Programming is an essential skill that all computing students must master. However programming can be difficult to learn. Compiler error messages are crucial for correcting errors, but are often difficult to understand and pose a barrier to progress for many novices. High frequencies of errors, particularly repeated errors, have been shown to be indicators of students who are struggling with learning to program. This study involves a custom IDE that enhances Java compiler error messages, intended to be more useful to novices than those supplied by the compiler. The effectiveness of this approach was tested in an empirical control/intervention study of approximately 200 students generating almost 50,000 errors. The design allows for direct comparisons between enhanced and non-enhanced error messages. Results show that the intervention group experienced reductions in the number of overall errors, errors per student, and several repeated error metrics. This work is important for two reasons. First, the effects of error message enhancement have been recently debated in the literature. This study provides substantial evidence that it can be effective. Second, these results should be generalizable at least in part, to other programming languages, students and institutions, as we show that the control group of this study is comparable to several others using Java and other languages.
Aqueous solvolyses of acyl derivatives of hydrates (water adducts) of anthracene and benzofuran yield carbocations which undergo competitive deprotonation to form the aromatic molecules and nucleophilic reaction with water to give the aromatic hydrates. Trapping experiments with azide ions yield rate constants k(p) for the deprotonation and k(H2O) for the nucleophilic reaction based on the "azide clock". Combining these with rate constants for (a) the H(+)-catalyzed reaction of the hydrate to form the carbocation and (b) hydrogen isotope exchange of the aromatic molecule (from the literature) yields pK(R) = -6.0 and -9.4 and pK(a) = -13.5 and -16.3 for the protonated anthracene and protonated benzofuran, respectively. These pK values may be compared with pK(R) = -6.7 for naphthalene hydrate (1-hydroxy-1,2-dihydronaphthalene), extrapolated to water from measurements by Pirinccioglu and Thibblin for acetonitrile-water mixtures, and pK(a) = -20.4 for the 2-protonated naphthalene from combining k(p) with an exchange rate constant. The differences between pK(R) and pK(a) correspond to pK(H2O), the equilibrium constant for hydration of the aromatic molecule (pK(H2O) = pK(R) - pK(a)). For naphthalene and anthracene values of pK(H2O) = +13.7 and +7.5 compare with independent estimates of +14.2 and +7.4. For benzene, pK(a) = -24.3 is derived from an exchange rate constant and an assigned value for the reverse rate constant close to the limit for solvent relaxation. Combining this pK(a) with calculated values of pK(H2O) gives pK(R) = -2.4 and -2.1 for protonated benzenes forming 1,2- and 1,4-hydrates, respectively. Coincidentally, the rate constant for protonation of benzene is similar to those for protonation of ethylene and acetylene (Lucchini, V.; Modena, G. J. Am. Chem Soc. 1990, 112, 6291). Values of pK(a) for the ethyl and vinyl cations (-24.8) may thus be derived in the same way as that for the benzenonium ion. Combining these with appropriate values of pK(H2O) then yields pK(R) = -39.8 and -29.6 for the vinyl and ethyl cations, respectively.
The equilibrium constant for keto-phenol tautomerisation of anthrone to 9-anthrol (K E = [phenol]/[ketone]) has been determined as pK E (Ϫlog K E ) = 2.10 from ratios of rate constants for ketonisation of anthrol and phenolisation of anthrone in aqueous acetic acid buffers at 25 ЊC. Combining this value with pK a = 10.0 for the ionization of anthrone, measured spectrophotometrically in piperazine and borate buffers, gives pK a = 7.9 for the phenolic hydroxy group of anthrol. Measurements of rate constants for tautomerisation showed acid catalysis by H 3 O ϩ in aqueous HCl but by the base component only in buffer solutions of weaker acids. The H 3 O ϩ -catalysed reaction is subject to a solvent isotope effect k H 3 O /k D 3 O = 4.8, consistent with protonation of 9-anthrol at the 10-carbon atom of the anthracene ring in the rate-determining step. Comparison with hydrolysis of the methyl ether of anthrol showed that ketonisation is faster by a factor of 3000. This large rate difference is consistent with NMR measurements which show that deuterium isotope exchange at the 10-position of the anthryl methyl ether occurs in competition with hydrolysis. This accounts for a 60-70 fold of the rate difference. The residue is attributed to (a) a normal difference of 16-fold in protonation rates of phenols and the corresponding methyl ethers and (b) a minor contribution from steric hindrance to resonance stabilisation of the anthracen-9-onium ion intermediate in the hydrolysis reaction from interaction of the conjugating methoxy group with the 1,8-hydrogen atoms of the adjacent phenyl rings.
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