Apply syn/anti elimination and stereoselectivity

An E2 prediction begins with the leaving group on the α-carbon and an actual hydrogen on an adjacent β-carbon.

GCE A-Level H3 Chemistry 9813-2027

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Learning objectives

Apply syn/anti elimination and stereoselectivity

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.

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.

The reacting bonds—not the largest groups—must be aligned. Rotate about Cα–Cβ until C–H and C–X are anti, then carry every group that remains attached directly into the alkene drawing.

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.

Try this

Draw the three E2 arrows and identify the stereochemical alkene consequence for the fixed anti conformer.

Check your answer

A strong answer should include 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.

Try this

For trans-1-bromo-2-methylcyclohexane, inspect both chairs and mark any Br/β-H trans-diaxial pair.

Check your answer

Only the chair with Br axial can eliminate. In it the trans methyl at C2 is also axial, so C2 has no axial H anti to Br; C6 supplies the trans-diaxial β-H. The other chair has Br equatorial and gives no anti pair. Do not erase the methyl group when checking C2.

Evaluate a constrained syn pathway

Supplied constraint: in a rigid substrate the only β-H is syn-periplanar (0°) to chloride; no anti-periplanar β-H is available.

Decide whether E2 elimination is possible and compare its expected rate with an otherwise comparable anti-periplanar pathway.

Try this

Assess the syn-periplanar pathway and explain what information is needed to draw its alkene stereoisomer.

Check your answer

Syn elimination is possible because the rigid frame holds H and Cl at 0°, but its eclipsed transition state makes it less favourable and usually slower than an otherwise comparable anti pathway. The positions of the remaining substituents are needed to draw the alkene stereoisomer; they are not specified here.

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.

Try this

Better reasoning: ‘Any β-H in 2-bromobutane gives the same E2 stereochemistry.’

Check your answer

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 more-substituted Zaitsev or more accessible Hofmann regioisomer under the stated conditions.

Show the Newman or chair conformation, mark the anti C–H/C–X pair, draw all three simultaneous arrows and derive the alkene geometry from that exact conformer.

Try this

List the evidence for a stereospecific anti-E2 prediction.

Check your answer

Your answer should include a fixed conformation, anti bond pair, concerted arrows, unchanged atoms and an E/Z conclusion derived from the remaining groups.