CHEMSUSCHEM
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Because benzylic protons next to a QA are highly reactive
(see the section on benzylic cations), enhanced stability is ex-
pected by increasing the distance between QA and the benzyl
group. The reason for the superior base resistance of benzyl
group-attached QAs with an alkyl-chain substituent is, howev-
er, less clear.
HTM the half-life surprisingly decreases again, dropping down
to ETM values between 10 and 16 carbon atoms, which sug-
gests that more than steric and inductive factors are involved.
Perhaps the same cause results in a decrease in the QA half-
life in membranes when compared to their simple-salt counter-
parts.[47,54,55] As in these experiments the OHꢀ concentration
was always the same, different hydration numbers can likely
be excluded as an explanation. A possible cause could be that
the elimination products from longer chains are removed
more quickly from the reaction equilibrium due to an increas-
ing tendency for phase separation with the aqueous phase. A
potential back reaction that can occur in the presence of
a strong base such as OHꢀ[56,57] may be thus inhibited.
A systematic look at the alkaline degradation of QAs with
alkyl chains reveals that the additional steric strain on the de-
composition reactions caused by longer chains diminishes for
chain lengths beyond the propyl group. This is because the ad-
ditional +I effect of longer chains is negligible, whereas steric
and stereoelectronic factors cause the chain to orient (or
“grow”) away from the nitrogen atom.[36,51] This is evident by
the fact that solvated molecules with a hydrophobic chain and
a charged head group (surfactants) stretch out and may even
form micelles.[52,53] Any stability trend directly depending on
the chain length should be easily discernible in a systematic
study, but unfortunately a more complex relationship is ob-
served (Figure 6).
A more probable reason is the formation of micelles, which
also increases in tendency with chain length and which is
known to influence the rate of chemical reactions[58] and also
potentially lead to decreased dissociation.[52,53,59,60] A lower
degree of dissociation would increase the local OHꢀ concentra-
tion around the QA and thus cause faster degradation (see the
section on ion solvation and temperature). Beyond a chain
length of six, micelle-induced decomposition may well be the
dominating factor and similar effects induced by the polymer
backbone are potentially responsible for accelerated decompo-
sition in HEMs.
The shortest possible side chain is the methyl group in TMA,
which exhibits a relatively high half-life of 62 h with nucleo-
philic substitution being the only possible degradation mecha-
nism. Adding only one additional CH2 group to create an ethyl-
trimethylammonium (ETM) group decreases half-life by a factor
of 20 due to considerably faster b-elimination.
Although no data between HTM (six carbons atoms) and
PTM (three carbon atoms) were measured, it is quite likely that
a stability maximum (or plateau) exists in this region, where b-
elimination is inhibited to the largest possible extend by steric
and inductive factors while chain length-induced accelerated
decomposition does not yet dominate. This is supported by
previous experimental and theoretical studies, which showed
that the elimination rate in QAs decreases with chain lengths
up to around four carbon atoms, whereas substitution at the
methyl group remains comparatively unaffected.[61–64] Such
a maximum was also calculated for the reaction free energy
barriers DG# by Long et al.[61] (unfortunately only calculated up
to HTM) although micelle formation was not taken into ac-
count.
This b-elimination proceeds most rapidly with a freely rotat-
ing CH3 end group devoid of steric interference, which be-
comes apparent when another CH2 group is added to create
a propyltrimethylammonium (PTM) group. The propyl chain
causes strain on the steric requirements of the elimination re-
action, increasing the transition state energy and with it the
half-life compared to ETM.
The next data point is that of a hexyltrimethylammonium
(HTM) with six carbon atoms, which exhibits basically the same
half-life as PTM within the margin of error. However, beyond
By calculating DG# from the degradation rate constants
using the Eyring–Polanyi equation for solutions (see the Sup-
porting Information) a direct comparison with the data by
Long et al.[61] was possible, showing that the data follow the
same relative trend, with absolute values differing by about
50 kJmolꢀ1 (12 kcalmolꢀ1).[65,66]
The maximum half-life of side chain-containing QAs with op-
timal length is considerably less then that of TMA, suggesting
that their primary degradation pathway is b-elimination, which
is to some extend suppressed by the chain itself. Removal of
b-protons may then conceivably lead to higher resistance
against nucleophilic attack; however, this prediction could not
be confirmed experimentally as neopentyltrimethylammonium
(NTM), which contains no b-protons, decomposed far more
rapidly than the PTM analogue with an equally long main
chain (see Table 3 and Figure 6). This implies other degradation
mechanisms (e.g., Stevens rearrangement) proceed more rap-
idly in NTM compared to the elimination in PTM.
Figure 6. Half-life of alkyltrimethylammonium functional groups containing
*
one alkyl chain of varying length in 6m NaOH at T=1608C ( ) and a neo-
&
pentyl chain at the same conditions ( ). Corresponding reaction free energy
#
*
barrier DG data ( ) was calculated by using the Eyring equation. DFT data
~
)
by Long et al. (
differ by about 50 kJmolꢀ1
[61] follows the same qualitative trend, but absolute values
.
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