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Ring Chlorination of BenzenoidCompounds Using CalciumHypochlorite [Ca(OCl)2]Stephen O. Nwaukwa a & Philip M. Keehn ba Department of Pure and Applied Chemistry, OndoState University, Ado-Ekiti, Ondo State, Nigeriab Department of Chemistry, Brandeis University,Waltham, Massachusetts, 02254-9110, U.S.A.

Available online: 24 Oct 2006

To cite this article: Stephen O. Nwaukwa & Philip M. Keehn (1989): Ring Chlorinationof Benzenoid Compounds Using Calcium Hypochlorite [Ca(OCl)2], SyntheticCommunications: An International Journal for Rapid Communication of SyntheticOrganic Chemistry, 19:5-6, 799-804

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SYNTHETIC COMMUNICATIONS, 19(5&6), 799-804 (1989)

RING CHWRINATION OF BENZENOID COMPOUNDS USING

CALCIUM HYPOCHLORITE [Ca(OCl)z]

Stephen 0. Nwaukwat and Philip M. Keehn*#

tDepartment of Pure and Applied Chemistry, Ondo State University, Ado-Ekiti, Ondo State, Nigeria and +Department of Chemistry, Brandeis University, Waltham, Massachusetts 02254-9110, U.S.A.

Abstract: Chlorination of activated benzenoid rings is efficiently effected at 0°C in aqueous acetone/HOAc using calcium hypochlorite as the chlorinating agent. Good to excellent yields of the chlorinated products are obtained.

Early work using calcium hypochlorite (Ca(0Cl)2) as an

aromatic ring chlorinating reagent focused mainly on its utility

in chlorinating N-alkylanilines and on defining the mechanism of

that transformation. In those studies'' it was demonstrated that

N-chloroaniline intermediates are initially generated giving rise

to ortho and para ring chlorinated products after their subsequent

rearrangement. Reaction conditions varied using as much as ten

equivalents of hypochlorite for a period of 1-12 hrs in CC1, or

buffered ethanol solution.

studies in using Ca(OCl), as a chlorinating reagent for other

aromatics, no further work was carried out in this regard.

Despite the promise engendered by the

799

Copyright @ 1989 by Marcel Dekker, Inc.

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8 00 NWAUKWA AND KEEHN

We have been studying the use of calcium hypochlorite as an

oxidizing agent3’4#5 and found that it is a reasonably good

reagent for the chlorination of aromatic compounds other than

anilines. Reactions are carried out under mild conditions at 0 ° C

in aqueous acetone/HOAc solutions. In most cases reactions are

complete in less than 1 hr, though 1 hr was used as a standard

reaction time. In general, one molar equivalent of calcium

hypochlorite is employed per mole of substrate. In a few cases

more than one molar equivalent was used. Yields of the

chlorinated compounds range from good to excellent depending on

how activated the aromatic ring is.

A typical experimental procedure follows for the chlorination

of naphthalene.

water condenser was added calcium hypochlorite (8.58 g , 0.039 mol;

Fisher Scientific, 67% calcium hypochlorite), water (100 mL) and

acetic acid (10 mL). The flask was then immersed in a ice/water

bath (0°C) and the contents stirred until the calcium hypochlorite

dissolved and a light yellow solution was obtained. A solution of

naphthalene (5.0 g, 0.039 mol) in acetone (100 mL) was then added

over a three minute interval. The resultant solution was then

stirred at 0 ° C for 1 hr. after which time the reaction mixture was

diluted with water (100 mL). At this time, insoluble chloronaph-

thalene could be seen as an oil settling at the bottom of the

reaction flask. The mixture was then extracted with ether

( 2 x 150 mL) and the combined ether extracts washed with saturated

sodium bicarbonate (2 x 100 mL). After drying, evaporation of the

To a flask equipped with a stirring bar and a

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RING CHLORINATION OF BENZENOID COMPOUNDS 801

ether afforded a yellow liquid which, on vacuum distillation, gave

pure 1-chloronaphthalene as a bright yellow oil (5.48 g, 87%;

bp 260-262"; lit. 258.2,,,6). The 'H NMR spectrum' was identical

with that of authentic material.

