Tutorial questions Chem320 Clayden Chapters 1 1. The pk a provides an indication of acid strength. Substituents on an aromatic ring have a profound effect on the acidity of an aromatic carboylic acid. Eplain this relationship between the pk a of an aromatic acid and the effect of substituents using the following eamples: () 2 pk a =.8 2 pk a = 3. ote: Base your argument on the ease of loosing a proton and the distribution of the partial charge on the carboylic oygen atoms: C C - 2. ammett (189-1987) discovered that para- and meta-substituents to aromatic carboylic acids display a well defined linear relationship. Eplain what the nature of this effect is (electronic, steric etc.) by discussing the reasons why the correations for different classes of substrates differ as shown in the plot below. () log k of ester hydrolysis (RCEt) = aliphatic acid/esters = para/meta aromatic acid/esters = ortho aromatic acid/esters pk a of RC R = aliphatic or ortho-, meta- and para-aromatic acids 3. ammett defined the substituent constant σ and rather the value for σ against log(k /k ). The formula to calculate σ is provided below: k a ( C6C) σ = log ka (C6C) where = substituent in para or ortho position. 1
Eplain what will happen to the value of σ for electron withdrawing substituents such as = 2. Motivate your answer by referring to the general trend in pk a values of 2 -C 6 - C (pka = 3.) and C 6 -C (pka =.2). (). Use the ammett equation above to eplain why the value of σ for benzoic acid (per definition) is one? (int: = for benzoic acid). (2). According to eisting eperimental results, the mechanism for the base catalysed hydrolysis of an ester is: - Et Et - Et - Et - Et (a) ow many transition states are present in the reaction above? (1) (b) Which of the intermediates (2, 3 or ) could potentially be a rate determining step? Motivate your answer. (3) (c) What is the difference/similarity between a transition state and the rate determining step in a reaction? (use the eample above to illustrate your answer). (2) (d) Can there be more than one rate determining steps in the reaction above? Eplain your answer. (2) (e) The ammett reaction constant ρ for the reaction above 2.6 [the slope of the plot of σ against log(pk /pk )]. Eplain which of the intermediates above would be the rate determining step when ρ = 2.6? Motivate your answer. Use the information provided below. -6 - - -3-2 -1 0 6 large negative ρ values 1. Positive charge on ring. 2. Positive change delocalized on the ring. moderate negative ρ values 1. Electrons flow out of TS. 2. Positive change near ring. 3. Loss of conjugation small ρ values 1. too far away. 2. o electron change. 3. Two ρ-values cancell out. moderate postive ρ values 1. Electrons flow into TS. 2. egative change near ring. 3. Loss of conjugation large postive ρ values 1. egative charge on ring. 2. egative change delocalized on the ring. (note: TS = transition state) 6. Eplain what the following convention implies? (draw all possible structures): (3) 2
7. Use the information in the question (c) and your general knowledge about chemistry to eplain what electronic reason can be use to rationalize the different ρ-values below. (6) - ρ = 1.0 (by definition) ρ = 0. ρ = 0.2 ρ = 0. - ρ = 2.3 3 2 ρ = 3.2 8. Below is a mechanism of a nucleophilic aromatic substitution reaction where a bromine is substituted with an amine. The reaction is a two step process involving two transition states. 3
- - - - - (a) - - - - - - If the transition state associated with rate determining step was found to be the transition state 2, what value of ρ would you suggest for the reaction [motivate your answer, using the data in question 3(c)]. (3) 9. Below is an eample of an intramoleculare Friedel-Crafts reaction. ote that the ortho- and para-substituents under investigation in terms of the ammett behaviour are positioned on the -group.
(fast) - 2 6 - (fast) ote that = 7 The ammett plot is not linear, when different substituents are used. See the below. log k ρ = 2.7 ρ = -2. σ = 0. on linear ammet plot for the reaction above. Indicate which of the two transition states above (3 or ) is associated with ρ = 2. and ρ = 2.7 respectively. Motivate why discuss the influence of para-substituents = 2 and = 2 on the value of ρ as well as the mechanism of the reaction. () σ 10. Similar questions on p1096-1100.