Orgo Final Nucleophilic Substitution – Flashcards
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Indicate whether the following situation would increase, decrease, or not affect the rate of an SN1 reaction. Answers should follow what you learned in the lab lecture.
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-Decrease temperature - decrease rate -change the leaving group from Br to F - decrease rate -Decrease concentration of nucleophile - no affect -change solvent from ethanol to water - increase rate -increase concentration of ally halide - increase rate -increase concentration of nucleophile - no affect -change solvent from water to acetone - decrease rate -decrease concentration of alkyl halide - decrease rate
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Indicate whether the following situation would increase, decrease, or not affect the rate of an SN2 reaction. Answers should follow what you learned in the lab lecture.
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-Increase temperature - increase rate -change the leaving group from Br to I - decrease rate -decrease concentration of nucleophile - decrease rate -change solvent from ethanol to acetone - increase rate -decrease concentration of alkyl halide - decrease rate -decrease temperature - decrease rate -change the leaving group from Br to Cl - decrease rate -increase concentration of nucleophile - increase rate -change solvent from acetone to water - decrease rate -increase concentration of alkyl halide - increase rate
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Why would a chiral starting material yield a 50:50 mixture of enantiomers when it undergoes SN1?
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because it has a planar carbocation intermediate.
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Why is it important that you use the exact same amount of nucleophile in each test tube for the SN2 reaction in order not to add another variable to the reaction?
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in SN2, the nucleophile affects rate
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You react 2-fluorobutane and 1-bromobutane with sodium iodide in acetone. Which alkyl halide would theoretically yield precipitate in this reaction faster? Provide reason.
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1-bromobutane, its primary ally halide-leaving group is Br, which is better than F
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Provide 2 reasons why it is important to add the same amount of alkyl halide to each test tube in the nucleophilic substitution experiment you carried out.
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1. amount of alkyl halide affects rate 2. alkyl halide can act as a solvent in which nucleophile is insoluble giving a false positive
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You react 2-iodo-2-methylpropane and 2-chlorobutane with silver nitrate in ethanol. Which alkyl halide would theoretically yield precipitate in this reaction faster? Provide reason why.
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2-iodo-2-methylpropane, it form a tertiary carbocation, the leaving group I is better
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Why are aprotic solvents preferable for the SN2 reactions?
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it does not solvate the nucleophile or it does not stabilize the reactants
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What is the reason a false positive may occur if you use too much alkyl halide in the nucleophilic substitution reaction?
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alkyl halide acts as a solvent in which nucleophile is insoluble
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Why don't vinyl halides undergo SN2?
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electrons in the nearby double bond repel the nucleophile
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The SN2 reaction of 2,2-dimethyl-1-bromopropane is significantly lower than bromoethane, even though both are primary alkyl halides. Why?
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the steric hindrance of 2,2-dimethyl-1-bromopropane
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Why are polar ptotic solvents preferable for the SN1 reactions?
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stabilizes transition state more than it does reactants
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In the reaction of an alkyl bromide with sodium iodide in acetone, why would the resulting alkyl iodide be attacked by a bromide ion?
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the bromide ion is not available since it precipitates as NaBr in acetone
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What compounds would form a precipitate when reacted with silver nitrate in ethanol
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2-bromocyclopentene and 2-chloro-2-methylpropane
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List the reason why aryl and vinyl halides do not undergo SN2
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electrons in the double bond repel the nucleophile
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Define reflux
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heating a reacting mixture to the boiling point of solvents for extended time without evaporation
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Why do the Br- and Cl- not attack the alkyl iodides that form the SN2 reaction carried out?
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because NaCl and NaBr are formed so there is not free Cl or Br ion to attack alkyl halide
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What factors promote SN2 in the NaI in acetone reaction?
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-not soluble in acetone and form a precipitate -iodine ion is excellent nucleophile -does not favor ionization of alkyl halide
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What factors hinder SN2 in the silver nitrate in ethanol reaction?
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the solvent (ethanol) favors ionization of the halide, and the nitrate ion is a very poor nucleophile so alkyl nitrates do not form by an SN2 reaction
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Recognizing alkyl halides that form carbocations that rearrange in SN1
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the carbocation is spy hybridized and trigonal-planar in structure with a vacant p orbital
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Why E2 will not occur in the sodium iodide in acetone reaction?
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it is a nonpolar solvent that will readily dissolve sodium iodine
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Why E1 will occur in silver nitrate in ethanol reaction?
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the carbon halogen bond is weakened because a molecule of insoluble silver halide is formed, thus promoting SN1 reaction of the alkyl halide
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What kind of alkyl halide will definitely undergo an SN1 reaction (if substitution reaction is the only possibility) even in the presence of a good nucleophile? Think about an alkyl halide that will definitely undergo SN1 but not SN2.
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a tertiary halide
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Describe the carbocation in terms of hybridization, structure and orbital characteristics.
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sp2 hybridized and trigonal-planar in structure with a vacant p-orbital
