Addition-elimination reactions second step

This is an example of the second step of an addition-elimination reaction mechanism converting an ester (methyl acetate) to an amide (/V-mcthylacetamide). Arrow pushing is illustrated below ... 

Comparison of SN2 and acyl addition-elimination reactions with methoxide as the leaving group. In the concerted SN2, methoxide leaves in a slightly endothermic step, and the bond to methoxide is largely broken in the transition state. In the acyl substitution, methoxide leaves in an exothermic second step with a reactant-like transition state The bond to methoxide has just begun to break in the transition state. 

The first step in a step reaction mechanism is the formation of esters or amides from the diols and diacids or diacids and diamines, respectively. From these intermediates, the polymerization reaction (second step) proceeds. Because the first step is a faster reaction than the second, the monomer is used up quickly. During the two steps of the reaction, small molecules such as H2O or CH3OH are eliminated. Water is the most frequent byproduct molecule, for example from the reaction between a diacid and a dialcohol. Unlike addition polymers, condensation polymers, because they incorporate functional groups, generally have noncarbon atoms (heteroatoms) as part of the main backbone chain. Examples are shown in Fig. 3.30. 

This reaction is a nucleophilic acyl substitution reaction because a nucleophile (Z ) has replaced the substituent (Y ) that was attached to the acyl group in the reactant. It is also called an acyl transfer reaction because an acyl group has been transferred from one group to another. Most chemists, however, prefer to call it a nucleophilic addition-elimination reaction to emphasize the two-step nature of the reaction a nucleophile adds to the carbonyl carbon in the first step, and a group is eliminated in the second step. 

A weak base attached to the acyl group also makes the second step of the addition-elimination reaction easier, because weak bases are easier to eliminate when the tetrahedral intermediate collapses. 

Addition-elimination reactions may also benefit from acid catalysis. The acid functions in two ways First, it protonates the carbonyl oxygen (Step 1), activating the carbonyl group toward nucleophilic attack (Section 17-5). Second, protonation of L (Step 2) makes it a better leaving group (recall Section 6-7 and 9-2). 

The scheme just shown omits the respective elimination steps of the two nucleophilic addition-elimination reactions. Show how the enzyme aids in accelerating them as well. [Hint Draw the result of the electron pushing depicted in the first (or second) picture of the scheme and think about how a reversed electron and proton flow might help.]... 

The rate-determining step is usually nucleophilic attack at the carbonyl carbon atom to form a tetrahedral intermediate. The loss of the leaving group occurs in a second, faster step. The overall process is an addition-elimination reaction. 

Aldehydes and ketones undergo reversible addition reactions with alcohols. The product of addition of one mole of alcohol to an aldehyde or ketone is referred to as a hemiacetal or hemiketal, respectively. Dehydration followed by addition of a second molecule of alcohol gives an acetal or ketal. This second phase of the process can be catalyzed only by acids, since a necessary step is elimination of hydroxide (as water) from the tetrahedral intermediate. There is no low-energy mechanism for base assistance of this... 

Hydrolysis of diphenyl phosphorochloridate (DPPC) in 2.0 M aqueous sodium carbonate is also believed to be a two-phase process. DPPC is quite insoluble in water and forms an insoluble second phase at the concentration employed (i.e. 0.10 M). It seems highly significant that the hydrophobic silicon-substituted pyridine 1-oxides (4,6,7) are much more effective catalysts than hydrophilic 8 and 9. In fact, 4 is clearly the most effective catalyst we have examined for this reaction (ti/2 < 10 min). Since derivatives of phosphoric acids are known to undergo substitution reactions via nucleophilic addition-elimination sequences 1201 (Equation 5), we believe that the initial step in hydrolysis of DPPC occurs in the organic phase. Moreover, the... 

It seems that no general mechanistic description fits all these experiments. Some of the reactions proceed via an addition-elimination mechanism, while in others the primary step is electron transfer from the arene with formation of a radical cation. This second mechanism is then very similar to the electrochemical anodic substitution/addition sequence. 

The first step in the cycle, analogous to the cross-coupling reactions, is the oxidative addition of an aryl (vinyl) halide or sulfonate onto the low oxidation state metal, usually palladium(O). The second step is the coordination of the olefin followed by its insertion into the palladium-carbon bond (carbopalladation). In most cases palladium is preferentially attached to the sterically less hindered end of the carbon-carbon double bond. The product is released from the palladium in a / -hydrogen elimination and the active form of the catalyst is regenerated by the loss of HX in a reductive elimination step. To facilitate the process an equivalent amount of base is usually added to the reaction mixture. 

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