Gas-solid reactions of single crystals: A study of the reaction of bromine with single crystals of trans-cinnamic acid and a range of its derivatives by infrared and Raman microspectroscopy

Article abstract of DOI:10.1039/b501427c

Single crystals of trans-cinnamic acid and of a range of derivatives of this compound containing halogen substituents on the aromatic ring have been reacted with 165 Torr pressure of bromine vapour in a sealed desiccator at 20°C for 1 week. Infrared and Raman microspectroscopic examination of the crystals shows that bromination of the aliphatic double bond, but not of the aromatic ring, has occurred. It is demonstrated also that the reaction is truly gas-solid in nature. A time-dependent study of these reactions shows that they do not follow a smooth diffusion-controlled pathway. Rather the reactions appear to be inhomogeneous and to occur at defects within the crystal. The reaction products are seen to flake from the surface of the crystal. It is shown, therefore, that these are not single crystal to single crystal transitions, as have been observed previously for the photodimerisation of trans-cinnamic acid and several of its derivatives. It is shown that there are no by-products of the reaction and that finely ground samples react to form the same products as single crystals. The Owner Societies 2005.

Full text of DOI:10.1039/b501427c

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R E S E A R C H P A P E R
Gas–solid reactions of single crystals: A study of the reaction of
bromine with single crystals of trans-cinnamic acid and a range of
its derivatives by infrared and Raman microspectroscopyw
Samantha L. Jenkins, Matthew J. Almond* and Peter Hollins
School of Chemistry, University of Reading, Whiteknights, Reading, UK RG6 6AD.
E-mail: m.j.almond@rdg.ac.uk
Received 27th January 2005, Accepted 9th March 2005
First published as an Advance Article on the web 18th March 2005
Single crystals of trans-cinnamic acid and of a range of derivatives of this compound containing halogen
substituents on the aromatic ring have been reacted with 165 Torr pressure of bromine vapour in a sealed
desiccator at 20 1C for 1 week. Infrared and Raman microspectroscopic examination of the crystals shows that
bromination of the aliphatic double bond, but not of the aromatic ring, has occurred. It is demonstrated also
that the reaction is truly gas–solid in nature. A time-dependent study of these reactions shows that they do not
follow a smooth diffusion-controlled pathway. Rather the reactions appear to be inhomogeneous and to occur at
defects within the crystal. The reaction products are seen to flake from the surface of the crystal. It is shown,
therefore, that these are not single crystal to single crystal transitions, as have been observed previously for the
photodimerisation of trans-cinnamic acid and several of its derivatives. It is shown that there are no by-products
of the reaction and that finely ground samples react to form the same products as single crystals.
attention upon photochemical dimerisation reactions of single
crystals of trans-cinnamic acid and its derivatives.1–3 An in-
tegrated approach using infrared and Raman microspectro-
Introduction
In recent years there has been a strong renewal of interest in the
reactions of solid crystalline organic substrates.1–5 Reactions of
this type are desirable from an environmental point of view in
scopy clearly demonstrates the topotactic nature of these
reactions where the form of the dimer obtained is dependent
that they obviate the need for large quantities of solvent as the
upon the orientation of the monomer molecules within the
reaction medium.4,6 At the same time it has become increas-
starting compound.2,3 We have also extended our studies to
ingly recognised that such reactions give considerable stereo-
chemical control, as the form of the reaction product may be
investigate the time-dependency of the dimerisation of mole-
cules of a-2,4-dichloro-trans-cinnamic acid within a single
crystal.1 The reaction is shown to follow strictly first order
directed by the arrangement of reactant molecules within the
crystal.2,3,7–11 Performing reactions in the solid state has been
kinetics, rather than the zeroth order kinetics predicted by
found in many examples to improve specificity.12 The explo-
some theoretical models.1
sion of interest in combinatorial chemistry where a wide range
of reagents are tested on solid beads of substrates provides a
further impetus to research in this area.
