 2·375,01 0,240 2-31

Anisaldehyde 0'51 0·1 2,761·05 0·1 2,612'55 0·1 2·655·09 0'1 2-897·38 0'1 3·39

11·08 0·1 3·3516'18 0'1 3·3324'00 0·1 3'79

m-Methoxy benzalde- 0·53 0·1 2'61hyde 2,50 0'1 2·60

11-14 0·1 3,22

Tolualdehyde 5'00 0,1 2'8610·09 0·1 2·5420'69 0·1 2,9529,08 0'1 3·1441-10 0·1 3·20

o-Chloro benzalde- 2·57 0·1 2·27hyde 5·29 0'1 2'85

11 '13 0'1 2'9120·00 0'1 3,0634'18 0'1 3'34

m-Chlorobenzalde- 5·38 0'1 2'58hyde 10·13 0'1 2'43

1)3

INDIAN J. CHEM., VOL. 17A, JANUARY 1979

TABLE 1 - DEPENDENCE OF RATE CONSTANT ON[SUBSTRATE] AND [NBS] IN AQUEOUS ACETIC ACID AND

PURE ACETIC ACID - contd

[Hg(OAc), = 0-005M; NaCAc = 0'02M; temp.= 60°]

Compound 10'[S), M 10'[NBS], M

HOAc-WATER(75: 25), % vlv

p -Nitro benzalde- 5·09 0·1hyde 7-61 0·1

10'19 0'120·50 0-1

o-H ydroxybenzalde- 0'53 0-1hyde 1·03 0·1

2'72 0'16'08 0·1

11·26 0-16·02 0-286·02 0-406,02 0·45

m-Hydroxybenzalde- 0,52 0-1hyde 1·12 0-1

2,74 0'15·00 0-1

12'28 0'1

p-Hydroxybenzalde- 5,09 0-1hyde

100% ACETIC ACID

o-H ydroxybenzalde- 2·75 0-1hyde 5·16 0'1

12,44 0-135,88 0-1

m-H ydroxybenzalde- 1·11 0-1hyde 5·13 0-1

10'83 0-121'62 0'1

p-Hydroxybenzalde- 5'18 0·1hyde

2-332-672-733·36

3-203-565-057-69

12-517-267-367-30

4-546-20

10-2415-6429-96

34-71

0,670·901·373,08

0-812·193-827-04

6·90

dependence on NBS in the concentration rangestudied.

The first-order rate constants at varying concen-trations of mercuric acetate remain almost constantup to eight-fold concentration of mercuric aceta~e(Table 2). This indic',l-tes ~hat the role of mercuncacetate in the reaction IS only to prevent theformation of Be formed in the course of reaction.

An increase in the concentration of sodium acetateincreases the rate (Table 2). The plot of log klvs log [NaOAc] is linear. A plot of log kl vs,PHis also linear indicating that the enhanced reactionsrate is due to the increase in pH of the medium.This is quite in consonance with the reaction ofaldehydes by molecular bromine. ..

An increase in the percentage of acetic aciddecreases the rate (Table 3), showing that decrease indielectric constant decreases the rate. Laidler equa-tion predicts a linear plot of log kl vs (D-l/2D+l),whereas Amis equation predicts a linear plot oflog kl vs liD (D is the dielectric constant). The

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TABLE 2 - DEPENDENCE ON [Hg(CAc),] A1\D [NaCAcJ INALDEHYDE-NBS REACTIONS IN AQUEOUS ACETIC ACID

[NBS = 0-001M; aq. acetic acid (75: 25, % v/v);temp. = 600J

10'[SJM

10'[Hg(OAc),J 10'[NaOAc]M M

102k,(min-t)

ANISALDEHYDE

5-10 0-50 1·94 2-895·10 1'00 1-94 2-405·10 2-52 1-94 2-405,10 4-03 1·94 2-912·55 0-50 1'94 2-652'56 0-50 4-06 4-012-56 0·50 10-07 8-322'56 0-50 20-66 9·43

O-CHLOROBENZALDEHYDE

5·105·105,105,105·195-185,18

0-501-002'524-030-500'500'50

2-062-062·062·064·06

10-0720'66

2-852-772-743-265-02

10-0712·75

m-HYDROXYBENZALDEHYDE

1-10 0-50 1·95 6,201-08 1-00 1-95 5-961·08 2·52 1-95 5-601·08 4-03 1,95 6·451-10 0-50 4,06 9-741,09 0-50 10-07 20'421,09 0-50 20-66 26·54

