266 Bulky Base Elimination

266 Bulky Base Elimination - The effect of a strong bulky...

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Unformatted text preview: The effect of a strong bulky base acetone CH3CH2CH2Br Primary + CH3S Na Good nucleophile Weak base CH3CH2OH CH3CH2CH2SCH3 SN2 H3C CH3CH2CH2 OCH2CH3 SN2 (91%) + H C C H E2 (9%) H CH3CH2CH2Br + CH3CH2O Na Primary Good nucleophile Strong base H3C CH3CH2CH2CH2Br + Primary H3C H3C Poor nucleophile Bulky Strong base C O Na (CH3)3COH H3C H3C H3C C OCH2CH2CH2CH3 SN2 (15%) H3CH2C + H C C H E2 (85%) H Br CH3CHCH3 Secondary + O acetone CH3C O Na Fair nucleophile Weak base Br CH3CH2CHCH2CH3 Secondary + H2O Poor nucleophile Weak base 80oC O C CH3 O CH3CHCH3 SN2 100% OH CH3CH2CHCH2CH3 SN1 (Major) + H3CHC CHCH2CH3 E1 (Minor) I NaOCH3 2o DMSO H H + 62% H H 21% H H 17% S H + 59% H H 20% H + H H 21% H3C DMSO Solvent CH3 + H O total internal double bond = 83% I (Bulky Base) NaOC(CH 3)3 2 o H DMSO total internal double bond = 79% Organic Chem: Jones and Fleming CH 3 CH 3CH 2CCH 3 3o Br CH3CH2ONa, CH3CH2OH, 70oC -HBr H3C CH3 C C CH3 + H3CH2C C H3C 30% 2-methyl-1-butene CH2 H 2-bromo-2-methylbutane CH 3 CH 3CH 2CCH 3 3o Br (Bulky Base) (CH 3) 3COK, (CH 3)3COH -HBr 70% 2-methyl-2-butene H3C C H C CH3 + CH3 H3CH2C C H3C 73% 2-methyl-1-butene CH2 2-bromo-2-methylbutane 27% 2-methyl-2-butene Dr. Steven Forsey 1 Substrate CH 3X RCH2X o Primary (1 ) R R CH X Methyl Possible Reaction Explanation SN2 SN2: Sterically unhindered electrophile easy access for nucleophilic attack An elimination reaction cannot occur SN2 E2 SN2: Main reaction with strong nucleophiles (e.g., RS, I, CN) or strong bases (e.g., RO). The alkyl halide is sterically unhindered and will readily undergo an SN2 E2: An E2 reaction will mainly occur with a hindered strong base (e.g., (R3CO) SN1/E1 does not occur because a primary carbocation will not form Tertiary (3o) SN1/E1 SN2, E2, SN1/E1 E2 SN2: main reaction when SN1: main reaction if a weak using good base/poor nucleophile (e.g., nucleophiles/weak bases ROH) is used in a polar (e.g., RS, I, CN, RCO2). An protic solvent (e.g., ROH). aprotic solvent. (e.g., Solvent is most likely the acetone, DMF,THF) enhances nucleophile. E1 minor the reactivity of a nucleophile products will also form. Remember to think about E2 internal double bond: regiochemistry and Main reaction when an stereochemistry. Anytime a strong base (e.g., RO) is carbocation forms used. Do not forget (E) and rearrangement can occur (Z) isomers may form E2: Main reaction with a SN1: main reaction if a weak strong base (e.g., RO, base/poor nucleophile (e.g., R3CO) Remember to think ROH) is used in a polar protic about regiochemistry and solvent (e.g., ROH). Solvent is stereochemistry most likely the nucleophile. An SN2 reaction can not E1 products will also form. occur because of steric Remember to think about interaction. The nucleophile regiochemistry and is completely blocked from stereochemistry. Anytime a a nucleophilic attack with a carbocation forms tertiary alkyl halide rearrangement can occur Secondary (2o) R R C R X Dr. Steven Forsey 2 Steric hindrance of nucleophiles in SN2 reactions For an SN2 reaction to advance to the transition state the nucleophile must get close to the electrophilic carbon before bonding can occur. Bulky groups adjacent to the nucleophilic atom will hinder this approach and slow down the rate of reaction. For example consider two oxygen nucleophiles, sodium tertbutoxide and sodium methoxide. The nucleophilic oxygen of the methoxide ion is unhindered by adjacent groups. However, the oxygen of the tertbutoxide ion is hindered by three methyl groups. methoxide ion H 3C O H3C C O ter t-butoxide ion H 3C H 3C Sodium tertbutoxide is a stronger base than sodium methoxide because of the electron donating alkyl groups adjacent to the negatively charged oxygen. However when these nucleophiles are reacted with the same electrophile we find that the rate of reaction is faster for the methoxide anion. + Br + H3C Fast H3C O H 3C O CH3 + Br H 3C H3C + Br CH 3 + C O + Slow H 3C Br C O H3C H 3C H3C H 3C Thus just like a solvent that impedes the approach of nucleophile and the formation of the transition state, bulky group adjacent to the nucleophilic atom will slow down the rate of reaction. The methoxide ion is therefore a stronger nucleophile than the bulky tert butoxide ion even though the tertbutoxide is a stronger base. You may ask wouldn't the bulkiness of the base effect it's base strength as well. In fact it might but in most cases we find that steric hindrance has little effect on basicity because basicity generally involves an attack on an unhindered proton. Dr. Steven Forsey 3 ...
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