-
— 1 —
Chemistry 2030 “Introduction to
Organic Chemistry”
Fall Semester 2014 Dr.
Rainer Glaser
Examination #3
“Nucleophilic Substitutions & Eliminations, Alcohols, and
Ethers”
Thursday, October 30, 2014, 8:25 - 9:15 am
Name:
Answer Key
Question 1. Mechanisms of Nucleophilic Substitution Reactions 20
Question 2. Reactions of Organic Halides with Nucleophiles 20
Question 3. Chemistry of Alcohols 20 Question 4. Oxidation of
Alcohols 20 Question 5. Chemistry of Ethers 20
Total 100 ALLOWED: Periodic System of the Elements (printed, w/o
handwriting on it). Molecular models (you can bring pre-made
models). Simple, non-programmable calculator (not really needed).
NOT ALLOWED: Books. Notes. Electronic devices of any kind (other
than a simple calculator).
-
— 2 —
Question 1. Mechanisms of Nucleophilic Substitution Reactions.
(20 points) (a) In the SN2 reaction, the nucleophile Nuc- attacks
the carbon that carries the leaving group L from the __________
(frontside, backside) and a ___Walden__ inversion occurs. This
reaction is fastest for H3C−L and __________ (primary, secondary,
tertiary) substrates. In the penta-coordinate transition state
structure, the C−L bond is _____________ (totally, partially, not
yet) broken and new C−Nuc bond is ____________ (totally, partially,
not yet) formed. Using dashed lines for partial bonds, provide a
perspective drawing of the transition state structure for the SN2
reaction of the cyanide nucleophile with the substrate propyl
bromide. Draw the C---Nuc and C---L bonds horizontally in the paper
plane and draw the C−C2H5 bond also in the paper plane and up. In
the SN2 transition state structure the ∠(H−C−H) angle is
approximately ______ (90°, 109°, 120°), the ∠(NC−C−Br) angle is
approximately ______ (90°, 109°, 120°, 180°), and the ∠(H−C−Br)
angle is approximately ______ (90°, 109°, 120°). (10 points)
(b) In the SN1 reaction, the weak nucleophile ______ (does, does
not) participate in the cleavage of the C−L bond. In the
intermediate of the SN1 reaction, the C−L bond is _____________
(totally, partially, not yet) broken and new C−Nuc bond is
____________ (totally, partially, not yet) formed.
Consider the SN1 reaction of (S)-3-bromo-3-methylhexane with
methanol. Draw the intermediate of the SN1 reaction; the ∠(Me−C−Et)
angle is approximately ______ (90°, 109°, 120°) and the
hybridization of the cationic center is ___ (sp, sp2, sp3). Provide
perspective drawings of the two stereoisomers formed in this
reaction. (10 points)
Intermediate Cation
Product, Stereoisomer #1
Product, Stereoisomer #2
-
— 3 —
Question 2. Reactions of Organic Halides with Nucleophiles. (20
points, 5 points each part) (a) Using abbreviated structural
formulas, show the substrate, the reagent, and the product of the
reaction of sodium acetylide (Na+ -CCH) with benzylbromide. Product
formed by _____ (SN1, SN2).
(b) Using abbreviated structural formulas, show the substrate
and the major product of the reaction of sodium methoxide with
2-bromo-2-methylpropane. The substrate is a ______________
(primary, secondary, tertiary) alkyl bromide. Major product formed
by ____ (SN1, SN2, E1, E2).
(c) Using abbreviated structural formulas, show the substrate
and the major product of the reaction of methanol with
2-bromo-2-methylpropane. Major product formed by ____ (SN1, SN2,
E1, E2 ).
