Organic Chemistry Chapters 6+7: Properties and Reactions of Haloalkanes
Unimolecular Elimination (E1)
A single, molecule reaction where a halide is removed along with a hydrogen from an adjacent carbon due to the former carbon having a positive charge. This coincides with the generation of a double bond.
Solvolysis
A transformation in which a substrate undergoes substitution by solvent molecules such as ethanol or water or many other compounds.
As one goes right on the periodic table, nucleophilicity decreases.
As one goes right on the periodic table, what happens to nucleophilicity?
With strongly basic nucleophiles with a compound with high steric bulk, elimination reactions become more prevalent.
As steric bulk increases, how do strongly basic nucleophiles affect the type of reaction pathway that haloalkanes may follow?
The bond strength decreases.
As the size of X (the halide) increases, what happens to the strength of the C-X bond?
During an Sn2 reaction, the nucleophile attacks the haloalkane with the simultaneous expulsion of the leaving group.
Describe the process of an Sn2 reaction.
They promote substitution reactions.
Do weakly basic nucleophiles promote substitution reactions, or elimination reactions?
Racemic products
During an Sn1 reaction, assuming favorable conditions, what kind of products are expected upon completion?
Polar solvents accelerate Sn1 reactions due to the highly polarized transition-state structure produced during the reaction.
How do polar solvents affect the Sn1 reaction and why?
Weaker bases make for better leaving groups.
How does basicity relate to nucleophilicity?
Changing the nucleophile does not affect the rate of an Sn1 reaction. However, when two or more nucleophiles compete for the capture of the intermediate carbocation, their relative strengths and concentrations may affect the product distribution.
How does changing the nucleophile affect the rate of an Sn1 reaction? What happens when two or more nucleophiles compete for a cation?
An increasing negative charge increases nucleophilicity.
How does charge relate to nucleophilicity?
Lengthening a carbon chain only reduces Sn2 activity after adding on one or two carbons. Any further addition has no effect on the reaction rate.
How does lengthening a carbon chain affect Sn2 reactivity?
Increasing polarizability improves nucleophilic power.
How does polarizability affect nucleophilic power?
Solvation impedes nucleophilicity
How does solvation affect nucleophilicity?
The leaving group ability is inversely related to base strength. Therefore, good leaving groups are the conjugate bases of strong acids.
How is a functional group's base strength related to it's leaving group ability?
The nucleophilicity of smaller ions increases much more than that of other ions.
How is the nucleophilicity trend of smaller anions, such as fluoride anions, affected while in aprotic solvents?
Branching at a reacting carbon decreases the rate of a Sn2 reaction due to steric hinderance.
How is the rate of an Sn2 reaction affected by branching at the reacting carbon.
The reactivity is raised manifold.
How is the reactivity of a nucleophile affected while in an aprotic solvent?
One would use a sequence of two Sn2 reactions with an intermediate nucleophile to change into the unwanted configuration.
How would you retain a certain configuration through a Sn2 reaction.
Charge, basicity, solvent, polarizability, and the nature of the substituents.
Name five factors that affect nucleophilicity.
It's favored when the substrate used has a very good leaving group, has a very poor nucleophile, and a protic solvent.
When is an Sn1 reaction favored in a secondary haloalkane?
Protic Solvents
Solvents which are capable of hydrogen bonding are called what?
They follow a first-order rate law, they are not stereospecific, and they are characterized by the opposite order of reactivity.
Solvolyses are characterized by how?
Leaving Group Ability
The relative rate at which a leaving group can be displaced.
Hyperconjugation
The result of overlap between a p orbital with a neighboring molecular orbital.
Bimolecular nucleophilic substitution (Sn2)
This a second-order substitution reaction where two reactants, a haloalkane and a nucleophile, interact in a single step, a.k.a. there is no intermediate step.
Unimolecular Nucleophilic Substitution (Sn1)
This is a first order substitution reaction where the rate determining step involves the dissociation of the haloalkane to an alkyl cation via heterolytic cleavage.
Nucleophilic Substitution
This is a process where a reagent attacks a compound, such as a haloalkane, and replaces the most electronegative functional group attached to the hydrocarbon, such as the halide.
Bimolecular Elimination (E2)
This occurs when strong bases attack haloalkanes prior to the formation of a carbocation. This leads to deprotonation by the base and the subsequent rehybridization of the reacting carbon centers from sp3 to sp2.
An inversion of configuration occurs, meaning that a Sn2 reaction is stereospecific.
What happens to the configuration of a stereocenter after a Sn2 reaction?
They could be strongly basic, in the example of a hydroxide ion, or they could be weak bases, such as an iodide anion.
What reasons could a nucleophile be so reactive?
The dissociation of the haloalkane and the formation of a carbocation.
What step of an Sn1 reaction is considered to be the rate-determining step?
They undergo both Sn1 and Sn2 reactions.
What substitution reactions do secondary syystems undergo?
The polarity of the solvent, the quality of the leaving group, and the strength of the nucleophile.
What three things determine whether a secondary haloalkane will undergo an Sn1 or Sn2 reaction?
There is no steric hinderances. Therefore, Sn2 reactions are more energetically feasable.
Why can't primary haloalkanes undergo Sn1 reactions?
