Halogen Derivatives
Halogen Derivatives tests classification, nucleophilic substitution (SN1 and SN2), elimination, Grignard reagents and named reactions such as Finkelstein and Swarts.
Core rules
- SN1 proceeds via a carbocation, so reactivity is 3° > 2° > 1° > methyl; it gives racemisation.
- SN2 is a one-step backside attack, so reactivity is methyl > 1° > 2° > 3°; it gives inversion.
- Elimination follows Saytzeff's rule: the more substituted alkene predominates.
- Aryl and vinyl halides are inert toward SN reactions because the C-X bond has partial double-bond character.
- Grignard reagents (RMgX) are prepared in dry ether and react with water, aldehydes, ketones and CO2.
Worked example
Arrange 1-bromobutane, 2-bromobutane and 2-bromo-2-methylpropane in order of SN1 reactivity. The tertiary halide forms the most stable carbocation, the secondary less so, and the primary least, so the order is 2-bromo-2-methylpropane > 2-bromobutane > 1-bromobutane. Reasoning: SN1 rate depends on carbocation stability, not on steric hindrance at the carbon under attack.
Traps
Reversing the SN1 and SN2 orders, forgetting that SN1 gives a racemic mixture from a chiral substrate, and using water instead of dry ether for Grignard preparation. Saytzeff's rule gives the more substituted alkene, not the less substituted one.