Hydroxide ions reaction with weak acid

One very important factor is of course the need to desolvate the nucleophile so that it can react. When hydroxide ion reacts with an acid, it can do so via a hydrogen-bonded complex HO" HX whose heat of formation will be at least equal to that of a hydrogen bond between HO" and a molecule HZ of a protic solvent, HO" HZ. In an S 2 reaction, however, at least one molecule of solvent must be removed from HO" in order that it should be able to approach the substrate RX. Since approach is from the backside of the R—X bond, and since alkyl groups are weakly polar, the interaction between HO" and RX will be small. This loss of solvation energy is indeed responsible for a major part of the activation energy of reactions. 

As long as the buffer solution contains acetic acid as a major species, a small amount of hydroxide ion added to the solution will be neutralized completely. Figure 18-1 shows two hydroxide ions added to a portion of a buffer solution. When a hydroxide ion collides with a molecule of weak acid, proton transfer forms a water molecule and the conjugate base of the weak acid. As long as there are more weak acid molecules in the solution than the number of added hydroxide ions, the proton transfer reaction goes virtually to completion. Weak acid molecules change into conjugate base anions as they mop up added hydroxide. 

Identify the major species in solution by assigning each of the points to one of the four characteristic regions of the titration curve. In the titration reaction, hydroxide ions react with molecules of weak acid ... 

Chelate materials of this type are products of reaction of a metal base, such as calcium hydroxide or metal oxide, with weakly acidic organic substances with at least two functional groups. The ones used clinically are typically hydrolytically unstable, and this is responsible for their therapeutic effects. Ions released have beneficial properties, reducing inflanunation, being bacteriostatic and stimulating the odontoblasts to form secondary dentine. Ideally, calcium hydroxide chelates of this kind dissolve completely with time, and thus have the maximum possible therapeutic effect. 

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. 

A base is any material that produces hydroxide ions when it is dissolved in water. The words alkaline, basic, and caustic are often used synonymously. Common bases include sodium hydroxide (lye), potassium hydroxide (potash lye), and calcium hydroxide (slaked lime). The concepts of strong versus weak bases, and concentrated versus dilute bases are exactly analogous to those for acids. Strong bases such as sodium hydroxide dissociate completely while weak bases such as the amines dissociate only partially. As with acids, bases can be either inorganic or organic. Typical reactions of bases include neutralization of acids, reaction with metals, and reaction with salts ... 

Neutralisation reactions, or addimetry and alkalimetry. These include the titration of free bases, or those formed from salts of weak acids by hydrolysis, with a standard acid (addimetry), and the titration of free acids, or those formed by the hydrolysis of salts of weak bases, with a standard base (alkalimetry). The reactions involve the combination of hydrogen and hydroxide ions to form water. 

Below is the titration curve for the neutralization of 25 mL of a base with a strong monoprotic acid. Answer the following questions about the reaction and explain your reasoning in each case, (a) Is the base strong or weak (b) What is the initial hydroxide ion concentration of the base (c) What is Kh for the base (d) What is the initial concentration of the base (e) What is the concentration of acid in the titrant (f) Use Table 11.3 to select an indicator for the titration. 

C04-0032. Carbonic acid, H2 CO3 (molecular model shown below), is a weak oxoacid that forms when carbon dioxide dissolves in water. Carbonic acid contains two acidic hydrogen atoms. Write the net ionic reaction that occurs when carbonic acid reacts with an excess of hydroxide ions. Draw a molecular picture of the process. 

In a weak acid or base, the backwards reaction (where ions join to form the acid or base) occurs more often than it does in a strong acid or base. Therefore, with a weak acid or base, some hydrogen and hydroxide ions are released, but there are many more molecules of intact acid or base than there would be with a strong acid or base. Most acids and bases are weak. They do not completely break down in water. 