We have utilised the techniques of infrared and Raman
We have now turned our attention to the reactions of single
crystalline organic substrates with gaseous reagents. Such
reactions are considered to be relatively environmentally
friendly and often they give yields at least as high as those
obtained by analogous solution reactions.4–6 Their full exploi-
microspectroscopy to monitor reactions of this type and to
characterise reaction products.1–3 The principal advantages of
tation, however, in synthesis is held back because of uncer-
vibrational microspectroscopy in this context are that single
tainty as to how controllable are the reactions and as to how
crystals may be monitored in situ and that the spectra give
the products of the reactions relate to products formed by
considerable structural information about reactants and pro-
analogous reactions in solution. As an example of such reac-
tions we selected for study the bromination of trans-cinnamic
coming from an alternative approach, which has been used
acid:
ducts. Such structural information is much less readily forth-
elsewhere,4,6,13,14 in which atomic force microscopy is em-
ployed. Here morphological changes to the crystals are seen
upon reaction, but it is difficult to relate these changes to the
chemical processes ongoing within the crystal. Diffraction
methods have, of course, also been used8,9,15 but are still
undergoing development for use in time-resolved monitoring
of solid state reactions.16 Single crystal methods may present
difficulties if the crystals under investigation degrade during the
experiment.17 In this context, however, it is interesting to note
and some of its halogenated derivatives.
a very recent single crystal diffraction study of the photodi-
merisation of the a0-polymorph of ortho-ethoxy-trans-cinnamic
acid in the solid state.18 Until now we have focused our
The reaction of solid aryl alkenes with bromine vapour has
been known for almost 150 years.19 While these conditions are
thought to give the same products as those formed in solution
by an ionic mechanism there is still uncertainty regarding the
course of the reaction. In the first place it has been proposed20
that bromination of the para-position of the aromatic ring as
w Electronic supplementary information (ESI) available: Tables S1–S6.
See http://www.rsc.org/suppdata/cp/b5/b501427c/
1966
P h y s . C h e m . C h e m . P h y s . , 2 0 0 5 , 7 , 1 9 6 6 – 1 9 7 0
T h i s j o u r n a l i s & T h e O w n e r S o c i e t i e s 2 0 0 5

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well as of the aliphatic double bond will occur. Secondly, there
has been some controversy as to whether the reaction is truly
gas–solid in nature.21,22 It has been suggested22 that the reac-
tion actually occurs in an adsorbed phase on the surface of the
crystal or in a film of solution formed by the aromatic
compound dissolved in liquid.
It is of interest to mention two studies which are related to
the work reported here.23,24 Garcia-Garibay and his co-work-
ers23 found that bromine gas induced an enantioselective
carbocation rearrangement upon reaction with crystalline bi-
benzobarrelene. The products were identified by GC and NMR
spectroscopy. When powders were reacted, yields of 85–90%
of one dibrominated product were obtained; by contrast reac-
tion of large single crystals (20–100 mg) gave a large number of
by-products. Reaction of moist chlorine gas with crystals of 4-
phenylthiazole-2(1H)-thione over a period of several weeks24
led to complete destruction of the starting material and the
formation of two products. The major product (as identified by
single crystal X-ray diffraction and NMR spectroscopy) was
chlorinated at the para position of the phenyl rings and oxi-
dation and ring-opening had led to the formation of (4-chloro)
phenacyl disufide in which two molecular units were linked by
an S–S bond.
It is thus apparent that there remain uncertainties in the
interpretation of these gas–solid reactions and that, with the
exception of a number of studies by single crystal X-ray
diffraction25 and by atomic force microscopy,26 few investiga-
tions of single crystals have been made. Vibrational micro-
spectroscopy has not, to our knowledge, previously been
employed to investigate the reaction of any organic single
crystal with a gaseous reagent. Given the current general
interest in such reactions and our previous success in utilising
vibrational microspectroscopy to monitor photodimerisation
reactions in single crystals we felt that the study reported here
was very timely. We felt, moreover, that our approach could
usefully address specific questions regarding these particular
reactions such as: (i) Are the aromatic and aliphatic bonds of
aryl alkenes brominated? (ii) Are the reactions homogeneous
or inhomogeneous, are they truly single crystal to single crystal
processes? (iii) Are they diffusion controlled?
