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5 V s−1 or higher. This scan-rate dependence provides insight into
the stability of the proposed radical-anion intermediate formed
during the one-electron cathodic process; double-potential-step
chronoamperometry revealed that this radical-anion undergoes
ring-opening and has a half-life of approximately 0.7 s. When
controlled-potential (bulk) electrolyses of phthalide were carried
out at a mercury pool cathode held at −1.60 V (very close to the
cathodic peak potential), the coulometric n value was found to
be 1. After the catholyte was partitioned between 10% aqueous
hydrochloric acid and diethyl ether, a subsequent gas chromato-
graphic analysis revealed the presence of 2-methylbenzoic acid
(50%) and phthalide (44%). In addition, bulk electrolysis of phthalide
excess of phenol as a proton donor proved to be a four-electron pro-
cess that affords a mixture of products [2-methylbenzoic acid (28%),
cis-hexahydrophthalide (30%), trans-hexahydrophthalide (6%), and
cis-1,2,3,6-tetrahydrophthalide (32%)] [7].
In later work [8], it was discovered that, if one performs a
bulk electrolysis of phthalide at a mercury pool in DMF–0.10 M
TBAP, under conditions similar to those just described, and then
injects an aliquot of the catholyte directly into a gas chromato-
graph, the products are found to be n-butyl 2-methylbenzoate and
the products seen after direct injection of catholyte into a gas
chromatograph consist essentially of a 1:1 mixture of n-hexyl 2-
methylbenzoate and phthalide.
In light of our previous research [7,8] concerning the electro-
chemical behavior of phthalide, particularly the peculiar pattern
of products seen as experimental conditions have been varied, we
have employed cyclic voltammetry and controlled-potential (bulk)
electrolysis in the present investigation, first, to probe more deeply
the electrochemical reduction of phthalide at a carbon cathode in
DMF containing either 0.10 M tetramethylammonium perchlorate
(TMAP) or tetra-n-butylammonium perchlorate (TBAP) as suppor-
ting electrolyte and, second, to compare our new results with
those obtained earlier for the reduction of phthalide at a mercury
cathode. In this report, some important questions are addressed.
First, how can one explain the presence of phthalide at the end
of an exhaustive electrolysis? Second, what process is responsi-
ble for the various 2-methylbenzoate esters found at the end of
an electrolysis? Third, what intermediate controls the exclusive
appearance of either 2-methylbenzoic acid or 2-methylbenzoate
ester as an electrolysis product? Fourth, what additional infor-
mation can be obtained from an electrolysis of phthalide in the
presence of deuterium oxide? Answers to these questions have
been deduced with the aid of additional experiments entailing
the use of (i) sodium 2-(hydroxymethyl)benzoate to account for
the presence of phthalide at the end of a bulk electrolysis, (ii) 1-
iodooctane as a trapping agent for 2-methylbenzoate, (iii) pure
sodium 2-methylbenzoate to elucidate the ester-forming process,
and (iv) D2O to track protonation–deprotonation reactions involv-
ing transient anions.
Chemicals), and hydrochloric acid (38%, Mallinckrodt Chemicals).
Deuterium oxide (99.9%) was purchased from Cambridge Isotope
Laboratories and was used for deuterium-labeling studies. Zero-
grade argon (Airgas) was used to deaerate all solutions prior
to electrochemical experiments. Tetramethylammonium perchlo-
rate (TMAP, >99%, GFS Chemicals) and tetra-n-butylammonium
perchlorate (TBAP, >99%, GFS Chemicals) were employed as sup-
porting electrolytes; each compound was recrystallized from ethyl
acetate–hexanes and stored in a vacuum oven at 70–80 ◦C prior to
use.
2.2. Cells, electrodes, procedures, and instrumentation
A description of the one-compartment cell used for cyclic
voltammetry has been previously reported [9]. We constructed
a circular, planar carbon cathode (geometric area of 0.071 cm2)
by press-fitting a short piece of glassy carbon rod (Grade GC-
20, 3.0-mm-diameter, Tokai Electrode Manufacturing Company,
Tokyo, Japan) into a machined Teflon tube; electrical connec-
tion was made by a thin stainless-steel pole that contacted the
glassy carbon and extended upward through the axis of the tube.
A coil of platinum wire was used as the auxiliary electrode for
cyclic voltammetry. Prior to each scan, the cathode was cleaned
in an ultrasonic bath. All potentials are reported with respect
to a reference electrode that consisted of a cadmium-saturated
mercury amalgam in contact with DMF saturated with both cad-
mium chloride and sodium chloride [10–12]; this electrode has a
(SCE) at 25 ◦C. Cyclic voltammetry experiments were performed
with a Princeton Applied Research Corporation (PARC) model 273A
potentiostat–galvanostat operated by PowerSuite® software.
Instrumentation used for controlled-potential (bulk) electrol-
ysis is described elsewhere [13,14]. Working electrodes in the
form of circular disks were cut from reticulated vitreous carbon
rods (RVC 2X1-100S, ERG Aerospace Corporation, Oakland, CA)
to have approximately a 2.4-cm diameter, a 0.4-cm thickness,
and a 200-cm2 geometric area. Preparing, cleaning, and storing of
RVC electrodes are described in a previous publication [15]. For
bulk electrolyses, the aforementioned cadmium-saturated mercury
amalgam reference electrode was utilized, and the auxiliary elec-
trode was a carbon rod immersed in a DMF-electrolyte solution
separated from the cathode compartment by a sintered-glass disk
backed by a methyl cellulose–DMF–0.10 M tetraalkylammonium
perchlorate plug. To perform the bulk electrolyses, we employed a
PARC model 173 potentiostat that was controlled with the aid of
LabVIEW software.
Simulations of cyclic voltammograms were carried out with the
aid of DigiElch 4.0, a software package for Digital simulation of
the effects of uncompensated resistance; however, the resistance
was not compensated electronically by the potentiostat. Voltam-
mograms recorded at seven scan rates (0.05, 0.1, 0.2, 0.5, 1, 2,
and 5 V s−1) were simulated according to an ECE-type scheme
(DISP1 mechanism [17]), which comprises an electrode reaction
(one-electron reduction of phthalide to its radical-anion, E-step),
ring-opening of the radical-anion to give the 2-carboxylatobenzyl
radical (C-step), and one-electron reduction of the ring-opened
radical-anion, either at the electrode (E-step) or reduction in solu-
tion by reaction with the phthalide radical-anion (DISP) with the
final reaction being sufficiently rapid to cause the C-step to be
2. Experimental
2.1. Reagents
Each of the following compounds was used, as received, with-
out further purification: 2-methylbenzoic acid (99%, Aldrich),
methyl 2-methylbenzoate (99%, Alfa Aesar), phthalide (98%,
Aldrich), dimethylformamide (DMF, 99.9%, EMD Chemicals), anhy-
drous diethyl ether (absolute, EMD Chemicals), n-hexadecane
(99%, Sigma), 1-iodooctane (98%, Sigma), 1-butanol (99%, J. T.
Baker), chloroform-d (99.8%, Aldrich), sodium hydroxide (97%, EMD