Reactivity patterns in nucleophilic substitution reactions

Chemical reactions are described on a "family" basis. Protic acids undergo acid-base reactions. Haloalkanes under nucleophilic substitution reactions. And so on.

When different members of a chemical family exhibit different tendencies to undergo a chemical reaction, we seek to understand why one member is ready to react and another is not. If we can find a correlation between molecular structure and reactivity, we say we have found a reactivity pattern.

Reactivity patterns can be based on thermodynamics or on kinetics. A pattern based on thermodynamics shows how the equilibrium constant for reaction (favorability) varies with molecular structure. A pattern based on kinetics shows how the rate constant (speed) varies with structure. Nucleophilic substitution reactions are normally evaluated in terms of their speed so we will use kinetics data to develop our reactivity pattern.

Mechanism

The speed of a chemical reaction depends, in part, on the energy that must be added to the reactants to achieve the rate-limiting transition state. However, the structure of this transition state (and its energy) depend on the reaction mechanism, and nucleophilic substitution reactions occur by two very different mechanisms.

This experiment tests two different reagents: NaI-acetone (Finkelstein reaction) and ethanolic AgNO₃. These reagents react with haloalkane (RX) substrates to give different products. More importantly, they often react by different mechanisms, so it may be possible to use each reagent to establish a reactivity pattern for a particular substitution mechanism.

NaI-acetone. Iodide is considered a strong nucleophile and acetone is considered a (moderately) polar aprotic solvent. When this reagent reacts with a bromo- or chloroalkane (RX), the product will be the iodoalkane (RI) and the mechanism will usually be S_N2. Therefore, the speed of the reaction will depend on the ability of the reactant to form a five-coordinate transition state in which the leaving group (halide ion) and the nucleophile (iodide ion) are both weakly bonded to the electrophilic carbon.

Ethanolic AgNO₃. Ethanol is the strongest nucleophile in this system. Therefore, when this reagent reacts with a bromo- or chloroalkane (RX), the product will be an alkyl ethyl ether (ROEt). Because ethanol is a very weak nucleophile, and a polar protic solvent, the mechanism will usually be S_N1. The speed of this reaction will depend on the ability of the reactant to ionize, i.e., dissociate into a leaving group (halide ion) and a three-coordinate carbocation.

Reaction speed

Reaction speed depends on substrate reactivity, but it is also affected by other factors: concentration, temperature, and medium. Since you want to determine (and explain) some reactivity patterns, you should try to hold the other factors constant from one substrate to the next. You should also try to use the same definition of "speed" for each reaction.

Both of the reagents used in this experiment produce a precipitate should substitution occur. NaBr and NaCl, the products of the Finkelstein reaction, are both insoluble in acetone. Likewise, AgBr and AgCl are both highly insoluble in ethanol. In addition, the time needed to form these ionic compounds from the separated ions is very short compared to the time needed to initiate nucleophilic substitution. Therefore, once bromide or chloride concentration rises to a certain level, a precipitate will form and you should be able to use the "time needed for precipitation" as an indicator of reaction speed.