Chemistry Question

STUDY GUIDE FOR THE SECOND EXAM IN ORGANIC CHEMISTRY ICHM 241, FALL 2022
Saturday, October 29, 2022, 9:00 AM
As for the first exam, the second exam will also be in open book format. Use of molecular
models and of a periodic table are allowed.
CHAPTER 5: CONFORMATIONS
1. Constitutional isomers and conformational isomers
2. Alkane chain conformations: at room temperature,  bonds allow for free rotation of the
atoms connected by the bond.
3. Eclipsed and staggered conformations
4. Drawing Newman projections – the dihedral angle
5. Determining relative stability of conformational isomers
6. Cycloalkane conformations – angle strain: cycloalkanes distort from planarity to alleviate
angle strain (except for cyclopropane)
7. Drawing cyclohexane chair forms: the axial (a) and equatorial (e) positions. Make sure
that in your drawing the (a) and (e) can be distinguished.
8. Stability of chair forms
9. Mono-and disubstituted cylcohexanes.
CHAPTER 6: CONFIGURATIONAL ISOMERS – STEREOCHEMISTRY
1. Two Major Classes of Isomers. Definition of isomers, constitutional isomers, and
stereoisomers. You should be able to identify constitutional and stereoisomers.
2. Introduction to stereoisomerism. Chiral and Achiral Molecules. Draw the enantiomer of a
given chiral molecule.
3. Chirality (or Stereogenic) Centers. You should be able to identify/locate all stereogenic
centers present in each molecule.
4. Enantiomers – what they are, and how do you recognize them?
5. Designating Configuration using the Cahn-Ingold-Prelog system. Should be able to label
stereogenic centers with R and S. For this, you should be able to apply the rules to assign
priority to various substituents.
Problems: Draw all possible stereoisomers for the compound below, label pairs of
enantiomers and diastereomers, and label the meso compound! a. CH3-CH(OH)-CH(OH)CH2-CH3; b. CH3-CH(Cl)-CH2-CH(Br)-CH3. Label compounds as R or S.
Study Guide for Exam II in CHM. 241, Fall 2022
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6. Diastereomers. Calculate the maximum number of stereoisomers for a given molecule (2n,
where n is the number of stereogenic centers).
7. Symmetry and chirality. Meso Compounds.
8. Optical Activity. Definition. How do enantiomers rotate plane-polarized light? Dextrorotatory
and levorotatory isomers. Specific rotation and its equation. Racemic mixture: definition,
properties, and how does it differ from the meso compound? The Table below was discussed
in class.
Racemic Mixture
Meso Compound
Stereogenic Centers
Present
Present
Optical Activity
None
None
Cancellation of
Optical Activity
Intermolecular (1:1, mol/mol mixture
of the enantiomers)
Intramolecular
Can you render it
optically active?
Yes! e.g., by chiral chromatography,
microbial activity
Impossible!
CHAPTER 7: INTRODUCTION TO REACTIONS
7.1. Enthalpy or heat of reaction
Steps and species involved in reactions:
Radicals, carbanions, and carbocations
Heat of reaction: exothermic and endothermic reactions. Energy diagrams (do not forget to label
the axes: Energy vs. Reaction coordinate)
7.2. Entropy: the state of randomness of a system. The entropy increases, when a gas is released
in a reaction, e.g., CH3–CH3 → CH2=CH2 + H2. The entropy decreases, when a cycle forms out
of an open chain molecule (e.g., X-CH2-CH2 CH2-CH2-Y forms a cyclobutene); the reverse of
this reaction, where the cycle cleaves, so that it gains more freedom of motion involves an
increase in entropy.
7.3. Gibbs free energy change (Go = Ho – T So) predicts whether a reaction is allowed or
not.
Thermodynamics: Equilibrium constant and change in free energy (Go). What does it mean,
when Go < 0 and when Go > 0? How can you get this information from an energy diagram?