Fig. 1 A single crystal of trans-cinnamic acid before (a) and after (b)
exposure to 165 Torr of bromine vapour for a period of 1 day. The
scale shows distance measured in mm.
Results and discussion
Reaction of trans-cinnamic acid with Br2 vapour
Single crystals of trans-cinnamic acid, whose infrared spectra
had previously been recorded were exposed in a sealed desic-
cator to a pressure of 165 Torr of Br2 vapour for a period of 1
week. At the end of that time the crystals were examined by
optical microscopy. It may be seen from the photographs in
Fig. 1 that the surface of the crystal has begun to disintegrate
upon exposure to Br2; it has become cracked and pitted. This
disintegration of the crystal may be contrasted with the
photodimerisation reactions of trans-cinnamic acid and several
of its derivatives where the crystals remain intact.1,2 Infrared
spectra recorded on a single crystal before and after exposure
to Br2 vapour (Fig. 2) show clearly that a chemical reaction has
occurred. The wavenumbers of the observed bands are listed in
Table 1, and the principal changes may be summarised as
follows.
Experimental section
Samples of all organic solids were used as supplied by Aldrich.
The stated purities of the reagents used were as follows: trans-
cinnamic acid (99þ%), 4-bromo-trans-cinnamic acid (98%), 4-
chloro-trans-cinnamic acid (99%), 4-fluoro-trans-cinnamic
acid (99%), 2-fluoro-trans-cinnamic acid (98%), 3-fluoro-
trans-cinnamic acid (98%), 9-methylanthracene (98%) and
hydantoin (99%). Bromine (99.5%þ purity) was used as
supplied by Aldrich.
(1) The band assigned to n(CQC) aliphatic at 1627 cmꢀ1 28
has disappeared, indicating reaction across the double bond.
Reactions of bromine vapour with organic substrates were
carried out in a sealed desiccator in which an open reservoir of
bromine and a watch glass with a single crystal of the reagent
under investigation were placed. The temperature of the system
was held at 20 1C at which Br2 has a vapour pressure of 165
Torr. After a period of 1 week the single crystal was extracted
and examined by infrared and Raman microspectroscopies.
For ‘bulk’ reactions a finely ground sample was reacted under
identical conditions and IR spectra were recorded on the
product in a KBr disc. NMR spectra were also recorded on
DMSO-d6 solutions.
The infrared (Brucker IRscope II attached to a Brucker
Equinox 55 FT interferometer) and Raman (Renishaw model
1000) microscopes which were used to record vibrational
spectra of single crystals have been described elsewhere.2,3,27
Infrared spectra of bulk samples were recorded using a Perkin
Elmer 1720-X FTIR, NMR spectra were measured on a
Bruker DPX 250 MHz spectrometer at 250 Hz.
Fig. 2 The infrared spectra of a single crystal of trans-cinnamic acid
before (below) and after (above) exposure to 165 Torr pressure of
bromine vapour for 1 week.
T h i s j o u r n a l i s & T h e O w n e r S o c i e t i e s 2 0 0 5
P h y s . C h e m . C h e m . P h y s . , 2 0 0 5 , 7 , 1 9 6 6 – 1 9 7 0
1967

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Table 1 Positions of infrared absorptions (cmꢀ1) of a single crystal of
trans-cinnamic acid before and after exposure to 165 Torr pressure of
bromine vapour for 1 week
in the regions 1015–1035, 990–1010 and 605–630 cmꢀ1 which
are indicative of a monosubstituted ring. Moreover, such a ring
is expected to show strong infrared bands in the regions 690–
710 and 730–770 cmꢀ1. Bands are seen before bromination at
702 and 770 cmꢀ1 and after bromination at 693 and 767 cmꢀ1
.
