Differences in reactivity

Differences in reactivity between amino and imino isomers toward CSj and diazonium salts (199. 165). 

The conversion of 4,5-dicyanothiazoles to diketones has been attempted (91). A difference in reactivity between the two cyano groups has been observed the least labile is the group in the 5-position. These Grignard reactions are limited and lead to 4-acetyl-5-cyanothiazole (Scheme 25). 

There is an overall 29 fold difference in reactivity of 1 chlorohexane 2 chlorohexane and 3 chlorohexane toward potassium iodide in acetone... 

Acrolein (H2C=CHCH=0) reacts with sodium azide (NaNj) in aqueous acetic acid to form a compound C3H5N3O in 71% yield Propanal (CH3CH2CH=0) when subjected to the same reaction conditions is recovered unchanged Suggest a structure for the product formed from acrolein and offer an explanation for the difference in reactivity between acrolein and propanal... 

However, when it is obtained by pyrolysis of diethylmagnesium or by reaction of diethylmagnesium and LiAlH (11), it is very reactive with both air and water. This difference in reactivity mainly results from the much finer particle size of the product obtained by the pyrolysis route. 

Mono- and diaLkylboranes obtained by controlled hydroboration of hindered olefins and by other methods can serve as valuable hydroborating agents for more reactive olefins. Heterosubstituted boranes are also available and used for this purpose. These borane derivatives show differences in reactivity and selectivity. 

Chemical methods to determine the crystalline content in silica have been reviewed (6). These are based on the solubility of amorphous silica in a variety of solvents, acids or bases, with respect to relatively inert crystalline silica, and include differences in reactivity in high temperature fusions with strong bases. These methods ate qualitative, however, and fail to satisfy regulatory requirements to determine crystallinity at 0.1% concentration in bulk materials. 

VEs do not readily enter into copolymerization by simple cationic polymerization techniques instead, they can be mixed randomly or in blocks with the aid of living polymerization methods. This is on account of the differences in reactivity, resulting in significant rate differentials. Consequendy, reactivity ratios must be taken into account if random copolymers, instead of mixtures of homopolymers, are to be obtained by standard cationic polymeriza tion (50,51). Table 5 illustrates this situation for butyl vinyl ether (BVE) copolymerized with other VEs. The rate constants of polymerization (kp) can differ by one or two orders of magnitude, resulting in homopolymerization of each monomer or incorporation of the faster monomer, followed by the slower (assuming no chain transfer). 

Miscellaneous Copolymers. VP has been employed as a termonomer with various acryUc monomer—monomer combinations, especially to afford resins usehil as hair fixatives. Because of major differences in reactivity, VP can be copolymerized with alpha-olefins, but the products are actually PVP grafted with olefin or olefin oligomers (151,152). Likewise styrene can be polymerized in the presence of PVP and the resulting dispersion is unusually stable, suggesting that this added resistance to separation is caused by some grafting of styrene onto PVP (153). The Hterature contains innumerable references to other copolymers but at present (ca 1997), those reviewed in this article are the only ones known to have commercial significance. 

Differences in reactivity of the double bond among the four isomers are controlled by substitution pattern and geometry. Inductive effects imply that the carbons labeled B in Table 3 should have less electron density than the A carbons. nmr shift data, a measure of electron density, confirm this. 

Side chain reactivity is also enhanced and is typified by the difference in reactivity of 2-methylpyrazine and 2-methylpyrazine 1,4-dioxide towards anion formation and subsequent condensation reactions. 2-Methylpyrazine undergoes condensation with benzal-dehyde at 180 °C, with zinc chloride catalysis, to yield the styrylpyrazine (58), whereas the corresponding reaction of 2-methylpyrazine 1,4-dioxide proceeds at 25 °C under base catalysis (67KGS419). 

Dihaloquinoxalines are extremely reactive and both halogen atoms are replaceable, on occasions explosively (59RTC5), whereas in the case of dihalopyrazines, and tri- or tetra-halopyrazines, there is frequently a considerable difference in reactivity of the halogen atoms. When 2,3-dichloropyrazine is treated with ammonia at 130 °C, only one chlorine atom is displaced, giving 2-amino-3-chloropyrazine (66FES799). 

The carbonyl reactivity of pyrrole-, furan-, thiophene- and selenophene-2- and -3-carbaldehydes is very similar to that of benzaldehyde. A quantitative study of the reaction of iV-methylpyrrole-2-carbaldehyde, furan-2-carbaldehyde and thiophene-2-carbaldehyde with hydroxide ions showed that the difference in reactivity between furan- and thiophene-2-carbaldehydes was small but that both of these aldehydes were considerably more reactive... 

Scheme 3.2 gives some data that illustrate the differences in reactivity between groups in axial and equatorial positions. It should be noted that a group can be either more or less reactive in an axial position as compared to the corresponding equatorial position. 

The reactions of the specific classes of carbonyl compounds are related by the decisive importance of tetrahedral intermediates, and differences in reactivity can often be traced to structural features present in the tetrahedral intermediates. 

