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Chemistry 303

First Hour Examination

October 10th, 1994

7:30 pm-9:30 pm

Name:_____________________________________________________________

Lab TA name (or time of lab)___________________________________________________________

This is an open book test. You may use anything that is not alive.

Write all of your answers in the space beneath each question. If you mess up the assigned space, you maychoose another space of equivalent size (e.g., on the back of a page) but it is your obligation to make clear wherethe answer is you wish graded. If it is not in the right place or carefully indicated, it will not be graded.

Turn in this exam and no other material at 9:30 pm.

READ EACH QUESTION CAREFULLY AND COMPLETELY. ANSWER ONLY WHAT IS ASKED!

Good luck!

Grade:

1__________/16

2__________/15

3__________/15

4__________/21

5__________/33

total score:

Pledge:_______________________________________________________________________________

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1. (16 pts) Consider the following trios of molecules with similar structure.  Within each trio circle the

molecule with the lowest bp and put a square around the one with the highest bp. For each trio, explain yourordering by giving the main reason for the difference between the 1st and 2nd, and then between the 2nd and 

3rd in your sequence.

 

CH3

N

CH3CH3

CH3

CH3 CH3

CH3

CH3

OH

A.

1   2   3

Explain:

________________________________________________________________________

 CH3

CH3 OH

CH3

CH3 CO2-

OHCH

3

B.

Na+

3 54Explain:

________________________________________________________________________

 CH3

CH3OH

CH3

NCH3

CH3O CH3

CH3

C.

31   6

Explain:

________________________________________________________________________

D. Which molecule in the series 1,2, and 3 is the strongest base? Explain your reasoning bydifferentiating it from the next most basic molecule in that group.

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2. (15 pts). Draw in all of the individual hydrogens on the structure A.

 

O

NH

OH

O

H2N

A. Identify the most acidic proton and label it (1).B. Identify the second most acidic proton and label it (2).C. Identify the third most acidic proton and label it (3).D. Explain carefully why #1 is more acidic than #2.E. Explain carefully why #2 is more acidic than #3

A

D. Explain #1 vs #2:

E. Explain #2 vs #3:

____________________________________________________________________________________3. (15 pts) Draw the molecule (not necessarily different for each case) with molecular formula C5H9ON whichbest satisfies the following criteria (explain your choice by describing the single most important effect).

A. most soluble in water

B. most soluble in cyclohexane

C. has the most electron deficient carbon atom

D. has the most basic nitrogen atom (measured in the gas phase)

E. has the most angle strain

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4. (21 pts) The molecule shown below is quite acidic, with a pKa ca 12. There is no formal charge on B.A. Draw in all of the hydrogens individually on the structure, and add the non-bonding electrons.

  B. Circle that hydrogen (or set of equivalent hydrogens) which you believe to be most acidic.  C. Draw the three most favorable resonance structures for the conjugate base of B. Show all non-

bonding electrons and formal charges.  D. Order the three resonance structures; specify which is most stable,which is least stable; why?.

 O N

[conjugate base]

1

-H+

B

32 45

E. What is the hybridization at C-1 in B? (sp3)  (sp2 + p) (sp + p + p)(circle best answer)

F. What is the hybridization at the N atom of B? (sp3)  (sp2 + p) (sp + p + p)

G. What is the hybridization at the N atom in the

conjugate base? (sp3)  (sp2 + p) (sp + p + p)

H. What is the hybridization at C-3 in the

conjugate base? (sp3)  (sp2 + p) (sp + p + p)

_______________________________________________________________________________________

5. (33 pts)  Hemoglobin is the key molecule in biological oxygen transport, and the key part of hemoglobin is

an Fe+3 ion as part of its structure. A simple model suggests that the iron ion acts as a Lewis acid, bonding

reversibly to oxygen. Think about the bonding of oxygen to the iron ion.

A. Draw the oxygen molecule using Lewis structures and show all bonds, non-bonding electrons and

formal charges. Then draw next to it a picture of oxygen bound to a Lewis acid; please use BF3  as your Lewis

acid instead of Fe+3 (the picture will be simpler). Show all non-bonding electrons and formal charges.

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cont...

Sodium cyanide (NaCN) and carbon monoxide are toxic because they compete with the oxygen in binding to

the iron ion.

B. There are two reasonable resonance structures for cyanide anion, one with two bonds and one with three.

Draw a large picture of one resonance structure of cyanide anion, showing all bonds, non-bonding electrons,

and formal charges. Use arrows to indicate which are sigma and which are pi bonds. Indicate the hybridizationat C and at N.

Then also write the other reasonable resonance structure, showing all bonds, non-bonding electrons, and

formal charges. You need not label sigma and pi bonds. State which structure you feel is the more stable (more

important in the hybrid), and explain the basis for your choice briefly.

C. There are two reasonable resonance structures for CO, one with two bonds and one with three. Draw a

large picture of one resonance structure of CO, showing all bonds, non-bonding electrons, and formal charges.

Use arrows to indicate which are sigma and which are pi bonds. Indicate the hybridization at C and at O.

Then also write the other reasonable resonance structure, showing all bonds, non-bonding electrons, and

formal charges. You need not label sigma and pi bonds. State which structure you feel is the more stable (more

important in the hybrid), and explain the basis for your choice briefly.

D. Draw a structure for cyanide anion bound to BF3. Note that there are two "ends" to cyanide anion.Explain why you chose one particular end over the other to bind the Lewis acid. Show all bonds, non-bondingelectrons, and formal charges. Indicate the hybridization at the boron atom.

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E. Draw a representation of carbon monoxide bound to BF3. Note that there are two "ends" to carbonmonoxide. Explain why you chose one particular end over the other to bind the Lewis acid. Show all bonds,non-bonding electrons, and formal charges.

F. Which do you predict will bind more strongly to BF3: oxygen, cyanide anion, or carbon monoxide?Explain your reasoning briefly.

_______________________________________________________________________________________

THE END

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