Infrared:
trans-cinnamic
acid/cmꢀ1
Infrared: trans-cinnamic
acid after exposure to
bromine vapour/cmꢀ1
Thus the vibrational spectra confirm that bromination has
occurred at the aliphatic double bond only and that the
aromatic ring remains unchanged. Identification of the ring
vibrations as above allows us to assign the infrared band of the
brominated product at 658 cmꢀ1 to n(C–Br). This corresponds
to the Raman band at 666 cmꢀ1. In addition to this we note the
correspondence between our product spectra and that of an
authentic sample of a,b-dibromohydrocinnamic acid (Fig. 4).
It is possible to make some further observations from these vi-
brational spectra regarding the shift in the position of n(CQO)
to high frequency. This shift demonstrates that the CQC
aliphatic bond has been converted to a C–C single bond,
resulting in loss of conjugation to, and hence a strengthening
of, the CQO bond. The magnitude of this shift (40 cmꢀ1) may
be compared with the corresponding shift in band position
when trans-cinnamic acid is photodimerised to the correspond-
ing dimer3 of 8 cmꢀ1 (from 1680 cmꢀ1 to 1688 cmꢀ1). The
much greater shift which occurs during the bromination reac-
tion is caused by the electron withdrawing ꢀI effect of the Br
atom in the a-position to the CQO bond.
Assignment
n(C–H) unsaturated 3025 br
(aliphatic and
aromatic)
3009 br
n(CQO) (carboxyl) 1682
n(CQC) (aliphatic) 1627
n(CQC) (aromatic)
1722 vs
C–H deformation
(unsaturated)a
n(C–O) (carboxyl)a
d(C–H) (aliphatic
and aromatic)a
Ring vibration
(aromatic)a
1494, 1450 s, 1422 s 1496 s, 1432 s
1316, 1209, 1205
1177, 1068, 1027,
935, 872 s
1280, 1220 s
1147 s, 1108, 1088,
1060, 913 vs
767, 693 s
770 s, 702 s, 677
n(C–Br)a
658
a
Bands in this spectral region are difficult to assign with certainty
because of coupling of vibrations and overlap of bands.
We have carried out time-dependent studies of the reaction
by extracting crystals, which had been exposed to 165 Torr of
bromine vapour under identical conditions for periods ranging
from 24 h to 1 week. These studies show that the reactions do
not follow diffusion control. Rather they proceed very slowly
in the early stages, with no discernable spectral changes in the
first 24 h of reaction, but much more rapidly once the crystal
starts to fragment. Under these circumstances it was not
possible to obtain any meaningful kinetic plots from these
data.
(2) The band assigned to n(CQO) of the carboxyl group at
1682 cmꢀ1 in trans-cinnamic acid28 has disappeared and has
been replaced by a band at 1722 cmꢀ1 in the brominated
product.
(3) The appearance of the spectrum in the region associated
with vibrations of the aromatic ring (650–1050 cmꢀ1 29
is
)
somewhat changed in appearance although the basic profile
of bands is similar.
(4) New bands are seen around 600 cmꢀ1 which may arise
from C–Br stretching vibrations.31
The Raman spectra (Fig. 3) show perhaps more striking
changes. Once again, the band arising from n(CQC) aliphatic
has disappeared and a new strong band at 666 cmꢀ1 which may
be assigned to n(C–Br) has appeared. The observed Raman
bands of a single crystal of trans-cinnamic acid before and after
exposure to Br2 vapour are listed in Table 2.
Our experiments demonstrate, beyond any reasonable
doubt, that bromination of the aliphatic double bond, but
not of the aromatic ring, has occurred upon exposure of trans-
cinnamic acid to Br2 vapour. In order to demonstrate that the
same reaction occurs in finely-ground bulk samples as in single
crystals we measured infrared spectra of a bulk sample (as a
KBr disc) and 1H and 13C NMR spectra [dH(DMSO-d6) 7.65–
7.33 (m, 5 H), 5.56–5.51 (d, 1 H), 5.34–5.30 (d, 1 H).
dC(DMSO-d6) 129.42, 129.03, 128.76, 52.25, 47.52, 31.06].
