CEM 251 (Sections 25-36) Midterm #1 September 26, 2019 8-9

CEM251, Fall 2019 Midterm #1

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Question Points Score 1 12 2 16 3 20 4 24 5 18 6 10

Bonus 10

Total 100

This is a closed book and note examination. If boxes are provided for your answers, only what is written in the boxes will be graded. You have 80 min to complete the test.

Name:___________________________ PID: ____________________________ Section:__________________________

Section 25 Thu 10:20 – 11:10 AM Pepe Section 26 Thu 10:20 – 11:10 AM Sophie Section 27 Mon 2:40 – 1:30 PM Emily Section 28 Mon 9:10 – 10:00 AM Emily Section 29 Tue 10:20 – 11:10 AM Emily Section 30 Thu 1:50 – 2:40 PM Rahele Section 31 Thu 4:10 – 5:00 PM Pepe Section 32 Tue 3:00 – 3:50 PM Rahele Section 33 Tue 4:10 – 5:00 PM Pepe Section 34 Thu 4:10 – 5:00 PM Sophie Section 35 Thu 3:00 – 3:50 PM Rahele Section 36 Thu 1:50 – 2:40 PM Sophie

CEM 251 (Sections 25-36)

Midterm #1 September 26, 2019

8-9:20 am


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1. Provide a name or a structure for the following compounds (Indicate cis/trans where appropriate)(12 pts, 3 pts each box) a. 4-chloro-3,7-dimethyl nonane b. c. 3-ethyl-2-methyl-4-nitroheptane d

trans 1ethyl, 3hydroxy cyclohexane

1-bromo-3-ethyl-6-methylcyclooctane

Include cis/trans

HO

Br

Cl

NO2


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2. Attached below are Tables 1.5 and 1.6 from the book, indicating the Electronegativity Values for some Atoms of the periodic table and defining the Classification of the Chemical bonds (based on difference of electronegativity between the bonded atoms) respectively.

a. Based on the information given, classify the bonds between the highlighted atoms below as

either “Nonpolar Covalent”, “Polar Covalent” or “Ionic”. (8 pts)

LiCl

CH3CH2Cl CH3CH2OCH3

(CH3)3SiH

Polar CovalentPolar Covalent

Non-Polar CovalentIonic


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b. Show work for any one of the bonds above and explain your chosen classification (show calculations and explain your conclusion). (4 pts)

c. Using Lewis structures as models, use any example you want to explain the difference between

a Covalent (Polar or Nonpolar) and an Ionic Bond. (Hint: Use or to indicate electrons originating from different atoms) (4 pts)

(CH3)3SiH

Based on the values in Table 1.5, the electronegativities for Si and H are:

1.8 2.1

2.1 - 1.8 = 0.3

Based on Table 1.6, since the difference in electronegativity is less than 0.5, this is a

non-Polar Covalent bond

Covalent Ionicvs

C NaClH * ***

HH

H

each atom contributes 1 valence electron to the bond and both electrons of the bond are shared.

* ** *

***

Na lost 1 electron, and it is left positive with a full valence octet.

Cl picked up 2 extra electron and it is now negative with a full octet.

The bond is based on electrostatic interactions, no electron sharing is happening.


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3. The following is the structure of Atorvastatin, marketed by Pfizer under the name Lipitor. Lipitor is used to treat high cholesterol by reducing the levels of LDL (Bad cholesterol) and triglycerides in the blood to lower the risk of stroke and heart attack.

a. Complete the Lewis structure of Lipitor, showing all missing valence electrons and any formal

positive or negative charges. [Remember , this is a stick structure so assume all the C-H bonds not shown are there!] (6 pts)

b. Describe each circled bond in terms of the overlap in atomic orbitals. [What orbitals from each

atom are overlapping to form each bond?] For multiple bonds, indicate the orbitals used in each (6 pts)

N – H sp3 – s C=C 1st bond: sp2 – sp2

2nd bond: p - p

N ON

F

OH OH OO

H


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c. Identify and name 3 functional groups that exist within the Lipitor chemical structure (6 pts

d. How many pi (p) electrons are there in the Lipitor structure? (2 pts) 26 pi electrons

or anything else that makes sense

N

O

N

OH

Amide

Alcohol

Amine


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4. a. Draw 3 valid resonance structures for each of the molecules A and B (18 pts, 3 pts / box)

[A]

[B]O

or anything else that makes sense!