(d) Using abbreviated structural formulas, show the substrate
and the major product of the reaction of (H3C)3C−O- K+ with
1-bromopentane. The substrate is _________ (primary, secondary,
tertiary) and the base is very bulky. Major product formed by ____
(SN1, SN2, E1, E2).
inconsistencies in textbooks and literature on this issue, fill
points for SN2 or E2
-
— 4 —
Question 3. Chemistry of Alcohols. (20 points) (a) Ethanol is
soluble with water in any ratio. In an aggregate formed by one
ethanol molecule and one water molecule, ethanol can act as
hydrogen bond acceptor or as a hydrogen bond donor. Draw the
complete Lewis structures (all atoms, all lone pairs) of both
aggregates. Use dotted line to indicate hydrogen bonds. (4
points)
Water Aggregate with Ethanol as HB Acceptor
Water Aggregate with Ethanol as HB Donor
(b) The pKa value of ethanol is about _16_ (give value) and,
hence, ethanol is __________ (much more, much less, about as)
acidic than/as water. Consequently, it _____ (is, is not) possible
to convert ethanol quantitatively to ethoxide by reaction of
ethanol with NaOH. Complete the following reactions for the
formation of sodium ethoxide with correct stoichiometry. (7
points)
H3C−CH2−OH + 1 Na à H3C−CH2−O- Na+ + 0.5 H2
H3C−CH2−OH + 1 NaH à H3C−CH2−O- Na+ + 1 H2 (c) Thionylchloride,
O=SCl2 is the dichloride of H2SO3, ________________ (sulfuric,
sulfurous, hyposulfurous) acid. H2SO3 occurs as O=S(OH)2 and
contains a _________ (pyramidal, trigonal planar) S-atom because
there is/are ___ (no, one, two) lone pair(s) at S. For the reaction
of 1-butanol with thionylchloride, show Lewis structures of the
inorganic ester formed between one butanol and O=SCl2, of the
organic product formed by the SN2 reaction of chloride ion with
this inorganic ester, and of the inorganic products of this SN2
reaction. (9 points)
Inorganic Ester Intermediate
Organic Product
Inorganic Products
-
— 5 —
Question 4. Oxidation of Alcohols. (20 points) (a) Provide
complete structural formulas of benzyl alcohol and of its oxidation
products. (6 points)
Benzyl Alcohol
Benzaldehyde
Benzoic Acid
(b) Jones reagent is a solution of _chromium_ _trioxide_ (write
name of CrO3) in dilute ___sulfuric___ acid. CrO3 is a dark __
red__ (give color) material. CrO3 adds water to form __chromic
acid_ (give name of H2CrO4). Draw the structure of H2CrO4. (5
points)
(c) Provide abbreviated structural formulas of propanol and of
the products of its oxidations with Jones’ reagent and with
pyridinium chlorochromate (PCC). (4 points)
Product of Jones’ oxidation
Product of PCC oxidation
(d) Phenol and its derivatives function as antioxidants. Briefly
explain (i) how phenol reacts with hydroxyl radical (show reaction
of phenol with HO•) and (ii) why the product of that reaction is
less harmful than hydroxyl radical (show resonance forms of the
product). (5 points)
-
— 6 —
Question 5. Chemistry of Ethers. (20 points) (a) Consider the
formation of methyl propyl ether by Williamson Ether Synthesis.
Show structures of the substrate, the reagent, the product and of
the inorganic product. (4 points)
Alternative and equally correct answer: Propoxide reacts with
methyl bromide
(b) Draw the complete structure of peroxyacetic acid (PAA) in
the box to the immediate right. Provide a perspective drawing of
the product formed by the reaction of 1-methylcyclohexene with PAA
in the box to the very right. (6 p.)
PAA
1-methylcyclohexene + PAA
(c) Ethers are prone to oxidation by air by way of radical
reactions. The O2 molecules in air are diradicals and they can
initiate radical chemistry by H-abstraction from a C-atom directly
connected to the ether-O. Provide the Lewis structures requested.
(4 points)
Radical formed by H-abstraction from Diethyl Ether
Hydroperoxide formed by Air Oxidation of Diethyl Ether
(d) Grignard reagents are made in dry ether solution. The ether
solvent is important to stabilize the Grignard compound. Ether acts
as a Lewis ________ (acid, base) when it complexes the Mg2+ ion of
the Grignard reagent. Provide a perspective drawing of ethyl
magnesium bromide solvated by two dimethyl ether molecules. (6
points)