There are large differences in reactivity among the various carboxylic acid derivatives, such as amides, esters, and acyl chlorides. One important factor is the resonance stabilization provided by the heteroatom. This decreases in the order N > O > Cl. Electron donation reduces the electrophilicity of the carbonyl group, and the corresponding stabilization is lost in the tetrahedral intermediate. 

Another factor which strongly affects the reactivity of these carboxylic acid derivatives is the leaving-group abihty of the substituents. The order is Cl > OAr > OR > NR2 > 0 so that not only does the ease of forming the tetrahedral intermediate decrease in the order Cl>0Ar>0R>NR2>0 , but the tendency for subsequent elimination to occur is also in the same order. Because the two factors work together, there are large differences in reactivity toward the nucleophiles. 

This difference in reactivity between the different classes of amines explains the difference in the primer performance on polyolefin substrates with ethyl cyanoacrylate-based adhesives [37J. Since primary and secondary amines form low molecular weight species, a weak boundary layer would form first, instead of high molecular weight polymer. Also, the polymer, which does ultimately form, has a lower molecular weight, which would lower adhesives strength [8,9]. 

The pathways described above lack description with regard to ring position effects. As mentioned earlier, each ring position has its own reaction mechanisms available and these will vary across other reaction conditions, e,g, pH, C studies by Grenier-Loustalot et al, and by Kim and co-workers have clearly shown differences in reactivity for the various ring positions in their various states of substitution [128,144],... 

Isolated tetrasubstituted double bonds do not react under these conditions and the saturation of trisubstituted double bonds is extremely slow, thus limiting the general utility of the method. This difference in reactivity is used to advantage for the selective deuteration of the -double bond in androsta-l,4-diene-3,17-dione (138). In homogeneous solution, saturation usually occurs from the a-side and consequently the deuterium labels are in... 

An illustration of the difference in reactivity of a-and / -halides is provided by the ready elimination of 1,4a-dibromo-5a-cholestan-3-one to 4a-bromo-5a-cholest-l-en-3-one in collidine at room temperature. Calcium carbonate in refluxing DMA is necessary to complete the dehydrobromination to the l,4-dien-3-one. ... 

The major difference in reactivity between CF3OF and FCIO3 lies in the capacity of the former to react with olefins without the benefit of an electron releasing group and even with electron deficient olefins such as a,y5-un-saturated ketones. Reactions with nonactivated double bonds indicate the presence of an oc-fluoro cationic intermediate [e.g., (64)] as exemplified by the reaction with the -3-ketone (63), which yields the fluorophenol (65). 

This difference in reactivity, especially toward hydrolysis, has an important result. We ll see in Chapter 27 that the structure and function of proteins are critical to life itself. The bonds mainly responsible for the structure of proteins are amide bonds, which are about 100 times more stable to hydrolysis than ester bonds. These fflnide bonds are stable enough to maintain the structural integrity of proteins in an aqueous environment, but susceptible enough to hydrolysis to be broken when the occasion demands. 

Do changes in geometries, charges and size and shape of the HOMO between the enolate anion and its lithium salt suggest differences in reactivities If so, what differences are to be expected ... 

An interesting use of the Camps quinoline synthesis is in the ring contraction of macrocycles. Treatment of 9 member ring 24 with sodium hydroxide in water furnished quinolin-4-ol 25, while 26 furnishes exclusively quinolin-2-ol 27 under the same reaction conditions (no yield was given for either reaction). The reaction does not work with smaller macrocycles. The authors rationalize the difference in reactivity based upon ground state conformation differences, but do not elaborate. 

Only within the past few years have serious attempts been made to estimate quantitatively the differences in reactivity between thiophene and benzene and between the 2- and 3-position of thiophene. Careful investigation on the acid-induced exchange of deuterium and tritium have shown that the ratios of the exchange rates in the 2- and 3-positions are 1045 61 for deuterium and 911 60 for tritium in 57% by weight aqueous sulfuric acid at 24.6°C. A kinetic isotope effect in the isotopic exchange has been found to be k-r/kr, = 0.51 0.03 in the 2-position and kr/kjy — 0.59 0.04 in the... 

The acid cleavage of the aryl— silicon bond (desilylation), which provides a measure of the reactivity of the aromatic carbon of the bond, has been applied to 2- and 3-thienyl trimethylsilane, It was found that the 2-isomer reacted only 43.5 times faster than the 3-isomer and 5000 times faster than the phenyl compound at 50,2°C in acetic acid containing aqueous sulfuric acid. The results so far are consistent with the relative reactivities of thiophene upon detritia-tion if a linear free-energy relationship between the substituent effect in detritiation and desilylation is assumed, as the p-methyl group activates about 240 (200-300) times in detritiation with aqueous sulfuric acid and about 18 times in desilylation. A direct experimental comparison of the difference between benzene and thiophene in detritiation has not been carried out, but it may be mentioned that even in 80.7% sulfuric acid, benzene is detritiated about 600 times slower than 2-tritiothiophene. The aforementioned consideration makes it probable that under similar conditions the ratio of the rates of detritiation of thiophene and benzene is larger than in the desilylation. A still larger difference in reactivity between the 2-position of thiophene and benzene has been found for acetoxymercuration which... 

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