Bases, weak, protonation

Organolithium compounds are sometimes prepared in hydrocarbon solvents such as pentane and hexane, but nonnally diethyl ether is used. It is especially important that the solvent be anhydrous. Even trace amounts of water or alcohols react with lithium to form insoluble lithium hydroxide or lithium alkoxides that coat the surface of the metal and prevent it from reacting with the alkyl halide. Furthennore, organolithium reagents are strong bases and react rapidly with even weak proton sources to fonn hydrocarbons. We shall discuss this property of organolithium reagents in Section 14.5. 

The shapes of the titration curves of weak electrolytes are identical, as Figure 2.13 reveals. Note, however, that the midpoints of the different curves vary in a way that characterizes the particular electrolytes. The pV, for acetic acid is 4.76, the pV, for imidazole is 6.99, and that for ammonium is 9.25. These pV, values are directly related to the dissociation constants of these substances, or, viewed the other way, to the relative affinities of the conjugate bases for protons. NH3 has a high affinity for protons compared to Ac NH4 is a poor acid compared to HAc. 

Autocatalysis may arise when the nucleophilic atom of the reagent is bound to a hydrogen atom which is eventually eliminated during the reaction. This occurs with neutral reagents such as primary or secondary amines, thiols, and alcohols. If the displaced group (usually an anion) is a sufficiently weak base, the proton is effectively transferred to any basic reactant. Hence, the best known examples of autocatalysis involve chloro-A-heteroaromatic compounds as the substrates. 

Acids are proton donors and bases are proton acceptors. Strong acids (eg, HCl or H2SO4) completely dissociate into anions and cations even in strongly acidic solutions (low pH). Weak acids dissociate only partially in acidic solutions. Similarly, strong bases (eg, KOH or NaOH)—but not weak bases (eg, CalOHJj)—are completely dissociated at high pH. Many biochemicals are weak acids. Exceptions include phosphorylated in-... 

The same molecular reasoning shows that a buffer solution can absorb added hydronium ions. Consider what happens when some hydronium ions are added to the acetic acid-acetate buffer solution described in Example. The hydronium ion is a strong acid, and the acetate anion is a weak base, so proton transfer from CH3 CO2 to H3 goes essentially to completion ... 

As long as the buffer solution contains a weak base as a major species, a small amount of hydronium ion added to the solution will be neutralized completely. Figure 18-2 shows two hydronium ions added to a portion of a buffer solution. When a hydronium ion collides with a weak base ion, proton transfer occurs, forming a water molecule and the conjugate acid of the weak base. Example examines another buffer solution at the molecular level. 

As described in Chapter 4, acid-base reactions that go to completion can be exploited in chemical analysis using the method of titration. Titrations can be understood in greater detail from the perspective of acid-base equilibria. Protonation of a weak base by a strong acid is a reaction that goes virtually to completion because of its large... 

Dissolution is based on protonation Compounds of weak acids in strong acids, or formation of volatile acids... 

If an amphiprotic solvent contains an acid and base that are neither mutually conjugate nor are conjugated with the solvent, a protolytic reaction occurs between these dissolved components. Four possible situations can arise. If both the acid and base are strong, then neutralization occurs between the lyonium ions and the lyate ions. If the acid is weak and the base strong, the acid reacts with the lyate ions produced by the strong base. The opposite case is analogous. A weak acid and a weak base exchange protons ... 

When sodium acetate is dissolved in water the acetate ion behaves as a base removing protons from solution. For a weak electrolyte in water KbxKa = Kw. If a 0.1 M solution of sodium acetate in water is considered ... 

Competition of water with weak acids and bases for proton acceptance and donation. 

Acetic acid is a very weak proton acceptor and thus does not compete effectively with weak bases for protons. Only very strong acids will protonate acetic acid appreciably according to the equation shown below ... 

A 1,3-dithiane is a weak proton acid (pKa = 32) which can be deprotonated by strong bases such as n-butyllithium. The resulting carbanion is stabilized by the electron withdrawing effect of the two sulphur atoms. 

To fully characterize and categorize the solute selectivities of GC stationary phases, Rohrschneider and McRe5molds pioneered one of the earliest characterization methods [5,6]. The Rohrschneider-McReynolds system is the oldest and widely accepted stationary phase classification systems that is based on the retention of five probe molecules namely, benzene, bufanol, 2-penfanone, nifropropane, and pyridine. Each probe molecule is used to represenf a disfincf or a combination of interactions with the stationary phase. Benzene measures dispersive interactions with weak proton acceptor properties butanol measures dipolar interactions with both proton donor and proton acceptor capabilities 2-pentanone measures dipolar interactions with proton acceptor but not proton donor capabilities nitropropane measures weak dipolar interactions and pyridine measures weak dipolar interactions with strong proton acceptor but not proton donor capabilities. 

Drugs cross biological membranes most readily in the unionised state. The unionised drug is 1000-10000 times more lipid-soluble than the ionised form and thus is able to penetrate the cell membrane more easily. Chemical compounds in solution are acids, bases or neutral. The Bronsted-Lowry definition of an acid is a species that donates protons (H+ ions) while bases are proton acceptors. Strong acids and bases in solution dissociate almost completely into their conjugate base and H+. Weak acids and weak bases do not completely dissociate in solution, and exist in both ionised and unionised states. Most drugs are either weak acids or weak bases. For an acid, dissociation in solution is represented by ... 

The conjugate acid, BH+, of the weak base transfers protons to the OH ions supplied by a strong base. 

An amide ion, a very strong base, removes a weak proton from the carbon that is alpha to the carbon bonded to the bromine. 

Alkanes are rather unreactive compounds and are sometimes called paraffins (from the Latin for Tittle affinity ). They have strong C—C and C—H bonds, which are hard to break. All their electron pairs are in relatively stable sigma bonding MOs. They have no polar sites or unshared electrons to attract Lewis acid or base reagents. They are extremely weak proton acids. Therefore, only a few of the reactions covered in this text involve alkanes. 

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