The infrared spectra are identical, within experimental error,
in band position to the corresponding spectra of single crystals
and are also identical to the spectrum of an authentic sample of
One of the questions which we wished to address is: has
bromination occurred only at the aliphatic CQC bond or has
bromination of the aromatic ring occurred also? The vibra-
tional spectra, which we have obtained allow us to explore this
point. Empirical observation has allowed the positions of
attachment at a benzene ring to be determined by inspection
of the pattern of Raman bands resulting from vibrations of the
ring.29 The spectra of trans-cinnamic acid before and after
exposure to Br2 vapour show bands (before bromination 1032,
1
a,b-dibromohydrocinnamic acid. The H NMR spectra serve
to confirm the findings. The position of the peak arising from
the aliphatic H atoms shifts from ca. d ¼ 6.6 to ca. d ¼ 5.5 ppm
upon bromination. The signal from the brominated sample
appears as a doublet of doublets with a 1 : 1 intensity ratio,
indicating coupling between the two H atoms which are non-
equivalent, but showing that coupling to the quadrupolar 79Br
and 81Br nuclei is not resolved. 13C NMR shows peaks arising
from the aliphatic carbon atoms of the brominated product at
52.25 and 47.52 ppm. These have shifted from around 120 ppm
in the starting material indicating the attachment of the
electronegative Br atoms. Spectra in the aromatic region are
more difficult to interpret fully, with a cluster of peaks being
seen between 7 and 8 ppm. We note, however, that the 1H and
13C NMR spectra of our product are identical to those of an
authentic sample of a,b-dibromohydrocinnamic acid.
1006, and 625 cmꢀ1, after bromination 1030, 1008, 627 cmꢀ1
)
Examination of these infrared and NMR spectra demon-
strates two important points. First, that the yield must be
100% (or very close to this figure) as all infrared and NMR
bands of the starting material have disappeared upon reaction.
At the same time all of the infrared and NMR bands that
appear and grow during reaction may be assigned to the
product a,b-dibromohydrocinnamic acid, showing that a clean
Fig. 3 The Raman spectra of a single crystal of trans-cinnamic acid
before (below) and after (above) exposure to 165 Torr pressure of
bromine vapour for 1 week.
1968
P h y s . C h e m . C h e m . P h y s . , 2 0 0 5 , 7 , 1 9 6 6 – 1 9 7 0
T h i s j o u r n a l i s & T h e O w n e r S o c i e t i e s 2 0 0 5

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Table 2 Positions of Raman bands (cmꢀ1) of a single crystal of trans-
cinnamic acid before and after exposure to 165 Torr pressure of
bromine vapour for 1 week
Raman: trans-
cinnamic
acid/cmꢀ1
Raman: trans-cinnamic
acid after exposure to
bromine vapour/cmꢀ1
Assignment
n(C–H) unsaturated
(aliphatic and
aromatic)
n(CQO) (carboxyl)
n(CQC) (aliphatic) 1643 s
n(CQC) (aromatic) 1605 s
1611
C–H deformation
(unsaturated)a
n(C–O) (carboxyl)a
1501, 1450
Fig. 5 Comparison of the infrared spectra of single crystals of trans-
cinnamic (above) and 4-bromo-trans-cinnamic acid (below) after ex-
posure to 165 Torr pressure of bromine vapour for 1 week.
