Ease of reaction

Diazines are considerably more reactive toward nucleophiles than pyridines and as the number of ring nitrogens increases the propensity for nucleophilic addition reactions increases still more. Many 1,2,4-triazines give addition products with various nucleophiles which are formally dihydrotriazines. 

The limit is reached with 1,3,5-triazine. This reacts very easily even with weak nucleophiles, and ring cleavage nearly always follows. Thus, it behaves as a formylating agent toward amines and other active hydrogen compounds. 

Pteridines also undergo nucleophilic addition reactions particularly easily, including covalent hydration, addition of bisulfite and others. 

The intrinsic reactivity of pyridine 1-oxides toward nucleophiles is little greater than that of pyridine the strongest nucleophiles react. However, after initial reaction with an electrophile at the N-oxide oxygen, subsequent attack by nucleophiles is easy see above discussion under cationic rings . 

---

The alkanoic acids, with the exception of formic acid, undergo typical reactions of the carboxyl group. Formic acid has reducing properties and does not form an acid chloride or an anhydride. The hydrocarbon chain of alkanoic acids undergoes the usual reactions of hydrocarbons except that the carboxyl group exerts considerable influence on the site and ease of reaction. The alkenoic acids in which the double bond is not conjugated with the carboxyl group show typical reactions of internal olefins. All three types of reactions are industrially important. 

The ease of reaction of halopyridazines is indicated by the exothermic nature of the reaction of 3,6-dichloropyridazine with sodium methoxide at room temperature to yield 3-chloro-6-methoxy-pyridazine. Displacement of the deactivated chloro group in the latter required heating (66°, < 8 hr) the reaction mixture. Competitive methoxy-dechlorination (20°, 12 hr) of 3,4,6-trichloropyridazine shows the superior reactivity of the 4-position the 3,6-dichloro-4-methoxy analog (296) was isolated in high yield. The greater reactivity of the... 

It is recognised that vitreous enamel possesses good acid resistance, but an exception occurs with hydrofluoric acid. This is due to the relative ease of reaction between this acid and the silica (which is the largest constituent in the frit) to form silicon tetrafluoride. This reaction is made use of in some de-enamelling plants. 

Studies of halogenation of the partially reduced systems (e.g., 148) have shown that the 6- (148) or 8-nitro-2,3-dihydrothiazolo[3,2-a]pyridinium bromides were brominated in hydroxylic solvents with a regiochemistry and ease of reaction consistent with the intermediacy of a pseudo base... 

The greater ease of reaction compared to nitrosodeprotonation may arise from the greater ease of carbon-carbon bond cleavage compared to carbon-hydrogen bond cleavage for in the latter reaction this is partly rate-determining and may so be in the decarboxylation as well. 

In benzene itself the charge at the point of attack will be not 1.00c, as has been assumed heretofore, but (1.00 + (43/108)5,)c. (Since 5,-is small, the results of the perturbation treatment can be applied directly.) In other molecules the ease of reaction at the ith carbon atom will then be greater than that of benzene if the charge at that point is greater than (1.00 + (43/108)5 )e, and conversely. 

Using a conventional dorm-room quality microwave oven, we have successfully performed Suzuki-Miyaura cross-coupling reactions catalyzed by Pd/C. Shorter reaction times are obtained (13 min of irradiation) using microwave irradiation compared to conventional methods of heating (> 1 h). Yields with relatively non-volatile aryl bromides range from 65 to 83%. Lower yields (15 - 27%) are obtained with relatively volatile aryl bromides substrates which may be evaporating during the course of the reaction. Ease of reaction set-up, rate enhancement from the microwave irradiation and facile work-up provided by the use of Pd/C makes this a very efficient procedure to mn. 

Intramolecular nucleophilic assistance is another distinct possibility for providing activation, and can be realized by coordination of the metal center by a heteroatom (O, N, etc.) within the ligand which transiently forms a chelate-like attachment to the metal. Such a mode of activation was supposed to account for the ease of reaction of vinyl triflates with (zp-isomers of alkenyl-stannanes bearing OH groups in the proximity of the reaction center (71).2 5... 

Whereas in examples 1 and 2 above it may be argued that the direction and ease of reaction is (partly) due to merging resonance stabilization during product formation, this cannot be a positive factor in cases where OH or OR function as the nucleophile. Valuable examples are seen in the reaction of some o- and p-halogenoanisoles. o-Fluoroanisole and especially p-chloroanisole give smooth photosubstitution upon illumination in the presence of various nucleophiles (Brasem and Lammers, 1972 den Heyer, 1973). 

Inhibition of H2 formation can be seen when the anode-volume is saturated with O2 application of an external bias up to 0.7 V can usually prevent this effect, increased yield of H2 when Pt is present as a cathode has been rationalized in terms of three factors (a) the removal of conduction band electron from Ti02 to Pt [equation (4.4.14)], (b) The ease of reactions (4.4.15) and (4.4.16) because of a low overpotential for H2 evolution from water at the Pt cathode, and (c) H atom migration to the Pt cathode. 

In contrast to the ease of reaction of ring nitrogen atoms in 77-deficient six-membered heterocycles with electrophiles, electrophilic heteroaromatic substitution at carbon of the unsubstituted compounds proceeds only under very drastic conditions and yields of products are usually very poor. This is also true with pyridinium, pyrylium and thiopyrylium salts,... 

Of Reactions 1 to 5, Reactions 2, 3, and 4 probably play the more important role because they favor the formation of the kinetically important peroxides and enhance their contribution to chain branching. Their ease of reaction is expected to decrease in the order HI > HBr > HC1, and this accounts for the fact that the order of effectiveness of the halogen compounds in promoting preflame oxidation is iodine > bromine > chlorine. 

Pyridine readily forms stable coordination compounds. Thus, boron, aluminum and gallium trihalides react at 0°C in an inert solvent to give 1 1 adducts (cf. 85). Steric factors are important, and a-substituents decrease the ease of reaction. This is illustrated by the heats of reaction of pyridine, 2-methylpyridine and 2,6-dimethylpyridine with boron trifluoride which are 101.3, 94.1 and 73.2 kJ mol-1, respectively. The marked decrease in exothermicity here should be contrasted with the small steric requirement of the proton as shown by the pA., values of substituted pyridines (see Section 3.2.1.3.4). 

The analogous reactions of pyridines with these electrophilic reagents at the lone pair on the nitrogen atom are well known. All neutral azoles contain a pyridine-like nitrogen atom and therefore similar reactions with electrophiles at this nitrogen would be expected. However, the tendency for such reactions varies considerably in particular, successive heteroatom substitutions markedly decrease the ease of reaction. One convenient quantitative measure of the tendency for such reactions to occur is found in the basicity of these compounds this is treated in Section 3.4.1.3.5 and 3.4.1.3.7. 

Walsh proposed ease of reaction in the hydrogen abstraction step was in the order 3° 2° > 1° (229, 230). Boord has applied this idea (20, 21, 24, 25). If this order were to be accepted, it would have to be assumed that the rate of the hydrogen abstraction step is not controlling but something else, such as stability of peroxides (228) formed, determines knock. 

Addition of DMAD to some S-oxoalkyl-s-triazines (278) leads to 280,357 and the ease of reaction is dependent on the substituents on the triazine ring. The stereochemistry of the adduct (279) from EP is in agreement with the cyclic mechanism leading to a cis addition of the dienophile to the enol,358 and bicyclic products were not obtained. 

---