Table 1 summarizes the results obtained in this study. One

can see from the table that ring chlorination with calcium hypo-

chlorite, like other electrophilic aromatic substitution reac-

tions, is enhanced in substrates having electron donating groups

in the ring (cf. toluene, xylenes, anisoles) and retarded in those

with electron withdrawing groups (cf. nitrobenzenes and chloro-

benzene). This accounts for the sluggish reactivity of benzene

and the non-reactivity of nitrobenzene under these reaction

conditions.

a ring containing a methoxyl group (entry lo), but, a group as

weakly withdrawing as "C1" does not allow for chlorination to

occur. Ca(OCl), seems to oxidize aniline quite readily as

previously noted for N-alkylanilines.'*Z The isolation of

o-chloroaniline from a five minute reaction using half a molar

equivalent of Ca(OCl), suggests that chlorination is quite rapid.

However, ring decomposition seems to follow, probably due to the

general ease of ring oxidation in aniline.

the amino group, however, (as in acetanilide, entry 12) allows

chlorination to proceed in the normal manner. The improved yields

in the naphthalene series over the benzene series might be

expected due to the increased reactivity of the naphthalene ring.

The CHO group does not seem to hamper chlorination in

Slight deactivation of

Though other reagents and methodologies'-" have been used

for aromatic ring chlorination (even of deactivated aromatics) few

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802 NWAUKWA AND KEEHN

Table. Ring Chlorination of Benzenoid Compounds at O'C

Using Calcium Hypochlorite.

Entry

1 2 3

4

5

6

7

8

9 10

11

12 13 14 15

Substrate

Benzene Toluene Naphthalene

o-Xylene m-Xylene p-Xylene Anisole 2-Methoxynaphthalene 4-Methylanisole 2-Methoxybenzaldehyde Aniline

Acetanilide Chlorobenzene Nitrobenzene 4-Nitrotoluene

Product

Chlorobenzene Chloro toluene 1- Chloronaphthalene 4- Chloro -0 -xylene 4-Chloro-m-xylene 2-Chloro-p-xylene

Chloroanisole 1-Chloro-2-methoxynaphthalene

2-Chloro-4-methylanisole ~-Ch~oro-2-methoxybenzaldehyde3

2-Chloroaniline 4-Chloroacetanilide

No reaction' No reaction' No reaction'

Isolated Yield(%l

10'

sob 87

80

86

76

6SC

94 87

93 3od

79

a When the reaction was carried out at dient temperature using 2 molar equivalents of calcium hypochlorite the yield of chlorobenzene was 22%.

Product comprised of 72% p-chlorotoluene and 28% o-chlorotoluene as calculated from integrated curves in the lH NMR spectrum of the mixture.

'Ihe reaction was carried out for 30 minutes. The mixture comprised of 86% p-chloroanisole and 14% o-chloroanisole identified as in (b) above.

The reaction was carried out for 5 minutes using half a molar equivalent of Ca(OCl),. Another product obtained w a s a black solid (presumably carbon), insoluble in both water and organic solvents suggesting the decomposition of aniline. obtained when the reaction w a s allowed to proceed for a longer time.

This black solid was the only product

' Reaction failed even at room temperature.

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RING CHLORINATION OF BENZENOID COMPOUNDS 803

reagents are as mild as Ca(OCl), and few procedures as

straightforward (e.g., the use of Cl,, C1,019, high temperature,

etc.) as what we describe here. Our work thus extends the use of

Ca(OCl), beyond the chlorination of N-alkylanilines alone and

further broadens the scope of useful organic oxidations that

Ca(OCI), can initiate.3z4*5

Acknowledeement: S. 0. N. wishes to thank the Ondo State

University for partial support for this work.

NIH (Biomedical Research Support Grant RR 07044) is also

gratefully acknowledged.

Support from the

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Recently, Effenberger, et. a1.20 reported a method of chlorination of benzenoid compounds containing electron withdrawing.groups (e.g. nitrobenzene) which involves the use of dichloromonoxide in conjunction with trifluoromethane- sulfonic anhydride in phosphorus oxychloride. Since calcium hypochlorite can be employed under mild conditions for the chlorination of activated benzenoid compounds, the methods compliment one another though the former uses a corrosive and dangerous chlorinating reagent.

Effenberger, F., Kussmaul, U. and Huthmacher, K., Chem. B e r . , 1979, 112, 1677.

(Received in USA July 28, 1988)

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