1331, 1298, 1273,
1217
1229 br
d(C–H) (aliphatic
and aromatic)a
Ring vibration
(aromatic)a
n(C–Br)a
1183, 1032, 1006,
882, 855
689 w, 625
1171, 1095, 1030,
1008 s, 844
791, 627 w
did proceed by addition across the aliphatic double bond and
by substitution of the 4-position of the benzene ring we would
expect the products of reaction of 4-bromo-trans-cinnamic acid
and of trans-cinnamic acid itself to be identical. Inspection of
the infrared spectra shown in Fig. 5 demonstrates clearly that
the products are not identical. This observation confirms that
reaction of trans-cinnamic acid only occurs at the aliphatic
double bond. Observed infrared and Raman band positions of
4-bromo-trans-cinnamic acid before and after exposure to Br2
vapour are listed in Table S1 which is available as supplemen-
tary material.w The main observations are:
666 vs
a
Bands in this spectral region are difficult to assign with certainty
because of coupling of vibrations and overlap of bands.
addition reaction has taken place. In some cases the crystals
were a little discoloured after reaction, suggesting the presence
of some dissolved bromine. However, this was at a level too
low to be detected by Raman spectroscopy. Upon exposure to
bromine vapour for periods in excess of 1 week (up to 1 month)
no new products were observed, indicating that the primary
products are unreactive towards bromine and that the reaction
stops at this point.
(1) That the infrared (1627 cmꢀ1) and Raman (1652 cmꢀ1)z
bands assigned to n(CQC) aliphatic disappear.
(2) That the infrared band assigned to n(CQO) (1688 cmꢀ1
)
disappears and is replaced by a band at 1725 cmꢀ1
(3) That a band assigned to n(C–Br) appears at 654 cmꢀ1
.
(infrared) and 659 cmꢀ1 (Raman).
We have also investigated the 4-chloro and 4-fluoro deriva-
tives. These derivatives were selected as having groups attached
to the 4-position on the benzene ring which are unlikely to be
replaced by Br as the enthalpies of reaction would be unfa-
vourable.30
Reactions of derivatives of trans-cinnamic acid with Br2 vapour
In the next stage of our work we extended our studies to a
number of derivatives of trans-cinnamic acid. The first deriva-
tive that we studied was 4-bromo-trans-cinnamic acid. In this
case a bromine atom is already present at the 4-position on the
aromatic ring. Thus if bromination of trans-cinnamic acid itself
The positions of infrared and Raman bands of 4-chloro-
trans-cinnamic acid and 4-fluoro-trans-cinnamic acid before
and after exposure to bromine vapour are listed in Tables S2
and S3 which are given as supplementary material.w The
principal changes observed are as follows. Firstly, the disap-
pearance of the band arising from n(CQC), aliphatic (4-fluoro-
trans-cinnamic acid: 1650 cmꢀ1 infrared, 1663 cmꢀ1 Ramanz;
4-chloro-trans-cinnamic acid: 1630 cmꢀ1 infrared, 1640 cmꢀ1
Raman). Secondly, the shift of the band arising from n(CQO)
(4-fluoro-trans-cinnamic acid 1680 cmꢀ1 replaced by 1721
cmꢀ1; 4-chloro-trans-cinnamic acid 1700 cmꢀ1 replaced by
1725 cmꢀ1). Thirdly the appearance of bands resulting from
n(C–Br) (4-fluoro-trans-cinnamic acid 665 cmꢀ1 infrared, 668
cmꢀ1 Raman; 4-chloro-trans-cinnamic acid 646 cmꢀ1 infrared,
659 cmꢀ1 Raman).
