SN1 and SN2 reactions. When do they take place at a saturated carbon atom?

Substitution in general is the replacement of one functional group by another. It has been observed that tertiary alcohols react rapidly with HBr to give tertiary alkyl bromides. Primary alcohols, on the other hand, react very slowly with HBr and are usually converted to primary alkyl bromides when they react with PBr3.
The mechanisms proposed for these reactions are known as S_N1and S_N2 respectively.
 

When do S_N reactions at saturated C atoms take place according to the S_N1 mechanism and when do they take place according to the S_N2 mechanism?

Substitution reactions take place with the S_N2 mechanism in the following cases:

• Almost always in sterically unhindered benzyl και allyl positions (except benzyl and allyl triflates that react according to S_N1)

• Always in substitutions in CH₃X and RCH₂X (primary C)

• In substitutions in R₂CHX (secondary C)

• Never in substitutions in tertiary and primary substrates of the type R₃CX or R₃CCH₂X

 Substitution reactions take place with the S_N1 mechanism in the following cases:

• Always in substitutions in tertiary substrates of the type R₃CX, Ar₃CX and secondary substrates of the type Ar₂HCX

• Always in substitutions on substituted and unsubstituted benzyl and allyl triflates

• In substitutions on R₂CHX when poor nucleophiles are used

• In substitutions on R₂CHX in the presence of strong Lewis acids such as in the substitution by aromatics

• Almost never in substitutions on CH₃X

 

References

R. Bruckner, “Advanced Organic Chemistry – Reaction Mechanisms”, 2^ndEdition, Elsevier, 2002

M.B. Smith & J. March “March’s Advanced Organic Chemistry”, 6^thEdition, Wiley-Interscience, 2007