If the substrate is primary, can we rule out SN1 and E1 entirely?

Question

Ava Andrews · Accepted Answer

No. You should assume that #E1# and #S_N1# may still happen to some extent, because they usually still do.

Many factors contribute to that occurring; for primary alkyl halides in particular, they may include:

nucleophile strength

nucleophile steric hindrance and bulkiness

solvent steric hindrance and bulkiness

solvent interactions (nonpolar, polar protic, polar aprotic)

reaction temperature

etc.
Let us take some examples that I'm pulling from one of my 2013 worksheets:

(The given product ratios were on the worksheet as-is.)

You can tell that the substrate each time is a primary alkyl halide, but clearly there are exceptions that allow for elimination to occur, even at normal temperatures.

In the first one, the phase-transferable (organic/aqueous solubility) tetrabutylammonium chloride (t-BuNCl) nucleophile is extremely strong and is not hindered by the organic polar aprotic solvent, promoting substitution. This has no #E1# or #E2#, but has an unknown mixture of #S_N1# and #S_N2#.

In the second one, the nucleophile is pretty strong but not extremely so, and interacting with an organic polar protic solvent via H-bonding decreases the nucleophilicity, decreasing the amount of substitution by a bit. This has #S_N2#, some #E2#, and little #S_N1# and #E1#, but still some.

In the third one, the steric hindrance of the nucleophile promotes elimination, particularly #E2# and #E1#, but also allows for some #S_N1#. Likely no #S_N2# because the bulkiness on the nucleophile prevents it from finding the substrate quickly.

Jacob Anderson · Answer

undefined If the substrate is primary, we can typically rule out SN1 (unimolecular nucleophilic substitution) reactions due to the lack of a stable carbocation intermediate. E1 (unimolecular elimination) reactions are also less favored with primary substrates compared to secondary or tertiary substrates.