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7.4. Equilibria. Equilibrium constant and free energy changes. You should be able to correlate
the magnitude of Keq with Go; Go = -2.303RT*log Keq. Remember that a reaction is
spontaneous, when Keq > 1, which translates into Go < 0. 7.5. Reaction Kinetics. Energy diagrams. Be acquainted we the physical measures represented on the two axes. Activation energy (Ea) and the thermal effect (Ho) 7.6. Reading Energy Diagrams. Energy diagram of a one-step reaction and of a two-step reaction. Identification of the rate determining step (rds): the highest energy of its transition state is the highest. Transition state. Kinetics. Rate equations: 1st order and 2nd order.) One-step and two-step mechanisms. Golden rule: one-step reaction is 2nd order, while 2-steps reaction is 1st order. Catalysts. Be able to provide a complete definition and to draw comparative energy diagrams for catalyzed and uncatalyzed reactions. Types of reactions: (i) Redox reactions (generic); (ii) for organic reactions: substitution, elimination, addition, and rearrangement 7.7. Nucleophiles and electrophiles. Ionic reactions. Nucleophiles contain an excess of electrons, which can be donated to another reactant: ➢ Charged nucleophiles (OH-, R-O-, SH-, CN-, CH3-COO-, etc.) ➢ Neutral nucleophiles (H2O, CH3-OH, NH3, R-NH2, CH3-COOH, etc.) Electrophiles have an empty orbital, where they can accept a pair of electrons 7.8. Reaction mechanisms Note that curved arrows always start from lone pair electrons or from an electron-rich multiple bond! Nucleophilic attack: is performed onto the substrate molecule by a charged or neutral nucleophile. Leaving groups: good leaving groups are: –I > –Br > –Cl; H2O
Poor leaving groups: –F, –OH, –R, –H
CHAPTER 8: SUBSTITUTION REACTIONS
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8.1. Alkyl halides. Aryl halides, vinyl halides, and benzylic halides.
Nomenclature.
Structure of alkyl halides
8.2. Possible mechanisms for substitution reactions: concerted (one-step) vs. sequential (2-steps)
mechanisms.
8.3. SN2 Mechanism
One-step process, 2nd order, rate law: r = k·[nucleophile][substrate]
How does the identity of the alkyl group (methyl, 1o, 2o, and 3o) affect the reaction?
Problem: Explain why (CH3)3C-CH2-Br, a 1o alkyl halide, undergoes SN2 reactions very slowly!
8.4. Stereospecificity of SN2 reactions: backside attack with inversion of configuration at a
stereogenic center, like flipping the umbrella (Walden inversion): R center → S center, and
conversely. This applies only when there is a stereogenic (chiral) center in the molecule.
8.5. SN1 Mechanism
Two-step process, 1st order, rate law: r = k·[substrate]
The stereochemistry of SN1: attack from both sides of the sp2 hybridized planar carbocation racemization. How does the identity of the alkyl group (methyl, 1o, 2o, and 3o) affect the
reaction?
How does the structure of the substrate impact this reaction? Explanation resides in the stability
of carbocation which forms as the intermediate
8.6. Drawing the Complete Mechanism of an SN1 reaction.
8.7. Drawing the Complete Mechanism of an SN2 reaction.
8.8. Determining which mechanism predominates: When is the Mechanism SN1 or SN2?
Be proficient with the effect of (i) the alkyl halide, (ii) the nucleophile, (iii) the leaving group,
and (iv): the solvent effect (polar protic or polar aprotic).
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NAME (print): ________________________________________
October 28, 2022
CHEM. 241, Dr. Daniel A. Lowy
EXAM II
DIRECTIONS:
– This test consists of seventeen (17) questions + one Bonus Question on nine (9) pages. Several
questions are multiple questions. Whenever detailed answers are required, please show all your
work in the provided space, as indicated to you. Pay attention to the lone pairs and arrows. For
your convenience, on page 10 I supplied a Periodic Table.
– If you have a question about something on the test, please send me a text message (via chat)
– Relax and good luck!
