1. Explain why the reagent NaI in acetone encourages SN2 reactions. Provide another nucleophile/solvent combination that encourages SN2. What characteristics will each have?
2. Write the complete mechanism for the reaction of (3S, 4R) 3-bromo-3,4-dimethylhexane with methanol. Will this be an SN1 or SN2 reaction? Use wedge-dash notation to show the products.
For the first part of the first question, is it enough to say that I anion is an excellent nucleophile and acetone has limited ability to stabilize carbocation intermediates? Also, NaI is soluble in acetone. But when the leaving group is either Br or Cl in an SN2 reaction, NaBr or NaCl precipitates as an
insoluble salt?
1. The reagent NaI in acetone promotes SN2 (substitution, nucleophilic bimolecular) reactions for several reasons. Firstly, NaI dissociates in acetone to form sodium cations (Na+) and iodide anions (I-). The iodide anion (I-) is an excellent nucleophile due to its large size and high polarizability. This makes it highly reactive and capable of attacking the carbon center in SN2 reactions.
Secondly, acetone acts as a polar aprotic solvent for the SN2 reaction. It has limited ability to stabilize carbocation intermediates, which is crucial for SN2 reactions that proceed via a concerted mechanism. A polar aprotic solvent like acetone solvates the reactants and helps to facilitate the nucleophilic attack without interfering with the reaction pathway.
Another example of a nucleophile/solvent combination that encourages SN2 reactions is potassium cyanide (KCN) in DMSO (dimethyl sulfoxide). KCN dissociates in DMSO to form potassium cations (K+) and cyanide anions (CN-). The cyanide anion (CN-) is a strong nucleophile that can attack the carbon center in SN2 reactions.
DMSO, similar to acetone, is a polar aprotic solvent that can solvate reactants without hindering the SN2 reaction pathway. This combination of a strong nucleophile and polar aprotic solvent promotes efficient SN2 reactions.
In summary, the characteristics of a nucleophile that encourages SN2 reactions are high reactivity, strong nucleophilicity, and ability to attack the carbon center. The characteristics of a solvent that encourages SN2 reactions are polar aprotic nature and ability to solvate the reactants without interfering with the reaction mechanism.
2. The complete mechanism for the reaction of (3S, 4R) 3-bromo-3,4-dimethylhexane with methanol determines whether it is an SN1 (substitution, nucleophilic unimolecular) or SN2 reaction. Unfortunately, without knowing the specific conditions of the reaction, it is difficult to determine the exact mechanism.
If the reaction is conducted under conditions favoring SN2 reactions (e.g., a strong nucleophile, a polar aprotic solvent, and low temperatures), it is more likely to proceed via an SN2 mechanism. In an SN2 reaction, the nucleophile attacks from the backside of the C-Br bond, leading to an inversion of stereochemistry. The products would be (3S, 4R)-3-methoxy-3,4-dimethylhexane and bromide ion.
However, if the reaction conditions favor SN1 reactions (e.g., a weak nucleophile, a polar protic solvent like water or alcohol, and high temperatures), it is more likely to proceed via an SN1 mechanism. In an SN1 reaction, the C-Br bond breaks first, generating a carbocation intermediate. Then, the nucleophile attacks the carbocation, leading to racemization or a mix of stereochemical products.
To provide a complete mechanism with wedge-dash notation, the specific reaction conditions are needed.