If the substrate is primary, can we rule out SN1 and E1 entirely?
No. You should assume that
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
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
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
In the third one, the steric hindrance of the nucleophile promotes elimination, particularly