Hydroxide ion as a base

A number of very interesting conclusions are arrived at in this study.86 (1) Reaction of hydroxide ion as a base on carbocation 77 to form epoxide (k 2[HO-]) is energetically more favorable than its reaction with 77 as a nucleophile to form diols. (2) The reaction pathway for formation of ketone 128 is completely separate from the stepwise mechanism for diol formation. (3) The observed kinetic deuterium isotope effect for reaction of 76-f>-d at pH 11 is 2.15 the kinetic deuterium isotope effect on the 1,2-hydrogen migration is estimated to be 4. Hydrogen migration must be occurring at the transition state for ketone formation. 

The reaction of hydroxide ion with carbocation 82 is expected to occur at the diffusion-controlled limit because proton transfer from the /i-OI I is thermodynamically favorable, and the activation barrier for collapse of the resulting zwitterion (108, Scheme 32) to reform the epoxide ring should be absent or very small. A concerted mechanism for this reaction is enforced if zwitterion 108 is too unstable to exist as an intermediate.104 The attack of hydroxide ion as a base on 82 as shown in Scheme 38 must be energetically more favorable than attack as a nucleophile to yield tetrol product. The exact yield of tetrol from direct combination of hydroxide ion with carbocation 82 could not be estimated, but is negligible or very small. 

Bases are compounds that produce hydroxide ions, OH, when dissolved in water. Most bases are ionic compounds containing the hydroxide ion combined with a Group 1A or 1IA metal ion. Ammonia, NH3, which is a gas at room temperature, is also a base because, when dissolved in water, it reacts weakly with water to produce hydroxide ion. As a base, it is called aqueous ammonia and symbolized NH3(aq), though many chemists still refer to it simply as ammonia. ... 

The Dow Process utilizes an elimination/addition reaction to convert chlorobenzene to phenol. The proposed mechanism for this reaction is shown in Figure 8-3. The high-temperature reaction begins with chlorobenzene and aqueous sodium hydroxide. Note that this mechanism starts with the hydroxide attacking as a base, beginning dehydrohalogenation to form benzyne. The second hydroxide ion attacks as a nucleophile to form a carbanion intermediate, which behaves as a base in the last step to yield the final product. 

An important feature of cyanohydrin formation is that it requires a basic catalyst. In the absence of base, the reaction does not proceed, or is at best very slow. In principle, the basic catalyst may activate either the carbonyl group or hydrogen cyanide. With hydroxide ion as the base, one reaction to be expected is a reversible addition of hydroxide to the carbonyl group ... 

The elimination of alkyl halides is done under basic conditions, the elimination of alcohols is done under acid conditions. Under basic conditions, an E2 elimination would require the loss of a hydroxide ion as a leaving group. Since the hydroxide ion is a strong base, it is not a good leaving group and so the elimination of alcohols under basic conditions is difficult to achieve. 

The reactions of stable carbocations with water are generally base catalyzed,109,110 and their reactions with hydroxide ion are slower than their reactions with azide ion and sulfhydryl ions. The less favorable reaction of hydroxide ion with carbocations has been attributed to the fact that deprotonation of a water molecule by hydroxide ion is not a thermodynamically favorable reaction, and activation energy to generate a desolvated hydroxide ion is required.110 These factors would also account for the less favorable reaction of hydroxide ion as a nucleophile with 82 to form tetrols instead of as a base to bring about epoxide ring closure. 

A modified interfacial mechanism has been proposed by Starks [1] assuming that tetraalkylammonium hydroxide, generated at the interphase by ion exchange between ammonium halide and sodium hydroxide, acts as a base for substrate deprotonation. This mechanism has been disputed by Mqkosza through competitive addition of CCl3 anions to A-alkylpyridinium salts and carbanion acceptors such as vinyl acetate [31]. 

The definition of acids and bases that is most commonly used in mineralogy is that an acid is a species capable of donating a hydrogen ion, ", and a base is a species capable of accepting a hydrogen ion. This definition was proposed independently by the Danish chemist Johannes Bronsted and the British chemist Thomas Lowry. In the reaction of HCl with NaOH, HCl is the acid because it donates a hydrogen ion and hydroxide ion is a base becanse it accepts the hydrogen ion. This reaction and most other acid-base reactions are carried ont in water. Water itself can act as both an acid and a base and is said to be amphiprotic (this word is much like ambidextrous—it denotes action possible in two different ways). 

Malachite and aurichalcite cannot be distinguished by using only precipitation reactions and Bronsted-Lowry add-base reactions. Another type of acid-base reaction, the Lewis reaction (reaction category 3), is necessary. Zinc ion will react with hydroxide ion to form insoluble zinc hydroxide (solubility rule 4), which will then react with excess hydroxide ion as a Lewis add to form the soluble Lewis adduct Zn(OH)4 Although the copper ion will react with hydroxide ion to form insoluble Cu(OH)2, this hydroxide does not ad as a Lewis acid and will not dissolve in excess hydroxide ion. 

Here is a tiny technical point that I really do need to introduce. Chemists now refer to a proton-accepting molecule and ion as a base . Thus, OH is a base. They keep the term alkali for bases dissolved in water. So, for instance, sodium hydroxide, NaOH, dissolves in water, separating into Na ions and OH ions. It is therefore a source of the base OH and the solution is an alkali. I shall use the term base from now on, because it is more general than alkali (a molecule or ion doesn t need to be present in water to be a base). 

