Page 34 - Arrow Pushing in Inorganic Chemistry A Logical Approach to the Chemistry of the Main Group Elements

A COLLECTION OF BASIC CONCEPTS
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• Associative and dissociative processes (Section 1.9)
• S 2-Si (Section 1.10)
N
• Two-step mechanisms: the S 1 and E1 pathways (Section 1.11)
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• Electrophilic addition to C–C multiple bonds (Section 1.12)
• Electrophilic substitution on aromatics: Addition–elimination (Section 1.13)
• Nucleophilic addition to C–heteroatom multiple bonds (Section 1.14)
• Carbanions and ylides (Section 1.15)
• Carbenes (Section 1.16)
• Oxidative addition and reductive elimination (Section 1.17)
• Migrations (Section 1.18)
• Ligand exchanges (Section 1.19)
• Radical reactions (Section 1.20)
• Pericyclic reactions (Section 1.21)

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1.7 BIMOLECULAR -ELIMINATION (E2)

A process that often competes with S 2 displacements for organic systems is E2 (which
N
stands for “elimination, bimolecular”). When there is one or more hydrogens with respect
to the leaving group (see reaction 1.8) and the incoming nucleophile is a strong enough
base, bimolecular elimination occurs, often in competition with nucleophilic displacement,
as shown below:
Me
−
C
Me O
Me H H H − Me Me H (1.8)
− Br
C C C OH + C C
Me β α Me
Me Br Me Me H

An important point is that, although an E2 reaction involves movement of three electron
pairs, it all happens as a concerted one-step process.
A sterically hindered carbon center, such as the tertiary carbon in a t-butyl halide, is
generally not conducive to an S 2 displacement.
N
Me

X
Me
Me
t-Butyl halide