### H-Br - Strong Brønsted acid with pKa = -9 - Strong electrophilic character - Very weak nucleophilic character ### Carbocations - Planar geometry, sp$^{2}$ hybridization at carbon - Positive charge on carbon (Pauling EN = 2.5) - high energy species! - Irresistibly attracted to nucleophiles, δ- species - Incomplete octet - Powerful electrophile: covets electron pairs! - Should react with even weak nucleophiles - Alkyl substitution on positively charged carbon reduces electrophilicity ### H$_{3}$O$^{+}$ - Strong Brønsted acid with pKa = -1.7 - Strong electrophilic character - No nucleophilic character ### Bromine and Chlorine - X-X bonds are very weak (F-F 38 kcal/mol, Cl-Cl 58, Br-Br 46, I-I 36) - X-X bonds are highly polarizable - X-X bonds have a low energy vacant σ* orbital - Br$_{2}$, Cl$_{2}$, and I$_{2}$ are relatively strong electrophiles; F$_{2}$ is untameably strong - Relatively low nucleophilicity due to high electronegativity of halogens ### Bromonium Ions - Positively charged species - highly electron deficient - Carbon atoms have significant δ+ character - High degree of angle strain in three-membered ring - Powerful electrophile - will react with even weak nucleophiles - Nucleophilic attack favored at carbon thus neutralizing + charge on bromine - Carbon-bromine bond has low energy vacant σ* orbital - Nucleophilic attack at carbon occurs anti to C-Br bond ("backside attack") ### Borane - Incomplete octet - only six electrons in valence shell - Vacant p orbital - Very strong electrophilic character and Lewis acidity - Behavior: ravenous desire for electron pairs! - Forms complexes with Lewis bases that then have anionic character on the hydrogens attached to boron ### Primary Alkyl Halides - Highly polarized C-X bond with δ+ character at carbon - C-X bond has low-energy vacant σ* orbital - Moderate electrophilic character at carbon of C-X bond - Very weak nucleophiles due to electronegativity of halogens and lack of negative charge ### Peroxyacids - Brønsted acids with pKa = 7-8 (less acidic than carboxylic acids) - Extremely weak O-O bond (BDE = ca. 35 kcal/mol) - O-O bond has low energy σ* orbital - Expected behavior: significant electrophilic character at the oxygen of the hydroxyl group ### Alkyl Sulfonates (tosylate R-OTs, mesylate R-OMs, triflate R-OTf) - Sulfonates are strong electron-withdrawing groups via inductive effects - Carbon attached to OSO$_{2}$R is highly electrophilic - Sulfonates are excellent leaving groups (Brønsted bases with conjugate acids pKa -3 to -13) - Sulfonate anions are very weak bases and very weak nucleophiles