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Exam_1-Chem35-2007-key

Exam_1-Chem35-2007-key - Name SISD Problem I(20 points For...

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Unformatted text preview: Name SISD Problem I. (20 points) For each molecule (parts A - C): (i) Draw the two possible cyclohexane chair conformations. (ii) Circle the H underneath the conformer that is Higher in energy (less stable). Circle the L underneath the conformer that is Lower in energy (more stable). Use the templates provided to draw the chair conformations. You must draw, unambiguously, the Methyl groups in the proper equatorial or axial positions. The molecules and the chair templates are numbered. Make sure to draw the chair conformations using the numbering scheme that matches the given structure. A) (5 points) 1 pt . all Me cis 3 pts - 6 unambiguous and correct axial & equitorial Me 2 pte - 5 correct 1 pt - 4 correct 1 pt correct H 8. L 2 A: HorQ B) (5 points) 1 pt - all Me cis 3 pts - 6 unambiguous and correct axial & equitorial Me 2 pts - 5 correct 1 pt - 4 correct 1 pt correct H & L Name SISD . 1 pt — all Me cis C) (5 pOII‘ltS) 3 pts - 6 unambiguous and correct axial & equitorial Me 2 pts - 5 correct 1 pt - 4 correct 1 pt correct H & L / I J}, * C: HorL C: HOI® D) (5 points) In parts A) — C) of this problem you have generated six cyclohexane chair conformations that are labeled A-H, A-L, B-H, B-L, OH and C-L. Arrange the si_x structures into the order of their relative stabilities. Place the labels corresponding to each conformer into the chart below. (Hint: Think about the number of gauche interactions that are present in each conformation). CL AL BL AH B_H CH (lowest energy) (highest energy) # equitorial Me 3 # axial Me 0 0 # Me - Me gauche 1 pt each CL, CH. AL, BL, AH Name SISD Problem 2 (20 points) In parts A - C, draw all lone airs and all formal char es. A) (7 points) Draw all resonance structures for the molecule shown below that have full octets on all atoms other than hydrogen. On the original structure that we provide, indicate the correct h bridization (sp3, spz, or sp) of all atoms other than hydrogen. 9 e K] «e e..\ m w ._.. we :H(.. O.‘/ H o\ / H 0‘ ®0\N”@ O._/ 'I4 T53 lb ® '1“ I G) 7‘9 veryminor 4 pts - all bonds and lone pairslformal charge In correct place on rung (4 5'33 _ Me on N C'R'S‘) 2 ll th C N 0 2 pts for all correct formal Sp ' a 0 er ' ' charge or 1 N+ error 1 pt for 2 FC error 1 pt ALL correct hybridization B) (6 points) Draw all resonance structures for the molecule shown below that have full octets on all atoms other than hydrogen. Circle the resonance structure that is lowest in energy (i.e. contributes more to the resonance hybrid). 0n the original structure that is given, indicate the correct h bridization (sp3, spz, or sp) for all atoms except hydrogen. 3 pts - correct double bond in the three CR8 2 pts - Formal charge all sp2 - B N F 0 correct or SP3 _ all 0.5 1 pt one Incorrect F.Charge 1 pt all correct hybridization or 1 error Name SISD C) (7 points) For the molecule shown below, draw all resonance structures that have a full octet of electrons on every atom (except for hydrogen). On the original structure that is given, indicate the correct h bridization (sp3, spz, or sp) for all atoms except hydrogen. 5 x 1 pt each completely correct CR8 1 pt for 2 con'ect sp hybridization 1 pt for all correct sp2 hybrid. sp - nitrile C St N sp3 - methyl C sp2 - all remaining 0, O. N. Name SISD Problem 3. (12 points) Provide structures of 9 distinct cyclic isomers that have the formula C4H30. For each isomer, all atoms must have complete octets and zero formal charge and th_e molecule must contain a ring. Cis and trans cyclic compounds are distinct isomers. (Do not generate configurational isomers — those involving stereogenic centers). Name SISD Problem 4. (20 points) The shape of a potential energy versus torsion angle diagram for rotation around a C—C bond depends on the groups attached to the carbons. For the following five molecules A) (15 points) Draw Newman Projections for the lowest energy conformer viewing along the C2—C3 bond (in each case, the C2 should be closer to you). (Br — Me eclipsing = 6.0 kcal/mole. Br — Me gauche = 0.5 kcal/mole) B) (5 points) Potential Energy Surface. Four distinct potential energy versus torsion angle curves are shown on the next page. BASED ON THE QUALITATIVE SHAPE of the curves on the next page, determine which potential energy surface type (A — D) best represents rotation about the C2—C3 bond for each of the above molecules. Indicate your selection for each molecule on the line below its Newman Pro'ection. 3 points each Newman Projection - one point for each proper anti-periplanar pair of 02-03 substituents 1 point each proper Potential Energy vs Torsion Angle Assignment Name SISD Potential Energy Plots TYPEA TYPEB an. 5 A 9 % 4-: % a E E r = 3-5 :- e If! 3 a u u 5 $5. 2: 5 4 >. >. 3 £1: : 0.5 I: 1 I“ 0 . . I“ o 0 60 120 180 240 300 360 0 60 120 180 240 300 360 Dlhedral Angle Dihedral Angle TYPEC TYPEB can 6 F 5 3 5 3 G a} 1 o I: m 0 I.I=J O 60 120 180 240 300 360 0 60 120 180 240 300 360 Dlhedral Angle Dihedral Angle Name— SISD Problem 5. (10 points) In the space provided next to each pair of structures, indicate if the structures are conformers, isomers, or identical (same conformer). A) (2 points) M:MH i_antical 2 —oints I B) (2 points) identical 2 points H Beg“ ”HT?“ H“" Br Me Me Br Br C) (2 points) F “\H H F ‘ M F : e H Me F D) (2 points) we E) (2 points) OH HO’fiI: Br "Me — 10 identical 2 points Name SISD Problem 6. (18 points) Assorted Problems 1) (9 points) Indicate whether structures A - E Q or are not Contributing Resonance Structures (CRS). BRIEFLY explain why you reject any structures. A B C 9 e N N m“;- tfibs Hm G) N“... \ / N / .._.. egos Hmhg D E Structure CRS ? {yes or no} Explanation if not a CRS A C - the imine (left) N lone pair is in an sp2 hybrid orbital that is perpendicular B to the pi system. The sp2 orbital can not overlap with the neighboring p-orbital. Thus, it can not make a pi bond C E - The tertiary amine is bonded to a methyl group (sp3 hybridized). This 1 pt each resonance structure has the nitrogen attached to a OH group. D 2 points each correct explanation E ii) (4 points) Indicate whether each of the following alkenes is a (Z) stereoisomer, an (E) stereoisomer or not a stereoisomer. \ OMe o H _ OMe MeO |\| OT E ”’l n 1 pt each correct answer 11 Name SISD iii) (5 points) Indicate whether structures F - J g or are not Contributing Resonance Structures (CRS). BRIEFLY explain why you reject any structure. F G H / / / HN HN | HN el 6) 9| /B / / "" "/B\ / ‘ ’/BM (’3 ‘ HN|/ ‘ HNI/ /B\/\@)\ /B\e/\)\ | J Structure CRS?1yes or no! Explanation if nota CRS Q F N/ G The only CRS is drawn --=~. \\/\ Every atom has an octet. None of the 5 RS (F-J) places . complete octet on every atom J is missing a + charge on the N 12 ...
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