Finally we investigated the reactions of 2-fluoro-trans-cin-
namic acid and 3-fluoro-trans-cinnamic acid. Both of these
compounds brominate across the aliphatic double bond. Posi-
tions of infrared and Raman bands before and after bromina-
tion are listed in Tables S4 and S5 (ESIw). Principal changes
observed are as follows. 2-fluoro-trans-cinnamic acid: (i) dis-
appearance of n(CQC) aliphatic (1652 cmꢀ1 infrared, 1652
cmꢀ1 Raman); (ii) shift of n(CQO) from 1679 to 1723 cmꢀ1
(infrared); and (iii) appearance of n(C–Br) 661 cmꢀ1 (infrared),
668 cmꢀ1 (Raman). 3-fluoro-trans-cinnamic acid: (i)
Fig. 4 Comparison of the infrared spectrum of a single crystal of
trans-cinnamic acid after exposure to 165 Torr pressure of bromine
vapour for 1 week (above) and of a single crystal of an authentic
sample of a,b-dibromohydrocinnamic acid (below). Small differences
between the two spectra arise from (i) the greater intensity of the upper
spectrum leading to saturation of the detector, especially around 1700
cmꢀ1, and (ii) slight differences in the structures of the two crystals
leading to solid-state effects. The crystal of a,b-dibromohydrocinnamic
acid is an authentic sample which has been recrystallised after pre-
paration while the crystal of the reaction product of trans-cinnamic
acid with bromine has been formed by a gas–solid reaction and has not
been recrystallised.
z The broadness of the infrared absorption probably accounts for the
discrepancy in the position of the infrared and Raman bands arising
from these vibrations.
T h i s j o u r n a l i s & T h e O w n e r S o c i e t i e s 2 0 0 5
P h y s . C h e m . C h e m . P h y s . , 2 0 0 5 , 7 , 1 9 6 6 – 1 9 7 0
1969

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disappearance of n(CQC) aliphatic 1658 cmꢀ1 (infrared), 1654
cmꢀ1 (Raman); (ii) shift of n(CQO) from 1684 cmꢀ1 to 1722
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,
(infrared) 690 cmꢀ1 (Raman).
1
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all of the derivatives studied are listed in Table S6.w
4
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Our experiments have demonstrated a number of important
points regarding the reaction of bromine vapour with single
crystals of trans-cinnamic acid and its derivatives. First the
reaction is shown to be quantitative and clean. This may be
contrasted with some previously studied halogenations of
crystals23,24 in which rather complex reactions were observed
and in which a number of by-products were seen. These
reactions included chlorination of the phenyl rings of 4-phe-
nylthiazole-2(1H)-thione,24 oxidation,23,24 fragmentation,24
and rearrangement23 processes. The reactions investigated in
the current work presented here proceed by bromination of
the aliphatic CQC bond, and once this bond is brominated the
reaction stops. There is no sign of any bromination of the
aromatic ring and no other by-product is formed even upon
prolonged exposure to Br2 vapour. The procedure may there-
fore find use in a synthetic context.
However, it is clear that these are not homogeneous single
crystal to single crystal transitions. The reactions do not follow
any known diffusion-controlled kinetics.31 Rather bromination
appears to occur at crystal defects and is greatly accelerated
once the crystal begins to fragment. This behaviour may be
contrasted with the photodimerisation reactions of the 2-
chloro-, 4-chloro- and 2,4-dichloro- derivatives of trans-cin-
namic acid.1 Here the crystals remain intact upon photolysis,
the reactions are topotactic and they follow a smooth conver-
sion from monomer to dimer, which may be modelled kineti-
cally.
10 Y. Ito, B. Borecka, J. Trotter and J. R. Scheffer, Tetrahedron Lett.,
1995, 36, 6083–6086.
11 Y. Ito, B. Borecka, J. Trotter and J. R. Scheffer, Tetrahedron Lett.,
1995, 36, 6087–6090.
12 See, for example: (a) M. A. Garcia-Garibay, S. Shin and C. N.
Sanrame, Tetrahedron, 2000, 56, 6729–6735; (b) G. Kaupp, H.
Frey and G. Behmann, Chem. Ber., 1988, 121, 2135–2141.
13 G. Kaupp and J. Schmeyers, Angew. Chem., Int. Ed. Engl., 1993,
32, 1587–1589.
14 G. Kaupp, Mol. Cryst. Liq. Cryst., 1992, 211, 1–15.
15 (a) R. S. Miller, I. C. Paul and D. Y. Curtin, J. Am. Chem. Soc.,
1974, 96, 6334–6338; (b) R. S. Miller, D. Y. Curtin and I. C. Paul,
J. Am. Chem. Soc., 1974, 96, 6340–6348.
16 J. M. Cole, P. R. Raithby, M. Wulff, F. Schoffe, A. Plech, S. J.
Teat and G. Bushell-Wiye, Faraday Discuss., 2003, 122, 119–129.
