Nucleophiles Lewis base

The ionization mechanism for nucleophilic substitution proceeds by rate-determining heterolytic dissociation of the reactant to a tricoordinate carbocation (also sometimes referred to as a carbonium ion or carbenium ion f and the leaving group. This dissociation is followed by rapid combination of the highly electrophilic carbocation with a Lewis base (nucleophile) present in the medium. A two-dimensional potential energy diagram representing this process for a neutral reactant and anionic nucleophile is shown in Fig. 

Our analysis of literature data will focus on two closely related questions about the influence of changes in the relative thermodynamic driving force and Marcus intrinsic barrier for the reaction of simple carbocations with Bronsted bases (alkene formation) and Lewis bases (nucleophile addition) on the values of ks/kp determined by experiment. 

The thermodynamic tendency of a substance to act as a Lewis base. The strength of a Lewis base depends on the nature of the acid with which the Lewis base forms a Lewis adduct. Hence, comparative measures of Lewis basicities are given by equilibrium constants for the formation of the adducts by a common reference acid. See Lewis Base Nucleophilicity Hard Bases Soft Bases Donor Number... 

Electron-pair donor (or Lewis base), NUCLEOPHILE ELECTRON SINK ELECTRON SPIN RESONANCE ELECTRON TRANSEER MARCUS EQUATION ELECTRODE KINETICS Electron transfer mechanism,... 

MEISENHEIMER ADDUCT LEWIS BASICITY DONOR NUMBER HARD BASES LEWIS BASE NUCLEOPHILICITY SOFT BASES L HOPITAL S RULE L Hospital s rule L HOPITAL S RULE... 

Proposed mechanisms for polycondensations are essentially the same as those proposed in the organic chemistry of smaller molecules. Here, we will briefly consider several examples to illustrate this similarity between reaction mechanisms for small molecules and those forming polymers. For instance, the synthesis of polyamides (nylons) is envisioned as a simple Sn2 type Lewis acid-base reaction, with the Lewis base nucleophilic amine attacking the electron-poor, electrophilic carbonyl site followed by loss of a proton. 

A Lewis acid (electrophile) shares an electron pair furnished by a Lewis base (nucleophile) to form a covalent (coordinate) bond. The Lewis concept is especially useful in explaining the acidity of an aprotic acid (no available proton), such as BFj. 

Label the reactants in these acid-base reactions as Lewis acids (electrophiles) or Lewis bases (nucleophiles). Use curved arrows to show the movement of electron pairs in the reactions. 

What kind of reactions do alkyl halides undergo The characteristic reactions of alkyl halides are substitution and elimination. Because alkyl halides contain an electrophilic carbon, they react with electron-rich reagents— Lewis bases (nucleophiles) and Brpnsted—Lowry bases. 

Again, the reaction sites are multiple bonds (double and triple bonds) polar bonds and Lewis acids (electrophiles) and Lewis bases (nucleophiles). 

Electrophiles (b), (c), (e), and (0. All four species lack filled outer shells they are all Lewis acids capable of chemical interaction with electron-rich species such as Lewis bases (nucleophiles). 

In this section, we will discuss the two major substitution reactions — S l and S 2 reactions. In nucleophilic substitution reactions involving alkyl halides as the substrate, a Lewis base (nucleophile) substitutes the halogen present in the alkyl halide. We will discuss nucleophilic reactions in which alkyl halides react with nucleophiles. A general representation can be done as follows ... 

Nucleophile An atom, ion, or molecule that has an electron pair that may be donated in bonding to an electrophile or Lewis add all nucleophiles are Lewis bases. Nucleophilicity, sometimes referred to as nucleophile strength, refers to a substance s nucleophilic character and is often used to compare the affinity of atoms. 

Use the curved-arrow notation to write the reaction that would take place between dimethylamine (CH3)2NH and boron trifluoride. Identify the Lewis acid, Lewis base, nucleophile, and electrophile and assign appropriate formal charges. 

Based on Pracejus s previous work with cinchona alkaloids, Bergson and Langstrom developed the Michael addition of p-ketoesters to acrolein catalyzed by 2-(hydroxymethyl)quinuclidine.Soon after, Wynberg developed severalorganocatalytic reactions using cinchona alkaloids as chiral Lewis base/nucleophilic catalysts [14]. 

FIGURE 2.27. Representative Lewis base (nucleophilic) organocatalysts. 

Just as with the pK and pKb scales for Bronsted acids and bases, other scales have been developed to relate the strength of one Lewis acid (electrophile) to another, or one Lewis base (nucleophile) to another. For example, scales based upon the relative reactivities of various nucleophiles and electrophiles in common organic reactions, such as Sn2 transformations, have been defined. We will cover these scales in Chapter 8 after we examine linear free energy relationships. Further, after examining the scales, we will discuss what factors make for a good nucleophile and electrophile. 

Hard, Moderate, and Soft Lewis Bases (Nucleophiles) and Lewis Acids (Electrophiles)... 

PROBLEM 1.31 Identify the Lewis base (nucleophile) and Lewis acid (electrophile) in each of these reactions. Write the molecule formed in each case. 

ANSWER In the second step of the reaction, chloride ion acts as the Lewis base (nucleophile) and reacts through one of the filled orbitals (the HOMO) containing the nonbonding pairs of electrons. The LUMO (electrophile) is easy to find it must be the empty 2p orbital of the carbocation. 

FIGURE 18.45 The acid-induced hydrolysis of a nitrile to a carboxylic acid. The nitrile is the Lewis base (nucleophile) in the first step. 

Step 2 This step is the combination of a cation (Lewis acid, electrophile) with an anion (Lewis base, nucleophile) and occurs rapidly. 

---