How to Work Through SN2/SN1 Questions on Your Exam

Here are a few SN2/SN1 Questions you might see you on your upcoming exam

Now that you are an expert by using substitution reactions- SN2 versus SN1, it is time to put your understanding about S_N1 and S_N2 into practice! Let’s work though a few examples…

Let’s look at the following for all examples:

• Type of carbon
• Solvent
• Nucleophile

In this first example, we have:

Type of carbon:

• Alkyl halide attached to a secondary carbon
• Fairly good a stabilizing carbocations

Solvent:

• Polar protic solvent mixture
• Good solvent system for ionization (carbocation formation)

Nucleophile:

• Poor nucleophiles (H₂O and methanol (CH₃OH))
• Therefore, we need assistance to make this reaction proceed (need to form a carbocation)

Looking at our assessment, it all leans towards S_N1:

OK, on to the second example:

Type of carbon:

• Alkyl halide attached to a primary carbon
• Good for backside attack
• Also, bromide is a good leaving group

Solvent:

• Polar aprotic solvent
• Good solvent for solubilizing counter ions (..like the Na⁺ in NaBr. This makes the CN^– more reactive, meaning more nucleophilic)
• Not a good solvent for stabilizing anions (CN^–); cyanide anion can react (attack)

Nucleophile:

• Strong nucleophile
• Speaks for itself

All of our information points to S_N2:

So what about our third example?

Type of carbon:

• Alkyl halide attached to a secondary carbon
• Good for both S_N1 and S_N2…this is that grey area in the last post (on algorithm)
• Bromide is a good leaving group

Solvent:

• Polar aprotic solvent
• Good solvent for S_N2 reactions; bad solvent for S_N1 reactions

Nucleophile:

• Strong nucleophile

Once again, it all points to S_N2:

So, I am pretty sure you all are experts by now. See if you can figure out these last two examples:

cyclic bromide product answer…click here

cyclic bromide mechanism answer…click here

alkyl bromide  product answer …click here

alkyl bromide  mechanism answer…click here