Conjugate bases reactions

This reaction type is called the ElcB mechanism, which stands for unimolecular elimination conjugate base reaction, because the conjugate base of the starting material is being formed as the reactive intermediate. It is sometimes called the carbanion mechanism. As this mechanism results from the removal of a proton, it is not surprising that it is favoured by those substrates that possess an acidic hydrogen atom. Thus, would you expect the ElcB mechanism to be more prevalent in reactions that result in a carbon/carbon double bond or in reactions that result in a carbon/carbon triple bond ... 

The ElcB (elimination, unimolecular, conjugate base) reaction... 

The overall rate of the reaction may be considered as made up of a first order solvation reaction plus a second order base solvolysis reaction. Precise rate constants are difficult to obtain for reactions of cobalt(IIl) complexes in ammonia, as has been noted by other investigators (2, 3). Addition of amide ion yields measurable quantities of the conjugate base complex of the product, which is then converted to the amine (or ammine depending on the complex) by the addition of ammonium ion. This observation along with the retardation of the reaction by ammonium ion suggests the importance of conjugate base reactions in liquid ammonia. 

Rate data are also reported for the base hydrolysis of three stereoisomers of the [Co(dien)(tmd)Cl] + ion and one isomer of [Co(dpt)(tmd)CI]2+ [structures (12)— (15)]. The results are compared with previous data in Table 24. Complexes with structures (13) and (15) in which the triamine is in a meridional position react 10 — 10 times faster than those in which the triamine is facial. Deprotonation of the secondary amine (NH) groups is indicated for these conjugate-base reactions. 

In Chapter 10 we looked at El reactions (two-step elimination reactions that form a car-bocation intermediate) and E2 reactions (concerted elimination reactions). The preceding base-catalyzed dehydration represents the third kind of elimination reaction— namely, an ElcB (elimination unimolecular conjugate base) reaction, a two-step elimination reaction that forms a carbanion intermediate. ElcB reactions occur only if the carbanion can be stabilized by electron delocalization. 

Product acids and bases such as those formed in this process are termed conjugate acids and conjugate bases. Thus, all acid-base reactions can be written as... 

The electrophilic character of boron is again evident when we consider the oxida tion of organoboranes In the oxidation phase of the hydroboration-oxidation sequence as presented m Figure 6 11 the conjugate base of hydrogen peroxide attacks boron Hydroperoxide ion is formed m an acid-base reaction m step 1 and attacks boron m step 2 The empty 2p orbital of boron makes it electrophilic and permits nucleophilic reagents such as HOO to add to it... 

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... 

In most biochemical reactions the pH of the medium is close to 7 At this pH car boxylic acids are nearly completely converted to their conjugate bases Thus it is common practice m biological chemistry to specify the derived carboxylate anion rather than the carboxylic acid itself For example we say that glycolysis leads to lactate by way of pyruvate... 

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

When an acid and a base react, the products are a new acid and base. For example, the acetate ion, C1T3COO-, in reaction 6.7 is a base that reacts with the acidic ammonium ion, N1T45", to produce acetic acid and ammonia. We call the acetate ion the conjugate base of acetic acid, and the ammonium ion is the conjugate acid of ammonia. 

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). 

Discernible associative character is operative for divalent 3t5 ions through manganese and the trivalent ions through iron, as is evident from the volumes of activation in Table 4. However, deprotonation of a water molecule enhances the reaction rates by utilising a conjugate base 7T- donation dissociative pathway. As can be seen from Table 4, there is a change in sign of the volume of activation AH. Four-coordinate square-planar molecules also show associative behavior in their reactions. 

The tautomeric 2-hydroxypyrroles undergo base-catalyzed reactions probably through deprotonation to the ambident conjugate base (Scheme 71). 2-Hydroxyfurans (e.g. 178) similarly exist in ketonic forms (179) and (180), the most favoured tautomer being the conjugated 2(5//)-furanone (180)... 

Specific acid catalysis is observed when a reaction proceeds through a protonated intermediate that is in equilibrium with its conjugate base. Because the position of this equilibrium is a function of the concentration of solvated protons, only a single acid-dependent term appears in the kinetic expression. For example, in a two-step reaction involving rate-determining reaction of one reagent with the conjugate acid of a second, the kinetic expression will be as follows ... 

Convenient syntheses of vinyl fluorides are of synthetic interest, fhe conjugate base of fluoromethyl phenyl sulfone reacts with carbonyl compounds to provide P-tluoro alcohols, which are used to prepare terminal vinyl fluorides [25] (equation 23) (Table 9) This reaction offers an alternative to the Winig reaction, which may be very sensitive to reaction conditions. 

Step 3 is new to us. It is an acid-base reaction in which the carbocation acts as a Br0n-sted acid, transferiing a proton to a Brpnsted base (water). This is the property of carbocations that is of the most significance to elimination reactions. Car bocations ar e strong acids they are the conjugate acids of alkenes and readily lose a proton to form alkenes. Even weak bases such as water are sufficiently basic to abstract a proton from a carbocation. 

Citing amine basicity according to the pK of the conjugate acid permits acid-base reactions involving amines to be analyzed according to the usual Brpnsted relationships. For exarnple, we see that amines are converted to anmoniurn ions by acids even as weak as acetic acid ... 

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