Why do tertiary alkyl halides react in an #"S"_N1# mechanism more easily than #"S"_N2#?

Question

Sarah Angotti · Accepted Answer

Because they are bulky (kinetically stable), and hence block against #"S"_N2# backside-attack, giving the alternative mechanism of #"S"_N1# a greater percentage of success than #"S"_N2#.

They also form the most thermodynamically stable carbocation.

#"S"_N1# is first-order nucleophilic substitution, so nucleophile strength is an important factor to consider.

Nucleophilicity is a kinetic phenomenon, so a good nucleophile is fast.

For #"S"_N1# reactions of alkyl halides, the #stackrel("central carbon")overbrace("C")-stackrel("leaving group")overbrace"LG"# bond is weak, blocked, and the nucleophile is slow. So the #"LG"# leaves first in an #"S"_N1# fashion, giving a first-order process.

Also, because tertiary carbocations are among the most thermodynamically stable, this departure is favorable (not nonspontaneous).
- For #"S"_N2# reactions of alkyl halides, the #stackrel("central carbon")overbrace("C")-stackrel("leaving group")overbrace"LG"# bond is weak, open, and the nucleophile is fast. So the nucleophile influences the #"LG"#'s leaving.
  
  As less substituted carbocations are less thermodynamically stable, the leaving group cannot easily depart on its own, so the joint process makes it a second-order process.
  
  A bulky alkyl halide like a #3^@# alkyl halide blocks the nucleophile from attacking the central carbon, decreasing the percentage of successful #"S"_N2# and increasing the percentage of success in the alternative choice, #"S"_N1#.
  
  Basically, the site to be attacked is heavily cluttered, which is hard to get past, and the significant thermodynamic stability of the carbocation promotes its formation during the #"S"_N1# process.

Carter Allen · Answer

undefined Tertiary alkyl halides react more easily in an SN1 (substitution nucleophilic unimolecular) mechanism compared to SN2 (substitution nucleophilic bimolecular) because in SN1 reactions, the rate-determining step involves the formation of a carbocation intermediate. Tertiary alkyl halides, which have more substituted carbons, form more stable carbocation intermediates due to the increased electron density around the positively charged carbon. This stability facilitates the formation of the carbocation, making the SN1 reaction pathway more favorable for tertiary alkyl halides. Additionally, the steric hindrance around the carbon bearing the leaving group in tertiary alkyl halides makes the SN2 pathway less accessible, further favoring the SN1 mechanism.