Striking Difference

Several examples illustrate how organometallic molecules are strikingly different from those encountered in classical inorganic and organic chemistry. Ligands1 of cyclic delocalized n systems (e.g., benzene or cyclopentadienyl) can team up with metal atoms to form sandwich compounds with a metal sandwiched between them. Sometimes atoms of other elements, such as phosphorus or sulfur, can be included as well. Examples of these double- and multiple-decker sandwich 

2The traditional notion of carbon forming bonds to at most four additional atoms must be reconsidered (a few examples of carbon bonded to more than four atoms are also known in organic chemistry). 

---

This behaviour also stands for functionalized [60]fullerene derivatives, with, however, a few striking differences. The most obvious parameter is the negative shift of the reduction potentials, which typically amounts to -100 mV. Secondly, the separation of the corresponding reduction potentials is clearly different. Wlrile the first two reduction steps follow closely the trend noted for pristine [60]fullerene, the remaining four steps display an enlianced separation. This has, again, a good resemblance to the ITOMO-LUMO calculations, namely, a cancellation of the degeneration for functionalized [60]fullerenes [31, 116, 117]. 

Enantiomers can have striking differences however m properties that depend on the arrangement of atoms m space Take for example the enantiomeric forms of carvone (R) (—) Carvone is the principal component of spearmint oil Its enantiomer (5) (+) carvone is the principal component of caraway seed oil The two enantiomers do not smell the same each has its own characteristic odor... 

There is a striking difference in the acidity of cyclopentadiene compared with cycloheptatriene Cycloheptatriene has a pK of 36 which makes it 10 times weaker m acid strength than cyclopentadiene... 

Consider again two detection elements, and suppose an ion beam has been dispersed in time such that ions of m/z 100 arrive at each of several elements (Figure 30.5). In this TOF mode, the next ion of m/z 101 has not yet arrived, and the ion of m/z 99 has arrived previously. Although the m/z ions are dispersed in time over a region of space and strike different elements of the detector, they are collected and monitored simultaneously because all of the microchannels are electronically connected. The operation of the microchannel plate is much easier than that of the array because all the elements are monitored as one at the plate, while each element must be monitored separately in the array. The microchannel plate detector is tremendously useful for those cases in which ions... 

Evidence for the formation of oxabicyclobutane (6) in the irradiation (S) was negative (72JA1193). The striking difference in photochemical behavior of enones such as (5) and the analogous conjugated dienes, which readily form bicyclobutanes, is attributed to the substantially greater endothermicity of the isomerization in the oxa series. 

The variable domains associate in a strikingly different manner. It is obvious from Figure 15.11 that if they were associated in the same way as the constant domains, via the four-stranded p sheets, the CDR loops, which are linked mainly to the five-stranded p sheet, would be too far apart on the outside of each domain to contribute jointly to the antigen-binding site. Thus in the variable domains the five-stranded p sheets form the domain-domain interaction area (Figure 15.11). Furthermore, the relative orientation of the p strands in the two domains is closer to parallel than in the constant domains and the curvature of the five-stranded p sheets is such that they do not pack... 

Methylcyclopropane shows strikingly different reactivity toward chlorination and bromination under radical-chain conditions. With chlorine, cyclopropyl chloride (56%) is the major product, along with small amounts of 1,3-dichlorobutane (7%). Bromine gives a quantitative yield of 1,3-dibromobutane. Offer an explanation for the difference. 

Simulated SWCNT ED patterns will be presented below. Tbe most striking difference with tbe MWCNT ED patterns is tbe absence of tbe row of sharp oo.l reflexions. In tbe diffraction pattern of ropes there is still a row of sharp reflexions perpendicular to the rope axis but which now corresponds to the much larger interplanar distance caused by the lattice of the tubes in the rope. The ho.o type reflexions are moreover not only asymmetrically streaked but also considerably broadened as a consequence of the presence of tubes with different Hamada indices (Fig. 3). 

The previous sections have dealt with stable C=N-I- functionality in aromatic rings as simple salts. Another class of iminium salt reactions can be found where the iminium salt is only an intermediate. The purpose of this section is to point out these reactions even though they do not show any striking differences in their reactivity from stable iminium salts. Such intermediates arise from a-chloroamines (133-135), isomerization of oxazolidines (136), reduction of a-aminoketones by the Clemmensen method (137-139), reductive alkylation by the Leuckart-Wallach (140-141) or Clarke-Eschweiler reaction (142), mercuric acetate oxidation of amines (46,93), and in reactions such as ketene with enamines (143). 

