How can we tell which mechanism to use? This question will get more complicated unfortunately, but for now we can use the following factors to answer this question.
When given a substitution reaction, use the following two factors to determine the mechanism:
Nucleophile Strength: SN1 = WEAK SN2 = STRONG
Leaving Group Substitution: SN1 = 3° > 2° SN2 = 0° > 1° > 2°
Problem: Predict the product of the reaction4m
Problem: Predict the product of the reaction6m