The instability of HOOF is clearly related to its unique structure with three highly electronegative atoms strung together with single bonds. Shown below is an E2 elimination with a hydroxide ion as the base that leads to O2, a product of the reaction. 

In the final step of the mechanism, water (not hydroxide) functions as a base and abstracts a proton from the oxonium ion. Like all proton transfer steps, this process requires two curved arrows. One curved arrow is drawn with its tail on a lone pair of water and its head on the proton, while the second curved arrow is drawn with its tail on the O—H bond and its head on the oxygen atom ... 

In 1884, Svante Arrhenius observed that all substances called acids contain hydrogen ions, H. Bases, on the other hand, always contained hydroxide ions, OH . An acid was thus identified as a substance whose water solution contains more hydrogen ions than hydroxide ions, and a base is a substance whose water solution contains more hydroxide ions than hydrogen ions. According to the Arrhenius theory of acids and bases, the properties of an acid are the properties of the hydrogen ion, and the properties of a base are the properties of the hydroxide ion. 

We have already encountered several base-catalyzed reactions, such as the interconversion of keto and enol tautomers (Section 18.3), the Claisen condensation (Section 18.13), and the enediol rearrangement (Section 21.5). A base catalyst increases the rate of a reaction by removing a proton from the reactant. For example, the dehydration of a hydrate in the presence of hydroxide ion is a base-catalyzed reaction. Hydroxide ion (the base) increases the rate of the reaction by removing a proton from the neutral hydrate. 

The souree of hydroxide ion is a base (alkali), such as lime (Ca(OH)2), sodium hydroxide (NaOH), or sodium carbonate (Na2C03). Most metal ions tend to produce basie salt preeipitates, such as basic copper(ll) sulfate, CuS04 3Cu(0H)2, which is formed as a solid when hydroxide is added to a solution containing Cu and S04 ions. The solubilities of many heavy metal hydroxides reach a minimum value, often at a pH in the range of 9-11, then increase with increasing pH values due to the formation of soluble hydroxo complexes, as illustrated by the following reaction ... 

By definition, hydroxide ion is called a specific base and, hence, SB catalysis involves hydroxide ion as a catalyst. SB-catalyzed reactions must be only inter-molecular, because in the intramolecular catalytic reaction, the catalyst (hydroxide ion) is required to be covalently or electrovalently attached with the substrate, and under such circumstances, substrate-bound O or OH group becomes the GB catalyst. There are two possible modes of SB catalysis (1) hydroxide ion acts as a base in an acid-base reaction of overall catalytic process as shown in Scheme 2.28, and (2) hydroxide ion acts as a nucleophile in the overall catalytic process as shown in Scheme 2.29. It is evident from Scheme 2.28 that Y must be a stronger base than HO" so that hydroxide ion could not be consumed during the... 

Since amine catalysis in the reactions of 2,4-dinitrochlorobenzene with amines can be demonstrated, it is logical to anticipate the possibility of other bases, such as hydroxide ion or acetate ion, functioning as catalysts. Hydroxide ion as a catalyst has been the subject of two studies. In the first, Bunnett and Pruitt claimed that the reaction of 2,4-dinitrochlorobenzene and piperidine in 50% dioxane--50% water was not catalyzed either by piperidine or by hydroxide... 

Hydroxide functions as a base and deprotonates the a position, giving a resonance-stabilized enolate (only the more significant resonance structure is drawn below) that is protonated to give an enol. The enol is then deprotonated to give another enolate ion, which is then protonated to give the product. 

Aqueous ammonia also acts as a base precipitating metallic hydroxides from solutions of their salts, and in forming complex ions in the presence of excess ammonia. For example, using copper sulfate solution, cupric hydroxide, which is at first precipitated, redissolves in excess ammonia because of the formation of the complex tetramminecopper(TT) ion. 

In reaction (18), one of the unshared pairs of electrons on the oxygen atom of the MOH group has accepted a proton, and so MOH can act as a base without actually releasing hydroxide ion. 

The difference between basicity and nucleophilicity is a difference of function. In other words, the hydroxide ion can function in two ways as a base (which means it is pulling off a proton and then running away with that proton) or as a nucleophile (latching onto a compound). In some cases, the hydroxide ion might function mostly as a base while in other situations, the hydroxide ion might function mostly as a nucleophile. To understand mechanisms weU, it is important to be able to distinguish between the two roles. Let s see an example. 

EXERCISE 8.25 Below you will find the first two steps of a mechanism. In each step, determine whether the hydroxide ion is functioning as a nucleophile or as a base ... 

Answer In the first step, the hydroxide ion is removing a proton, so it is functioning as a base. In the second step, it is attacking the C=0 and latching on to the compound, so it is functioiung as a nucleophile. 

The ability of water to ionize, while shght, is of central importance for life. Since water can act both as an acid and as a base, its ionization may be represented as an intermolecular proton transfer that forms a hydronium ion (HjO ) and a hydroxide ion (OH ) ... 

In Figure the hydronium ion acts as an acid because it donates a proton to a base. The hydroxide anion acts as a base because it accepts a proton from an acid. When a hydronium ion with charge +1 transfers a proton to a hydroxide ion with charge -1, the two resulting water molecules have zero charges. The pair of charges becomes a neutral pair. A proton transfer reaction such as this one, in which water is one product and a pair of charges has been neutralized, is called a neutralization reaction. 

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