trans-cyclooctene 4-fluoro-1-butanol 2-cyclohexenol 4. (5 pts) Provide structural formula for each of the following molecules: Br OH Cl OH

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1 CEMISTRY MIDTERM # 2 answer key March 12, 2009 Statistics: Average: 72 pts (72%); ighest: 98 pts (98%); Lowest: 21 pts (21%) umber of students performing at or above average: 22 (50%) umber of students performing at or below 55%: 8 (18%) 1. (8 pts) Mark as true (T) or false (F) the following statements. Do not explain! (T) All E1 reactions involve formation of carbocations; (T) More stable carbocations are generated faster; (T) Carbocations are electrophiles; (T) Carbocations are electron deficient; (T) Free radicals are electron deficient; (T) Alcohols are ønsted bases; (F) The rate-determining step is always the fastest elementary step; (F) More stable alkenes have larger heats of hydrogenation; 2. Circle ALL that apply: A. (3 pts) Free-radical brominations: a. Are chain reactions; b. ccur via carbocations; c. Require an initiation step; d. Are selective for tertiary positions; B. (3 pts) E2 reactions: a. Require strong bases; b. ccur faster at lower temperatures; c. Are subject to stereoelectronic control; d. Generate alkyl halides; C. (3 pts) The following reactions can be used to prepare alkenes: a. Free-radical chlorination of alkanes; b. ydroboration oxidation of alkenes; c. Dehydration of alcohols; d. ydrogenation of alkenes; 3. (3 pts) Provide an acceptable name for each of the following molecules: F trans-cyclooctene 4-fluoro-1-butanol 2-cyclohexenol 4. (5 pts) Provide structural formula for each of the following molecules: I Vinyl chloride Methylenecyclopropane (E)-2-Penten-1-ol Allyl iodide Trans-2-bromocyclopentanol 5. (5 pts) Assign E or Z configuration to each of the following alkenes. Do not explain! F C C C 2 C C 2 3 C C C Z E E E C 2

2 6. (3 pts) For the species below, complete the Lewis structure and draw two more valid resonance structures. early show, using the curved arrow notation, the shift of electron pairs. Rank the resonance structures (4 pts) Arrange the following alkenes in order of increasing stability. A. Cis-2-pentene; B. Trans-2-pentene; C. 2-Methyl-2-butene; D < A < B < E < C 2 D. 1-Pentene; E. 2-Methyl-1-butene; 8. (2 pts) Select the alcohol which is expected to have the fastest reaction with hydrobromic acid. A. 1-Pentanol; B. eopentyl alcohol C. 3-Pentanol; D. 2-Methyl-2-butanol; 9. (20 pts) Predict the organic product in each of the following reactions. If more than one product is formed, indicate the component. 2 3 P 4 i ac 2 5 C 2 5 RR 1) B 3.TF 1) 3 2) 2 2, - 2) 2, Zn 1 eq. 2 hν 100% 2 S 4 2 P 3 S 3 2 hν, 3 C a C 2 S 2 pyridine ac 2 5 C 2 5

3 10. (4 pts) In each of the following cases, suggest the appropriate starting material for the indicated transformation. C 2 2 1) 3 2) ( ) 2 S 11. (4 pts) Each of the following carbocations can rearrange to a more stable one. Propose structures for the likely rearranged species. In each case clearly indicate the type of rearrangement (i.e. hydride shift, methyl shift, etc.). hydride shift methyl shift 12. (6 pts) Provide one example of each of the following: A. An anti-addition reaction to an alkene; 2 2 B. A concerted reaction; C. An E1 reaction; C P (2 pts) Provide the structure of a 5-carbon alkyl iodide, which yields a single alkene upon reaction with ac 2 5 /C 2 5. I 14. (4 pts) For each of the reactions below, select the appropriate starting alkyl bromide that would yield the indicated alkene as the exclusive product. a a 15. When trans-2-methylcyclohexanol is subjected to acid-catalyzed dehydration, the product is 1-methylcyclohexene. owever, when trans-1-bromo-2-methylcyclohexane is subjected to dehydrohalogenation with ac 2 / C 2, the only product is 3-methylcyclohexene. A. (4 pts) Provide equations for the above outlined reactions; 2 S 4 ac 2 5 heat C 2 5 only

4 B. (4 pts) Account for the difference in products in the two reactions (Show explicit structures; I will not accept a verbal only explanation!! More structures, fewer words!). Dehydration: It occurs via E1 mechanism. A carbocation is formed as an intermediate and it generates the alkene products according to the Zaitsev Rule, yielding predominantly the most substituted, 1-methylcyclohexene. C minor shift C2 minor Dehydrobromination: It occurs via E2 mechanism (i.e. a concerted process). The type and quantity of product(s) is subject to stereoelectronic control. Abstraction of proton can occur only from C β bonds that are coplanar to the C bond (best is anti-coplanar, or at least syn-coplanar). The starting material can exist in two conformations, of which only the diaxial has E C β bond, which is anti-coplanar to the C bond. nly this particular proton can be abstracted by the base, yielding a single product: 3-methylcyclohexene. 3 C C C no coplanar C - bonds E anti-coplanar C - bond 16. (2 pts) Provide the structure of an octane isomer, which yields only one monochlorinated product upon free-radical chlorination. 17. (4 pts) The addition of to 1-isopropylcyclohexene yields a rearranged product as shown below. ffer a detailed mechanism to account for the reaction outcome. -shift

5 18. Suggest synthetic sequences for each of the following transformations: A. (3 pts) Conversion of 1-propanol to 2-propanol. 3 P B. (4 pts) Conversion of cyclopentane to. 2 hν a C (2 pts) BUS PRBLEM (In order to receive credit for this problem, it has to be solved entirely!!). There are other reagents, besides, that can add to alkenes following a free-radical mechanism, in the presence of peroxides. Suggest a plausible mechanism and predict the product of free-radical addition of methanethiol ( S) to propene. S RR?? Initiation: 1) R R 2 R 2) R S R S Propagation: 1) S S 2) S S S S verall: S RR S