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2311A and B Practice Problems to help Prepare for Final from Previous Marder Exams.

Disclaimer.: Use only to help learn what you need to know and don’t expect the final to be in the

same form.

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1. Short answer questions.

a) Underline the compound with 2 (two) (and only two!) stereocenters ( 5 points)

b) Which of the following equilibria are favored to the left?: (5 points)

CH3CH2SH

CH3CH2OH

CH3CH2OH

CH3CH2O–

CH3CH2O–

CH3CH2S–

CH3CH2– CH3CH3

HCl Cl–

CH3CH2OH

CH3CH2O–

CH3CH2O–NH3 NH2–

CH3CH2OH

(i) I, II (ii) II, III (iii) III, IV (iv) I, III

c) What are the pKa’s (±3 units) of the following molecules (1 point each)

(i) HI

(ii) Acetic acid

(iii) Ammonia

(iv) Water

(v) Ethylene

(vi) Ammonia

CH3

BrCH3

CH3H3C

CH3 OOH

HOHO

HOOH

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d) Underline the least stable carbocation (5 points)

CH3 CH3 CH3 CH2F

e) Underline 5,5-Dimethyl-2-hexyne. (5 points)

HC CH2CH2CH(CH3)CH(CH3)2

CH3C CCHCH(CH3)3

CH3

f) Indicate the formal charge of nitrogen and oxygen in the following structure. If an atom is

formally neutral, indicate a charge of zero. (4 points)

N O

N: ________ O: ________

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g) Circle the energy- reaction coordinate diagram that represents the reaction reaction shown

below. (5 points)

DMSO

E

Reaction Coord.

E

Reaction Coord.

E

Reaction Coord.

E

Reaction Coord.

SH–OH OH

–SH

h) Consider a compound known as allene: CH2=C=CH2. (4 points)

(i) How many carbon-carbon s-bonds does this molecule contain? _________

(ii) How many p-bonds does this molecule contain? __________

(iii) What is the hybridization of the central carbon of allene? __________

(iv) What are the H-C-H angles in allene? _______

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i) Which of the following compounds would react most rapidly with NaCN in an SN2 reaction?

(5 points)

(1) iodocyclopentane.

(2) 1-iodo-1-pentene.

(3) 1-flouropentane.

(4) ) 1-iodobutane.

(5) Neopentyl iodide.

j) Which of the following IS an nucleophile? (4 points)

(1) BF3

(2) CH3O–

(3) Br2

(4) All are electrophiles.

(5) None is an electrophile..

k) Identify the following as either Z or E configurations. (4 points each)

OHO

OHBr

OH

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2. Designate the mechanism (SN1, SN2, E1, E2) that is most consistent with the following

statements. Some of these statements are consistent with more that one symbol! Indicate so! (14

points)

______ (i) Alkyl bromides react slower than alkyl iodide in reactions that proceed by

these mechanisms.

______ (ii) Benzyl halides react with sodium ethoxide in ethanol by this mechanism.

______ (iii) These reaction mechanisms cannot involve carbocation intermediates.

______ (iv) Rearrangement reactions are possible for reactions proceeding by these

mechanisms.

______ (v) Very bulky bases and sterically hindered sites with leaving groups attached

favor this type of reaction.

______ (vi) These reactions are often run in polar aprotic solvents.

______ (vii) These reactions have second order kinetics.

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3(a) For the molecule allyl anion ,shown below, on the lef, write another resonance form in the boxprovided. (3 points)

(b) The normal carbon-carbon single bond length between two sp2 carbons (for example between

carbons 2 and 3 in butadiene) is 1.46Å and the normal carbon-carbon double bond length betweento sp2

carbons is 1.34Å. What would you expect the carbon-carbon bond length between thecarbons to be in allyl anion shown above? (3 points)

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4. For the reaction shown below fill in the boxes with either the product or reagents as

required. (4 points for each answer)

O

1. Mg, ether2.

H3O+

CH3OH Na CH3I

OH CN

CH3Br

OTs

1. di-isoamylborane

2. H2O2, OH

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5. Consider the following five pairs of structural formulas. For each pair, indicate whether the

structures represent identical molecules, constitutional isomers, enantiomers, or diastereomers.

