Organic Chemistry Week 4 Notes
Organic Chemistry Week 4 Notes CHEM141A
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This 2 page Class Notes was uploaded by Hannah Malcomson on Saturday September 24, 2016. The Class Notes belongs to CHEM141A at University of Vermont taught by Wurthmann in Fall 2016. Since its upload, it has received 6 views. For similar materials see Organic Chemistry 1 in Chemistry at University of Vermont.
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Date Created: 09/24/16
Hannah Malcomson Week 4: Orgo Notes Acids and Bases: Cont. pH= environment of a reaction pKa= value, defines acidity of compound Ex- H-A pKa= 10 H-X pKa= 22 pKa values can be used to added the relative acidity of two compounds. The pKa of H-A tells you that H-A is more acidic than H-X, which indicates that H-A will be a better H+ donor. Since the more acidic molecule is more likely to donate an H+, it is less stable. The weaker acid will be more stable, and is therefore the favored side of the equilibrium. Larger pKa= weaker acid= favored side of the equilibrium Keq= [products]/[reactants]= [H O3][A-]/[H O]2HA] If we assume this is a dilute solution, then 2 O is a constant- therefore we convert Keq to Ka. Ka= [H 3+][A-]/[HA] The Ka for acids vary by large numbers. To harmonize the numbers, we use pKa= -log(Ka) Process for Assessment: (use pKa’s to find favored side of equilibrium) Identify the base (H+ acceptor, frequently negative charge or lone pairs) Identify acidic hydrogens (label ∂+ and ∂-) Draw curved arrows Draw structures of conjugate acids and conjugate bases based on the mechanism. Assign pKa values, and predict favored side. Higher pKa value= favored side Strong acid + strong base ↔ weaker conjugate base + weaker conjugate acid What to do without pKa values? Estimate acid/ base strength. An acidic molecule will only donate an H+ if the receiving base molecule has a viable way to host the electrons as a conjugate acid- an electronegative atom is well suited to host a negative charge and will frequently do so. A weak conjugate base if a good host of electrons Estimating Acid/ Base strength: Acidity increases across the row of the Periodic Table Down a column of the Periodic Table, acidity increases. Larger atom- more effective distribution of electrons= more stable Resonance- sharing of electrons (negative charge) over multiple atoms stabilized charge. Induction- electronegative atoms create a polar covalent bond which can deplete another atom of it’s electron density. Depleting a negative charge stabilized a molecule. Hybridization- the negative charge in an sp-hybrid orbital is more spherical compared to an sp or sp –hybrid orbital. This pulls the electrons closer to the nucleus of the carbon atom, stabilizing the negative charge.