CHEM10006 Study Guide - Final Guide: Racemic Mixture, Walden Inversion, Sn2 Reaction
12 Jun 2018
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Reactions of the Carbonyl Functional Group
●Large electronegativity difference between C & O:
strongly polarized bond
Primary alcohols: Oxidized to aldehydes or carboxylic acids,
conversion to aldehyde uses milder conditions, can sometimes
be isolated; can be oxidized directly to the acid by KMnO
4
(powerful oxidant)
Secondary alcohols: Can be oxidized to ketones, can’t be
oxidized further
Tertiary alcohols: do not oxidize
Nucleophilic addition: Attack of a nucleophile leads to the loss
of a pi bond and formation of a new sigma bond, becomes SP
3
●Addition of hydride (H
-
) affords an alkoxide:
protonation of the alkoxide affords an alcohol
●Reduction process: hydrogens added in separate
fallen off by itself, vs in SN2 where it attacks initially to
remove the leaving group; departure of the leaving group in
SN1 creates an open face that only requires a weak
nucleophile to attack
Occurs via spontaneous dissociation of the alkyl halide to give
a planar carbocation intermediate which can be attacked from
either face: chiral substrate gives rise to a racemic mixture
Ease of S
N
1 reactions depend on carbocation stability: tertiary
easiest, methyl hardest, preferred because tertiary carbocation
intermediate is very stable and S
N
2 substitution is sterically
hindered
Alkyl halide: Haloalkanes, one or more hydrogen atoms in an
alkane have been replaced by halogen atoms e.g. CCl
4
●S
N
2: Concerted:
Single step reaction, second order, rate depends on both
substrate and nucleophile concentration , bimolecular,
breaking and making bonds is simultaneous and concerted ;
rate of reaction faster with primary/methyl groups rather than
tertiary because they are sterically accessible and because the
resulting carbocations are so unstable
Biomolecular: A mechanism in which two reacting species
combine in the transition state of the rate-determining step
E/Z Nomenclature helps distinguish major & minor products
Walden Inversion: An umbrella-like inversion of the
stereochemistry of S
N
2 reactions:
●an optically pure reactant will be converted to an
optically pure product: sample contains only one
enantiomer , also known as homochiral
●SN2 reactions therefore create one inversion isomer
●SN1 reactions produce two isomers (racemic
mixtures) of inversion & retention (not 50/50 ratio,
usually more of the inverted form), occurs because
nucleophiles can attach from both sides & it is
easier to attach from one side than another
(slow) & deprotonation (fast) , loss of leaving group precedes
loss of a proton
E2: Second order elimination reaction, a one step reaction
involving loss of a leaving group at the same time as loss of a
proton , bimolecular, rate depends on concentrations of
substrate and base, concerted
The transition state dictates the stereochemical outcome
An antiperiplanar arrangement (both axial, in line) makes
elimination easier as the nucleophile has better access to the
carbon that it is attacking
