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 atom o 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 donor o The acidic proton in A H is pulled off by the base and this rxn leads to cleavage of the covalent bond b w A and H w transfer of those 2 electrons from the A H bond to A o 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 proton o The weaker the A H bond is the stronger the base will be o 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 Assume the larger value of Ka will correlate w a stronger acid Equilibrium lies to the right 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 reactive o The rxn w CA is slow and equilibrium is pushed to the right Ka is larger o o o Formation of a more stable CB is associated w a stronger acid If A is less reactive there will be a higher concentration of A and HB products 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 reactive o The CB easily reacts easily reacts w the CA o Concentration of products is lower and the concentration of the reactants is greater o 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 degree o 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 degree o This polarization is most often observed when H is attached to a heteroatom The O H unit is found in alcohols and carboxylic acids o B c O is more electronegative than H the O H unit is polarized such that O is and H 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 water o Most alcohols have pKa values of 16 18 o The H atom of an alcohol is polarized so it s a Bronsted Lowry acid and reacts w a base A base chosen to react w the alcohol should be stronger than the alkoxide The CB of this rxn is an alkoxide RO 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 acidity o The C next to the dipole on O suggests that the electron density is distorted o toward O so C is considered to be electron releasing If the O atom in an alcohol draws electron density away from C less electron density must 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 water o 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 and the 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 water o 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 acid when NaNH2 is the base However when methanol is mixed w NaNH2 there s a significant o Other factors influence acidity of a given acid concentration of CB in the rxn 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 E o G free E change H enthalpy change S entropy change Enthalpy H 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 G assumes the rxn is done under standard conditions 1M and 1 atm Entropy S measures the disorder of a system o o If the of particles for a rxn remains the same or decreases the change in entropy is small If the of particles greatly increases during the course of a rxn then entropy increases A negative value of H indicates a spontaneous exothermic rxn exergonic rxn o The bond making and bond breaking processes of the rxn generate more E than is A positive value of H 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 required to initiate the rxn It s not a or H that determines whether a rxn is endothermic or exothermic but rather a or value of G If the S is very small and H is large then the above statements about or values of H then using on the values of H to estimate G are reasonable However if S is large and or H 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 endothermic It is possible to follow the E changes for a …