KINETICS OF OXIDATION OF ETHANOLAMINES BY BROMAMINE-B IN ALKALINE BUFFER
481
chloramine-T (CAT), which is a byproduct in saccha-
rin manufacture, and is a well-known analytical re-
agent for the determination of diverse substrates. The
benzene analogue ofchloramine-T is chloramine-B
(CAB). Mechanistic aspects ofmany ofthese reactions
have been documented [7,8]. The bromine analogue
bromamine-B (sodium-N-bromobenzenesulfonamide;
C6H5SO2NBrNa · 1.5H2O or BAB) is becoming im-
portant and is found to be a better oxidizing agent than
the chloro compound. The compound can be easily
prepared by the bromination ofCAB. A survey oflit-
erature indicates limited information [9–11] on the
mechanisms ofoxidation reactions ofBAB. Hence, it
was thought interesting to investigate the oxidative be-
havior ofBAB toward ethanolamines. As a part ofour
broad program on the mechanistic studies ofethanol-
amines with organic haloamines, we report the kinetics
ofoxidation ofmonoethanolamine (MEA), diethano-
lamine (DEA), and triethanolamine (TEA) by BAB in
alkaline buffer medium (pH 8.7–12.2) at 40ЊC in the
present communication. Attempt has been made to
identify the most probable reactive species of the ox-
idant in alkaline medium. Activation parameters have
been calculated and an isokinetic relationship has also
been deduced.
ofpurity, they were used without further purification.
An aqueous solution ofdesired strength was prepared
freshly each time. All other chemicals used were of
analytical grade. Heavy water (D2O 99.2%) was sup-
plied by the Bhaba Atomic Research Centre, Mumbai,
India. The ionic strength (I) ofthe system was main-
3
tained at a constant high value (0.5 mol dmϪ ) using
a concentrated solution ofsodium perchlorate “to
swamp” the reaction. The dielectric constant or per-
mittivity (D) ofthe reaction medium was altered by
the addition ofmethanol in varying proportions (% v/
v) and values ofD ofmethanol–water mixtures re-
ported in literature [13] were employed. Standard buf-
fer systems (NH2CH2COOH–NaOH) were employed
[14]. Triply distilled water was used in preparing all
aqueous solutions.
Kinetic Measurements
Reactions were carried out under pseudo-first-order
conditions ([substrate]o ϾϾ [oxidant]o) at constant tem-
perature (e.g., 40ЊC) in glass stoppered Pyrex boiling
tubes coated black on the outside to eliminate photo-
chemical effects. The oxidant and requisite amounts
ofsubstrate, NaClO , and buffer solutions taken in
4
separate boiling tubes were thermostated for 30 min
at 40ЊC. The reaction was initiated by the rapid addi-
tion ofa measured amount ofBAB to the mixture and
was shaken intermittently for uniform concentration.
The progress ofthe reaction was monitored by iodo-
metric titration ofunconsumed BAB in known ali-
quots (5 ml each) ofthe reaction mixture withdrawn
at regular time intervals. The reactions were followed
up to 70% completion. The psuedo-first-order rate
constants (kЈ) calculated from the linear plots of
log[BAB] vs time were reproducible within Ϯ3–5%.
EXPERIMENTAL
Materials
Bromamine-B (BAB) was prepared by partial debrom-
ination ofdibromamine-B (DBB) in NaOH medium
[12]. Pure chlorine was bubbled through an aqueous
solution ofchloramine-B (30 g in 560 ml ofwater)
and liquid bromine (6 ml) was added dropwise with
constant stirring. The yellow precipitate ofDBB
formed was thoroughly washed with water, filtered un-
der suction, and dried in a vacuum desiccator. Dibro-
mamine-B (31.5 g) was digested in small batches with
constant stirring in 50 ml of4.0 mol dm Ϫ3 NaOH. The
solution was cooled in ice, filtered under suction, and
the product was dried over anhydrous calcium chlo-
ride. The purity ofBAB was checked iodometrically
through its active bromine content and the compound
was further characterized by its 13C FT-NMR spectrum
(obtained on a Bruker WH 270-MHz nuclear magnetic
resonance spectrometer) with D2O as solvent and TMS
as the internal standard (ppm relative to TMS) 143.38
(C-1, carbon attached to S atom), 134.30 (C-4, para to
the hetero atom), 131.26 (C-2, 6), and 129.31 (C-3, 5).
An aqueous solution ofBAB was standardized iodo-
metrically and preserved in brown bottles to prevent
its photochemical deterioration.
Stiochiometry
Different sets of reaction mixtures containing different
amounts ofoxidant and substrate ([BAB] ϾϾ [EA]) in
an alkaline buffer medium (pH ϭ 9.69) were kept for
24 h at 40ЊC. Determination ofthe unreacted oxidant
showed the following stoichiometric reactions (eqs.
(1–3)):
H2N(CH2CH2OH) ϩ 2RNBrϪ ϩ OHϪ ϩ H2O !:
HCHO ϩ HCOOϪ ϩ NH3 ϩ 2RNH2 ϩ 2BrϪ (1)
HN(CH2CH2OH)2 ϩ 4RNBrϪ
ϩ 2OHϪ ϩ 2H2O !:
2HCHO ϩ 2HCOOϪ ϩ NH3
Because ethanolamines were ofthe accepted grades
ϩ 4RNH2 ϩ 4BrϪ (2)