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Chemistry 303

fall, 2007

FIRST EXAMINATION

7:00 PM, OCTOBER 15TH, 2007

Duration: 2.0 hr

Name________________________KEY____________________________________ 

Lab TA___________________________________________________________

(if you do not know his/her name, give day of lab section—not BJ, Casey, nor David)

This is an "open book" examination; you may use anything that is not alive [or connected to the Web]

Note: if you do not know the complete or specific answer, give a partial or general answer—

We love to give partial credit.

If there seems to be more than one good answer, explain your thinking.

If you invoke resonance delocalization as part of your answer, draw the relevant resonance structures

WRITE SOMETHING

BUT: write legibly! We will not work too hard to decipher sloppy writing.

READ each problem carefully!

Write only in the space provided for each question.  

Score:

I___________/11 II___________/14 III___________/18 IV___________/12

V___________/12 VI___________/15 VII___________/08 VIII___________/10

Total: /100

There are 9 pages in this exam; please check now to be sure you have a complete set.

Pledge:_________________________________________________________________________________

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I. (11 pts). Consider the molecules, A, B, C.

A   B   C

O

O

N

CH3N

O

CH3H3C

 

A. (03 pts). Write here the list of molecules in order of increasing bp: 

__C___ < ___B__ < __A___

lowest highest

B. (04 pts). Give the single most important reason for the difference in bp for the one with highest bp and the one that is second

highest. Use words and pictures.

A B

O

N

CH3N

O

CH3

N

O:

CH3

A'

O

N:

B'

CH3

Dipolar structure A' is a stronger contributor to A than dipolar structure B' is to B, because chargesare localized on atoms with more favorable EN: (-) on O and (+) on N.  Therefore,A is a more polar molecule, stronger dipole-dipole interaction, and higher bp.  

C. (04 pts).  Give the single most important reason for the difference in bp for the one with second highest bp and the one that is

lowest. Use words and pictures.

CB

O

N

CH3

O O:

C'

O

N:

B'

CH3

Dipolar structure B' is a stronger contributor to B than dipolar structure C' is to C. Therefore,B is a more polar molecule, stronger dipole-dipole interaction, and higher bp. Resonance structureC' has one fewer bonds than C, and is therefore a small contributor.  

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II. (14 pts). Consider the molecule M for which the resonance stucture M1 has the connectivity represented below.

(all bonds and non-bonding electrons shown; no net charge)

 A. (02 pts) Draw in on the structure all formal charges, clearly showing the location.

H3C Ca

H3C

N   Cb   CH3

M1  

 B. (08 pts) Using the arrow formalism, draw three additional “good” resonance structures for M;  label them M2, M3, and M4. 

Evaluate the structures and circle the most stable one [if more than one is "best", indicate both]. Explain your choice.

H3C Ca

H3C

N   Cb   CH3H2C Ca

H3C

N   Cb   CH3H3C Ca

H3C

N Cb   CH3H3C Ca

H3C

N   Cb   CH3

M1  M2   M3   M4

OR:H2C Ca

H3C

N   Cb   CH3H3C Ca

H3C

N   Cb   CH3

M2M3

H3C Ca

H3C

N Cb   CH3

M1

H3C Ca

H3C

N Cb   CH3

M4

Both M2 and M3 have one additional bond. The charges are stabilized in the same way in both, so these structures are

not distinguishable in terms of stability.

M1 and M4 have no charges, which is stabilizing, but they also have one fewer bonds which is the dominant factor.

C. (04 pts) What is the hybridization at C a? sp3  sp2  sp  (circle best answer)

 At C b? sp3  sp

2  sp 

 At N? sp3  sp

2  sp 

What is the geometry about the C a-N-C b bonds— bent or linear? 

III. (18 pts) Consider the acidity of the three molecules, X, Y, and Z.

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VII. (08 pts) Compare the three alcohols here,  F, G, and H

H

CH

H

OH

H3C

CH3C

H3C

OH

F

CF

F

OH

F   G   H  

A.  Which is the best H-bond donor? Explain by comparing with each of the other two.

H

CH

H

O

H3C

CH3C

H3C

OHF

CF

F

OF   G

H

H

sterically accessible,polar -O-H bond.Good H-bond donor

sterically a little lessaccessible but probably notsignificant at the site of H-bond donation,polar -O-H bond.Good H-bond donor

H

Inductive effect of the CF3 group stronglyenhances the dipole character of the -O-H bond,makes the H more !+ and a better H-bond donor

B.  Which oxygen is the best H-bond acceptor? Explain by comparing with each of the other two

H

CH

H

O

H3C

CH3C

H3C

OHF

CF

F

O

F   GH

H

sterically accessibleoxygen, polar -O-Hbond. Good H-bondacceptor.

sterically less accessibleoxygen, weakerH-bond acceptor

H

Inductive effect of the CF3 group stronglyenhances the dipole character of the -O-H bond,makes the O less !- and a poorer H-bond acceptor

!+

!-

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VIII. (10 pts) Pick that member of the following pairs which is more basic, draw the conjugate acid, and give the single

most important reason for your choice with a brief explanation. [resonance, inductive, hybridization, steric, other]

A.

N CH3   N CH3

1   2

 

N CH3

N CH3

1

2

special hybridization in the 3-membered ring. More "p" character in the ring bonds, and therefore more "s" character inthe external bonds and orbital with lone pair. Therefore the lone pair "feels" a stronger stabilizing interaction with the nucleusand is less available as a Lewis base.

Normal tertiary amine, sp3 (approximately) hybridization

B.

N   N

3   4  

N N

3   4

N

N

etc

H H

Extensive delocalization, resonance structures;lone pair is dispersed, stabilized, and a weakerLewis base.

no delocalization possible;lone pair remains localized;normal amine base

 

end exam