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Questions and Answers
What is a radical species in chemistry?
A species that contains unpaired electrons.
How is the structure of an alkyl radical similar to a carbocation?
Both have a carbon atom that is sp2 hybridized and have an unpaired electron in the p-orbital.
What is the order of stability of radicals?
Tertiary > secondary > primary > methyl.
What are the three fundamental steps in a radical reaction?
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How can weak bonds be broken to initiate a radical reaction?
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What is the bond strength of the O-O bond in organic peroxides?
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What is the major product of the propagation step in a radical reaction?
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What is the significance of the 'fishhook' arrows in radical reactions?
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What is the purpose of the initiation step in a radical reaction?
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What happens during the termination step of a radical reaction?
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Study Notes
SN2 Reaction
- SN2 reactions result in inversion of configuration at stereocenters due to back-face attack.
- The reaction is influenced by the substrate structure, nucleophile strength, leaving group quality, and solvent used.
SN2 in Biochemistry
- DNA, as a biomolecule, contains nucleophiles prone to SN2 reactions, especially with alkylating agents.
- Alkylation can result in altered DNA shape, reduced hydrogen bonding, and potentially destructive mutations.
SN1 Reaction
- SN1 stands for Substitution Nucleophilic Unimolecular, involving one reactant in the rate-determining step (RDS).
- This makes SN1 reactions first-order, relying on substrate concentration alone.
SN1 Reaction Mechanics
- Rate of SN1: Rate = k[substrate]; varying nucleophile concentration does not affect the rate.
- Important leaving groups include I-, Br-, Cl-, RSO2-, and OH2, while F-, HS-, NC-, HO-, and CH3O- are less effective.
Carbocation Stability
- Carbocations are electron-deficient and stabilized by electron-donating groups, particularly alkyl groups exhibiting a positive inductive effect (+I).
- Negative inductive effects (-I) occur when carbon is connected to an electronegative element.
Elimination Mechanisms
- Elimination can occur through E1 (unimolecular) or E2 (bimolecular) mechanisms.
- In E1, both nucleophilic and basic characters can compete, often rivaling with SN1 processes.
Saytzeff Elimination
- Saytzeff's rule states that the more substituted alkene is the preferred product when eliminating hydrogen atoms.
E2 Mechanism Details
- E2 requires an anti-periplanar alignment between the C-H bond and the C-LG bond for elimination.
- Bulky groups hinder nucleophilic attack, favoring elimination over SN2 reactions.
Reaction Types Overview
- Main reaction types include Substitution, Elimination, Addition, and Radical processes.
Addition Reactions
- Addition of H-X to alkenes produces significant regioisomers, with the major product formed preferentially.
- Markovnikov’s rule states that the hydrogen from hydrogen halides attaches to the alkene carbon with the most hydrogen atoms.
Bromination Mechanism
- Addition of Br2 to alkenes results in vicinal dihalides through mechanisms involving bromonium ion intermediates.
- The mechanism features polarisation of the Br2 molecule leading to formation of a three-membered bromonium ion during the addition process.
Conclusion
- Understanding the subtleties between SN1, SN2, E1, and E2 reactions, as well as the mechanisms and factors influencing these reactions, is crucial in organic chemistry.
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Description
Understand the SN2 reaction mechanism and its consequences on stereocentres. Learn how the reaction affects the substrate, nucleophile and leaving group. Test your knowledge on the reactivity details of SN2 reactions.
