Very slow reactions

Meakins and Morris recently reinvestigated the reaction of 5a-cholestan-3-one (lb) with an excess of diazomethane and observed the same results reported by Nelson and Schut. Using preformed diazomethane in ether-methanol, Meakins and Morris found the reaction very slow. However, in the presence of potassium hydroxide, ring expansion proceeds smoothly. The role of the base in markedly increasing the reaction rate has not been explained. 

Figure 16.7 Free-energy profiles for glyceraldehyde oxidation followed by acyl-phosphate formation. (A) A hypothetical case with no coupling between the two processes. The second step must have a large activation barrier, making the reaction very slow. (B) The actual case with the two reactions coupled through a thioester intermediate.

Hydrolysis of peptide bond favored energetically, but uncatalyzed reaction very slow... 

The third aromatic ring in the anthraquinone derivatives possibly renders all secondary reactions very slow in comparison with the timescale for radical decay. Hydroxyl radicals react wi OH-substituted quinones or other hydroquinones mainly by addition, followed by acid or base catalysed water elimination [55,72]. 

The Landolt reaction (iodate + reductant) is prototypical of an autocatalytic clock reaction. During the induction period, the absence of the feedback species (Irere iodide ion, assumed to have virtually zero initial concentration and fomred from the reactant iodate only via very slow initiation steps) causes the reaction mixture to become kinetically frozen . There is reaction, but the intemiediate species evolve on concentration scales many orders of magnitude less than those of the reactant. The induction period depends on the initial concentrations of the major reactants in a maimer predicted by integrating the overall rate cubic autocatalytic rate law, given in section A3.14.1.1. 

The catalysis was very strong, for in the absence of nitrous acid nitration was very slow. The rate of the catalysed reaction increased steeply with the concentration of nitric acid, but not as steeply as the zeroth-order rate of nitration, for at high acidities the general nitronium ion mechanism of nitration intervened. 

Note 4. At lower temperatures part of the reagent may crystallize out and the reaction becomes very slow or even stops. 

Carboxylic acids react with butadiene as alkali metal carboxylates. A mixture of isomeric 1- and 3-acetoxyoctadienes (39 and 40) is formed by the reaction of acetic acid[13]. The reaction is very slow in acetic acid alone. It is accelerated by forming acetate by the addition of a base[40]. Addition of an equal amount of triethylamine achieved complete conversion at 80 C after 2 h. AcONa or AcOK also can be used as a base. Trimethylolpropane phosphite (TMPP) completely eliminates the formation of 1,3,7-octatriene, and the acetoxyocta-dienes 39 and 40 are obtained in 81% and 9% yields by using N.N.N M -tetramethyl-l,3-diaminobutane at 50 in a 2 h reaction. These two isomers undergo Pd-catalyzed allylic rearrangement with each other. 

The reaction of 2 2 dimethyl 1 propanol with HBr is very slow and gives 2 bromo 2 methyl propane as the major product... 

Chlorination is carried out m a manner similar to brommation and provides a ready route to chlorobenzene and related aryl chlorides Fluormation and lodmation of benzene and other arenes are rarely performed Fluorine is so reactive that its reaction with ben zene is difficult to control lodmation is very slow and has an unfavorable equilibrium constant Syntheses of aryl fluorides and aryl iodides are normally carried out by way of functional group transformations of arylammes these reactions will be described m Chapter 22... 

If the rate of chemical reaction is very slow compared to the rate of group diffusion (kj. kj, k ), then Eq. (5.8) reduces to... 

Positive-Tone Photoresists based on Dissolution Inhibition by Diazonaphthoquinones. The intrinsic limitations of bis-azide—cycHzed mbber resist systems led the semiconductor industry to shift to a class of imaging materials based on diazonaphthoquinone (DNQ) photosensitizers. Both the chemistry and the imaging mechanism of these resists (Fig. 10) differ in fundamental ways from those described thus far (23). The DNQ acts as a dissolution inhibitor for the matrix resin, a low molecular weight condensation product of formaldehyde and cresol isomers known as novolac (24). The phenoHc stmcture renders the novolac polymer weakly acidic, and readily soluble in aqueous alkaline solutions. In admixture with an appropriate DNQ the polymer s dissolution rate is sharply decreased. Photolysis causes the DNQ to undergo a multistep reaction sequence, ultimately forming a base-soluble carboxyHc acid which does not inhibit film dissolution. Immersion of a pattemwise-exposed film of the resist in an aqueous solution of hydroxide ion leads to rapid dissolution of the exposed areas and only very slow dissolution of unexposed regions. In contrast with crosslinking resists, the film solubiHty is controUed by chemical and polarity differences rather than molecular size. 

Production of Acrolein Dimer. Acting as both the diene and dienoplule, acrolein undergoes a Diels-Alder reaction with itself to produce acrolein dimer, 3,4-dihydro-2-formyl-2id-pyran, CgHg02 [100-73-2], At room temperature the rate of dimerization is very slow. However, at elevated temperatures and pressures the dimer may be produced in single-pass yields of 33% with selectivities greater than 95%. 

Dimerization of acrolein is very slow at ambient temperatures but it can become a mnaway reaction at elevated temperature (ca 90°C), a consideration in developing protection against fire exposure of stored acrolein. 

The equations of combiaed diffusion and reaction, and their solutions, are analogous to those for gas absorption (qv) (47). It has been shown how the concentration profiles and rate-controlling steps change as the rate constant iacreases (48). When the reaction is very slow and the B-rich phase is essentially saturated with C, the mass-transfer rate is governed by the kinetics within the bulk of the B-rich phase. This is defined as regime 1. 

In contrast to reaction of ozone with nucleophilic haUde and hypohaUte ions, reaction of ozone with electrophilic hypohalous acids is very slow. 

The reaction is very slow in neutral solution, but the equiUbrium shifts toward the lactam rather than glutamic acid. Under strongly acidic or alkaline conditions, the ring-opening reaction requires a very short time (10). Therefore, neutralization of L-glutamic acid should be performed cautiously because intramolecular dehydration is noticeable even below 190°C. 

Hydrolysis. The hydrolysis of dialkyl and monoalkyl sulfates is a process of considerable iaterest commercially. Successful alkylation ia water requires that the fast reaction of the first alkyl group with water and base be minimised. The very slow reaction of the second alkyl group results ia poor utilisation of the alkyl group and gives an iacreased organic load to a waste-disposal system. Data have accumulated siace 1907 on hydrolysis ia water under acid, neutral, and alkaline conditions, and best conditions and good values for rates have been reported and the subject reviewed (41—50). 

Direct reaction of oxygen with most organic materials to produce radicals (eq. 13) is very slow at moderate temperatures. Hydrogen-donating antioxidants (AH), particularly those with low oxidation—reduction potentials, can react with oxygen (eq. 14), especially at elevated temperatures (6). 

This reaction is very slow in acid but rapid above pH 10. Chlorine dioxide solutions are also decomposed by light. 

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