Substitution And Elimination Reactions & Related Concepts Flashcards
Why do carbocation rearrangements occur?
They occur to produce a more stable carbocation. For example, a secondary carbocation will become a tertiary carbocation through a hydride shift.
(Weak/strong) bases make good leaving groups.
Weak
A good prediction is that a good leaving group is also a ___________.
good nucleophile
How do sterically hindered substrates affect the type of substitution reaction that occurs?
Sterically hindered substrates undergo SN1 reactions; they don’t undergo SN2 reactions.
SN2 reactions are compatible with (polar protic/polar aprotic) solvents.
Polar aprotic solvents
SN1 reactions are compatible with (polar protic/polar aprotic) solvents.
Polar protic solvents
What are the stereochemical outcomes of a SN2 reaction?
An SN2 reaction is stereospecific because the nucleophile attacks from the back, this causes the product to change R/S configuration from the original reactant.
What are the stereochemical outcomes of an SN1 reaction?
Racemic mixture of enantiomers given that the initial reactant has a stereo enter (chiral carbon)
What does it mean when a SN2 reaction is said to be “second order”?
It means that the rate of the reaction is dependent upon both the type of substrate and the type of nucleophile used.
True or false:
For an SN1 reaction, the rate of the reaction depends on the substrate (alkyl halide) and the nucleophile used.
False, an SN1 reaction is first order which means that reaction rate will only be dependent on the substrate used.
Which reactant will have a faster rate when undergoing an SN1 reaction?
1. CH3C(CH3)2CH2Br
2. CH3CHCH3CH2Br
- Because it is more sterically hindered and go undergo a methyl shift via carbocation rearrangement to form a more stable tertiary carbocation.
Name, examples of polar protic solvents and polar aprotic solvents.
Polar protic - water, alcohols (any solvent with OH), IPA, ethanol, methanol
Polar aprotic - DMSO, acetone, DMF, THF, ACN
How do inductive atoms (such as F or Cl) on adjacent carbons affect stability of the carbocation?
Inductive atoms, such as fluorine or chlorine are very electronegative which means that they withdraw electrons from the nearby electrophile, making the subsequent carbocation that forms less stable.
True or false:
Sterically hindered substrates (even hinderance/substituents present on the beta carbon) next to the alkyl halide affect the reaction rate of an SN2 reaction.
True. For example, methyl groups that are present on the beta carbon next to the bromine affect the reaction rate of an SN2 reaction.
How does the type of nucleophile affect the rate of an SN1 reaction?
Nucleophile choice doesn’t affect SN1 reactions because they are first order and only dependent on type of substrate used. Therefore, a sterically hindered or non-hindered nucleophile has the same effect on rate (doesn’t change).
If you are reacting a primary alkyl bromide with a sterically hindered nucleophile, how does this affect the rate of the reaction?
A sterically hindered nucleophile for an SN2 reaction causes a decrease in rate of the reaction.
Substrates that undergo SN1:
Secondary and tertiary alkyl halides, allylic alkyl halide (resonance forming)
Substrates that undergo SN2
Primary alkyl halides
What are the factors that determine the strength/reactivity of a nucleophile?
- Negative charge
- Higher electronegativity
- Bigger size
- No resonance
- No steric hindrance
Why are alkyl fluorides not used for SN2 reactions?
Because the C—F bond is to strong, so it doesn’t make a good leaving group.
What is the rate limiting step of SN1 reaction?
Formation of carbocation intermediate (1st step)
What is the rate limiting step of SN2 reaction?
Nucleophilic attack
Name the nucleophiles used in SN1 reactions. Are they weak or strong nucleophiles?
H2O, MeOH, and EtOH
WEAK
Name the nucleophiles used in SN2 reactions
Strong nucleophiles ONLY; I-, Br-, Cl-, OH-, EtO-, MeO-
What makes a good leaving group? What is the best LG out of all halogens?
They must be able to stabilize the negative charge when the nucleophile attacks. Therefore, out of all of the halogen LGs, Iodide is the best one because it has the largest size (atomic radius) and can stabilize the negative charge best.
