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Preparation of Alkyl Halides (schematic)

Preparation of Alkyl Halides (schematic). Alkanes - Alkyl Halides (free-radical halogenation) Alkenes - Alkyl Halides (hydrohalogenation via HX or allylic bromination using NBS) Alkenes - Alkyl Dihalides ( vic dihalogenation via X 2 ) Alkenes - Mixed Substituted Alkyl Halides

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Preparation of Alkyl Halides (schematic)

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  1. Preparation of Alkyl Halides (schematic) • Alkanes - Alkyl Halides (free-radical halogenation) • Alkenes - Alkyl Halides (hydrohalogenation via HX or allylic bromination using NBS) • Alkenes - Alkyl Dihalides (vic dihalogenation via X2) • Alkenes - Mixed SubstitutedAlkyl Halides (via X2/ competing nucleophile) • Alcohols - Alkyl Halides (via various reagents)

  2. Preparation of Alkyl Halides (schematic)

  3. General Scheme for the Nucleophilic Aliphatic Substitution:

  4. Substitution Reactions. Nucleophilicaliphatic substitutions at the sp3 carbon are divided into the following sub-categories: 1.Nucleophilicaliphatic substitution at the sp3 carbon, bimolecular (SN2). In this case the mechanism is often (but not always) a single step and the rate determining step involves both the nucleophile and substrate. 2.Nucleophilicaliphatic substitution at the sp3 carbon, unimolecular (SN1). In this case the mechanism is multi-step and the rate determining step involves only the substrate.

  5. The SN2 Reaction

  6. The SN2 Reaction • Kinetics: Rate = k [RX] [Nu:-]. Both RX and Nu:- are involved in the rate-determining step. • Nucleophile: Negatively-charged (strong nucleophile) work best; occasionally neutral nucleophiles can be used. • Reactivity of alkyl halides: CH3 > 1° > 2° due to steric effects that hinder backside attack. 3° halides do notreact. Allyl (CH2=CH-CH2X) and benzyl (C6H5CH2X) halides are unusually reactive (comparable to or more reactive than methyl).

  7. The SN2 Reaction • Solvents: Wide variety can be used, but polar, aprotic solvents are favored and usually cause 2° halides to react by this mechanism. • Rearrangements: Do not occur • Stereochemistry: Complete inversion of configuration via pentacoordinate carbon TransitionState.

  8. Nucleophiles and Bases. 1.Weak, charged nucleophiles (soft bases) (Favor SN2). OH-, CN-, I-, RS- 2.Weak, neutral nucleophiles (soft bases) (Favor SN1 and elimination) ROH, H2O, RSH, R3P 3.Strong nucleophiles (hard bases) (Favor SN2 and elimination) Cl-, RNH2, RCO2- (mainly SN2) F-, RO-, HO-/alcohol, NH3, CO32- (mainly elimination)

  9. The SN2 reaction is concerted, with all bonds being made and broken at the same time.

  10. The Solvents 1 Polar, aprotic solvents (no H-bonding, but good dipoles) (Favor SN2 and elimination).

  11. The SN1 Reaction

  12. The SN1 Reaction Kinetics: Rate = k [RX]. Only RX is involved in the rate-determining step. Nucleophile: Neutral (weak nucleophile) work best. Reactivity of alkyl halides: 3° > 2° due to the relative stabilities of carbocations that form (electronic effects). Methyl and 1° halides do notreact. Allyl and benzyl halides are unusually reactive due to the exceptional stabilities of the allyl and benzyl carbocations (resonance effects).

  13. The SN1 Reaction Stereochemistry: Partial racemization via intimate and solvent-separated ion pairs. Rearrangements: Occur if a more stable carbocation can form. Solvents: Wide variety can be used, but polar, protic solvents are favored and usually cause 2° halides to react by this mechanism.

  14. The SN1 reaction occurs in several steps, with formation of the carbocation beinginvolved in the first, rate-determining step

  15. Carbocations are involved in this mechanism, with some features of carbocations being : Stability: 3° > 2°. The more stable an intermediate is, the more easily it is formed; hence the relative reactivities of and halides. Methyl and 1° carbocations are seldom formed. Allyl and benzyl carbocations form readily because of resonance stabilization of the positive charge. Rearrangements: If possible, a carbocation may rearrangeto a more stable one by an alkylgroup shift or a hydride shift. The most common rearrangement is from a 2° to 3° ion. Primary carbocations, if formed (and they seldom are), could rearrange to either a 2° or 3° cation.

  16. Example of an SN1 reaction of a 2° alkyl halide proceeding with carbocation rearrangement.

  17. The Solvents 2 • Polar, protic solvents (good H-bonding donors and acceptors) (Favor SN1 and elimination). ROH, H2O, RNH2, liq. NH3, CH3CO2H

  18. Elimination. The E2 Reaction The E2 reaction is a concerted reaction, with all bonds being made and broken at the same time.

  19. Elimination. The E2 Reaction Kinetics: Rate = k [RX] [B:-]. Both RX and B:- are involved in the slow, rate-determining step. The base: Strong bases, such as KOH/alcohol or NaOCH2CH3 (NaOEt), are required. Reactivity of alkyl halides: 3° > 2° > 1°. With a strong base, alkyl halides undergo elimination by the E2 reaction. Rearrangements: Do not occur

  20. Elimination. The E2 Reaction Solvents: Reaction can occur in a variety of solvents. The use of a strong base is the driving force for elimination to occur. Reactants: 1) Must have one or more b hydrogens. 2) When the eliminated H and the leaving group are anti to one another and in the same plane as the two carbons to which they are attached the elimination is fast. Otherwise, the gauche elimination is slow. Products: If two or more alkenes are produced, the one having the greater or greatest number ofcarbon-containing substituents attached to the C=C bond is usually the major product. The reason for this trend is based upon the stabilities of various alkenes, with the more or most stable alkene being formed more easily and thus is the major product.

  21. Elimination. The E1 Reaction The E1 reaction occurs in several steps, with formation of the carbocation beinginvolved in the first, rate-determining step

  22. Elimination. The E1 Reaction Kinetics: Rate = k [RX]. Only RX is involved in the slow, rate-determining step. The base: Weakly basic substances, such as H2O, CH3OH, and CH3CH2OH can cause elimination to occur by the E1 reaction. Reactivity of alkyl halides: 3° > 2°. 1° halides usually do not undergo E1 reaction. Note: If a strong base is present, E2 reaction occurs.

  23. Elimination. The E1 Reaction Rearrangements: If possible, a carbocation may rearrangeto a more stable one by an alkylgroup shift or a hydride shift. The most common rearrangement is from a 2° to 3° ion. Primary carbocations, if formed (and they seldom are), could rearrange to either a 2° or 3° cation. Rearrangement occurs more rapidly than does the loss of a proton from a carbocation, so rearrangement products usually predominate. Solvents: Reaction is favored in polar, protic solvents. Reactants: 1) Must have one or more b hydrogens. 2) Unlike the E2 reaction, no special geometry is required.

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