H H H H S H H H NO 2 O H H 3 C. OMe CH 3

Transcription

1 Fall Workshop #9 Question #1 Using the tables on pages of SAM, the table inside the back cover of Jones, and the figure we created in class: (a) Report an estimated pka value for each hydrogen atom shown in RED in each structure (b) Describe which of the following effects contribute to the acidity of the indicated hydrogen atoms: the formal charge, electronegativity, inductive, and resonance effects (c) For each compound, circle the most acidic hydrogen (d) Using a generic base, deprotonate the most acidic hydrogen atom and draw its conjugate base anion. Are there features that can contribute to delocalization of charge? Cl S Me 2 3 C C Me S C 3 F

2 Question #2 Rank the compounds in each of the series below from STRGEST (1) to WEAKEST (4) acid. Make sure that you locate the MST acidic hydrogen in each structure and estimate a pka value before you rank. 2 2 C 3 C 2 C 2 C 2 C 3 C 2 CBrC 2 ClC 2 C 2 C 2 C 2 C 3 C 2 CCl 2 C 2 C 2 (C 2 C 2 5 ) 2 C3 CC 2 C 2 C 2 5 (C 3 C) 2 C 2 RC C Question #3 In this problem you are asked to draw structural formulas for the most favored conjugate acid for each of the following bases. Consider the reaction of each with a generic acid (+) which will lead you to the conjugate acid forms of these bases. Most of the conjugate acids here are stabilized by charge delocalization, so you should draw formulas for all important contributing structures (also called canonical or resonance forms). The KEY ASPECT of this problem is determining where to protonate.

3 Question #4 ften times, amines are converted to their acid salts in order that they can be stored for long periods of time without degrading. ne such example is shown below in which mescaline exists as a hydrochloride salt. Consider the reaction shown below between the protonated form of mescaline and a. Using the pair problem solving method (as explained by your workshop leader), answer the following directives. ote: each directive has been labeled with an A or B to indicate the task of each person in the pair. A: Show all hydrogen atoms in protonated mescaline and report the pka values (from the tables in SAM ). Circle the most acidic hydrogen and explain your choice. B: Realizing that a dissociates into ions, draw the Lewis structure for hydroxide anion. A: The hydroxide anion exists as a major reactive species in solution and functions as the Bronsted-Lowry base in the forward direction. Use the curved-arrow formalism to show the flow of electrons as the reaction proceeds from reactants to products (forward direction) A: Draw the structures for the expected products of this reaction based upon the mechanism you derived. B: Which species is your nucleophile (Lewis base) and which is your electrophile (Lewis acid) in the forward direction? Briefly explain your choices. A: Report the appropriate pk a value to use for the other acid (on the product side of the of your equilibrium reaction). B: Designate each of the species in your reaction as a strong acid (SA), strong base (SB), weak acid (WA), or weak base (WB). Explain your rationale. B: Show the conjugate acid-base pairs. A: Calculate the overall pk a (from the individual pk a s) and convert to the K a. pka (overall) = pka (forward rxn) - pka (reverse rxn) B: Use the K a from the previous answer to find ΔG rxn at room temperature (23 C). B: Which species (products or reactants) are favored at equilibrium? Explain.

4 Question #5 Sketch reaction energy diagrams that accurately represent each of the following criteria. Be sure to appropriately label the axes, reactants, products, intermediates, transition states, ΔG, ΔG, Δ, and Δ. (a) a two-step reaction in which the first step is endergonic, the second step is exergonic, and the overall reaction is spontaneous (b) a three-step reaction in which the first step is endothermic, the second and third steps are exothermic, and the overall reaction is exothermic (c) a two-step reaction that is spontaneous and whose first step is faster than its second step For each of the reaction energy diagrams, answer the following and discuss your reasoning: Which species does EAC transition state most closely resemble? Which is the fastest step in the forward direction? Which is the fastest step in the reverse direction? Which step in each reaction is the rate-determining step (forward direction only)? Question #6 Complete the table below in which you rank the carbocations shown in DECREASIG order of stability. Be sure to indicate the type(s) of special effects that are contributing to the relative stability of each ion -- Recall our discussion about the electron donating capability of attached ALKYL groups (I referred to this in class as hyperconjugation, a term you may want to define before solving this problem see Jones p and the Purple Book p for more info). For those ions that exhibit a resonance stabilization effect --- show ALL possible resonance structures. Sketch a corresponding reaction energy diagram for each dissociation being sure to show the relative magnitude of energetic change rder of stability Stabilizing Effects (Reason for ranking) (most stable) = (least stable) =

5 Question #7 Based upon your knowledge of acid-base chemistry, rank the following R-X compounds in terms of their ability to dissociate (of the type shown in the generic reaction scheme below). Rank in order of MST to LEAST endothermic and sketch a corresponding reaction energy diagram for each dissociation being sure to show the relative magnitude of energetic change. What is the overwhelming reason why you ranked these compounds as you did? R X R + + X - 3 C Ts 3 C Cl 3 C 3 C 2 TE: Ts refers to the tosylate group -- often organic chemists react alcohols with para-toluene sulfonyl chloride (ptscl) to form the tosylate group which converts from a poor leaving group to a good leaving group ptscl pyridine Ts good LG poor LG S