Voltammetry of organic halogen compounds. IV. The reduction of organic chlorides at the vitreous (glassy) carbon electrode

tetrahedron

Letters

VOLTAMMETRY

No. 36,

pp 3231 - 3234,

Printed

Pergamon Press.

1974.

OF ORGANIC HALOGEN COMPOUNDS.

in Gre8t

Britain.

IV. THE REDUCTION OF ORGANIC CHLORIDES

AT THE VITREOUS (GLASSY) CARBON ELECTRODE’ Frank L. Lambert* Department (Iboeired

of Chemistry,

in USA 1 3uly

electrode

(GCE) , a process

(Aromatic

chlorides

in electrochemical than simply

College,

synthesis,

California

29 July

at the dropping mercury electrode.)

of organic

(glassy)

electrode

carbon 283

(DME).

In view of the increasing

with a comparatively

of the polarography

90041

1974)

at the vitreous

our reduction of chloroalkanes

of a novel electroreduction investigation

publication

of alkyl monochlorides

reduced at this carbon

in organic

Los Angeles,

ir UK for

which has not been effected

techniques

In their pioneering

reoeived

reduction

are also readily

an example

Occidental

1974;

We wish to report the general

and Glynnis B. Ingall

may be of greater import

unused electrode

halides,

interest

system.

von Stackelberg

and Stracke

n

found that alkyl bromides only monochlorides chloride

reducible

in which the chlorine

using tetraethylammonium despite

of all types were readily

many variations

at the DME were methyl chloride is activated.

bromide

research

Subsequently,

as the supporting

in conditions

been able to reduce non-activated In our current

on the electrochemistry

Subsequent work with l-chloropentane

the reduction

for a number of chlorides

are reproduced

on the next page to show the characteristics

ventional

coulometry

at controlled

potentials

but still clearly

3231

at the glassy

of bromine

chlorobenzene (DMF)4 but nor we have

indicated

and that the process Voltammetric

of the electrode

curves

electrode,

5

that this

was general. for typical

Halfcompounds

response.

were experimentally indicate

carbon

in 1-bromo-5-chloropentane.

alkyl monochlorides

are given in Table I.

potentials

in reducing

in N, N-dimethylformamlde

of bromoalkanes

in such molecules

studies at these very negative

such as ally1 or benzyl

neither other investigators3

and other non-activated

of the chlorine

they showed that the

other than methyl chloride.

wave potentials

Coulometric

we succeeded

electrolyte

alkyl monochlorides

However,

and compounds

using the DME system,

we noted a second very negative wave following

wave was due to the reduction

reduced at the DME. ‘

more difficult

that the reduction

than con-

mechanism

cannot

NO. 36

3232

FIGURE

1

FIGURE 2 A. Blank B. 1 -Chloropentane C. 1-Bromo-Schloropentane

-i.3

FIGURE

volts -i.7~

-2: Iv

-2: 5v

-2: 9v

3

A. Blank C: B Chlorobenzene

,

! :

,III

-1: 3 volts Voltammetric

Reduction

-i.7v of Organic

-2: Iv Chloro Compoulids

-2:5v at the Glassy

Carbon Electrode

-2.9v

be the simple traditional2

two-electron

for its reduction

volts and gave hexane as the principal

at -2.90

hexene ordodecane.

3_Chloropentane,

only 1.2 electrons the possible electrons

per molecule

dimer.

for reduction

Proportionate

reduction

chloride, tertiary

several

of these classes Some steric (viz.,

qualitative

chlorides

methyl

increase members dropping

of primary

importance

deductions

have markedly

electrode.

Acknowledgment. which made this research

volts required

classes)

volts required

1.7

and 66 per cent of 2-methylbutane. process

or secondary

is an important, Any free radical

chlorides

but not exclusive, which might be

does not subsequently

dis-

conditions.

of this process

for the controlled

potentials

to the reduction

and neopentyl (-2.83)

secondary

than simple

potentials chloride

secondary

reduction

of any alkyl mono-

shown in Table I.

to reduce than comparable

0.03 volts in going from the simplest

mercury

only 1.7 electrons

at -2.80

at -2.80

can be drawn from the results

may be contributing

of their respective

amyl chloride,

carbon in DMF.

less negative reduction

(-2.76), pentyl (-2.80)

of ca,

at glassy

are only somewhat easier

hindrance

tertiary

under our experimental

In addition to the general

under coulometry

data, a one-electron

of alkyl chlorides

nor di merize

chloride,

required

product with less than one per cent of

34 per cent of 2-methylbutenes

from the coulometric

by such a one-electron

1-Cblorohexane

but also formed less than one per cent of pentene or 3,4_dietbylhexane,

but formed

pathway for the reduction

process.

a secondary

2-Chloro-2-methylbutane.

Quite obviously

formed

stepwise

chlorides,

primary

of chloro

compounds

but it is far less than in the reduction

chloride

but both

chloro

half -wave potentials,

or tertiary

At the GCE

compounds.

at the GCE as well as the

to the more complex

of bromo

compounds

a.t the

1

We wish to thank the Research

Corporation

for a Cottrell

possible. References

F. L. Lambert,

J. Org.

1.

Paper III.

2.

M. von Stackelberg

3.

For example,

4.

F. L. Lambert

and K. Kobayashi,

5.

F. L. Lambert

and G. B. Ingall,

and W. Stracke,

W. M. Moore

Chem.,

31, 4184 (1966). CITr

Z. Elektrochem.

and D. G. Peters, J. Org.

, ,53_,118 (1949).

Tetrahedron

Chem.,

Lett.,

23, 773 (1953). M

paper in preparation.

454 (1972).

College

Science

Grant