It conjugation

With each random choice of y and its conjugate momentum Py, one can have a separate trajectory with a different final wave function. After a series of calculations, the energy and state resolved cross-sections are obtained. 

Because of the mentioned leveling effect of the solvent (or excess acid itself acting as such) the acidity cannot exceed that of its conjugate acid. In the case of water the limiting acidity is that of HsO. Proton-ated water, H30 (hydronium ion), was first postulated in 1907, and its preeminent role in acid-catalyzed reactions in aqueous media was first realized in the acid-base theory of Bronsted and Lowry. Direct experimental evidence for the hydronium ion in solution and in the... 

The significance of the possible diprotonation of water under extremely acidic conditions directly affects the question of acid strength achievable in superacidic systems. The leveling effect mentioned above limits the acidity of any system to that of its conjugate acid. Thus, in... 

Nitration is almost always carried out under acidic conditions. If the compound being nitrated is basic, the problem arises of deciding whether the free base or its conjugate acid is being nitrated, or if both of these species are reacting. 

If the observed second-order rate constant is greater than calc., reaction via the free base is precluded. If /taobs. is less than k czlc., reaction via the conjugate acid or the free base is possible. The first compound reported to be nitrated via its conjugate acid, and yet to have 2 calc. > obs. at the acidities concerned, was pyrazoleother examples are mentioned later ( 9.3 10.4.2). 

Among the W-oxides of this series of compounds isoquinoline 2-oxide shows the simplest behaviour on nitration. The acidity dependence of the rate of nitration (table 8.1), and comparison with the 2-methoxyiso-quinolinium cation (v, R = Me) (table 10.3) show the oxide to be nitrated as its conjugate acid (v, i = H) in 76-83 % sulphuric acid. The... 

Comparison of the behaviour of cinnoline 2-oxide (vi, i = O) with that of 2-methoxycinnolinium (vi, R = OMe) suggests that at high acidities the former is nitrated as its conjugate acid (vi, R = OH), but that as the acidity is lowered the free base becomes active. At high acidities 5- and 8-nitration are dominant, but as the acidity is lowered 6-nitration becomes increasingly important. The 5- and 8-nitro compounds are probably formed mainly or wholly by nitration of the conjugate acid, and the 6-nitro compound wholly or mainly from the free base. ... 

In the equation shown the base uses an unshared pair of electrons to remove a proton from an acid The base is converted to its conjugate acid, and the acid is converted to... 

Its conjugate base A base and its conjugate acid always differ by a single proton Like wise an acid and its conjugate base always differ by a single proton... 

The stronger base is derived from the weaker conjugate acid Therefore add a proton to ammonia to give its conjugate acid (ammonium ion) and a proton to pyridine to give its conjugate acid (pyridimum ion) then look up the values for each... 

Example A common misconception is that the conjugate base of a weak acid is strong This is sometimes but not always true It is true for example for ammo nia which is a very weak acid (pK 36) Its conjugate base amide ion (H2N ) is a much stronger base than HO It is not true however for acetic acid both acetic acid and its conjugate base acetate ion are weak The conjugate base of a weak acid will be strong only when the acid is a weaker acid than water... 

Cyanide is a weaker base than hydroxide because its conjugate acid HCN (pK 9 1) IS a stronger acid than water ipK 15 7)... 

Notice too that strongly basic leaving groups are absent from Table 8 8 In gen eral any species that has greater than about 2 for its conjugate acid cannot be a... 

Amide ion is a much stronger base than acetylide ion and converts acetylene to its conjugate base quantitatively... 

Alkylation of acetylene involves a sequence of two separate operations In the first one acetylene is converted to its conjugate base by treatment with sodium amide... 

In spite of S—H bonds being less polar than O—H bonds thiols are stronger acids than alcohols This is largely because S—H bonds are weaker than O—H bonds We have seen that most alcohols have pA s of 16-18 The corresponding value for a thiol IS about 10 The significance of this difference is that a thiol can be quantitatively con verted to its conjugate base (RS ) called an alkanethiolate, by hydroxide Conse quently thiols dissolve m aqueous base... 

Base promoted cyclization of vicinal halohydrms (Section 16 10) This reaction is an intramolecu lar version of the Williamson ether synthesis The alcohol function of a vicinal halohydrin is con verted to its conjugate base which then displa ces halide from the adjacent carbon to give an epoxide... 

From this equation it can be seen that when [CH3C02 ] = [CH3CO2H] then the second term is log 1 = 0 and pH = pK This means that when the pH of a solution is equal to the pK of a weak acid the con centration of the acid and its conjugate base are equal This is a relationship worth remembering... 

The more basic the amine the weaker its conjugate acid... 

Some ammo acids have side chains that bear acidic or basic groups As Table 27 3 indicates these ammo acids are characterized by three values The third pK reflects the nature of the side chain Acidic ammo acids (aspartic and glutamic acid) have acidic side chains basic ammo acids (lysine arginine and histidine) have basic side chains The isoelectric points of the ammo acids m Table 27 3 are midway between the pK values of the zwitterion and its conjugate acid Take two examples aspartic acid and lysine Aspartic acid has an acidic side chain and a pi of 2 77 Lysine has a basic side chain and a pi of 9 74... 

Solvolysis reaction (Section 8 7) Nucleophilic substitution m a medium m which the only nucleophiles present are the solvent and its conjugate base... 

The most common charge types for the acid HB and its conjugate base B are CH3COOH = H+ -f CH3C00 (acetic acid, acetate ion)... 

Tabulating Values for K and Kb A useful observation about acids and bases is that the strength of a base is inversely proportional to the strength of its conjugate acid. Consider, for example, the dissociation reactions of acetic acid and acetate. 

The equilibrium constant for equation 6.13 is K. Since equation 6.13 is obtained by adding together reactions 6.11 and 6.12, may also be expressed as the product of Ka for CH3COOH and Kb for CH3COO-. Thus, for a weak acid, HA, and its conjugate weak base, A-,... 

A more challenging problem is to find the pH of a solution prepared from a polyprotic acid or one of its conjugate species. As an example, we will use the amino acid alanine whose structure and acid dissociation constants are shown in Figure 6.11. 

Adding as little as 0.1 mb of concentrated HCl to a liter of H2O shifts the pH from 7.0 to 3.0. The same addition of HCl to a liter solution that is 0.1 M in both a weak acid and its conjugate weak base, however, results in only a negligible change in pH. Such solutions are called buffers, and their buffering action is a consequence of the relationship between pH and the relative concentrations of the conjugate weak acid/weak base pair. 

Equation 6.44 is written in terms of the concentrations of CH3COOH and CH3COO- at equilibrium. A more useful relationship relates the buffer s pH to the initial concentrations of weak acid and weak base. A general buffer equation can be derived by considering the following reactions for a weak acid, HA, and the salt of its conjugate weak base, NaA. 

Acid-base reactions occur when an acid donates a proton to a base. The equilibrium position of an acid-base reaction is described using either the dissociation constant for the acid, fQ, or the dissociation constant for the base, K, . The product of and Kb for an acid and its conjugate base is K (water s dissociation constant). 

Adding NaOH converts a portion of the acetic acid to its conjugate base. 

Chemical Limitations to Beer s Law Chemical deviations from Beer s law can occur when the absorbing species is involved in an equilibrium reaction. Consider, as an example, an analysis for the weak acid, HA. To construct a Beer s law calibration curve, several standards containing known total concentrations of HA, Cmt, are prepared and the absorbance of each is measured at the same wavelength. Since HA is a weak acid, it exists in equilibrium with its conjugate weak base, A ... 

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