(100 Points + 4 Points)
1. Draw the structure of 1-ethyl-2-propyl-2,3-dimethylcyclohexane
(3 points)
2. Draw the structure of 3(R)-methyl-4,4-dimethyloctane
(3 points)
3. Write the IUPAC name of the following molecules
(4 points)
Second Exam in CHM 241, Fall 2022
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4.a. Identify and mark with asterisk the stereogenic centers in the molecules below (3 points)
4.b. For each molecule calculate the maximum number of stereoisomers!
(2 points)
4.b. Indicate the maximum number of stereoisomers for the 3 molecules:
A:
B:
C:
5.a. Establish the relationship (enantiomers, diastereomers, or identical) between the four
molecules shown below! Consider them by pair: A-B, A-C, A-D, B-C, B-D, and C-D.
(5 points)
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5.b. For the molecules below identify the stereogenic centers (if any), and draw the enantiomers;
determine for each, whether it has an R or S center?
(4 points)
6. Briefly explain the similarities and differences between a meso compound and a racemic
mixture!
(4 points)
7. Below are conformational isomers of butane. Determine which the highest energy conformer
is, and which is the lowest energy conformer. Label them as “highest” and “lowest” and
“intermediate,” respectively.
(3 points)
CH3
H3C CH3
H
H
H
H
H
H
Second Exam in CHM 241, Fall 2022
CH3
CH3
H
H
H
H
CH3
H
H
3
8. Draw a Newman projection for the most stable and the least stable conformer of
1-bromopropane looking down the C1-C2 bond. Clearly label which is which. (5 points)
9. Write the combustion reaction of the following alkanes:
(a) Tetradecane (C14H30)
(b) Methylcyclopentane!
Hint: Make sure that that you balance the chemical equations correctly!
10.a. What change in entropy do you expect for the addition reaction below?
Explain why!
(3 points)
(3 points)
(2 points)
CH3-CH=CH2 (g) + HCl (g) → CH3-CH(Cl)-CH3 (l)
10.b. Assuming that at a certain temperature this reaction is slightly endothermic, do you expect
it to be spontaneous or nonspontaneous?
(2 points)
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11.a. A student conducts a chemical reaction in a flask. When the student touches the flask with
one finger, senses through the rubber glove that the flask is cold. Is this reaction exothermic
or endothermic? What algebraic sign will the enthalpy change have (is H positive or
negative)?
(3 points)
11.b. The student notices gas evolution (i.e., gas bubbles formed) in the reaction. What entropy
change do you expect (is S positive or negative)?
(2 points)
12. For reactions a. and b. below
(a) Write the substitution product(s)
(b) Show the mechanism and stereochemistry of the reaction (if applicable)
(c) Draw the energy diagram for each
(d) Provide the rate equation (rate law) for each
(4 points)
(6 points)
(3 points)
(3 points)
a.
+ CH3O-
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b.
+ CH3OH
13. Write the IUPAC name for the organic reactant (i.e., the alkyl halide) in Question #12!
(2 points)
14. Name 3 poor leaving groups and 2 good nucleophiles!
Second Exam in CHM 241, Fall 2022
(5 points)
6
15. For each reaction below the reaction center, where the substitution/elimination occurs, is of S
configuration. Do the followings:
(1) Write the products of substitution reaction
(6 points)
(2) Name the compounds, and
(2 points)
(3) Provide the expected mechanism for each reaction!
(6 points)
Hint: designate clearly, whether the mechanism is SN1 or SN2
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16. Give one example for each of the followings (write chemical formulas, rather than just
names):
(3 points)
(a) Benzylic halide
(b) A tertiary halide
(c) Vinyl halides
17. Draw the product(s) for reactions a and b below, including stereochemistry. Provide the
reaction mechanism!
(9 points)
a.
+ CN-
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b.
+ CH3CH2OH
BONUS QUESTION: Briefly describe what a chiral center means and provide 2 examples
(write molecules)!
(4 points)
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Second Exam in CHM 241, Fall 2022
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