Name:
ORGANIC CHEMISTRY I (CHEM 323)
EXAM 1
September 20, 2011
There are five pages, NINE questions, and a total of 110 points in this exam, which gives an extra
10 points on top of that required for a “perfect” paper. Please read each question carefully and possibly
more than once. Good luck…
(7)
1.
The molecule below is an artificial sweetener called aspartame (let’s start the exam on a sweet
note).
Although the bond indicated with an arrow is a single bond, it does not undergo free rotation as
easily as other single bonds.
Using resonance theory explain why this is so.
(i.e. provide a resonance structure and write a brief narrative explanation based on this resonance
structure. Use curly arrows for any electron movements; show formal charges where needed)
the bond shown has double bond character; thus does not undergo rotation
freely. Double bonds require a - and a -bond. Since -bonds form as a
result of a sideways overlap of p-orbitals, rotation around the C-C axis
would easily break them.
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2.
Write a skeletal (zig-zag) structure of a molecule with the molecular formula C 6H14 which does
not have any secondary carbons.
2.1.
What type of relationship (if any) does 2,2-dimethylbutane have with molecule you have
written above?
Constitutional (structural) isomer
2.3.
Draw the structure of 2,2-dimethylbutane and circle all equivalent carbons.
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3.
Circle the best correct answer (only ONE) for each of the multiple choice questions given
below:
3.1.
What is the electron configuration for boron?
(1) 1s2 2s2 2p6
(2) 1s2 2s2 2p1
(3) 1s2 2s2 2p4
(4) 1s2 2s2 2p3
3.2.
Lewis Theory cannot predict one of the following to any extent.
(1) The reactivity of the molecule (2) The stability of the molecule
(3) The geometry of the molecule
(4) Relative strength of the bonds in the molecule
3.3.
Which of the following is most unstable?
(1) A
3.4.
(2) B
(3) B and C
(4) A and D
(5) C
Which of the following structures shown in question 3.3. is/are inter-convertible
into another/others?
(1) None of them can be converted into the others shown
(2) Only A into D
(3) Only C into A, B, or D
(4) Only A, B, and D into C
(5) They are all inter-convertible into one another
3.5.
How many lone pairs (pairs, not individual electrons) of electrons are there in the
following molecule?
H
O
(1) none
3.6.
(2) one
(3) two
(4) three
Which of the following statements related to the molecules below is TRUE?
(1) the carbon connectivity is different.
(2) they are inter-convertible through conformational mobility (ring flip) of the ring.
(3) they are different in the spatial arrangement of their atoms, and they are not
inter-convertible.
(4) One of the structures above is too unstable to exist.
3.7.
Which of the following has the higher heat of combustion, Hcomb?
(1) trans-1,2-dimethylcyclohexane
(2) trans-1,3-dimethylcyclohexane
(3) their heat of combustion values must be the same, since both are trans
(4) cannot be determined since each molecule is conformationally mobile
Page 2
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4. In the boxes under the structures given below write “1” for the molecule with the highest
boiling point and “4” for the molecule with the lowest boiling point.
1
4
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5.
Cyclohexane templates in chair form are given below. Place substituents in appropriate
positions as specified in 5.1 to 5.3.
5.1.
cis-1,3-dimethylcyclohexane
5.2.
the more stable conformation of trans-1,2-dimethylcyclohexane
5.3.
cis-1-tert-butyl-4-methylcyclohexane
Page 3
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6.
Nitromethane and methyl nitrite both have the same molecular formula, CH 3NO2; however, the
atom connectivity of each compound is different. In addition, nitromethane is significantly more
acidic than methyl nitrite.
6.1. Write Lewis structures for nitromethane and methyl nitrite (at this stage you do not need to
identify which is which). Show all formal charges and lone pairs (if any).
6.2. Identify nitromethane, the more acidic of the two compounds (pK a 10.2), by going through
the following process: (no guessing)
(i)
Write the conjugate base structure for each compound
(ii)
Choose the more stable conjugate base by using one of the following criteria:
element factor; resonance; inductive effect; hybridization effect.
Explain your thought process clearly (show different resonance structures if
applicable)
Lone pair on carbon cannot be delocalized
(iii)
charge on carbon delocalized
Identify nitromethane and briefly rationalize your choice.
The structure where the charge is delocalized by resonance is the more stable (weaker)
conjugate base; thus when protonated is the stronger acid. Nitromethane is the structure
on the right.
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7.
Draw the skeletal (zig-zag) structure of the molecule whose Newman Projection
Representation is given below.
7.1.
Name the molecule according to the IUPAC rules of naming:
2-ethyl-3-methylheptane
Page 4
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8.
Complete the reactions below and identify the structures of the missing products. Use curly arrows
to show electron transfers, bond formations, and/or bond cleavages. Show all formal charges (if
any).
8.2.
8.2.
Which of the following features characterizes BF3 most accurately (only one)?
(a) a Lewis base (b) a Lewis acid (c) a nucleophile
(d) an alkyl halide
8.3.
8.4.
Which of the following features characterizes C8H16 most accurately (only one)?
(a) a Lewis base (b) a Lewis acid (c) an electrophile (d) an alkane
8.5.
balance the equation
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9.
Identify the overlapping orbitals which form the indicated bonds below:
9.1.
9.2.
sp3 – sp2
Identify two sets of orbitals for each
type of bond in the double bond
-bond: sp2 – sp2
-bond: p - p
Page 5