Quiz - Alkyl Halides - SN Reactions 2 | Alkyl Halides - Nucleophilic Substitutions

Organic Chemistry 1 - Quiz - Alkyl Halides - SN Reactions 2


What is a unimolecular nucleophilic substitution (SN1)?

SN1 reactions are nucleophilic substitutions that proceed through a carbocation intermediate ⇒ SN1 reactions are unimolecular with a bond-breaking step followed by a bond-making step


Which of the following factors does not significantly affect the rate of SN2 reactions?

SN2 reactions are bimolecular, with the rate depending on both the concentration of the nucleophile and the substrate. The rate is greatly affected by steric hindrance, the strength of the nucleophile, and the nature of the solvent (particularly polar aprotic solvents). The presence of electron-withdrawing groups can actually affect the rate, but not as significantly compared to SN1 reactions where carbocation stability is critical.


Which type of carbon-halogen bond would typically undergo nucleophilic substitution reactions most readily?

The C-I bond undergoes nucleophilic substitution more readily than other carbon-halogen bonds due to iodine's larger atomic radius, which makes the bond weaker and more susceptible to attack by nucleophiles.


What is the specific rotation of the alcohol produced in the bimolecular nucleophilic process shown below?

A bimolecular nucleophilic process and NaOH, a strong nucleophile, mean that the reaction is a SN2. A SN2 is a one-step process with inversion of configuration. The alcohol produced is the enantiomer (mirror image) of the alcohol drawn with a specific rotation of -5.6. The correct answer is therefore [α]D = +5.6


What is the specific rotation of the alcohol produced in the following nucleophilic substitution?

The tertiary alkyl halide and the poor nucleophile (H2O) mean that the reaction is an SN1. In an SN1 process, the leaving group leaves, forming a carbocation, and then the nucleophile attacks the carbocation to give the product. Because of the planar carbocation intermediate, SN1 reactions result in a racemization of the stereochemistry in the reaction center (nucleophiles can attack from both sides). Therefore, the specific rotation of the mixture is [α]D = 0.0


Which of the following solvents would typically favor an SN1 reaction for a tertiary alkyl halide?

Polar protic solvents stabilize carbocations and are therefore favorable for SN1 reactions, which involve a carbocation intermediate.


What reagent could accomplish this reaction:   


According to the reaction, the nucleophile is N3-. The only possible reactants are IN3 and NaN3. The electronegativity of Na is 0.93, that of I is 2.66 and that of N is 3.04. The difference in electronegativity is greater between Na and N3 than between I and N3, which makes NaN3 the best reagent for this reaction


What is the transition state of the reaction of CH3I with CH3OK in CH3OH?

CH3I is a primary alkyl halide and CH3OK is a strong nucleophile. The reaction is an SN2, a bimolecular with simultaneous bond-making and bond-breaking steps:


Which factor affects the SN1 and SN2 reactions the most?

The nature of the alkyl halide is the most important factor in determining whether a reaction follows the SN1 or SN2 mechanism:

  • Methyl halides (CH3X) and primary halides (RCH2X) ⇒ SN2 reactions only
  • Tertiary halides (R3CX) ⇒ SN1 reactions only
  • Secondary halides (R2CHX) ⇒ both SN1 and SN2 reactions. Other factors such as the strength of the nucleophile and the polarity of the solvent determine the mechanism

Why might a negatively charged species be a good nucleophile?

A negatively charged species possesses an excess of electrons, making it a good nucleophile capable of donating electron pairs in nucleophilic substitution reactions.