H3 Chemistry 9813 · Study focus: H3 Chemistry: Apply syn/anti elimination and stereoselectivity
H3 Chemistry: Apply syn/anti elimination and stereoselectivity
Start from the governing chemical model, test it against evidence, then transfer the reasoning to an unfamiliar case.
Your success criteria
- Apply syn/anti elimination and stereoselectivity
- Use named chemical evidence.
- Transfer the governing reason to an unfamiliar case.
Expose the reacting bonds
An E2 prediction begins with the leaving group on the α-carbon and an actual hydrogen on an adjacent β-carbon.
Draw the Cβ–Cα bond as a Newman projection or a cyclohexane chair before claiming which alkene stereoisomer can form.
Periplanar elimination
Anti-periplanar means the β-C–H and Cα–leaving-group bonds have a 180° dihedral; syn-periplanar means they align at 0°.
Anti elimination is normally favoured because its staggered geometry reduces eclipsing and permits continuous orbital overlap during concerted C–H cleavage, C=C formation and leaving-group departure.
In one E2 step, a base removes the aligned β-H, the C–H electron pair forms the π bond, and the C–X bond electron pair moves to X.
Because the reacting conformation fixes the non-reacting substituents, different stereoisomeric substrates or accessible anti conformers can give different E/Z product proportions.
Text alternative: The text names front/rear atoms, every substituent, the 180° angle, axial directions and each curved-arrow source and destination.
Translate conformation into product
In one E2 step, a base removes the aligned β-H, the C–H electron pair forms the π bond, and the C–X bond electron pair moves to X.
Because the reacting conformation fixes the non-reacting substituents, different stereoisomeric substrates or accessible anti conformers can give different E/Z product proportions.
Read an anti Newman projection
View CH3–CH(Br)–CH(D)–CH3 along C2→C3 with Br on C2 anti to H on C3; D remains attached to C3.
If the two methyl groups lie opposite in this reacting Newman projection, concerted anti-E2 gives the alkene in which the higher-priority methyl-bearing groups are opposite.
- Draw the three E2 arrows and identify the stereochemical alkene consequence for the fixed anti conformer.
Open the feedback checkpoint after attempting
- Credit base→H, C–H→C2=C3 and C–Br→Br arrows, with the product geometry inherited from the drawn conformer.
Check a cyclohexane chair
E2 on a cyclohexyl halide requires the leaving group and β-H to be trans-diaxial, which is the ring expression of anti-periplanar geometry.
A ring flip can make an equatorial halide axial, but it simultaneously exchanges every other axial/equatorial position while preserving up/down configuration.
- For trans-1-bromo-2-methylcyclohexane, inspect both chairs and mark any Br/β-H trans-diaxial pair.
Open the feedback checkpoint after attempting
- Accept elimination only from a chair containing axial Br and an axial β-H anti to it; do not erase the methyl group when checking the site.
Evaluate a constrained syn pathway
A rigid substrate may lack an anti-periplanar β-H yet present a syn-periplanar β-H and leaving group.
A proposed syn elimination must show the 0° alignment, greater eclipsing cost and the stereochemical product that follows; syn is possible in constrained cases but not assumed equal to anti.
- Assess a rigid drawing in which the only β-H is syn-periplanar to chloride.
Open the feedback checkpoint after attempting
- Require explicit syn geometry and a cautious prediction of a less favourable, constraint-enabled pathway.
Do not predict from a flat formula
A structural formula confirms β-hydrogens but does not reveal their dihedral angles.
Anti describes the two bonds broken in elimination, not whether two arbitrary large substituents happen to be opposite.
- Repair: ‘Any β-H in 2-bromobutane gives the same E2 stereochemistry.’
Open the feedback checkpoint after attempting
- Reject it; rotate to each accessible H–Cβ–Cα–Br anti arrangement and carry the remaining substituents into the product.
Audit stereochemical evidence
Name α and β atoms, label the removed H, state a 180° or 0° dihedral, draw all three arrows and preserve every remaining substituent in the alkene.
Next decide which β-site gives the thermodynamic Zaitsev or kinetically accessible Hofmann regioisomer.
- List the evidence for a stereospecific anti-E2 prediction.
Open the feedback checkpoint after attempting
- Require a fixed conformation, anti bond pair, concerted arrows, unchanged atoms and an E/Z conclusion derived from the remaining groups.