Alkyl Halide Ir

Question: What are the major objectives of this experiment?

Answer: Part 1: Comparison of SN1 and SN2 reactions Conversion of alcohols to alkyl halides.
Part 2: Reactivity of Alkyl halides. Infrared (IR) Spectroscopy evaluation, Micro-scale extraction techniques.

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Question: Overview of Nucleophilic Substitution

Answer: •Prepare a Stock solution of NH4Br/H2SO4.
• Prepare this stock solution ON ICE initially
• Reflux for 75 min with the primary or secondary alcohol.
• Cool carefully, perform micro scale workup.
• CH2Cl2/extract with water, NaHCO3, dry with
Na2SO4.
• Record the IR spectrum.

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Question: Objectives of Part A SN2 of a primary alcohol with NH4Br/strong acid

Answer: • Prepare Stock solution of NH4Br/H2SO4
• Prepare this stock solution ON ICE initially
• React at RT with the tertiary alcohol for about 15 min (constant shaking).
• Micro scale workup (CH2Cl2, extract with water, sodium bicarbonate, dry with Na2SO4)
• Record the IR spectrum

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Question: Objectives of Part B: SN1 with NH4Br/strong acid

Answer: • React a series of ten alkyl halides with NaI in acetone to test reactivity vs structure. Start at RT then heat to 50 °C if no reaction observed.
• React a series of ten alkyl halides with AgNO3 in ethanol. Start at RT then heat if no reaction observed.

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Question: Objective of Part 2: Reactivity of alkyl halides

Answer: This method is used for distinguishing between primary secondary, and
tertiary halides. The test depends on the fact that both sodium chloride and sodium bromide are not very soluble in acetone, whereas sodium iodide is. The reactions that occur are SN2 substitutions in which iodide ion is the nucleophile; the order of reactivity is primary > secondary > tertiary.

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Question: Nucleophilic Substitution Bimolecular, SN2

Answer: Tertiary halides are more reactive in an SN1 reaction than secondary halides, which are in turn more reactive than primary halides. Differing rates of silver halide precipitation would be expected from halogen in each of these environments, namely, primary < secondary < tertiary. Alkyl bromides and iodides react more rapidly than chlorides, and the latter may require warming to produce a reaction in a reasonable period. Aryl halides are unreactive toward the test reagent, as are any vinyl or alkynyl halides generally. Allylic and benzylic halides, even when primary, show reactivities as great as or greater than tertiary halides because of resonance stabilization of the resulting allyl or benzyl carbocations.

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Question: Reaction of 1-Butanol, NH4Br in H2SO4

Answer: The more alkyl groups connected to the reacting carbon, the slower the reaction

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Question: Apparatus for the reaction 7A

Answer: tertiary alkyl halides react extremely slowly in SN2 reactions. But tert-butyl bromide reacts with water 1,000,000 times faster than methyl bromide.
• Tertiary alkyl halides react rapidly in protic solvents by a mechanism that involves departure of the leaving group prior to addition of the nucleophile
• Called an SN1 reaction - occurs in two distinct steps while SN2 occurs with both events in same step
• If nucleophile is present in reasonable concentration (or it is the solvent), then ionization is the slowest step

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Question: IR spectrum of 1-butanol shown

Answer: NaI in acetone because Iodide is a good nucleophile, and if it displaces bromide or chloride, NaBr or NaCl will precipitate (these are much less soluble in acetone than NaI).

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Question: Reaction of Ammonium bromide with 2-methylbutan-2-ol

Answer: AgNO3 in ethanol because Ethanol is a polar protic solvent and can promote ionization of certain organo-halides. If halide ion is released a precipitate of AgCl or AgBr will form

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Question: Reactivities of Alkyl halides: Sodium iodide in Acetone

Answer: Using simple vs. complex 3º did make a large difference in reaction time. This makes sense because the complex 3º had more carbons attached, and therefore was a larger molecule, which may have made a difference in reactivity. Also, the carbocation intermediate could not have formed an sp2 hybridization because of the other bonds, and was less stable, slowing the reaction. Bromobenzene did not react with any solution, as aryl halides are unreactive, as are vinyl and alkynyl halides. Nucleophiles are negatively charged and benzene has a pi-cloud, which is electron rich. The pi-cloud repels the nucleophiles.

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Question: How does the use of Sodium iodide in Acetone as a reagent affect reactivity

Answer: Secondary, Experimentally required heating to 50 degrees celsius in NaI and 100 degrees celsius in AgNO3

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Question: Reactivities of Alkyl halides: Alcoholic Silver Nitrate

Answer: Secondary, Experimentally required heating to 50 degrees celsius in NaI and did not require heating in AgNO3

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Question: Alcoholic Silver Nitrate promotes SN1 reactions. Why?

Answer: Primary, Experimentally required heating to 50 degrees celsius in NaI and 100 degrees celsius in AgNO3

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Question: SN2 Reaction Mechanism

Answer: Primary, Experimentally did not require heating to in NaI and in AgNO3

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Question: SN2 Reaction: stereochemistry

Answer: Tertiary, Experimentally did not react after heating to 50 degrees celsius in NaI and did not require heating in AgNO3 (Reaction was completed with AgNO3 and not with NaI)

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Question: SN2 Reaction: substrate structure (Alkyl bromides react with KI in Acetone at 25°)

Answer: Primary(stabilized by resonance), Experimentally did not required heating in NaI and in AgNO3
(NaI-formed colored percipiatied)

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Question: what is the order of Reactivity in SN2 reactions?

Answer: Primary(stabilized by resonance), Experimentally did not required heating in NaI and in AgNO3
(NaI-formed colored percipiatied)

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Question: Oxygen Nucleophiles (SN2)

Answer: Secondary(stabilized by resonance), Experimentally did react in NaI and in AgNO3

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Question: What is the relative nucleophilicity trend

Answer: Secondary,
Experimentally required heating in NaI and did not require heating in AgNO3

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Question: SN2 Reaction: What is the importance of a having a good leaving group present

Answer: Secondary,
Experimentally required heating in NaI and did not require heating in AgNO3

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