CHM 233 : Fall 2018 Quiz #10 - Answer Key

Size: px

Start display at page:

Download "CHM 233 : Fall 2018 Quiz #10 - Answer Key"

Fay Bishop
5 years ago
Views:

1 M 233 : Fall 2018 Quiz #10 - nswer Key Question 1 M20a Which of the following is The strongest Bronsted cid? R- 2 is a carboxylic acid and R-S 3 is a solfonic acid, you will want to draw these two as Lewis structures and also draw the conjugate base anions to answer this question F 3 S S 3 3 S 3 B D The strongest acid has the most stable conjugate base anion 3 more acidic, lower pka 3 more stable less acidic, higher pka B 3 3 B less stable Bronsted acidity is controlled by the following factors: 1. Electronegativity of the atom with the negative charge. 2. The size of the atom bearing the negative charge. 3. Resonance. 4. Substituent effects. 2 3 carboxylic acid 3 S S S S sulfonic acid Sulfonic acids are generally stronger than carboxylic acids since the conjugate base is stabilized by 3 resonance structures compared to 2 for the carboxylic acids, therefore the answer is not B. Increasing Stability F 3 electronegative group S > S > 3 resonance donating group The remaining difference is the substituents. The electron withdrawing F 3 group (3 electronegative F's) stabilizes the negative charge and thus the anion. Later we will see that a resonance electron donating group such as -3 destabilizes the negative charge and thus the anion. S

2 acetone pk a = 19 Question 2 M20c Which of the following statements about the equilibrium below is true? Stronger Stronger Weaker Weaker cid Base Base cid allyl anion acetone enolate propene pk a = 43 I the equilibrium will lie on the left II the equilibrium will lie on the right III the allyl anion is the strongest base in the equilibrium IV propene is the weakest acid in the equilibrium I and IV only II, III and IV only B II and IV only D I, III and IV only acetone and propene are the acids on each side of the equilibrium, the two anions are the bases. The strongest acid is the one with the lower pka the stronger acid has the weaker conjugate base and vica versa, thus the allyl anion is the strongest base. The strongest acid and the strongest base "push" the equilibrium to the RIGT because they react faster than the weaker acid and base react in the reverse direction both of these answers are easy to obtain since the pka's are given, the smaller pka corresponds to the stronger acid you could also have worked this out based on electron energies. cetone is the stronger acid because it "makes" the more stable conjugate base, acetone enolate. omparing resonance structures of the two anions, the negative charge is "shared" by two carbons in the allyl anion, but by a carbon and a more electronegative oxygen in the enolate, the electrons are lower in energy in the enolate, the enolate is less reactive, is a weaker base. acetone enolate allyl anion

3 Question 3 M20d Which of the following is the product of the acid/base reaction between aluminum trichloride (ll 3 ) and acetone ( 3 3 )? l l l l ll 3 ll 3 ll 3 ll 3 B D although ll3 has lots of non-bonding electrons, they are on the electronegative chlorines and are thus not very high in energy, not very reactive. the nonbonding electrons on acetone are also on an electronegative atom, oxygen, and whether acetone reacts as a base or an acid depends upon what it reacts with L l l l LB l in this case the key point is that aluminum has only 3 valence electrons, thus with 3 bonds to chlorine it can easily accept a pair of electrons from oxygen, thus forming a bond without having to break a bond, this is an ideal Lewis acid/base reaction. note that if the pi-electrons in the acetone had ben used, a bond would have been broken in the reaction, thus D is not a good answer the oxygen will develop a formal positive charge since is loses a half share of the the non-bonding electrons, and the aluminum gains a formal negative charge since it gains a half share of the non-bonding pair the electrons in the new bond are considerably lowered in energy, we expect this Lewis acid/base reaction to be exothermic l l l l

4 Question 4 M20e Give the major product of the following Lewis cid/base reaction. ote that the non-bonding electrons are T shown on the fluorines in either the reactants or products (hint, you will need to draw a minor resonance contributor of the ester to determine the most reactive pair of nonbonding electrons in the ester) BF 3 LB BF 3 L BF 3 BF 3 B D BF 3 B and D are impossible, boron does not have a pair of electrons that it can use to form a new bond To decide between and, consider the following minor resonance contributor R, consider the following bond dipole moments is bigger than because of the pi electrons in the double bond Either way it is clear that there is a larger partial negative charge on the carbonyl (=) oxygen, thus the electron energy will be higher on this oxygen, hence these electrons are more reactive, these represent the Lewis base site.

5 Question 5 M20i Which of the following reactions is most likely to occur? makes a σ-bond, breaks a π-bond and puts negative charge on oxygen, this is the best. B gives electrons to hydride, nowhere for them to go, very bad reaction! makes a σ-bond, breaks a π-bond and puts negative charge on carbon, which is not as good as. D gives electrons to hydride, nowhere for them to go, very bad reaction! B D

6 Question 6 M20k Which is the most acidic proton in this molecule, a, b, c or d (not all of the - bonds are shown in this molecule)? hint, draw all of the conjugate base anions and decide which anion has the lowest energy electrons) a b c d a B b c D d a b c d b c d b c d 2 resonance contributors to the anion structure a b c d a c d X no other resonance contributors to the anion structure a b c d a b d X no other resonance contributors to the anion structure a a a a b b b b c d c c c TREE resonance contributors for the anion structure, most delocalized lowest energy nonbonding electrons, most stable conjugate base anion, most acidic proton

7 Question 7 M20l Which is the weakest base? (hint, look for minor resonance contributors) B D X no minor resonance contributors to the structure, non-bonding electrons are not particularly stabilized or low in energy B X no minor resonance contributors to the structure, non-bonding electrons are not particularly stabilized or low in energy minor resonance contributor to the structure shows how the nonbonding electrons are somewhat stabilized by delocalization, and this lower in energy than in or B D two minor resonance contributors to the structure shows how the non-bonding electrons are even more stabilized by delocalization than in, these non-bonding electrons are thus lowest in energy of all, are thus least reactive, D is thus the weakest base

8 Question 8 M20b What are the most likely products of the following Bronsted cid/base reaction? 3 2? I II 3 3 III 3 IV 2 2 V = I & IV B = I & III = II & IV D = II & V highest energy, most reactive electrons are non-bonding on nitrogen, the amine will be the Lewis base L/B LB/BB making a bond to the hydrogen requires breaking this - bond, which generates a resonance stabilized anion on oxygen, this is the bst reaction for the Lewis acid since these are the lowest electron energy products that can be obtained from the Lewis acid also, it isn't cheating just to know that a carboxylic acid will donate the proton on the oxygen when reacting with a base!!! in addition to being a Lewis acid base reaction the base provides the electrons to form a new bond to one of the atoms on the acid, LB and L), this is also a Bronsted acid base reaction, since overall a proton is transferred from teh acid to the base, thus the reaction is labelled as L/B and LB/BB.

CHEM 109A Organic Chemistry

CHEM 109A Organic Chemistry https://labs.chem.ucsb.edu/zakarian/armen/courses.html Chapter 2 Acids and Bases Central to Understanding Organic Chemistry Draw the conjugate acid of each of the following: