For example, enolate ions are common carbon nucleophiles in biochemical reactions, while the cyanide ion ( C N −) is an example of a carbon nucleophile commonly used in the laboratory (Figure 7.5).įigure 7.5 Carbon Can Act as a Nucleophile Under Certain Circumstances. Under certain circumstances carbon atoms can also act as nucleophiles. In both laboratory and biological organic chemistry, the most common nucleophilic atoms are oxygen, nitrogen, and sulfur, and the most common nucleophilic compounds and functional groups are water/hydroxide ion, alcohols, phenols, amines, thiols, and sometimes carboxylates. The upper and lower diagrams show faster reacting and slower reacting nucleophiles, respectively. When we evaluate nucleophilicity, we are thinking in terms of kinetics – how fast does the nucleophile react with a reference electrophile?įigure 7.4 Nucleophiles. The upper diagram and lower diagram show the equilibrium positions of strong and weak bases, respectively.Ī nucleophile shares its lone pair of electrons with an electrophile – an electron-poor atom other than a hydrogen, usually a carbon. Remember that when we evaluate basicity – the strength of a base – we speak in terms of thermodynamics: where does equilibrium lie in a reference acid-base reaction?įigure 7.3 Brønsted-Lowry Bases. What, then, is the difference between a base and a nucleophile?Ī Brønsted-Lowry base uses a lone pair of electrons to form a new bond with an acidic proton. The same, however, can be said about a base: in fact, bases can act as nucleophiles, and nucleophiles can act as bases. In heterolytic cleavage both shared electrons are retained by one of the atoms from the shared bond, whereas in homolytic cleavage one electron is retained by each atom of a shared bond forming radical intermediates.Ī nucleophile is an atom or functional group with a pair of electrons (usually a non-bonding, or lone pair) that can be shared. This process is also known as homolytic fission or radical fission.įigure 7.2 Examples of Heterolytic and Homolytic Bond Cleavage. In homolytic cleavage, or homolysis, the two electrons in a cleaved covalent bond are divided equally between the products. This process is also known as ionic fission. Thus, a fragment gains an electron, having both bonding electrons, while the other fragment loses an electron. In heterolytic cleavage, or heterolysis, the bond breaks in such a fashion that the originally-shared pair of electrons remain with one of the fragments.

In general, there are two classifications for bond cleavage: homolyticand heterolytic, depending on the nature of the process (Figure 7.2). This can be generally referred to as dissociation when a molecule is cleaved into two or more fragments. Enzymes have high substrate specificity, and can even show regiospecificity that leads to the generation of stereospecific products.įigure 7.1 Effect of an enzyme on reducint the activation energy required to start a reaction where (a) is uncatalyzed and (b) is an enzyme-catalyzed reaction.īond cleavage, or scission, is the splitting of chemical bonds. Enzymes, like other catalysts, are also not used up during the reaction. Note that enzymes do NOT alter the ΔG of the reaction and do NOT have any affect on the spontaneity or equilibrium position of the reaction. They facilitate the formation of the transition state species within the reaction and speed up the rate of the reaction by a million-fold in comparison to non catalyzed reactions. Recall from Chapter 6, that enzymes are biological catalysts that reduce the activation energy required for a reaction to proceed in the forward direction (Figure 7.1). In this section, we will review some fundamentals of organic and biological chemistry that are helpful in understanding enzyme reaction mechanisms. Chapter 7: Catalytic Mechanisms of Enzymes 7.1 Concept Review for Enzyme Reactions

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