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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).

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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

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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

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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

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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)

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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)