Acetylene is sufficiently acidic to allow application of the gas-phase proton transfer equilibrium method described in equation l7. For ethylene, the equilibrium constant was determined from the kinetics of reaction in both directions with NH2-8. Since the acidity of ammonia is known accurately, that of ethylene can be determined. This method actually gives A f/ acid at the temperature of the measurement. Use of known entropies allows the calculation of A//ac d from AG = AH — TAS. The value of A//acij found for ethylene is 409.4 0.6 kcal mol 1. But hydrocarbons in general, and ethylene in particular, are so weakly acidic that such equilibria are generally not observable. From net proton transfers that are observed it is possible sometimes to put limits on the acidity range. Thus, ethylene is not deprotonated by hydroxide ion whereas allene and propene are9 consequently, ethylene is less acidic than water and allene and propene (undoubtedly the allylic proton) are more acidic. Unfortunately, the acidity of no other alkene is known as precisely as that of ethylene. 

Alternatively, some conclusions can be derived from the relative reactivities of car-banions. For example, DePuy and colleagues13 made use of a clever method involving reactions of silanes with hydroxide ion to deduce acidities of such weak acids as alkanes and ethylene. The silane reacts with hydroxide ion to form a pentacoordinate anion that ejects a carbanion held as a complex with the hydroxysilane rapid proton transfer gives the stable silanoxide ion and the carbon acid (equation 5). 

In the process of a weak acid or weak base neutralization titration, a mixture of a conjugate acid-base pair exists in the reaction flask in the time period of the experiment leading up to the inflection point. For example, during the titration of acetic acid with sodium hydroxide, a mixture of acetic acid and acetate ion exists in the reaction flask prior to the inflection point. In that portion of the titration curve, the pH of the solution does not change appreciably, even upon the addition of more sodium hydroxide. Thus this solution is a buffer solution, as we defined it at the beginning of this section. 

The catalytic effect of quaternary ammonium salts in the basic liquid liquid two-phase alkylation of amines [1-3] is somewhat unexpected in view of the low acidity of most amines (pKfl>30). Aqueous sodium hydroxide is not a sufficiently strong base to deprotonate non-activated amines in aqueous solution and the hydroxide ion is not readily transferred into the organic phase to facilitate the homogeneous alkylation (see Chapter 1). Additionally, it is known that ion-pairs of quaternary ammonium cations with deprotonated amines are decomposed extremely rapidly by traces of water [4]. However, under solidrliquid two-phase conditions, the addition of a quaternary ammonium salt has been found to increase the rate of alkylation of non-activated amines by a factor of ca. 3-4 [5]. Similarly, the alkylation of aromatic amines is accelerated by the addition of the quaternary ammonium salt the reaction is accelerated even in the absence of an inorganic base, although under such conditions the amine is deactivated by the formation of the hydrohalide salt, and the rate of the reaction gradually decreases. Hence, the addition of even a weak base, such as... 

If a salt consists of the cation of a strong base and the anion of a weak acid, such as NaCHsCOO, only the anion reacts significantly with water. The reaction produces hydroxide ions. Therefore, the solution will have a pH that is greater than 7. Salts of strong bases and weak acids dissolve in water and form basic solutions. 

However, attempts to make an aqueous solution of the base sodium amide would result in the formation of sodium hydroxide and ammonia. The amide ion is a strong base and abstracts a proton from water, a weak acid. The reverse reaction is not favoured, in that hydroxide is a weaker base than the amide ion, and ammonia is a weaker acid than water. Take care with the terminology amide the amide... 

The mechanism of the el Q + H20 reaction is not clear. It is evident that the reaction may take place only with solvated electrons, and it requires the solvation of the hydroxide ions to make it thermodynamically feasible (66). As will be seen later the reaction cannot be regarded as a proton transfer by a weak Bronsted acid to e aq. Adhering to the formal scheme e aq + AB - AB- - A + B- one would expect an electron to be absorbed in one of the higher unoccupied orbitals of H20. A dissociative state of H20 - exists at about 4 e.v. (87) which yields H + OH - however,... 

In a basic solution, the principle is similar. The conjugate acid of the weak base combines with excess hydroxide ion to drive the reaction toward the weak base. The same factors govern the process the amount of the conjugate acid and the Kb of the base. 

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