CHEM 0310 Lecture Notes - Lecture 6: Sn2 Reaction, Nucleophilic Aromatic Substitution, Nucleophilic Substitution

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6 Feb 2017
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Chapter 6: Properties and Reactions of Haloalkanes
Physical Properties of Haloalkanes
-Main differences between haloalkanes and alkanes: bond strength, bond length, molecular polarity,
and boiling point
-The bond strength of C-X decreases as the size of X increases
-As the size of the halogen increases, there is a growing mismatch in size between p orbitals, thus
bonding overlap diminished along the series leading to longer and weaker bonds
-The C-X bond is polarized and partial charges are given to C and the halogen (negative)
-Haloalkanes have higher boiling points than the corresponding alkanes
-This is due to the columbic attraction between the partial charges (dipole-dipole interaction)
-BP also rises with increasing size of X because the greater London interaction
-The more polarizability, the stronger the London interactions, and the higher the BP
Nucleophilic Substitution
-Haloalkanes contain an electrophilic carbon atom, which may react with nucleophiles
-In a nucleophilic substitution, the reagent attacks the haloalkanes and replaces the halide
-There are two types of nucleophilic substitutions:
1. A negative nucleophile gives a neutral product
2. A neutral nucleophile gives a positively charges product (as a salt)
-In both cases, the halide ion is displaced so it is called the leaving group
-Substrate: the organic starting material that is the target of attack by the nucleophile
-Nucleophilic substitutions changes the functional group in a molecule
Reaction Mechanism
-Nucleophilic substitution is an example of a polar reaction: it includes charged species and polarized
bonds
-Opposite charges attract nucleophiles are attracted to electrophiles
-Curved arrows show how nucleophilic substitution occurs when a lone pair of electrons on a
nucleophile is transformed into a new bond with an electrophilic carbon, pushing a bonding pair of
electrons away from that carbon onto a leaving group
Rate Law: Biomolecular
-Measuring the kinetics of a reaction allows us to establish the rate law
-In a bimolecular nucleophilic substitution (CH3Cl and NaOH), the mechanism of reaction is consistent
with a second-order reaction
-This means doubling the concentration of both increases the rate by a factor of 4
-Abbreviated as SN2, this one-step transformation occurs when the nucleophile attacks the haloalkane,
with simultaneous expulsion of the leaving group
Concerted Reaction
-Bond making occurs at the same time as bond breaking, this is known as a concerted reaction
-There are two possibilities for this concerted reaction that are stereochemically different:
1. Frontside Displacement: the nucleophile approaches the substrate from the same side as the leaving
group, one group exchanging for the other (does not occur)
2. Backside Displacement: the nucleophile approaches carbon from the side opposite of the leaving
group
-Note that the formation of the transition state is not a separate step; the transition state only describes
the geometric arrangement of the reacting species as they pass through the maximum energy point of a
single-step process
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