Esters relative stability

Similarly, carboxylic acid and ester groups tend to direct chlorination to the / and v positions, because attack at the a position is electronically disfavored. The polar effect is attributed to the fact that the chlorine atom is an electrophilic species, and the relatively electron-poor carbon atom adjacent to an electron-withdrawing group is avoided. The effect of an electron-withdrawing substituent is to decrease the electron density at the potential radical site. Because the chlorine atom is highly reactive, the reaction would be expected to have a very early transition state, and this electrostatic effect predominates over the stabilizing substituent effect on the intermediate. The substituent effect dominates the kinetic selectivity of the reaction, and the relative stability of the radical intermediate has relatively little influence. 

There are very few examples of naturally occurring, fully unsaturated azepines.100 Surprisingly, in view of the relative stabilities of the azepine tautomers (vide supra), the 2/7-azepine, chal-ciporone (24) and the related propanoate ester, norchalciporyl propanoate (25), are the pungent components of the peppery mushroom, Chalciporus piperatus,40... 

The most widely used reagent for partial reduction of esters and lactones at the present time is diisobutylaluminum hydride (DiBAlH).83 By use of a controlled amount of the reagent at low temperature, partial reduction can be reliably achieved. The selectivity results from the relative stability of the hemiacetal intermediate that is formed. The aldehyde is not liberated until the hydrolytic workup and is therefore not... 

Both Z and E isomeric enol lactones undergo photoisomerization to yield mixtures of isomers (5,14,87) in which the thermodynamically more stable one prevails. It is the Z form in hydrastine series (5) and the E isomer in the more hindered narcotine series (87). Relative stabilities of isomeric enol lactones (98 versus 99 and 101 versus 102) were determined by comparing their rates of methanolysis. Keto esters of type 126 were formed (87). It turned out that both ( )-N-methylhydrastine enol lactone (99) and (Z)-narceine enol lactone (101) solvolyzed faster than their geometric partners. 

The instability of these chiral monolayers may be a reflection of the relative stabilities of their bulk crystalline forms. When deposited on a clean water surface at 25°C, neither the racemic nor enantiomeric crystals of the tryptophan, tyrosine, or alanine methyl ester surfactants generate a detectable surface pressure, indicating that the most energetically favorable situation for the interfacial/crystal system is one in which the internal energy of the bulk crystal is lower than that of the film at the air-water interface. Only the racemic form of JV-stearoylserine methyl ester has a detectable equilibrium spreading pressure (2.6 0.3dyncm 1). Conversely, neither of its enantiomeric forms will spread spontaneously from the crystal at this temperature. 

In the latter bonding situation the overall rate enhancement over that for the noncoordinated ester will depend on the relative stabilities, and acidities, of tetrahedral intermediates, provided these are formed. There have been no detailed mechanistic studies on systems incorporating metals at a distance (23, 24 Scheme 21) but [(NH3)5Co(GlyOEt)]3+ undergoes hydrolysis some 102 times faster than free GlyOEt (44), and the effectiveness of the Co(III) center in this case is similar to that of protonation (see Table III). In other situations Co(III) centers are somewhat less effective than H+, but the sig-... 

The similarity in the rate laws does not allow a clear choice to be made between mechanisms, but Mechanism A is required in H20 by the observation of general base catalysis. However, the relative stability of the (red) T° intermediate in Me2SO (this is dependent on the nature of the AA side chain, cf. Section III,C) in the absence of proton-ated amine makes us prefer Mechanism B for reaction in this solvent, since the solvent is unable to assist the departure of MeOH. The similar catalytic rate constants found for B = imidazole, Af-methylimidazole (26) suggest that transfer of the proton from T+ to the alcohol function remains stepwise (i.e., via T°) since N-methylimidazole cannot carry out a concerted transfer. Such general acid-catalyzed loss of MeOH from T° supports a suggestion made many years ago by Burnett and Davies relating to purely organic esters (62). 

