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Chapter 7: Haloalkanes

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Chapter 7: Haloalkanes 188 Chapter 7: Haloalkanes Chapter 7: Haloalkanes Problems 7.1 Write the IUPAC name for each compound: CH3 (a) (b) Br Cl 1-chloro-3-methyl-2-butene 1-bromo-1-methylcyclohexane Cl Cl (c) (d) CH3CHCH2Cl 1,2-dichloropropane 2-chloro-1,3-butadiene 7.2 Determine whether the following haloalkanes underwent substitution, elimination, or both substitution and elimination: O (a) KOCCH Br 3 O +KBr CH3COOH O In this reaction, the haloalkane underwent substitution. The bromine group was replaced by an acetate (CH3COO−) group. NaOCH CH (b) 2 3 + +NaCl CH3CH2OH Cl OCH2CH3 In this reaction, the haloalkane underwent both substitution and elimination. In the substitution, the chlorine group was replaced by an ethoxy (CH3CH2O−) group. Chapter 7: Haloalkanes 189 7.3 Complete these nucleophilic substitution reactions: (a) – + + – Br +CH3CH2S Na SCH2CH3 +NaBr O O (b) – + + – Br + CH3CO Na OCCH3 +NaBr 7.4 Answer the following questions: (a) Potassium cyanide, KCN, reacts faster than trimethylamine, (CH3)3N, with 1-chloropentane. What type of substitution mechanism does this haloalkane likely undergo? The rate of this reaction is influenced by the strength of the nucleophile, suggesting that the mechanism of substitution is SN2. The cyanide ion is a much better nucleophile than an amine. (b) Compound A reacts faster with dimethylamine, (CH3)2NH, than compound B.What does this reveal about the relative ability of each haloalkane to undergo SN2? SN1? Br Br A B Compound A is more able to undergo SN2, which involves backside attack by the nucleophile, because it is sterically less hindered than B. Both compounds A and B have about the same reactivity towards SN1, because both A and B can ionize to give carbocations of comparable stability. 7.5 Write the expected product for each nucleophilic substitution reaction, and predict the mechanism by which the product is formed: Br (a) SN2 +Na+SH– SH + – acetone +NaBr SH− is a good nucleophile and the haloalkane is secondary. Acetone is a polar aprotic solvent. These factors favor an SN2 reaction. Because the mechanism of reaction is SN2, an inversion of stereochemistry at the carbon occurs. 190 Chapter 7: Haloalkanes O (b) Cl O OCH SN1 + +HCl CH3CHCH2CH3 HCOH formicacid CH3CHCH2CH3 The solvent, formic acid, is highly ionizing and is also a weak nucleophile. The reaction therefore proceeds by SN1. 7.6 Predict the β-elimination product formed when each chloroalkane is treated with sodium ethoxide in ethanol (if two products might be formed, predict which is the major product): Cl (a) CH3CH2ONa CH2 CH3 CH3 + CH3CH2OH minor major CH Cl CH CH ONa CH (b) 2 3 2 2 CH3CH2OH Cl CH CH CH ONa CH CH (c) 3 3 2 3 3 + CH3CH2OH equal amounts 7.7 Predict whether each elimination reaction proceeds predominantly by an E1 or E2 mechanism, and write a structural formula for the major organic product: Note that if there are two or more possible products, the major product is the one that is most stable. Alkenes that are more substituted are more stable, as per Zaitsev’s Rule. trans Alkenes are also more stable than cis alkenes. Br (a) E2 +CHO–Na+ + 3 methanol major minor Sodium methoxide is a strong base, and the reaction will proceed by E2. Chapter 7: Haloalkanes 191 Cl (b) E2 +Na+OH– acetone The hydroxide ion is a strong base, and the reaction will proceed by E2. 7.8 Predict whether each reaction proceeds predominantly by substitution (SN1 or SN2) or elimination (E1 or E2) or whether the two compete, and write structural formulas for the major organic product(s): Br OCH (a) 3 +CHO–Na+ + 3 methanol The haloalkane is secondary, and methoxide is both a strong base and a good nucleophile. E2 and SN2 are likely to be competing mechanisms, with the elimination product being more favorable than the substitution product. Cl OH (b) +Na+OH– + acetone The haloalkane is secondary, and hydroxide is both a strong base and a good nucleophile. E2 and SN2 are likely to be competing mechanisms. Chemical Connections 7A. Provide IUPAC names for HFC-134a and HCFC-141b. F F H Cl FCC H HCC F F H H Cl HFC-134a HCFC-141b 1,1,1,2-tetrafluoroethane 1,1-dichloro-1-fluoroethane 192 Chapter 7: Haloalkanes 7B. Would you expect HFA-134a or HFA-227 to undergo an SN1 reaction? An SN2 reaction? Why or why not? F F F F F FCC H FCC C F F H F H F HFA-134a HFA-227 Neither HFA-134a nor HFA-227 is expected to undergo an SN1 or SN2 reaction. First, fluoride is a very poor leaving group. Second, the carbocation generated in an SN1 reaction would be extremely unstable; the other fluorine atoms would act as strong electron- withdrawing groups and destabilize the carbocation. Quick Quiz 1. An SN1 reaction can result in two products that are stereoisomers. True. An SN1 reaction involves a trigonal planar carbocation intermediate that can be attacked by the nucleophile from either face of the trigonal plane. 2. In naming halogenated compounds, "haloalkane" is the IUPAC form of the name while "alkyl halide" is the common form of the name. True. In IUPAC nomenclature, the halogen is named as a substituent bonded to a parent alkane. 