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Chapter 11

Ch.11 Summary.docx

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University of Toronto St. George
Barb Morra

CHM 247 CH.11 Reactions ofAlkyl Halides: Nucleophilic Substitutions and Eliminations • Carbon-halogen bond in an alkyl halide is polar and the carbon atom is electron-poor. Thus alkyl halides are electrophiles. • Alkyl halides do one of two things when they react with a nucleotide/base, such as hydroxide ion: either they undergo substitution of the X group by the nucleophile, or they undergo elimination of HX to yield an alkene. 11.1 The Discovery of Nucleophilic Substitution Reactions • German chemist Paul Walden found that pure enantiomers of malic acid could be introvconverted through a series of simple substitution reactions. • • Because (-)-malic acid was converted to (+)-malic acid, some reactions in the cycle must have occureed with a change, or inversion, in configuration at the chirality centre. • Nucleophilic substitution reaction: A reaction in which one nucleophile replaces another attached to a saturated carbon. • • • The inversion of stereochemical configuration must therefore take place in step 2, the nucleophilic substitution of tosylate ion by acetate ion. • From this and other similar reactions, workers concluded that the nucleophilic substitution reaction of a primary or secondary alkyl halide or tosylate always proceeds with inversion of configuration. 11.2 The S N Reaction • Kinetics: referring to reaction rates. Kinetic measurements are useful for helping to determine reaction mechanisms. • Direct relationship between the rate at which the reaction occurs and the concentrations of the reactants. • Second-order reaction: A reaction whose rate limiting step is bimolecular and whose kinetics are therefore dependent on the concentration of two reactants. • • S N reaction: A bimolecular nucleophilic substitution reaction. • SN2 is short for substitution, nucleophilic, bimolecular.An essential feature of anNS 2 mechanism is that it takes place in a single step without intermediates when the incoming nucleophile reacts with the alkyl halide or tosylate from a direction opposite the group that is displaced. • 11.3 Characteristics of the S 2 Reaction N • The rate of a chemical reaction is determined by the activation energyΔG‡, the energy difference between reactant ground state and transition state. • Achange in conditions can affect the activation energy either by changing the reactant energy level or by changing the transition-state energy level. • Lowering the reactant energy or raising the transition-state energy increases the activation energy and decreases the reaction rate; raising the reactant energy or decreasing the transition-state energy decreases the activation energy and increases the reaction rate. The Substrate: Steric Effects in the N 2 • The first N 2 reaction variable to look at it the structure of the substrate.A hindered substrate should prevent easy approach of the nucleophile, making bond formation difficult. The transition state for a sterically hindered substrate is higher in energy and forms more slowly than the corresponding transition state for a less hindered substrate. • • Vinylic halides and aryl halides are not shown on this reactivity because they are unreactive towards S 2 displacement. N The Nucleophile • The nature of the nucleophile has a major effect on the S 2Nreaction.Any species that it neutral or negatively charged can act as a nucleophile (as long as it has an unpaired set of electrons and is a lewis base). • If the nucleophile is negatively charged, the product is neutral; if the nucleophile is neutral, the product is positively charged. • Reactivity of a given nucleophile can change from one reaction to another. • Nucleophile strength: Nucleophilicity roughly parallels basicity. Nucleophilicity usually increases going down a column of the periodic table. I > Br > Cl. Negatively charged nucleophiles are usually more reactive than neutral ones. The Leaving Group • Another variable that can affect the S N reaction is the nature of the group displaced by the incoming nucleophile. The leaving group is expelled with a negative charge in most S 2 reactions, so the best leaving groups are those that best stabilize the negative charge N in the transition state. • The greater the extent of charge stabilization by the leaving group, the lower the energy of the transition state and the more rapid the reaction. • Weak bases makes good leaving groups. • - - • Weak bases such as Cl, and Br and tosylate ions make good leaving groups. • Alkyl fluorides, alcohols, ethers, and amines do not typically undergo S 2 Neactions. • Converting an alcohol to a better leaving group is a good leaving group will allow a reaction to occur. • • • Ethers don’t typically undergo S 2Nreactions, but epoxides, due to angle strain, are much more reactive than other ethers. • The Solvent • Rates strongly affected by the solvent. Protic solvents ( -OH or –NH) are generally worst for SN2 reactions while polar aprotic solvents are the best. • Protic solvent: A solvent such as water or alcohol that can act as a proton donor. • Solvation: The clustering of solvent molecules around a solute particle to stabilize it. • Protic solvents slow down the reaction by salvation of the reactant nucleophile. The solvent molecules hydrogen bind the nucleophile and form a cage around it. • Polar aprotic solvents increase the rate of N 2 reactions by raising the ground-state energy of the nucleophile. • ASummary of S 2 RNaction Characteristics • Substrate: Steric hindrance raises the energy of the N 2 transition state, increasing the activation energy and decreasing the reaction rate.As a result, N 2 reactions are best for methyl and primary substrates. Secondary substrates react slowly, and tertiary substrates do not react by an SN2 mechanism. • Nucleophile: Basic, negatively charged nucleophiles are less stable and have a higher ground-state energy than neutral ones, decreasing the activation energy and increasing the S N reaction rate. • Leaving group: Good leaving groups (more stable
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