There remains, of course, the question why apparently isobutene (and perhaps other aliphatic olefins) do not polymerise by the pseudo-cationic mechanism - or do so much less readily than, say, styrene. Probably the short answer lies in the relative stabilities of the esters in the polymerisation conditions, (e.g., perchlorate stabilised by co-ordination of styrene). The long answer will have to be based on a detailed understanding of all the factors which determine this stability and thus govern the equilibrium between ester and ions. 

Ethanolamine is used to displace the adduct from the borinate ester. The facility with which the transfer of acetylenic groups occurs is associated with the relative stability of the. v/ -hybridizcd carbon. This reaction is an alternative to the more common addition of magnesium or lithium salts of acetylides to aldehydes. 

The relative stability of the oxiranes to bases at room temperature allows the selective ammonolysis of carboxylic ester groups with ammonia in methanol,49 and, conversely, free hydroxyl groups may be esterified with acid chlorides or anhydrides in pyridine without affecting the oxirane ring. Hydroxyl groups may also be converted into alkyl ethers by using either the Haworth50 or the Purdie-Irvine procedure.5152... 

O2N.dcH2.CH2.C00Na, yel crysts(from MeOH), mp 97°(dec). Later, Klager(Ref 2) patented the prepn of the Na salts of methyl, ethyl butyl esters of 4,4-dinitrobutyric acid. These and other Na salts of poly nitro compds or nitrated new aliphatic compds were proposed as expl ingredients because of their uniformity relative stability Refs l)Beil-not found 2)K.Klager, JOC 16, 162-3(1951) CA 45, 6576(1951) 3)K.Klager,... 

Let us mention that there are still other relations between conformation and dipole moments. Relative stability of conformational isomers was often brought in connection with their dipole moments. For instance, the preferred sp conformation of esters (3a), or of carboxylic acids, was often explained by the lower dipole moment6 compared to the conformation 3b, or in other words by the electrostatic repulsion7-9 of partial dipoles in 3b. Reasoning of this kind is usually based on anticipated rather than measured moments that of 3b cannot be directly measured. Interaction of more distant dipoles is commonly neglected, viz both its effect on the conformation and on the total /z (see, for example, Klinot and coworkers10). This is in fact the fundamental assumption underlying all analyses of dipole moments. 

In the ester function, it was proposed that two types of electronic effects referred to as the primary and the secondary electronic effects (1-3), are present. The role and the relative importance of these two types of electronic effects can be understood by analyzing them in relation to the geometry and the relative stability of the two forms, Z and E, of the ester function. 

There is no direct experimental evidence to show the importance of the secondary electronic effects in the ester function except for the relative stability of the Z over the form. However, the relative stability of the different forms of dialkoxycarbonium ions can be explained by considering these electronic effects. Since dialkoxycarbonium ions are alkylated derivatives of esters, the result can be used as evidence to support the importance of the secondary electronic effects in the ester function. It is known from X-ray evidence and supported by calculations (8, 9) that dialkoxycarbonium ions like esters are planar and that they can exist theoretically in three different forms, the ZZ ( 0), the EZ (VO, and the EE (12) forms. The two oxygens of 10 each have one non-bonded electron pair antiperiplanar to a polar C-0 bond, 0 has one, whereas 2 has none. Thus, 10, 11, and 12 have respectively two, one and zero secondary electronic effects, and on that basis, their relative stability should follow in this order. In the ZZ form ( ] 0), there is a severe steric repulsion between the two R groups thus, with the exception of cases where the two R groups are part of a ring, this form must be eliminated. The EZ form (VO must therefore represent the most stable form of dialkoxycarbonium ions. 

The reactions so far discussed in this section have been characterized by stabilization of the transition state on transfer from protic to dipolar aprotic media. In principle the opposite situation can also obtain, when the transition state is relatively stabilized by the protic medium. A good illustration of this effect is provided by alkaline ester hydrolysis, a reaction which has been the focus of a great deal of attention over the years (Euranto, 1969 Ingold, 1969). 

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