Chapter 6: Acids, Bases, Nucleophiles, and Electrophiles- A normal chem rxn in which the base donates 2 electrons to an electron-deficient center (proton of A:H)- The A—H bond will be polarized w/a + H atomo Polarization weakens the A—H bond, making it easier to break, and there’s an electrostatic attraction b/w the + H and the electron rich base- The Bronsted-Lowry definition states that an acid is a proton donoro The acidic proton in A:H is “pulled off” by the base, and this rxn leads to cleavage ofthe covalent bond b/w A and H w/ transfer of those 2 electrons from the A—H bond to Ao Rxn generates an electron-rich A- (CB)- If H—A has a weak bond, it’s more reactive b/c it’s easier for the base to react w/ the protono The weaker the A—H bond is, the stronger the base will beo If A—H reacts w/ the base to a greater extent, there’s more product and Ka is larger (pKa is smaller) HA is more acidic Equilibrium lies to the right Assume the larger value of Ka will correlate w/ a stronger acid- If A—H bond is rather strong, it’s more difficult to break, and in an acid-base rxn there will be a lower concentration of the products- So, Ka will be smaller and the equilibrium will lie to the left- If the CB (A-) is more stable, it’s less reactiveo The rxn w/ CA is slow and equilibrium is pushed to the right (Ka is larger)o Formation of a more stable CB is associated w/ a stronger acido If A- is less reactive, there will be a higher concentration of A- and HB (products)o If the concentration of products is larger and Ka is larger, these data are interpreted to mean that HA is more acidic- If CB is less stable, it’s more reactiveo The CB easily reacts easily reacts w/ the CAo Concentration of products is lower, and the concentration of the reactants is greatero Smaller Ka weaker acid- If the charge is dispersed due to a larger size of the species, it’s more difficult to donate charge and the CB is more stable.o If the CB is resonance stabilized, it’s also less reactive.- If H+ is taken to be an acid, then any H atom that is polarized + should be acidic to some degreeo This + polarization is most often observed when H is attached to a heteroatom- If H+ is taken to be an acid, the any H+ atom that is polarized + should be acidic to some degreeo This + polarization is most often observed when H is attached to a heteroatom.- The O—H unit is found in alcohols and carboxylic acidso B/c O is more electronegative than H, the O—H unit is polarized such that O is – andH +o The presence of the C group in an alcohol leads to a smaller dipole for the O—H bond, so the H atom is less polarized (a smaller +) and this is expected to make an alcohol slightly less acidic in watero Most alcohols have pKa values of 16-18o The H atom of an alcohol is polarized +, so it’s a Bronsted-Lowry acid and reacts w/ a base The CB of this rxn is an alkoxide (RO-) A base chosen to react w/ the alcohol should be stronger than the alkoxide product- A pKa greater than 16-18- Anything making the O—H bond in methanol is more polarized should enhance acidity, but anything that makes it less polarized should diminish acidityo The + C next to the – dipole on O suggests that the electron density is distorted toward O, so C is considered to be electron releasingo If the O atom in an alcohol draws electron density away from C, less electron densitymust be w/drawn from the + H on an O, making the O—H bond less polarized when compared to O—H in water This means that the H of the proton in an alcohol has a smaller + than the proton in water, which doesn’t have the electron-releasing C group- Most alcohols are less acidic than watero If the new base is water rather than the amide anion, the O atom of water constitutes the base b/c it has unshared electrons The hydronium ion is a quite potent acid (much stronger than methanol) andthe equilibrium is shifted to the left, which means that Ka is much smaller.- If the acid is weaker relative to the CA, then water is a rather weak base in this rxn—much weaker than sodium amide- If water is a weak base relative to sodium amide, then Ka is larger for the rxn w/ NaNH2 ad small for the rxn w/ watero The strength of the acid depends on the strength of the base. Methanol is a very weak acid when water is the base, but it’s a stronger acidwhen NaNH2 is the base- However, when methanol is mixed w/ NaNH2, there’s a significantconcentration of CB in the rxno Other factors influence acidity of a given acid Solvent, stability of the ionic products formed, solubility, and the nature of the C group attached to the heteroatom- The progress of a rxn can be correlated w/ changes in Eo ΔG—free E change; ΔH—enthalpy change; ΔS—entropy change- Enthalpy (ΔH) is the bond E for the acid A—H, but the bond E of all bonds that are made or broken must be examined to determine the change in enthalpy- Standard Free Energy (ΔG): assumes the rxn is done under standard conditions (1M and 1 atm)- Entropy (ΔS) measures the disorder of a systemo If the # of particles for a rxn remains the same or decreases, the change in entropy is smallo If the # of particles greatly increases during the course of a rxn, then entropy increases- A negative value of ΔH indicates a spontaneous exothermic rxn (exergonic rxn)o The bond making and bond breaking processes of the rxn generate more E than is required to initiate the rxn- A positive value of ΔH indicates a nonspontaneous endothermic rxn (endergonic)o The bond making and bond breaking processes of the rxn generate less E than is required to initiate the rxn- It’s not a + or – ΔH that determines whether a rxn is endothermic or exothermic, but rather a + or – value of ΔGo If the ΔS is very small and ΔH is large, then the above statements about + or – valuesof ΔH, then using on the values of ΔH to estimate ΔG are reasonable.  However, if ΔS is large and/or ΔH is small, then entropy can’t be ignored- The fact that E must be applied to a rxn to initiate it is independent of whether the rxn is exothermic or endothermico It is possible to follow the E changes for a chem rxn and relate them to ΔG and exothermic/endothermic behavior- Energy Curveso The left of the curve is where the starting materials have not yet begun to react, and this E represents the E inherent to those compoundso The far right of the curve is where the rxn has been completed and the E inherent to