(20 points)

CH3

HO H

H CH3

H CH3

H OH

H3C H

H

and(a)

(b)

and

(c)

and

(d)

(e)

and

and

H3C CH3 H3C CH3

H

H

H HBr

Br

Br Br

CH3

O

CH2H3C

CH3

O

H2C CH3

CH2 CH2

H CH3 H3C H

BrBr

\

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6. Provide a curved arrow mechanism (3 steps) for the following reaction. (10 points)

OI

I2HO

HI

7. Show a complete mechanism for the reaction shown below using the curved arrow

formalism. (10 points)

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8. Provide a synthesis to get from the starting material on the left to the product on the right. To

receive full (or partial credit) show the product of each step of the synthesis with the reagents

required to effect each step. (10 points each)

(a) SCH3

(b)

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(c)

OH

CN

plus enantiomer

Note trans stereochemistry between methyl and CN!!

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Extra Credit. Using resonance structures, explain why benzylbromide can undergo an SN1reaction even though it is a primary halide (hint: think about why there can be an unusually stable

carbocation form as an intermediate). (10 points)

Additional Problems

. (a) Outline a sequence of reactions that is suitable for the preparation of the following compoundfrom the indicated starting material and any needed organic or inorganic reagents. The synthesiswill require at least three steps. (15 points)

(b) Outline a sequence of reactions that is suitable for the preparation of the following compoundfrom the indicated starting material and any needed organic or inorganic reagents. The synthesiswill require at least two steps. (10 points)

OCH3

SCH3

+ enantiomer

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CN

. Draw the product for the reaction of propene and each of the reagents shown below(3 points for each reagent)

. For each of the following reactions draw in the major products that will be formed in the boxes.• Be clear to indicate both the regiochemistry and expected stereochemistry. • If more than one product is expected indicate the stereochemistry of each using dashes and

wedges. • For each chiral carbon in the molecule (whether or not it is created by the addition to the

alkene), assign the carbon as being R or S. • Then, determine the relationships between the molecules that have been created. Are they

enantiomers, diastereomers, or identical. • If a compound is meso, label it as meso.Note. There will not necessarily be four products formed. There should be no rearrangementsnecessary. (15 points)

Example:

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CH3

CH3

CH3

CH3

BrBr

CH3

CH3R

S S

Sdiastereomers

HBr

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a.

Work space (no information here will be graded).

CH3HOBr

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b.

HBr, peroxides (trace)

Work space (no information here will be graded).

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9. For the reactions below draw: i) the initial carbocation intermediate formed, ii) the most stableintermediate formed (if a rearrangement is likely to occur and, use the curved arrow formalismto show how the rearrangement occurs) and iii) the product(s) of the reaction. If more than oneproduct is expected to be formed in significant yield indicate which you expect to be the majorproduct and why. (12 points each)

HBr

CH2Br H+ /H2O

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Extra Credit (4 points). Do this problem only if you have completed and checked the rest of the exam.

Label each asymmetric center clearly and unambiguously as either R or S (10 pts).

HBr, peroxides(trace)

1. Hg (CH3COO)2, CH3OH

Br2, H2O

1. BH3

2. H2O2, OH

H+ H2O

2. NaBH4

OCH3

HN

O

NH3C

O

H3C HH

H

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Chart of Useful Information

Polar Aprotic Solvents Polar Protic Solvents

Dimethyl formamide (DMF; (CH3)2NCHO) Water (H2O)

Dimethyl sulfoxide (DMSO; (CH3)2SO) Methanol (MeOH; CH3OH)

Acetonitrile (CH3CN) Ethanol (EtOH; CH3CH2OH)

Acetone (CH3COCH3) Acetic Acid (CH3CO2H)