3. A substitution reaction results in the formation of an alkene. False. The formation of an alkene is indicative of an elimination reaction. − + 4. Sodium ethoxide (CH3CH2O Na ) can act as a base and as a nucleophile in its reaction with bromocyclohexane. True. Ethoxide is both a strong base and a strong nucleophile, and bromocyclohexane can eliminate (E2) or substitute (SN2). However, elimination is more likely than substitution, because the haloalkane is secondary. 5. The rate law of the E2 reaction is dependent on just the haloalkane concentration. False. An E2 reaction is bimolecular at the rate-determining step, so the rate law will also depend on the concentration of the base. 6. The mechanism of the SN1 reaction involves the formation of a carbocation intermediate. True. The formation of the carbocation is the rate-determining step of the reaction. 7. Polar protic solvents are required for El or SN1 reactions to occur. True. Polar protic sovents stabilize the carbocation intermediates involved in these two reactions. 8. OH− is a better leaving group than Cl−. False. As a result of its atomic size, chloride is a much more stable anion than hydroxide. Chapter 7: Haloalkanes 193 9. When naming haloalkanes with more than one type of halogen, numbering priority is given to the halogen with the higher mass. False. Regular IUPAC convention applies, and the numbering priority is given to the halogen first encoutered in the chain. 10. SN2 reactions prefer good nucleophiles, while SN1 reactions proceed with almost any nucleophile. True. The nucleophile is not involved in the rate-determing step of an SN1 reaction, so the strength of the nucleophile is much less important. 11. The stronger the base, the better is the leaving group. False. Better leaving groups are groups that are more stable, meaning that they are weaker bases. 12. SN2 reactions are more likely to occur with 2° haloalkanes than with 1° haloalkanes. False. Haloalkanes that are sterically more hindered will react slower in an SN2 reaction. 13. A solvolysis reaction is a reaction performed without solvent. False. In the context of substitution, a solvolysis reaction is where the solvent itself acts as the nucleophile. 14. The degree of substitution at the reaction center affects the rate of an SN1 reaction but not an SN2 reaction. False. Both SN1 and SN2 reactions are affected by the degree of substitution at the carbon center. Substitution affects the stability of the carbocation intermediate involved in an SN1 reaction, but it also affects steric hindrance, which is important in an SN2 reaction. 15. A reagent must possess a negative charge to react as a nucleophile. False. Nucleophiles can be neutrally charged, but they must possess a nonbonding pair of electrons. 16. Elimination reactions favor the formation of the more substituted alkene. True. Alkenes that are more substituted are more stable (Zaitsev’s rule). 17. The best leaving group is one that is unstable as an anion. False. The opposite is true: the best leaving group is one that is very stable. 18. In the SN2 reaction, the nucleophile attacks the carbon from the side opposite that of the leaving group. True. Backside attack also results in stereochemical inversion. 19. Only haloalkanes can undergo substitution reactions. False. Other compounds containing good leaving groups can also undergo substitutioon reactions. 20. All of the following are polar aprotic solvents; acetone, DMSO, ethanol. False. Ethanol, CH3CH2OH, is polar but is capable of hydrogen bonding; it is a polar protic solvent. 194 Chapter 7: Haloalkanes End-of-Chapter Problems Nomenclature 7.9 Write the IUPAC name for each compound: Br Br (a) Cl (b) I (c) (d) Br F F (e) CHCl (f) 3 (a) 3-chloropropene (b) 2,2-dibromo-4-methylpentane (c) 4-fluorocyclohexene (d) 1-bromo-2-iodo-2-methylpentane (e) trichloromethane (f) 1-fluorocyclopropane 7.10 Write the IUPAC name for each compound (be certain to include a designation of configuration, where appropriate, in your answer): F Br (d) (a) Br (b) (c) I F Cl (a) (E)-1-bromopropene (or trans-1-bromopropene) (b) (R)-2-chloro-3-methylbutane (c) trans-1,3-difluorocyclopentane (d) cis-1-bromo-2-iodocyclohexane (e) (E)-2-fluoro-5-iodo-2-pentene (f) (R)-1-bromo-3-chlorobutane Chapter 7: Haloalkanes 195 7.11 Draw a structural formula for each compound (given are IUPAC names): (a) 1-iodo-2,4-dimethylpentane (b) trans-1-bromo-2-methyleyclohexane (c) 6-bromo-2-chloro-3-ethylheptane (d) cis-6-chloro-5-methyl-2-hexene (e) (R)-2-fluoro-4-methylpentane (f) meso-2,3-difluorobutane Br Br (a) I (b) (c) Cl F F F (d) (e) (f) Cl 7.12 Draw a structural formula for each compound (given are common names): (a) allyl chloride (b) sec-butyl fluoride (c) tert-butyl bromide (d) methylene iodide (e) chloroform (f) vinyl fluoride Br CHCl (a) Cl (b) (c) (d) CH2I2 (e) 3 (f) F F 7.13 Which compounds are 2° alkyl halides? (a) Isopropyl bromide (b) sec-butyl chloride (c) 3-fluoropropene (a) & (b).
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