For each pair, predict the stronger nucleophile in the SN2 reaction (using an alcohol as the solvent). Explain your prediction.
a. (CH₃CH₂)₃N or (CH₃CH₂)₂NH
b. (CH₃)₂O or (CH₃)₂S

Verified step by step guidance

1. Step 1: Understand the concept of nucleophilicity in SN2 reactions. Nucleophilicity refers to the ability of a species to donate a pair of electrons to an electrophile. In SN2 reactions, nucleophilicity is influenced by factors such as charge, electronegativity, steric hindrance, and the solvent used.
2. Step 2: Analyze the solvent's role. Alcohol is a protic solvent, meaning it can form hydrogen bonds with nucleophiles. Protic solvents tend to stabilize charged nucleophiles through hydrogen bonding, which can reduce their nucleophilicity. Neutral nucleophiles are less affected by this stabilization.
3. Step 3: Compare the nucleophiles in part (a). (CH3CH2)3N is a tertiary amine, while (CH3CH2)2NH is a secondary amine. Tertiary amines are more sterically hindered, making it harder for them to approach the electrophile in an SN2 reaction. Secondary amines, with less steric hindrance, are typically stronger nucleophiles in SN2 reactions.
4. Step 4: Compare the nucleophiles in part (b). (CH3)2O is an ether, while (CH3)2S is a thioether. Sulfur is less electronegative than oxygen, meaning it holds onto its electrons less tightly and is more polarizable. This makes (CH3)2S a stronger nucleophile than (CH3)2O in an SN2 reaction, especially in a protic solvent like alcohol.
5. Step 5: Summarize the predictions. For part (a), (CH3CH2)2NH is the stronger nucleophile due to reduced steric hindrance. For part (b), (CH3)2S is the stronger nucleophile due to sulfur's lower electronegativity and higher polarizability compared to oxygen.

Key Concepts

Nucleophilicity

Nucleophilicity refers to the ability of a species to donate an electron pair to an electrophile during a chemical reaction. Stronger nucleophiles are typically negatively charged or have lone pairs that can be readily donated. Factors influencing nucleophilicity include charge, electronegativity, and steric hindrance, with less steric hindrance generally leading to stronger nucleophiles.

SN2 Mechanism

The SN2 mechanism is a type of nucleophilic substitution reaction where the nucleophile attacks the electrophile from the opposite side of the leaving group, resulting in a concerted reaction. This mechanism is characterized by a single transition state and is sensitive to steric factors; less hindered substrates favor SN2 reactions, making the choice of nucleophile and solvent critical for predicting reaction outcomes.

Solvent Effects

The choice of solvent can significantly influence nucleophilicity and reaction rates in SN2 reactions. Protic solvents, like alcohols, can stabilize nucleophiles through hydrogen bonding, potentially reducing their reactivity. In contrast, aprotic solvents can enhance nucleophilicity by not stabilizing the nucleophile as much, thus affecting the overall reaction dynamics and the strength of the nucleophiles involved.