Organic Chemistry CHE230-001 EXAM1 key Page 1 of 2 1. (15 pts) Draw line structures (as the lowest-energy resonance structures) for the following compounds. Include all lone pairs and indicate all formal charges (unless the formal charge is zero). Indication regarding how to connect the atoms is present in the molecular formulas given below at left. 1a. S(CH3)3+ 1b. OCHCHCH2(-) 1c. CH3NO2 2a. (5 pts) Compute the degrees of unsaturation in C2H4O; show you work. My answer for 2c: __DU = 2-4/2+1 ______ 2bc (10 pts) Draw two isomers possessing molecular formula C2H4O. 3. (10 pts) Think about the electron-based mechanism of molecular absorption of light. Question: Which should absorb light of lowest energy, saturated or unsaturated hydrocarbons and why? My answer: One or two sentences! No credit without an explanation. ___________________ __Unsaturated hydrocarbons absorb lower energy light because the filled bonding and the empty antibonding orbitals are closer in energy than in saturated molecules because pi bonds overlap less efficiently than sigma. ___________________________________________________________________________ 1a S 1c NOO 1b OHHHHOHHHHOHHHHOHHHHABCCbest2b 2c O OOrganic Chemistry CHE230-001 EXAM1 key Page 2 of 2 4a. (10 pts) Draw the Newman projection of 2,3-dimethylbutane in its most stable conformation. 4b. (5 pts) The lowest-energy conformer of n-butane is 0.9 kcal/mol less in energy than the second lowest conformer. With this information you should be able to estimate the difference in energy between the structure you drew in 4a and the second lowest conformer of 2,3-dimethylbutane. There are 3 gauche interactions in the next highest conformer whereas there are two in the lowest energy conformer of 2,3-dimethylbutane. Each is worth 0.9 kcal/mol so the answer is 0.9 kcal/mol. 5a. (10 pts) Name the following structure. 7a: _____3-ethyl-3-hexene__________________ 5b. (10 pts) How many signals in the 13C NMR spectrum will the structure in 5a have? Answer: 5a_8___ How many signals in 5b?__3__ (5b) 6. (10 pts) In molecular formula C4H4O2 Each carbon atom is connected to at least two and only two carbon atoms by single bonds. Draw a structure that maximizes the dipole moment and draw a structure that minimizes the dipole moment of the molecule. 7 (15 pts) (5 pts) Rank the molecules according to stability. (10 pts.) Explain your answer for the most stable structure and the least stable structure (one or two words will be enough for each). Most stable _D_, _B_, _A_, least stable _C_. ABCD 4a HCH3CH3CH3HCH3 Maximum Dipole Minimum Dipole OO OO 7. Most stable: __Most substituted__ Least stable_trans-double bond in a six-membered ring is strained. Poor orbital overlap. _ 4b work space
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