J Amer Oil Chem Soc (2007) 84:573–578
577
Scheme 4 Hydrogen-bonded
and free radical structures
Hydrogen-bonded and Free radical structures
-
n C9H19
-
H
H
O
n C9H19
O
t
t
S
S
C8H17
C8H17
S
R
R
OH
OH
O
O
.
.
(12)
(13)
(14)
such as (10, R = t-C9), and the 2,2¢-isomer was less
effective, (3, R = n-C15H31).
Antioxidancy and corrosion tests are practical evalu-
atory tests, but the orientation of the lipidic chains in the
thiobisphenol molecule at a metal surface may also be
significant. X-ray structural information on certain of the
4.4¢-thiobisphenols could be informative, but suitable
crystals for this determination could not be prepared.
Although n-alkyl-4,4¢-thiobisphenols, available from
intermediates from natural sources, have useful antioxi-
dancy but rather less effective corrosion resistance, they
have potentially greater biodegradability [10] compared to
4,4¢-branched chain compounds from petrochemical C8 and
C9 intermediates. These, by contrast, afford radical stabil-
ity, as with the ‘‘hindered’’ phenols, resulting in antioxidant
ability and they also provide good corrosion resistance.
Thiobisphenols have useful properties other than as
lubrication additives; for example as fungicides and bac-
tericides [20].
Table 2 indicates that in terms of antioxidant ability, the
4,4¢-thiobisphenols (2, R = n-C9) and (6, R = t-octyl)
appeared to be more effective antioxidants than the com-
mercial 2.2¢-product 10 (R = t-C8H17), while notably (6,
n-C9H19), was less effective. In tarnish/corrosion testing,
only 6 (R = t-C8H17) and 10 (R = t-C8H17) gave corrosion-
free results. In the n-alkyl series, a peak action for anti-
oxidancy may occur in the middle of the range, around C9.
With the n-C9 isomer series, the order of antioxidant
activity is 3- (2) > 4- (10) > (6), 2-alkyl substitution.
In tarnish/corrosion testing, the order of isomer activity
is different. Instability of (2, R = n-C9), to the test condi-
tions may be involved, namely that of the C-S bond,
perhaps due to some loss of coplanarity and cleavage.
Generally, in this test, n-alkyl 4,4¢-thioibisphenols were
less effective than branched alkyl chain thiobis compounds.
Thus, by contrast, the 4,4¢-thiobisphenol 6, (R = t-octyl,
2-(1,1,3,3-tetramethyl)-butyl), possessed both effective
antioxidant and anticorrosion properties. Branched chain
structures appear more effective than the n-alkyl.
The antioxidancy results can be partly interpreted in
terms of current theory that effective compounds (AH)
provide a source of stable free radicals (A•) by the reac-
tion, AH + R• = RH + A• and act as radical scavengers in
the system.
Acknowledgments We thank the SERC and Esso Research, (later,
Exxon Chemical Technology Centre), Abingdon, Oxfordshire, UK,
for a CTA and CASE award from 1985–1988. Mr J. Marsh and Dr P.
Skinner, now of Infineum Ltd, are thanked for evaluatory help,
(viscosity and corrosion testing), with compounds. Thanks are due to
Dr P. Kirby who arranged fungicide testing at Shell Sittingbourne,
Kent.
References
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