The crystal structure of many compounds is dominated by the effect of H bonds, and numerous examples will emerge in ensuing chapters. Ice (p. 624) is perhaps the classic example, but the layer lattice structure of B(OH)3 (p. 203) and the striking difference between the a- and 6-forms of oxalic and other dicarboxylic acids is notable (Fig. 3.9). The more subtle distortions that lead to ferroelectric phenomena in KH2PO4 and other crystals have already been noted (p. 57). Hydrogen bonds between fluorine atoms result in the formation of infinite zigzag chains in crystalline hydrogen fluoride... 

One of the most striking differences between benzene and thiophene chemistry is the instability and difficult accessibility of the thiophene analogs of such important, well-known, and easily available compounds as phenol, aniline, and thiophenol. 

Like the chloronitrobenzenes, a chloroquinoline reacts faster with sodium p-tolylsulfide when the chloro group is para to the aza-group than when it is orthoy the factor involved being about 10. However, a strikingly different behavior is noted in the much lower BS-/ BO- ratio which is 2.5 for 4-chloroquinoline ( para isomer) and 0.24 for 2-chloroquinoline ( ortho isomer). For p-chloronitro-benzene this ratio is 38, and for 2,4-dinitrochlorobenzene it is 1950. Thus far there is no case in which the reaction of a chloronitrobenzene derivative with sodium methoxide is faster than that with sodium phenylsulfide. 

As already mentioned, there is a striking difference in the reactivity of 1- and 3-chloroisoquinoline the former reacts about 10 times faster than the latter with both piperidine and ethoxide ion at room temperature. The lower rate of ethoxy-dechlorination of the 3-isomer is due to an E which is 10 kcal higher. It is not justified to conclude that this isomer is virtually unactivated when its rate of ethoxylation is 100,000 times that of 2-chloronaphthalene and the E for this reaction is markedly decreased (by 7 kcal) relative to that of 2-chloronaphthalene. A direct comparison of reactivity with piperidine has not been made, but a rate ratio of 500 1 can be estimated by using a factor of one-fortieth (Table X, lines 1 and 4) to make the... 

After a full transformation cycle, i.e. one austenite - martensite and one martensite - austenite transition, we find a striking difference between the perfect configurations and those with vacancies Only the systems with defects regain the initial cubic form of the simulation box. 

Equation 35 is very similar to Eq. 33, although the correlation is not as sharp. The most striking difference between Eqs. 35 and 36 is that the latter does not contain a term in E. This means that para substituents have no effect on hydrolysis through... 

The most striking difference between the mass spectra of methyl 3-deoxy- and 4-deoxy-/ -D-f/ireo-pentopyranosides, 4 (Figure 1) and 5 (Figure 2), respectively, is the prominence of m/e 104 and 44 in the former and of m/e 60 [62] in the latter. The loss of C-2-C-3 from 5 leads to m/e 60 (see Equation 9), whereas its loss from 4 leads to m/e 44 (see Equation 7). The peak at m/e 104 [105] in Figure 2 shows that the elimination of C-3-C-4 also occurs from isomer 5. 

The most striking difference between alkenes and alkynes is that terminal alkynes are weakly acidic. When a terminal alkyne is treated with a strong base, such as sodium amide, Na+ -NH2, the terminal hydrogen is removed and an acetylide anion is formed. 

One of the most striking differences between conjugated dienes and typical alkenes is in their electrophilic addition reactions. To review briefly, the addition of an electrophile to a carbon-carbon double bond is a general reaction of alkenes (Section 6.7). Markovnikov regiochemistry is found because the more stable carbo-cation is formed as an intermediate. Thus, addition of HC1 to 2-methylpropene yields 2-chloro-2-methylpropane rather than l-chloro-2-methylpropane, and addition of 2 mol equiv of HC1 to the nonconjugated diene 1,4-pentadiene yields 2,4-dichloropentane. 

Perhaps the most striking difference between conjugated and nonconjugated dienes is that conjugated dienes undergo an addition reaction with alkenes to yield substituted cyclohexene products. For example, 1,3-butadiene and 3-buten-2-one give 3-cycIohexenyl methyl ketone. 

It is not unusual in halogen chemistry to find striking differences in the chemistry of acidic and... 

Chemical transformations at the macroeyclic chromophorc of expanded porphyrins are still not known. The complexation behavior of expanded porphyrins is very different from that of nonexpanded porphinoid macrocycles. The coordination hole of the expanded porphyrins is often too big for the complexation of a single metal ion, so in fact two metal ions can be chelated. With some expanded porphyrins, anion binding is observable, a striking difference to the nonexpanded porphyrins. The complexation behavior and the host-guest chemistry of expanded porphyrins is a rapidly growing field of research. The work in this field has been reviewed. Ie f... 

A strikingly different behavior was found with formyl ester 3 which bears two additional methyl groups a to the aldehyde functionality. In this case, addition of dibutylcuprate leads to the predominant formation of the /ranv-disubstituted y-lactone 441. 

---