Estimations acyl derivatives

A quite different and complimentary approach is to assume that addition of a nucleophile to an acyl derivative (RCOX) would follow the linear free energy relationship for addition of the nucleophile to the corresponding ketone (RCOR, or aldehyde if R=H) if conjugation between X and the carbonyl could be turned off, while leaving its polar effects unchanged. This can be done if one knows or can estimate the barrier to rotation about the CO-X bond, because the transition state for this rotation is expected to be in a conformation with X rotated by 90° relative to RCO. In this conformation X is no longer conjugated, so one can treat it as a pure polar substituent. Various values determined by this approach are included in the tables in this chapter. 

J. Fastrez, Estimation of the Free Energies of Addition of Nucleophiles to Conjugated Carbonyl Compounds and to Acyl Derivatives, J. Am. Chem. Soc. 1977, 99, 7004. 

When the substance under estimation is either an acyl derivative of an alcohol or phenol which is not affected by alkali and air, or an acyl derivative of a volatile base, the estimation may be carried out as for esters with alcoholic potash (p. 509), any free base being removed by distillation before titrating the excess of alkali. 

Exchange experiments arc also the basis of our estimate of the relative importance of the two steps differences in rate of hydrolysis of acyl derivatives depend chiefly on how fast intermediates are formed, and also on what fraction of the intermediate goes on to product. As we have said, the rate of formation of the intermediate is affected by both electronic and steric factors in the transition state, a negative charge is developing and carbon is changing from trigonal tow ard tetrahedral. 

Table 20.1 summarizes the stabilizing effects of substituents on carbonyl groups to which they are attached. In addition to a qualitative ranking, quantitative estimates of the relative rates of hydrolysis of the various classes of acyl derivatives are given. A weakly stabilized carboxylic acid derivative reacts with water faster than does a more stabilized one. 

CgHs-N C CHs+CHaC OH In the above equations, it should be noted that acetyl chloride does not convert aniline completely into the acetyl derivative since the by-product, aniline hydrochloride, is formed and this does not act readily with the reagent. On the other hand, with acetic anhydride the amine is converted quantitatively into the acyl derivative and therefore this latter reagent is of more importance in connection with the preparation of derivatives. It is also of value in quantitative estimations of the amine group, the excess of acetic acid which remains after the reaction being determined volumetrically. Benzoyl chloride, benzenesulfonyl chloride, and other acyl halides that may be used in aqueous solution may also convert the amine completely into an acyl derivative for the reason that they are usually used in the presence of alkali which will combine with the hydrochloric acid generated in the reaction. When benzoyl chloride is used, a small amount of benzoic acid may be formed, due to the following side-reaction ... 

Partial rate factors may be used to estimate product distributions in disubstituted benzene derivatives The reactivity of a particular position in o bromotoluene for example is given by the product of the partial rate factors for the corresponding position in toluene and bromobenzene On the basis of the partial rate factor data given here for Fnedel-Crafts acylation predict the major product of the reaction of o bromotoluene with acetyl chlonde and aluminum chloride... 

Hydrazinocarbonylpropyl)-6,7-dimethoxy-l-methyl-2(177)-quinoxalinone (126) undergoes acylation by fatty acids (EtN=C=NCH2CH2CH2NMe2, H2O, pyridine, 20°C) to afford highly fluorescent derivatives that are useful for the detection and estimation of such fatty acids. ... 

If we make the assumption that the reverse of reaction 15.5 is diffusion-controlled and assume that the activation enthalpy for the acyl radicals recombination is 8 kJ mol-1, the enthalpy of reaction 15.5 will be equal to (121 - 8) = 113 kJ mol-1. This conclusion helps us derive other useful data. Assuming that the thermal correction to 298.15 K is small and that the solvation enthalpies of the peroxide and the acyl radicals approximately cancel, we can accept that the enthalpy of reaction 15.5 in the gas phase is equal to 113 kJ mol-1 with an estimated uncertainty of, say, 15 kJ mol-1. Therefore, as the standard enthalpy of formation of gaseous PhC(0)00(0)CPh is available (-271.7 5.2 kJ mol-1 [59]), we can derive the standard enthalpy of formation of the acyl radical Af//°[PhC(0)0, g] -79 8 kJ mol-1. This value can finally be used, together with the standard enthalpy of formation of benzoic acid in the gas phase (-294.0 2.2 kJ mol-1 [59]), to obtain the O-H bond dissociation enthalpy in PhC(0)0H DH° [PhC(0)0-H] = 433 8 kJ mol-1. 

HIV infection in humans is now pandemic. As of January 2006, the Joint United Nations Programme on HIV/AIDS (UNAIDS) and the World Health Organization (WHO) estimate that AIDS has killed more than 25 million people since it was first recognized on December 1, 1981, making it one of the most destructive pandemics in recorded history. Antiretroviral treatment reduces both the mortality and the morbidity of HIV infection, but routine access to antiretroviral medication is not available in all countries. An alternative therapy to circumvent this problem is the use of polyanionic substances, which demonstrated a number of promising features as potential anti-HIV drug candidates. In this respect, various SPs (e.g., heparin, DS, dextrin sulfate, cyclo dextrin sulfate, curdlan sulfate, pentosan polysulfate, mannan sulfate, sulfoevernan, and fucoidan) and derivatives thereof (e.g., O-acylated heparin, polyacetal polysulfate, polyvinyl alcohol sulfate, and modified cyclo dextrin sulfates) have been found to inhibit HIV replication in vitro at concentrations that are up to 10 000-fold lower than the cytotoxic concentration [2,71]. 

From the relative abundances of the two ions derived from the loss of a,(S-unsaturated fatty acids specific at the sn-2 position for each of the other RRAc in castor oil, we estimated the content of the 1,3-diricinolcoyl-2-acyl-sn-glycerol species among the three isomers as follows RLR (95%), RLnR (96%), RSR (96%), RPR (78%), and RLsR (31%). These values fall consistently within +3% in repeated experiments. Both MS3 and CAD-MS2 were used to measure the content of each l,3-diricinoleoyl-2-acyl-5,n-glycerol among the three possible stereoisomers with equivalent results (data not shown). 

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