O

OO


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b. The following is a non-valid attempted resonance structure of molecule B. Explain what makes this structure not valid. Support your arguments using scientific principles. (3 pts)

Electrons were moved from the more electronegative Oxygen to less electronegative Carbon, leaving the Oxygen without a full octet.

c. Why I CANNOT draw a resonance structure through an sp3-hybridized carbon? What rule would that violate? Explain supporting your argument with scientific principles. (3 pts) Pushing a bond towards an sp3-C would violate the octet rule. An sp3-C already has 4 sigma bonds, and you cannot push a 5th bond without breaking a bond (the C would have 10 e), and you

O


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5. a. Draw the all possible distinct Newman projections [3 eclipsed and 3 staggered] of 2,3,5 trimethyl hexane looking down the C3-C4 bond as you are rotating that bond from 0-360 o. [The Newman Projections were named after American chemist Melvin Spencer Newman (1908-1993), a professor at The Ohio State University, who introduced them.] (12 pts, 2 pts / box)

b. Which of these Newman projections (A-F) is the most stable (lowest Energy)? Explain your choice and your reasoning indicating the evidence behind it. (4 pts) [Hint: You can redraw your structure and use it along with words to explain your argument] For my example, the lowest energy conformation is E, since it is the conformation that allows for more space around the largest groups on each carbon by putting them in a staggered, right across each other, c. What is the name used for the most stable Newman Projection? (2 pts) Anti

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2,3,5-trimethylhexane

Eclipsed

Staggered

A B C

D E FH

H

H

H

HH

H

H

H

H

H

H

H

HH

HH

H


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6. Choose (circle) the correct answer for the following questions (there is ONLY ONE correct answer for each). (2 pts each, 10 pts) a. Which of the following is the structure of an “ester”?

______________________________________________________________________________ b. What is the formal charge of the nitrogen atom in the following structure?

______________________________________________________________________________ c. Consider the following movement, represented by the full arrow: Then, consider the following statements.

i. Two electrons are moving from less electronegative A to more electronegative B. ii. Two electrons are moving from electron rich A to electron poor B. iii. A charge (positive or negative) is being moved from less electronegative A to more

electronegative B. iv. A charge (positive or negative) is being moved from electron rich A to electron poor B.

Which of the 4 answers is correct? (i) (iii) (i + ii) (iii + iv) ______________________________________________________________________________ d. Which of the following is the structure of a “cyanide/nitrile”?

______________________________________________________________________________ e. Which of the following is the structure of an “iso-butyl” group?

A B

N

i. 0

ii. +1

iii. -1

iv. +2

O

OH

O

Cl

O

O

O

NH

O N3 CN O


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Bonus Points (10 pts) a. Many reactions involve a change in hybridization of one or more atoms in the starting material.

For each reaction below, identify the atoms (in both starting material and product) that change hybridizations during the reaction and indicate the old and new hybridization. (6 pts) [At this point you don’t have to worry about how the actual reactions work!]

b. Convert the given Newman projections into the corresponding stick structures (4 pts).

H C C C HH

H+ H2O H C C

OCH

HH

H

H

H C C C CH

H

H

H

OH

H

H

HH H C C C C

H

H

H

H

O H

HH

C CH

H

H

CH

H HH C C C H

H

H

H

OH H+ H2O

H

sp sp sp2sp3

sp2sp3

sp2sp2

sp3

sp3

OH

HHBr

OH Br


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Scratch Sheet: Needs to be turned in with the test (if missing, the test will not be graded)


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Documents

CEM 251 (Sections 25-36) Midterm #1 September 26, 2019 8-9