17 B. M. Kariuki, D. M. S. Zin, M. Tremayne and K. D. M. Harris,
Chem. Mat., 1996, 8, 565–569.
18 M. A. Fernandes and D. C. Levendis, Acta Crystallogr., Sect. B:
Struct. Sci., 2004, B60, 315–324.
19 A. Schmitt, Ann. Phys. Chem., 1863, 127, 319.
20 R. E. Buckles, E. A. Hausman and N. G. Wheeler, J. Am. Chem.
Soc., 1950, 72, 2494–2499.
21 E. Hadjoudis, E. Kariv and G. M. J. Schmidt, J. Chem. Soc.,
Perkin Trans. 2, 1972, 1056–1062.
22 M. M. Labes, H. W. Blakesbee and J. E. Bloor, J. Am. Chem. Soc.,
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23 M. Garcia-Garibay, J. R. Scheffer, J. Trotter and F. Wireko,
Tetrahedron Lett., 1988, 29, 1485–1488.
24 V. Nalini and G. R. Desiraju, J. Chem. Soc., Chem. Commun.,
1987, 1046–1048.
25 See, for example, refs. 8, 9, 15 and 16.
The crucial difference between these processes is that upon
bromination a large amount of additional matter is being
added. This clearly causes too large a distortion to the crystal
to be accommodated and leads to disintegration. It has been
reported elsewhere27 that where intermolecular spacing be-
tween monomers of trans-cinnamic acid derivatives are too
large, photodimerisation will either not proceed or leads to
fragmentation. In the bromination of an aliphatic double
bond, the highly exothermic nature of the reaction (ca. ꢀ280
26 See, for example, refs. 4, 6, 13 and 14.
27 (a) S. D. M. Atkinson, PhD thesis, University of Reading, 2001;
(b) G. M. J. Schmidt, J. Chem. Soc., 1964, 2014–2017; (c) M. D.
Cohen, G. M. J. Schmidt and F. I. Sonntag, J. Chem. Soc., 1964,
2000–2013.
kJ mol )
ꢀ1 30 presumably drives the reaction to completion, even
28 (a) W. Kemp, Organic Spectroscopy, Macmillan, Basingstoke, 3rd
edn., 1991; (b) D. H. Williams and I. Fleming, Spectroscopic
Methods in Organic Chemistry, McGraw-Hill, Maidenhead, 5th
edn., 1996; (c) N. B. Colthrup, L. H. Daly and R. B. Wiberly,
Introduction to Infrared and Raman Spectroscopy, Academic Press,
London, 3rd edn., 1990; (d) K. Nakamoto, Infrared and Raman
Spectra of Inorganic and Coordination Compounds, Wiley, New
York, 4th edn., 1986.
at the expense of fragmentation of the crystal.
Our experiments show the usefulness of infrared and Raman
microspectroscopies in monitoring reactions of this type. This
approach is especially useful in a situation where the single
crystal fragments upon reaction, making single crystal X-ray
diffraction studies impossible.
29 J. G. Grasselli, M. K. Snavely and B. J. Bulkin, Chemical
Applications of Raman Spectroscopy, John Wiley and Sons, New
York, 1981.
Acknowledgements
30 J. G. Stark and H. G. Wallace, Chemistry Data Book in SI, John
Murray (Publishers) Ltd., London, 2nd edn., 1997.
31 Chemical Kinetics, in Reactions in the Solid State, ed. C. H.
Bamford and C. F. H. Tipper, Elsevier Scientific Publishing
Company, Amsterdam, vol. 22, 1980.
We thank the University of Reading Research Endowment
Trust Fund for the award of a studentship to SLJ. We are also
grateful to Rachel Porter for carrying out some of the pre-
liminary experiments.
1970
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