TABLE 3 - EFFECT OF VARYING COMPOSITION OF THESOLVENT ON REACTION RATE

[NBS = 0-001M; Hg(OAc), = O-OOSM; NaOAc = 0'02M;temp. = 600J

10' [5J HOAc: 10'k, 10'[5JM water (min-") M

(%, v/v)

HOAc: 10'k,water (min-v).(%, v/v)

BENZALDEHYDE O-HYDROXYBENZALDEHYDE

5,02 so: SO 12·28 2·71 50:50 21·445,02 60:40 7·36 2·71 60:40 13·245,01 75: 25 2-56 2-72 75: 25 5-055'02 90: 10 1-15 2·71 90: 10 2-01

2-71 100:00 0-67

ANISALDEHYDE m-HYbROXYBENZALDEHYDE.

5·10 50:50 9,33 1,10 50 50 18-255·10 60:40 6·05 HO 60 40 11·745'09 75: 25 2·61 H2 75 25 6-205·09 90: 10 H5 1·11 90 10 2-39i-n 100 00 0-82

TOLUALDEHYDE

5·025·025,005·02

so SO60 4075 2590 10

10·936·232'861-00

RADHAKRISHNAMURTI & SAHU: HALOGENATION OF SUBSTITUTED BENZALDEHYDES

+NBS+H+=> JBSH ... (1)

fastC6HsCHO+_-BS --')- C6HsCOBr+Succinimide ... (2)

Ifast I HPt

C6H5COOH+HBr ... (3)

Scheme 1

plots of log kl vs (D-1[2D+1) and log kl vs l[Dfor the different aldehydes studied are not linearbut exhibit a distinct curvature at high percentagesof acetic acid, thereby showing that the theoreticalapproaches of Amis and Laidler are not applicableunder the present conditions. The reaction bet-ween benzaldehydes or hydroxybenzaldehydes andNBS is essentially a dipole-dipole reaction.

Structure and reactivity - The different aldehydes,viz. benzaldehyde, anisaldehyde, m-methoxybenzal-dehyde, tolualdehyde, o-chlorobenzaldehyde, m-chlorobenzaldehyde and p-nitrobenzaldehyde, byNBS are oxidized at almost the same rate at varioussubstrate concentrations (Table 1). This shows thataldehyde is participating in the fast step.

The order of reactivity of hydroxybenzaldehydesis p-hydroxybenzaldehyde >m-h ydroxybenzaldehyde>o-hydroxybenzaldehyde. .

Mechanism - Benzaldehyde and substitutedbenzaldehydes undergo exocyclic halogenation. ofthe aldehyde giving rise to the correspondmg

carbonyl halide leading finally to the correspondingcarboxylic acids via hydrolysis in the fast steps.(Scheme 1).

Step (1) shows that a decrease in the proportionof sodium acetate the reaction medium will retardthe reaction and vice-versa. The equilibrium (1)will shift to the left with increase in the concen-tration of sodium acetate.

The sequence of reactions in Scheme 1explains thefirst order in NBS and a zero order dependenceon the substrate.

In the case of hydroxybenzaldehydes the presenceof hydroxyl group activates the nucleus and hencenuclear bromination occurs predominantly. Thebromine enters the position para to the hydroxylgroup and if para is occupied, then ortho bromi-nation occurs", These reactions are also first orderin NBS. The incursion of the second step into thereaction rate explains the fractional order in subs-trate observed for these substrates.

References

1. RADHAKRISHNAMURTI, P. s. & SAHU, B., Indian J. Chem..•15A (1977), 700.

2. RADHAKRISHNAMURTI, P. S. & SAHU, B., Proc. 65th Indian-Sci. Cong., 1978, Part III, Paper 161, 66.

3. BAILAR, J. C., The chemistry of coordination compounds(Reinhold, New York), 1956, 4.

4. MUSHRAN, S. P. & BOSE, A. K. & TIWARI, J. N., u».Chem., 107 (1976), 1021.

S. GINSBERG, D., J. Am. chem, Soc., 73 (1951), 702.

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