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Unformatted text preview: CHM 2120 MIDTERM EXAMINATION OCTOBER 17, 2009 TIME: 2H TONY DURST __ANSWERS AND MARKING SCHEME Please Note: 1. The total marks for the exam is 75 points. The marking scheme is a guide and should be considered approximate. It is subject to small changes 2. Answer all questions. Read the questions carefully and answer what is asked. 3. Part marks are given for reasonable, but incomplete answers. Question 1. 6 points Name each of the following structures using either the IUPAC system or an acceptable trivial name OR supply a structure to a name. Stereochemistry, if shown, should be recognized either in the name or in the structure drawn. H F H CH 3 a) b) CO 2 H H 3 C H 3 C c) (E, Z)- 2,4-hexadiene d) para-nitrophenol e) the allyl carbocation e) OH Cl MARKING; 1 POINT FOR A CORRECT UNAMBIGUOUS ANSWER. 0.5 POINTS[ one time only] for an answer that has a minor flaw. The TA marking this made the choice and was consistent. The answers shown are acceptable answers. Others may also be OK but they must follw one of the acceptable systems and be unambiguous (Z)-1-fluoro-1-butene 3,4-dimethylbenzoic acid. the numbering can be clowise or counterclockwise to get the smallest numbers.. The carboxyl carbon must be #1. H 3 C H CH 3 H OH O 2 N H H (+) In th end we accepted any allyl cation trans-2-chloro-cyclohexanol remeber E and Z are used in alkenes. cis and trans are use to define stereochemistry rings in Question 2. 7 points MARKING. ZERO POINTS FOR GUESSES RE THE DIRECTION. 2 POINTS FOR A RESAONABLE JUSTIFICATION. Part marls were considered, but not if the answer had serious flaws. Indicate the direction of the equilibrium in each of the following and EXPLAIN the reason for your choice. H 3 C N + a) CH 3 CH 3 H (+) OH H 3 C N CH 3 CH 3 O H H (+) + Left. Quite strongly. The simple reason is that a protonated alcohol is much more acidic than a protonated amine. Nitrogen carries a positive charge much more readily than oxygen due to its lower electronegativity. OR: amines, for example ammonia are stronger bases than alcohols and water. Thus you can easily conclude Weaker acid weaker base stronger base stronger acid. **************************** C C H H 3 C + Na (+) (-) OtBu C C (-) Na(+) H 3 C + HOtBu b) Strongly left. Again electronegativity gives the easy answer: Carbon is much less electronegative than oxygen therefore an a carbon anion R3C(-) will be less stable than an RO(-). Or You could have quoted pKa values: the terminal acetylene has a pKa of 25 and the tert-butanol has a pKa of 19. Thus the alcohol is the stronger acid. Many of you remembered that t-butoxide is a very strong hindered base. It is the strongest of the RO(-) bases but not strong enough to deprotonate a terminal alkyne....
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This note was uploaded on 01/16/2011 for the course CHM 2120 taught by Professor Dr.flynn during the Fall '10 term at University of Ottawa.
- Fall '10
- Organic chemistry