Nucleophiles - Organic Chemistry - Lecture Slides, Slides of Organic Chemistry

Download Nucleophiles - Organic Chemistry - Lecture Slides and more Slides Organic Chemistry in PDF only on Docsity! Nucleophiles, Electrophiles and Leaving Groups docsity.com Nucleophiles • Translation: Nucleus-loving • Chemical meaning: Reacts with positively- charged (or partially positive) atoms (electrophiles: electron-loving) • Characteristics: Nucleophilic atoms will have either lone pairs or pi bonds that can be used to form new bonds to electrophiles docsity.com Relative Nucleophile Strength - Solvent • Hydrogen-bonding solvents (protic solvents) reduce nucleophilicity by interacting with the free electrons in the nucleophile • This effect is particularly strong for small atoms with concentrated charges, thus larger atoms are more nucleophilic in protic solvents (~opposite basicity) HS- > HO- I- > Br- > Cl- > F- O H H N H H H O H H O H H O H H docsity.com Relative Nucleophile Strength – Aprotic Solvents • In solvents that can accept, but not donate hydrogen bonds, nucleophiles are not solvated (but the cations providing countercharges are) • Thus the nucleophilicity and basicity are more closely correlated: F- > Cl- > Br - > I- docsity.com Aprotic Solvent Examples O CH3CH2 CH2CH3 diethyl ether O C H N CH3 CH3 N,N-Dimethylformamide (DMF) O C H3C N CH3 CH3 Dimethylacetamide (DMA) O S H3C CH3 Dimethylsulfoxide O P(H3C)2N (H3C)2N N(CH3)2 Hexamethylphosphoramide (HMPA) docsity.com Example • Example: Hydroxide ion is a stronger base than cyanide ion, but cyanide ion is a stronger nucleophile (regardless of solvent) HO- + H2O H2O + HO - NC- + H2O HCN + HO - pKa = 15.7 pKa = 10 H CH H BrHO- + H CH H BrNC- + H CH H OH H CH H CN faster docsity.com Problem • Rank the following nucleophiles from strongest to weakest when dissolved in diethyl ether: Group I: H2O HO- H2N- H3N Group II: CH3Li NaCN CH2=CH2 docsity.com Electrophiles • Translation: Electron-loving • Chemical meaning: Reacts with sources of electrons (nucleophiles: nucleus-loving) • Characteristics: Electrophilic atoms will have – Positive charge, a partial positive charge, or be very polarizable – An empty orbital or a heterolytically breakable bond (to a leaving group) docsity.com Electrophile Strength - I • Given the same electrophilic atom, a greater degree of positive charge gives a stronger electrophile C H H H H C Br H Hδ − δ+ O C H3C CH3 δ+ δ− OH C H3C CH3 δ+ δ− CH3 is a better electrophile than is a better electrophile than docsity.com Electrophile Strength - II • The strength of electrophiles without empty orbitals (to which a bond must be broken before another can form) is also influenced by the nature of the group to which the bond will be broken (leaving group) docsity.com Leaving Groups • Leaving groups are the fragments that retain the electrons in a heterolytic bond cleavage: O H2C CH2 H H2C CH2 O H H C Br H Hδ − δ+ C H H H + Br- I I docsity.com