good nucleophilemoderate nucleophilepoor nucleophileLecture 15Outline of Last Lecture I. SN2 examplesII. Energy Diagramsa. Transition stateb. Activation energy (ΔG+)c. ΔG = ΔH - TΔSCHEM 333 1st Editiond. ΔG = -RTlnKeqi. Exergonic reactionii. Endergonic Reactiona. Enthalpy (ΔH)i. Exothermicii. Endothermicb. Rate determining stepI. SN1 vs. SN2 - Experimental Evidencea. Kineticsb. Solvent for nucleophilic reactionsi. Proticii. Aproticiii. Solvent Polarity1. Dielectric constant2. Non-polar solvent (nps)3. Polar solvent (ps)4. Border line solventOutline of Current LectureI. Solvent role in nucleophilicitya. Polar aprotic solventb. Polar protic solventII. Good vs. poor nucleophilesIII. Other rules of nucleophilicityIV. Alkyl halide effectV. Allylic halides (allylic carbocations)VI. Carbocation stabilityVII. Summary of SN1 and SN2VIII. Leaving groupsCurrent LectureI. Solvent role in nucleophilicitya. Polar aprotic solvent: solvate cations well but non anions. Therefore, anions unsolvates left as good nucleophiles.i. Nucleophilicity parallels basicity:F- > Cl- > Br- > I-F- is less polarizable. I- holds negative charge better.b. Polar protic solvent (acidic H’s): opposite reactivity (nucleophilicity)F- < Cl- < Br- < I-Anions are highly solvated, therefore, less able to participate in nucleophilic reactions.F- is small: hard to escape solvent. Charge is held tight, thereore, solvent is tightly associated. akes a solvent shell that F- can’t escape from.I- is large: solvent is less associated, so I- can escape and be a nucleophileII. Good vs. poor nucleophiles: determined by a standard SN2 reaction on an alkyl halidegood nucleophile moderate nucleophile poor nucleophileBr -I -NH3ROHHO -CH3 - SH H2ORS -CN -N3 -III.IV. Other rules of nucleophilicitya. Within a row - right to left - increase in nucleophilicity as basicity increasesb. Anions are better nucleophiles than nueatral with regards to nucleophilic atomi. Example: HO - > H2O, MeO - > HOMe, - NH2 > NH3, - S-R > H-SRc. Nucleophiles with same atoms: weaker acid = stronger conjugate base = stronger nucleophilei. Example:< HO - = RO -d. Polarizable molecules are more nucleophileici. N3 - I -- N = N + = N -e. Sterics around attacking atoms reduce nucleophilicityi. Example:V. Alkyl halide effecta. SN1: electronics of alkyl halide is importanti. Most stable carbocation? 3oii. Because of what effects? Induction and hyperconjugationiii. SN1 forms a carbocation - need alkyl groups to stabalize1. 3o alkylhalides: will undergo SN1 reaction2. 2o alkylhalides: less likely to undergo SN1 reaction3. 1o alkylhalides: will only undergo SN1 reactions in special casesb. SN2: sterics of alkyl halide is importantiii. Nucleophile must react with alkyl halide through backside attack. Nucleophile must reach carbon that leaving group is attached to.iv. Sterics (SN2 vs. B (beta) branching)1.3o alkylhalides and 3 B-brances are too sterically hindred to undergo SN2 reactions - nucleophile cannot do backside attackc. Alkyl halides (SN1 vs SN2):iii. 3o alkylhalides: only SN1 reactionsiv. 2o alkylhalides: can do both. Depends on solvent and nucleophile.v. 1o alkylhalides and methyl: only SN2 reactionsVI. Allylic halides (allylic carbocations)a. 1o allylic carbocation:b. 2o allylic carbocation:c. 3o allylic carbocation:VII. Carbocation stabilityVIII.Summary of SN1 and SN2a. SN1: polar protic promotes MeOH/H2O - solvent is nucleophileb. SN2: polar aprotic solvent with good nucleophileIX. Leaving groups: stable anions/conjugate base of strong acids are good leaving groupsa. Never a good leaving group: HO - , RO -, - NH2, H - , - CH3b. Best leaving groups are weak bases after they