Lecture 5: Another Way to S: SN1 – Substitution Nucleophilic ...

Lecture 5: Another Way to S: SN1 – Substitution Nucleophilic Unimolecular; Spontaneous dissociation of LG in slow, rate limiting step generates carbocation intermediate  reacts with nucleophile in a fast step  loss of charged species gives neutral product. [Nucleophile] has no effect on reaction rate (key aspect is carbocation’s stability). Local minimum point in energy diagram refers to intermediate carbocation. Protonation is always a low energy process. SN1 Mechanism; Key Features; it’s a two-step substitution via a planar carbocation intermediate. Starting enantiomer undergoes racemisation (chiral substrate and carbocation intermediate allows nucleophile two paths equal energy so products equally likely). SN1 Effect of Substrate; SN1 reactivity correlates with carbocation stability (factors that stabilise carbocations will stabilise developing carbocationic character in T.S. 1 lowering EA & increasing reaction rate). SN1 Effect of Substrate - Hyperconjugation; Substituted carbocations (20, 30) are stabilised by hyperconjugation. More sigma bonding electrons in the tertiary carbocations leads to greater stabilisation. Effect of Leaving Group; Good SN1 leaving groups are stable anions. Reactions can occur under acidic conditions converting poor leaving groups into good ones (e.g. OH- into H2O) through protonation. SN1 Effect of Nucleophile; [Nucleophile] is not in the rate equation so not involved in rate determining step. Nucleophile does impact reaction pathway. Good nucleophile = SN2 reaction but poor nucleophile = SN1 reaction. SN1 Effect of Solvent; Polar protic solvents solvate both positive and negative charges which stabilises T.S. 1 and so the EA decreases. Polar solvents stabilise carbocation intermediates in the SN1 reaction. H-bonding stabilise nucleophiles as they’re cleaved off and also once left isolated in solution. Lecture 5 – Another Way To S: Sn1 – Substitution Nucleophilic Unimolecular;

Sn1 Mechanism;

Sn1 Effect Of Substrate;

Sn1 Effect Of Substrate – Hyperconjugation;