Basic Organic Chemistry

SAMPLE MIDTERM - February 15, 2000 - Dr. John C. Vederas

Organic Chemistry : Chemistry 161 - Dr. John C. Vederas

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(Answers in RED)

I. Structure and Nomenclature - 56 Points -

A. Draw structures for which names are given, or name the given structures by any correct (systematic or common) nomenclature. Be sure to give cis or trans (or if appropriate Z or E) or R or S assignment to the isomer where indicated by asterisks (***). (4 points each)

1. chloroform

2. vinyl chloride

3.

4.

*** 5.

*** 6.

*** 7.

*** 8. cis-3,4-dipentylcyclobutene

B. Determine whether the following pairs of structures are identical (i.e. different pictures of the same molecule), structural isomers, or stereoisomers (4 points each).

1.

 

2.

3.

4.

C. Conformation (4 points each)

1. Draw the most stable conformation of n-propyl bromide using Newman projection

2. Draw the most stable conformation of:

II. Quantitative Analysis and Definitions - 35 Points

A. Define briefly (15 words or less) each of the following concepts -
(3 points each)

1. London Forces

Intermolecular attraction due to temporary dipoles (induced dipoles)

 

2. sigma orbital

Molecular wave function made by combination of atomic orbitals having s character

 

3. electronegativity

Desire of an atom for electrons (negative charge)

B. Extracts of the coffee plant, Coffea arabica, were first introduced into Arabia from Ethiopia about 900 AD to keep both Christians and Moslems awake during long religious services. An average cup of coffee contains 100 milligrams (mg) of caffeine, a stimulant which is also medicinally useful against overdoses of barbiturates or alcohol. Caffeine contains only C, H, N and O. Quantitative analysis gave: C, 49.48%, H, 5.19%, N, 28.85%.

1. Show how to calculate the empirical formula (15 points)

100% - 49.48% - 5.19% - 28.85% = 16.48% Oxygen

 

49.48% C divided by 12 (at wt C) = 4.12 crude ratio

5.19% H divided by 1 (at wt H) = 5.19 crude ratio

28.85% N divided by 14 (at wt N) = 2.06 crude ratio

16.48% O divided by 16 (at wt O) = 1.03 crude ratio

 

Divide all crude ratios by the smallest to get refined ratios:

4.12 divided by 1.03 = 4

5.19 divided by 1.03 = 5

2.06 divided by 1.03 = 2

1.03 divided by 1.03 = 1

Therefore the Empirical Formula is C4H5N2O

2. If the molecular weight is 194, what is the molecular formula ? (2 points)

To get the empirical weight multiply each number from empirical formula against atomic weight and add

4 x 12 (at wt C) = 48

5 x 1 (at wt H) = 5

2 x 14 (at wt N) = 28

1 x 16 (at wt O) = 16

48 + 5 + 28 + 16 = 97 is the empirical weight

Divide molecular weight given as 194 by empirical weight (calculated as 97 ) = 2

Therefore the molecular formula is twice the empirical formula: C8H10N4O2 is the molecular formula

3. Suppose you completely burned 194 mg of caffeine to CO2, H2O and nitrogen oxides. Show how to calculate the volume of CO2 produced at standard pressure and temperature (STP) (4 points)

C8H10N4O2 + O2 gives 8 moles of CO2

At STP 1 millimole ( 1 mmol) occupies 22.4 milliliters (mL)

8 x 22.4 mL = 179 mL

4. Show how to calculate the volume of the CO2 in part 3 would occupy if the temperature were raised to 100 C, but the pressure was kept constant.

PV=nRT (Gas Law) and P, n and R are constant in this problem, T is in degrees Kelvin (273 + degrees C)

Hence PV1=nRT1 which can be divided by PV2=nRT2

This gives V2 = V1 x (T2 divided by T1 in degrees Kelvin)

Thus V2 = 179 mL x ((100 + 273) divided by (273)) = 244.8 mL

III. Atomic and Molecular Structure - 22 Points

A. Acetylcholine is an important neurotransmitter (chemical messenger between nerve cells) in the body. Interference with its formation or cleavage is the primary means by which many nerve gases act. Its structure is shown below:

1. Determine the formal charge on the two atoms indicated by arrows labelled formal charge. Use any method but show calculations. (6 points)

For the Oxygen add the following:

8 protons = +8

2 electrons in 1s shell = -2

4 electrons unshared = -4

one half of 4 electrons shared = -2

Formal Charge on oxygen is therefore 0

 

For the Nitrogen add the following:

7 protons = +7

2 electrons in 1s shell = -2

one half of 8 electrons shared = -4

Formal Charge on nitrogen is therefore +1

2. Label the hybridization and approximate bond angles in acetylcholine which are indicated by arrows: (4 points)

Angle 1 is 109

Angle 2 is 120

Hybridization 1 is sp3

Hybridization 2 is sp2

B. Examine the structure of camphor shown below

1. Redraw the structure in flat projection. Use the six membered ring provided as part of your structure. Be sure to indicate three dimensional shape with dark and dashed lines. (3 points)

2. Draw in all of the hydrogen atoms on the structure of camphor shown below (4 points)

3. Circle one of the isoprene units in the above structure (2 points)

4. The molecular formula of camphor is (3 points): C10H16O

IV. Physical Properties and Reactions - 37 Points

A. Circle those compounds shown below which have a permanent dipole moment. (5 points)

B. Draw an energy diagram depicting all of the atomic orbitals of fluoride anion and indicate the number of electrons in each (5 points)

C. The compound shown below reacts with bromine ( 1 equivalent) in the presence of light to produce an inorganic and an organic product.

1. Show the structures of the two products (4 points)

2. Use arrows to help clearly label the quaternary and secondary carbons in the structure of the starting material below. Use "Q" for quaternary and "S" for secondary. (4 points)

3. Write the propagation step(s) for the above bromination reaction: (6 points)

4. Write a termination step for the reaction: (3 points)

Termination is always the combination of two radicals

D. Explain the Hammond Postulate in less than 20 words by drawing and comparing the energy diagrams for an exothermic reaction and for an endothermic reaction. Be sure to label axes in both directions. Also show for each diagram the activation energy (Ea), the delta G, and the transition state (TS) positions. (10 points)

EXTRA CREDIT (3 Points): Name one person who won this year's Nobel Prize in Chemistry

Sample Exams From Professor E. Blackburn's Sections

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