Strengthening effect

A comparison of the relative basicities of pyrrole, furan and thiophene may be made by comparing the pK values of their 2,5-di-t-butyl derivatives, which were found to be -1.01, —10.01 and —10.16, respectively. In each case protonation was shown by NMR to occur at position 2. The base-strengthening effect of alkyl substitution is clearly apparent by comparison of pyrrole and its alkyl derivatives, e.g. A-methylpyrrole has a pKa. for a-protonation of -2.9 and 2,3,4,5-tetramethylpyrrole has a pK of 4-3.7. In general, protonation of a-alkylpyrroles occurs at the a -position whereas /3-alkylpyrroles are protonated at the adjacent a-position. As expected, electron-withdrawing groups are base-weakening thus A-phenylpyrrole is reported to have a p/sTa of -5.8. The IR spectrum of the hydrochloride of 2-formylpyrrole indicates that protonation occurs mainly at the carbonyl oxygen atom and only to a limited extent at C-5. 

NMR data for 4-methyloxazole have been compared with those of 4-methylthiazole the data clearly show that the ring protons in each are shielded. In a comprehensive study of a range of oxazoles. Brown and Ghosh also reported NMR data but based a discussion of resonance stabilization on pK and UV spectral data (69JCS(B)270). The weak basicity of oxazole (pX a 0.8) relative to 1-methylimidazole (pK 7.44) and thiazole (pK 2.44) demonstrates that delocalization of the oxygen lone pair, which would have a base-strengthening effect on the nitrogen atom, is not extensive. It must be concluded that not only the experimental measurement but also the very definition of aromaticity in the azole series is as yet poorly quantified. Nevertheless, its importance in the interpretation of reactivity is enormous. 

In addition it may be seen that the strengthening effect of the fibres is only observed (i.e. a u > volume fraction is greater than a certain... 

On the other hand, fiber reinforcement makes the composite strong. Moreover, the matrix can be reinforced in the required direction by design, which ensures the maximum utilization of the fiber-properties. A disadvantage in fiber composites is that the fibers are able to transmit loads only in the directions of their axis and there is less strengthening effect in the direction perpendicular to the axis, and in some cases even weakening may occur. 

A simple qualitative model of the three-electron hemibond in [X.. X], based on the Hiickel approximation, has been proposed by Gill and Radom [122]. This qualitative model predicts that the strength of the hemibond should vary in proportion to the Hiickel parameter a, which can be replaced by the HOMO energy in X because a good correlation is found between Eho-Mo(X) and De(X-X ). This model readily rationahzes the marked substituent effect on the strength of the hemibond. In particular, electron-withdrawing substituents are found to have a strengthening effect. 

In deriving these equations no account was taken of the strengthening effect of the bottom and top rings of the bowl or basket so the equations will give estimates that are on the safe side. Strengthening hoops or bands are used on some basket designs. 

The marked acid-strengthening effect of p-NOy is usually attributed to the influence of the electron-attracting inductive effect (+/), augmented by a small electron-attracting mesomeric or resonance effect (+R). The smaller acid-strengthening effect of m-NOy is explained as the resultant of the inductive effect and a small relayed influence of the resonance effect. If op is regarded simply as a sum of oj and or (Section II.B) and a/ is taken as 0.67 (Section III.A), a value of 0.78 — 0.67 = 0.11 is indicated for or. The relay factor of 0.33 for the resonance effect accounts reasonably well for the value of om as ay + 0.33or = 0.67 + 0.04 = 0.71 cf 0.71 above. 

One final anomaly we have already commented on the enhanced value for the effect of para-substituents on the ionization of 2,6-di-fert-butylphenol in 1 1 v/v Et0H-H20 at 25 °C (Section III.D). In this solvent phenol itself has a p/fa value of 11.16, and the values for the 4-NO and 4-NO2 derivatives are 6.9 and 7.49, respectively134, i.e. NO has a greater acid-strengthening effect on phenol than NO2 does, as we have already seen above for solutions in water. However, the p/fa values of 2,6-di-fert-butylphenol and its 4-NO and 4-NO2 derivatives in 1 1 v/v Et0H-H20 are 14.22, 9.41 and 7.49, respectively134, i.e. the acid-strengthening effects of NO and NO2 are reversed. The apparent a value of p-NO is now about 0.9. Presumably there is a considerable amount of quinonoid form in this system. 

Here again the base-strengthening effect of methyl groups is clearly marked, as may be seen by comparing 2-methyl-butene-2 and 2,3-dimethylbutene-2 in Table 16. 

Values of pKa for some selected derivatives are also listed to give some idea of how the position and nature of a substituent affects basicity. In quinoline 2- and 4-methyl groups have the greatest base-strengthening effect. Attachment of nitro groups at any position in either system has a strong base-weakening effect. 

Upon addition of water, the hydration reactions initiate, and the hydraulic cement begins to gain strength. This process is very complex, but the strengthening effect is due basically to the formation of three types of hydration products colloidal products such as C2S xH20, which have a size of less than 0.1 p.m submicrocrystalline products such as Ca(OH)2, Al +, Fe +, and S04 phases with sizes from 0.1 to 1 tim and microcrystalline products, primarily of Ca(OH)2, with particle sizes greater than 1 p,m. The most common type of hydraulic cement, Portland cement, usually contains mostly colloidal products. 

In general, H-bonding involving the acidic H has an acid-weakening effect H-bonding in the conjugate base has an acid-strengthening effect. 

In summary, both 2- and 4-methoxy-5-nitropyrimidines undergo attack at a CH position rather than at the COMe position. A major factor for this behavior is believed to be the COMe bond-strengthening effect of the conjugation of the methoxy substituent with the ring. This is especially evident in the formation of adduct 16 resulting from attack of the reagent at the more hindered position 4 relative to position 2.41... 

The strengthening effect of a monobloc cylinder due to autoffettage practically is achieved by two effects First the introduction of the compressive (tangential) residual stresses which extend the elastically admissible internal pressure and second the increased available material strength by strain hardening. The maximum admissible pressure for optimum autoffettaged cylinders based on perfectly elastic-plastic materials, completely elastic stress (plus/minus) conditions at the inner bore diameter and the assumption of the GE-hypothesis can be calculated as [11]. 

Figure 8-34 An end-on view of a crystallite of hydroxylapatite. The shaded atoms of Ca, P, and O represent an underlying layer. The OH- groups form a longitudinal H-bonded array in the center. From J. A. Weatherell and C. Robinson,705 p. 55. A small amount of Mg2+ is present in place of Ca2+ and a very small fraction of the OH- is replaced by fluoride ion which has a bone strengthening effect.

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