Chem 112a Homework D Key 41462d

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

X

new pi bond must be adjacent to original lone pair location

new pi bond must be adjacent to original pi bond location

new pi bond must be adjacent to original pi bond location

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 f ollowing criteria: (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 f ormal charge and/ or more than one negative f ormal charge. [H2COH]

[H2CCN]

H H

H OH

H

minor major (N more electronegative than C)

minor major (major contributor has filled octets) [H2CNO2]

[(CH3)C(OH)2]

H H O O O N N N H H H O O O major minor minor (there is no way to have full octets and only one negative charge with this ion. The directions should have said "or (c) have more than one extra set of formal charges relative to the best resonance structure." H

H

H

H NH2

H2N

H

H2N

H

O

O H

H

H

O H

OH C H3C OH

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

H

NH2

O

OH C OH H3C

OH C H3C OH

NH2 H2N H H H minor minor major (major contributors are equal importance, both have filled octets) O

H2C C N

H2C C N

OH

H

H

H2N

O

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

NH2 H major

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

35 H NH

16 B

O

4-5 H

B

H O

20 O

H

B

10 (this is from p. 5 of Evans table)

H (deprotonate either site) NH

O

O

O

O

O

N

O

H B

O

O

O

O

N

N

O

O

b) For each of the molecules below, identify the most basic site and draw an arrow-pushing mechanism for reaction with a generic acid, H-A. Next, estimate the pKa of each of the products that you drew (the conjugate acids of the starting molecules). Consult a pKa table as needed. NH2

OH

NH3

HO

OH2

O

O

HO

OH

O

OH

N

O

HO

N

O

approximate pKa 10 -2 <0 (-8?) <0 (-6?) <0 (-12?) (see the "protonated species" column on the first page of the Evans pKa table - values in parentheses are for the corresponding molecule with a phenyl substituent instead of one of the ethyl groups, which will have an influence on the exact numbers.) Note: The other option f or protonation of the carboxylic acid is on the lef t oxygen. T his leads to a positive charge on that oxygen which cannot be resonance stabilized (while the structure above can be resonance stabilized - draw this resonance to convince yourself ). T heref ore the one shown above is more f avorable.

c) For each of the following molecules, draw as many significant resonance structures as you can, with electron-pushing arrows to show interconversion between your structures. NH2

OH

HO

O

O

O

N

O

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

d) Consider the resonance structures you drew in part c and the reactions that you drew in part a. For each molecule below, predict whether is it more 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 not possible, state why) NH2

OH

more acidic (lower pKa) conjugate base is stabilized by resonance

HO

O

O

O

N

O

very little effect on acidity extra pi bonds are not in the correct location to stabilize the negative charge on conjugate base)

e) Consider the resonance structures you drew in part c and the reactions that you drew in part b. For each molecule below, predict whether is it more 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)? (Limit your answer to the same site of reactivity - if the correponding reaction with an acid is not possible, state why) NH2 less basic, higher pKa minor resonance structure places partial positive charge on the atom that is acting as a base

OH

HO

O

O

slightly more basic: minor resonance structure places partial negative charge on the atom that is acting as a base

O

N

O

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 the corresponding arrow-pushing mechanisms for reaction with an acid or base.

NH2

HO

OH

O

O

O

O

N

H

H H-A

H-A B

OH

NH2

O

A

A

B

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

pKa (H2O)

pKa (DMSO)

NH4+

9.2

10.5

11

11.1

H N C C C N

CH3OH

15.5

27.9

CH3O

H2O

15.7

31.2

HO

CN

O

N

O

H

H

NC

B

conjugate base NH3

B

O S DMSO

N C C C N H

Propose an explanation for the strong solvent-dependance of the pKa of methanol and water, while the other two acids 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 and each solvent, and how those interactions af f ect stability) Water (as a solvent) can participate in H-bonding as either a donor or acceptor. DMSO does not have any H's attached 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. This stabilization, which is a type of solvation, effectively delocalizes some of the negative charge density onto solvent molecules. DMSO cannot do this. This s for the large pKa dependance on solvent for these two 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 s for smaller pKa solvent dependance. The conjugate base of the second example allows the negative charge to be delocalized by resonance onto both nitrogen atoms as well as the carbon. This delocalization weakens the potential H-bonding interactionv with solvent, ing for a smaller pKa solvent dependance.

Error: Eclipsed conformation is highest in energy