Substitution and Elimination reactions Flashcards
Nucleophilic Substitution Bimolecular (SN2)
Key points:
•Nucleophilic substitution is swapping of one functional group for another •Structure of alkyl halide is key: primary > secondary»_space; tertiary •Mechanism proceeds through backside attack
•Stereochemistry proceeds with inversion
•Rate depends on concentration of both nucleophile and alkyl halide •Reaction occurs faster in polar aprotic solvents
The SN2 Mechanism: why does the Backside Attack Lead to Inversion?
The nucleophile (electron pair) attacks the empty antibonding orbital which is on the backside of the carbon-leaving group bond
The Rate Limiting Step of the SN2 Is……
Bimolecular (Doubling the concentration of either component will double the rate.)
which solvents are the best for the SN2?
Polar aprotic (Because of the charges involved in the SN2 mechanism polar solvents are better than non polar solvents.
two types of polar solvents
polar protic (have O-H or N-H bonds) Polar aprotic (has dipoles but cannot hydrogen bond
why are polar aprotic solvents best for SN2? why not polar protic?
Polar protic solvents make a “jacket” around nucleophiles through hydrogen bonding, decreasing the reactivity of the nucleophile.
What Makes A Good Leaving Group?
One main thing: Good Leaving Groups are Weak Bases
How The Substrate Influences Reactivity: primary alkyl (1°)
SN2 (unhindered)
E2 (provided a proton is on the β carbon)
SN1/E1: not observed
How The Substrate Influences Reactivity: secondary alkyl (2°)
N2 (slower than primary) E2 (provided a proton is on the β carbon) SN1/E1 : can occur when LG is OH and a strong acid is added watch out for rearrangements! Generally poor when LG is a halide.
How The Substrate Influences Reactivity:
SN2: Not observed
E2 (provided a proton is on the β carbon)
SN1/E1 : strongly favored
if LG = halide, good reaction if LG = OH, requires acid
carbo cation stability
primary - least stable, secondary - more stable, tertiary - most stable
What Factors Influence Nucleophilicity?
Charge: The conjugate base is always a better nucleophile than the conjugate acid
Electronegativity: The less tightly held a pair of electrons are, the more readily they can be donated.
solvent: Polar protic solvents will hydrogen bond with nucleophiles (see column on left)
which decreases their nucleophilicity.
Steric bulk: If a sterically bulky group surrounds the nucleophilic atom, this will decrease its
Nucleophilicity increase……
as we go to the LEFT along the periodic table: C:>N:>O:>F:
Nucleophilicity increases as we go DOWN the periodic table:
Elimination, Bimolecular (E2)
Key points:
• Elimination is the loss of a leaving group from the α-carbon and a hydrogen from the β-carbon resulting in the formation of a double bond
•The major product is the more substituted alkene (Zaitsev product) and the bulkiest groups will be trans
•Use of a bulky base will result in formation of more of the less substituted alkene •Requires strong base
•The leaving group and the hydrogen must be oriented anti-periplanar
•Can occur with primary, secondary, tertiary substrates provided there is a hydrogen on the β-carbon that can orient itself anti to the leaving group
•Rate determining step is bimolecular
•Solvent is generally polar protic
•Reaction is favored with heat
what is the major product?
the Most Substituted Alkene - Because alkenes become more thermodynamically stable as C-H bonds are replaced with C-C bonds
