2 2 (I) Write one example of each of the following (3 pts each) (1) A sec-butyl group CH3-C-CH2-CH3H (2) A molecule with a tertiary carbon CH3CH3HCH3 (3) The conjugate base of water HO- (4) Cyclopropane OR (5) A molecule which contains an sp hybridized nitrogen. H3C C N C CCHHHHHH3 3 (6) A carbene HCH (7) An initiation reaction in free radical halogenation. Cl2Cl Cl+hv (8) A compound with a bridgehead carbon. (9) Trans-decalin HH (10) An ethyl group CH2CH34 4 (II) (A) Fill in the boxes to indicate the hybridization state of the labeled heavy atoms (3 pts each). (B) For each of the following. (i) Circle the contributing valence bond resonance structure that makes the largest contribution to the observed structure of the molecule (2 pts). (ii) Explain your answer (3 pts.) OCH3SOCH3S(1) Negative charge is more highly stabilized by oxygen than by sulfur. CH3OCH3CH3CH3OCH3CH3(2) The structure that places full octets of electrons at all heavy atoms is preferred. OO(3) The structure with minimum charge separation is preferred. CH3COCH2C C NCH2COHsp3spsp2sp2sp25 5 (III) 5 points each (A) Circle the compound with the highest boiling point (B) Circle the compound which has a dipole moment of greater than zero. FCFFFF FF FFF (C) Circle the statement that must be true for the following reaction, if ΔGo is less than zero at 25 oC. (a) At chemical equilibrium, the concentration of reactant A will be greater than the concentration of the product B. (b) At chemical equilibrium, the concentration of reactant A will be less than the concentration of the product B. (c) At chemical equilibrium, the concentration of reactant A will be equal to the concentration of the product B. (d) The value of ΔSo for the reaction must be greater than 0. (e) The value of ΔHo for the reaction must be greater than 0. (D) Circle the statement below that correctly describes the chair conformations of trans-1,4-dimethylcyclohexane. (a) ΔGo = 0 for interconversion of the two chair forms. (b) The most stable chair form contains two axial methyl groups. (c) The most stable chair form contains two equatorial methyl groups. (d) The most stable conformer is a boat form of the cycloalkane. (e) Trans-1,4-dimethylcyclohexane rapidly isomerizes to cis-1,4-dimethylcyclohexane at room temperature and 25 oC. AB6 6 (E) A value of K = 10 and ΔGo = -1.36 kcal/mole was determined for the conversion of A to B. Calculate the value of ΔGo for a reaction where K = 0.1. (5 pts) + 1.36 kcal/mole (F) Circle the most unstable conformer of 2-methylbutane. MeHHMeMe HHHMeMe HMeHMeMe HHMe (IV) (A) Write the full structure of the following compound. (5 pts) 2,2,4,4-tetramethylhexane (B) (1) Number clearly the chain that you will use when naming the following compound (2 pts). (2) Circle the substituents that lie at this chain (2 pts). (3) Give the full IUPAC name for this compound (6 pts). 5-butyl-2,3,4,6,8-pentamethylnonane or 5-n-butyl-2,3,4,6,8-pentamethylnonane AB1234567897 7 (C) Draw the Newman projection for the fully planar form of cyclobutane (5 pts). H HHHHHHH (D) Draw the two chair forms of trans-1,2-dimethylcyclohexane (5 pts). HHCH3CH3HCH3CH3HHHCH3CH3AND OR Two axial methyl groups or two equatorial methyl groups. Ring flipping converts axial groups to equatorial and equatorial groups to axial. (E) Draw the Newman projection for ClCH2CH2Cl in which the two chlorine atoms are gauche to one another (5). HClHHClH8 8 (V) (1) Briefly explain the difference between a homolytic and heterolytic reaction (5 pts). Heterolytic: Both electrons in the bond move to the same fragment during bond cleavage to give ionic products: A BA+ + B- Homolytic: Electrons in the bond partition between the two segments to give radical products: A• + B•A B (2) The arrows show the bonds with the given bond dissociation energy. Circle the C-H bond that undergoes the most rapid halogenation by Cl2 in a free radical chain process (5 pts). Briefly explain your answer (5 pts). HHHH MeH H 98 kcal/mole 95 kcal/mole 92 kcal/mole 85 kcal/mole The weakest C-H bond undergoes the most rapid hydrogen atom abstraction by chlorine radical in free radical chlorination reactions because it is easier to break.9 9 (3) Draw the transition state for the following reaction (5 pts). Does this transition state occur early or late along the reaction coordinate? Explain your answer (5 pts). MeHHFH An early reactant like transition state will be observed for the strongly thermodynamically favorable hydrogen atom abstraction by the fluorine radical. (4) Set up the two equations that you would use to calculate the expected relative yields of products A and B from chlorination of 1,2,3 trimethylcyclopropane by a free radical chain process, if the relative reactivity of the primary methyl H and the tertiary cyclopropyl H is 1:3 (10 pts). There was an error in this question and product B should have had only one chlorine in the primary position and not one in the tertiary position. Partial credit was given for this question. Dr. Richard will make an announcement about this question in class. MeHHHFMeHHH F+ +HCH3CH3HCH3HHCH3CH3ClCH3HHCH2ClCH3ClCH3HAB10 10 VI (Bonus). In the presence of Br2, the following light-promoted reaction was observed. Write a mechanism for this reaction that explains the formation of both of the products of this reaction (15 pts). Initiation: Br2Br• Br•hν+ Propagation: HBrCH3CH2Br2CH3CHHBr•CH3CH2BrBr2Br•CH3CH3CH3CH2BrBr•++resonance delocalized radicalcan react at two postions++