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 SN1 and SN2 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 SN1:

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 SN2:

So what about our third example?

Type of carbon:

  • Alkyl halide attached to a secondary carbon
  • Good for both SN1 and SN2…this is that grey area in the last post (on algorithm)
  • Bromide is a good leaving group

Solvent:

  • Polar aprotic solvent
  • Good solvent for SN2 reactions; bad solvent for SN1 reactions

Nucleophile:

  • Strong nucleophile

Once again, it all points to SN2:

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

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