1. Only one structure below has a correct set of curved arrows to show generation of an alternate resonancestructure. Each of the other structures has at least one incorrect use of curved arrows for this purpose. Crossout any incorrect arrow, and explain why that arrow would not lead to a valid resonance structure.

Guidelines for drawing electron-pushing arrows to generate resonance structures:

a. A lone pair of electrons becomes a bond to an adjacent atom.

b. A pair of electrons in a bond moves to become a lone pair on one atom in that bond.

c. A pair of electrons in a bond moves to become a bond to an adjacent atom.

2. For each of the following molecules or ions, draw as many significant resonance structures as you can, with

electron-pushing arrows to show interconversion between your structures. Label them "major contributor" and "minor

contributor" based on their relative importance in describing the overall electron distribution for the molecule or ion.

The bond connectivity is implied by the condensed structural formaulas. Make sure your answers are all

resonance stuctures of each other (not constitutional isomers).

(Note: Resonance structures are considered "not signif icant" if they meet any of the following criter ia: (a) have

an empty orbital (unf illed octet) on an electronegative atom (N, O, F) or (b) have more than one atom with

unf illed octets or (c) have more than one positive formal charge and/or more than one negative formal charge.

[H2COH]

[H2CNO2]

[H2CCN]

H2N NH2

[(CH3)C(OH)2]

H

O

H

O

new pi bondmust be adjacentto original lonepair location

X X

new pi bondmust be adjacentto original pibond location

X

new pi bondmust be adjacentto original pibond location

H OH

H

H OH

H

minor major(major contributor has filled octets)

H2C C N H2C C N

minor major(N more electronegative than C)

N

H

H

O

Omajor minor minor

(there is no way to have full octets and only onenegative charge with this ion. The directions shouldhave said "or (c) have more than one extra set of formalcharges relative to the best resonance structure."

N

H

H

O

O

N

H

H

O

O H3CC

OH

OH H3CC

OH

OH H3CC

OH

OH

minor major major(major contributors are equal, have filled octets)

H2N NH2

H

H

H

H

HH

H2N NH2

H

HH

H2N NH2

H

HH

minor minor major major(major contributors are equal importance, both have filled octets)

H

H

O

H

O

HH

O

H

O

Hminor major major(O is more electronegative than C, major contributors are equal importance)

3. Acidity, Basicity, and Resonancea) For each of the molecules below, identify the most acidic site and draw an arrow-pushing mechanism forreaction with a base, B:. Next, estimate the pKa of each starting molecule. You may wish to consult one ormore of the resources listed in the recent bSpace announcement. If you can't f ind enough information tomake a reasonable estimate, propose a reason why the inf ormation you looked for is not available.

b) For each of the molecules below, identify the most basic site and draw an arrow-pushing mechanism forreaction with a generic acid, H-A. Next, estimate the pKa of each of the products that you drew (theconjugate acids of the starting molecules). Consult a pKa table as needed.

NH2 OHHO O O

NO O

NH OO O O

NO O

NH3 OH2HO OH OH

NHO O

approximate pKa 10 -2

c) For each of the following molecules, draw as many significant resonance structures as you can, withelectron-pushing arrows to show interconversion between your structures.

d) Consider the resonance structures you drew in part c and the reactions that you drew in part a. For eachmolecule below, predict whether is itmore acidic or less acidic than the analogous molecule from part a.Does this correspond to a higher pKa or a lower pKa?(Limit your answer to the same site of reactivity - if the correponding reaction with a base is notpossible, state why)

e) Consider the resonance structures you drew in part c and the reactions that you drew in part b. For eachmolecule below, predict whether is itmore basic or less basic than the analogous molecule from part a.Does this correspond to a higher pKa (of its conjugate acid) or a lower pKa (of its conjugate acid)? (Limityour answer to the same site of reactivity - if the correponding reaction with an acid is not possible,state why)

NH2 OHHO O O

NO O

NH2 OHHO O O

NO O

NH2 OHHO O O

NO O

less basic, higher pKaminor resonancestructure places partialpositive charge on theatom that is acting asa base

slightly more basic:minor resonancestructure places partialnegative charge on theatom that is acting as abase

A partial answer key is included here (parts c, d, and e) to help get you startedon this problem. A full answer key will be posted separately to allow you touse these hints before looking up the answer.

more acidic(lower pKa)conjugate baseis stabilized byresonance

very little effect on acidity -extra pi bonds are not inthe correct location tostabilize the negativecharge on conjugate base)

f) Consider the resonance structures you drew in part c. Do any of the molecules have an alternate acidic site

and/or and alternate basic site that can be predicted based on these resonance structures? If so, draw thecorresponding arrow-pushing mechanisms for reaction with an acid or base.

4. Consider the following pKa data collected in different solvents:

acid pKa (H2O) pKa (DMSO)

NH4+ 9.2 10.5

11 11.1

CH3OH 15.5 27.9

H2O 15.7 31.2

Propose an explanation for the strong solvent-dependance of the pKa of methanol and water, while the other twoacids listed here show a much smaller solvent dependance.(Hint: think about what intermolecular interactions can take place with the acid and/or it's conjugate base andeach solvent, and how those interactions af fect stability)

NH2 OHHO O O

NO O

CNNC

S

O

DMSO

Water (as a solvent) can participate in H-bonding as either a donor or acceptor. DMSO does not have any H'sattached to N, O, or F, so it cannot act as a H-bond donor (it can still act as an H-bond acceptor).

Water can stabilize an anion such as hydroxide or methoxide by very strong hydrogen bonding. Thisstabilization, which is a type of solvation, effectively delocalizes some of the negative charge density ontosolvent molecules. DMSO cannot do this. This accounts for the large pKa dependance on solvent for thesetwo examples.

The conjugate base of NH4+ is neutral. While ammonia can still participate in H-bonding, it is not as crucial to

stabilize it since there is no negative charge. This accounts for smaller pKa solvent dependance.

The conjugate base of the second example allows the negative charge to be delocalized by resonance ontoboth nitrogen atoms as well as the carbon. This delocalization weakens the potential H-bonding interactionvwith solvent, accounting for a smaller pKa solvent dependance.

conjugate base

NH3

N CHC C N N C C

HC N

CH3O

HO

HH

BB

H-A H-A

NH2

H

OH

H

A A

ON

O O

B B

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Chem 112A Homework D keyChem+112A+-+Homework+D+key+-+Part+2