Organic reaction mechanism acid—base reactions

Our first three chapters established some fundamental principles concerning the structure of organic molecules and introduced the connection between structure and reactivity with a review of acid-base reactions In this chapter we explore structure and reactivity m more detail by developing two concepts functional groups and reaction mechanisms A functional group is the atom or group m a molecule most respon sible for the reaction the compound undergoes under a prescribed set of conditions How the structure of the reactant is transformed to that of the product is what we mean by the reaction mechanism... 

Because the activation energies are small and the stereoelectronic requirements are not difficult to meet, most acid-base reactions are very fast in comparison to other types of organic reactions. Therefore, it is usually not necessary to be concerned with the rates of acid-base reactions. In organic reactions that have mechanisms involving several steps, including an acid-base step, one of the other steps in the mechanism usually controls the rate. 

Many mechanisms in organic chemistry start with an acid/base reaction. This may be just a simple Bronsted-Lowry protonation of a hydroxyl group, which results in the activation of a C-OH bond or it may be a Lewis acid/base reaction as, for example, when aluminium trichloride complexes with a halogenoalkane in the first step of the Friedel-Crafts reaction. In each case, the initial intermediate usually reacts further and leads to the desired product. In inorganic chemistry, the acid/base reaction may be all that is of interest, e.g. the treatment of a carbonate with an acid to liberate carbon dioxide. However, it is unusual in organic chemistry for the acid/base reaction to be an end in itself. It is for this reason that acid/base characteristics are normally considered as a property of the molecule, similar to the nucleophilic and electrophilic properties to which they are closely related, rather than as a fundamental reaction type as is the case in inorganic chemistry. 

The determination of the metal ions affinity for the organic bti.ses is important to elucidate the acid-base reaction mechanism as well as to obtain information about fundamental biochemical processes (i.e. synthesis, replication structural integrity and cleavage of DNA and RNA) [ 108J. 

Common organic reaction mechanisms, such as nucleophilic substitution and general acid-base catalysis, are known to play roles in enzymatic catalysis. 

The recovery of naturally acidic oils by alkaline flooding fits into the phase alteration category. The recovery mechanisms of these floods are varied since the surface active salts, which are formed by the in situ acid-base reaction, can adsorb onto the oil-water interface to promote emulsification or can absorb onto the rock surface to alter wettability. The exact recovery mechanism, recently reviewed by Johnson (3) depends on the pH and salinity of the aqueous phase, acidity of the organic phase and wettability of the rock surface (4,5). In this study an additional alkaline recovery mechanism is explored. This mechanism. Emulsification and Coalescence, depends on the valency of the electrolyte as well as the pH and salinity of the aqueous phase. The Emulsification and Coalescence mechanism for the recovery of acidic oils is similar to the Spontaneous Emulsification mechanism suggested by Schechter et al. (6) for the recovery of nonacidic oils with petroleum sulfonate solutions. 

These four mechanisms concur that alkaline flooding enhances the recovery of acidic oil by two-stage processes. In the first and common stage of these alkaline recovery mechanisms, surface active salts are formed by the in situ acid-base reaction between the alkali contained in the floodwater and the organic acid present in the residual oil. The surfactants can adsorb onto the oil-water interface to lower the interfacial tension and thus promote emul-sificati6n under the action of surface driven forces (spontaneous) and/or under the action of shear driven forces (external and in-... 

In this example, the initial products of nucleophilic substitution bear a positive charge. They readily lose a proton to form neutral products. The overall process with a neutral nucleophile thus has three steps the first two constitute the two-step 8 1 mechanism (loss of the leaving group and attack of the nucleophile), and the third Is a Bransted-Lowry acid-base reaction leading to a neutral organic product. 

Acids and bases Chapter 2 on acids and bases serves two purposes. It gives students experience with curved arrow notation using some familiar proton transfer reactions. It also illustrates how some fundamental concepts in organic structure affect a reaction, in this case an acid-base reaction. Since many mechanisms involve one or more acid-base reactions, 1 emphasize proton transfer reactions early and come back to this topic often throughout the text. 

---