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Unit 4 – Alkyl Halides, Nucleophilic Substitution, and Elimination Reactions

Unit 4 – Alkyl Halides, Nucleophilic Substitution, and Elimination Reactions. Nomenclature and Properties of Alkyl Halides Synthesis of Alkyl Halides Reactions of Alkyl Halides Mechanisms of S N 1, S N 2, E1, and E2 Reactions Nucleophilicity, Substrate, and Leaving Group Effects.

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Unit 4 – Alkyl Halides, Nucleophilic Substitution, and Elimination Reactions

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  1. Unit 4 – Alkyl Halides, Nucleophilic Substitution, and Elimination Reactions • Nomenclature and Properties of Alkyl Halides • Synthesis of Alkyl Halides • Reactions of Alkyl Halides • Mechanisms of SN1, SN2, E1, and E2 Reactions • Nucleophilicity, Substrate, and Leaving Group Effects

  2. Alkyl Halides • Alkyl halide: • a compound with a halogen atom bonded to one of the sp3 hybridized carbon atoms of an alkyl group • Two types of names: • IUPAC system • Common names

  3. Nomenclature • IUPAC System: • Alkyl halides are named as an alkane with a halo-substituent: • Review the rules for naming alkanes covered in Unit 2 1-chloropropane bromocyclohexane

  4. Nomenclature • Common Names: • alkyl group name + halide n-propyl chloride Cyclohexyl bromide

  5. Nomenclature • Special common names: • CH2X2 = methylene halide • CHX3 = haloform • CX4 = carbon tetrahalide CH2Cl2 CHCl3 Methylene chloride dichloromethane chloroform trichloromethane CCl4 Carbon tetrachloride tetrachloromethane

  6. Types of Alkyl Halides • Alkyl halides can be classified by the type of carbon atom the halogen is bonded to: • primary halide (1o): • halogen attached to a 1o carbon • secondary halide (2o): • halogen attached to a 2o carbon • tertiary halide (3o): • halogen attached to a 3o carbon

  7. Types of Alkyl Halides • Geminal dihalide: • 2 halogens bonded to the same carbon atom • Vicinal dihalide: • 2 halogens bonded to adjacent carbon atoms

  8. Other Organic Halides • Aryl halide: • halogen is attached directly to an aromatic ring • Benzylic halide • halogen is attached to a carbon that is attached to a benzene ring thyroxine benzylic carbon benzylic chloride

  9. Other Organic Halides • Allylic halide: • halogen is attached to a carbon that is attached to a C=C Allylic carbon Allylic chloride

  10. Other Organic Halides • Vinyl Halide: • halogen attached to a carbon that is part of a C=C Monomer for PVC Monomer for teflon

  11. Uses of Alkyl Halides • Anesthetics: • Chloroform (CHCl3) • toxic • carcinogenic (causes cancer) • Solvents: • CCl4 • formerly used in dry cleaning • CH2Cl2 • formerly used to decaffeinate coffee • liquid CO2 used now

  12. Uses of Alkyl Halides • Freons: • Freon-12: CF2Cl2 • Freon-22: CHClF2 • Freon-134a: • Pesticides: DDT Chlordane (termites)

  13. Physical Properties • Boiling Point: • Compounds with higher MW’s and greater surface area (more linear) tend to have higher BP. • BP increases as size of halogen increases • F < Cl < Br < I • BP decreases as branching increases

  14. Physical Properties • Density: • Alkyl chlorides are common solvents for organic reactions. • CH2Cl2 • CHCl3 • CCl4 More dense than water

  15. Preparation of Alkyl Halides • Alkyl halides can be prepared from a variety of starting materials including alkanes, alkenes, alkynes, alcohols, and other alkyl halides. • You are responsible for knowing and applying the synthesis of R-X by: • free radical halogenation reactions • free radical allylic bromination reactions

  16. Preparation of Alkyl Halides • Free Radical Halogenation of Alkanes alkane + X2 alkyl halide(s) + HX • Poor selectivity and moderate yields often limit usefulness. • Bromination is more selective and gives the product formed from the most stable free radical. • Chlorination is useful when only one type of reactive hydrogen is present hu or D

  17. Preparation of Alkyl Halides • Useful Examples: hu 50 %

  18. Preparation of Alkyl Halides • The following free radical halogenation is doomed to failure! • The following addition reaction occurs instead:

  19. Preparation of Alkyl Halides • Free Radical Allylic Bromination: • where NBS = N-bromosuccinimide hn NBS

  20. Preparation of Alkyl Halides • NBS is used to generate low levels of Br2 in situ. • Minimizes addition of bromine across the C=C • Allylic bromination is highly selective and occurs in the allylic position due to resonance stabilization of the resulting free radical.

  21. Preparation of Alkyl Halides • Examples: hu hu

  22. d+ d- Reactions of RX • Most reactions of alkyl halides involve breaking the C-X bond. • Nucleophilic substitution • Elimination • The halogen serves as a leaving group in these reactions. • the halogen leaves as X-, taking the bonding electrons with it

  23. Reactions of RX • Nucleophilic substitution: • reaction in which a nucleophile replaces a leaving group • Nucleophile: • electron pair donor • Leaving group: • an atom or group of atoms that are lost during a substitution or elimination reaction • retains both electrons from the original bond

  24. Reactions of RX • General Equation for Nucleophilic Substitution • The nucleophile can be neutral or negatively charged, but it must have at least one lone pair of electrons. • Example:

  25. CH3O- Reactions of RX • Elimination Reaction: • two substituents are lost from adjacent (usually) carbons, forming a new p bond • Dehydrohalogenation: • an elimination reaction in which H+ and X- are lost, forming an alkene

  26. Reactions of RX • There are two common types of nucleophilic substitution reactions: • SN1 reactions • substitution, nucleophilic, unimolecular • 3o, allylic, benzylic halides • weak nucleophiles • SN2 reactions • substitution, nucleophilic, bimolecular • methyl and 1o halides • strong nucleophiles

  27. Reactions of RX

  28. Reactions of RX • Common strong nucleophiles: • hydroxide ion • alkoxide ions • many amines • iodide and bromide ions • cyanide ion • Common weak nucleophiles: • water • alcohols • fluoride ion

  29. SN2 Reactions • The reaction between methyl iodide and hydroxide ion is aconcerted reactionthat takes places via anSN2 mechanism nucleophile Leaving group substrate product • Substrate: • the compound attacked by a reagent (nucleophile)

  30. SN2 Reactions • Concerted reaction: • a reaction that takes place in a single step with bonds breaking and forming simultaneously • SN2: • substitution, nucleophilic, bimolecular • transition state of rate-determining step involves collision of 2 molecules • 2nd order overall rate law • Rate = k[RX][Nuc]

  31. SN2 Reactions • SN2 Mechanism: • Nucleophile attacks the back side of the electrophilic carbon, donating an e- pair to form a new bond • Since carbon can only have 8 valence electrons, the C-X bond begins to break as the C-Nuc bond begins to form

  32. SN2 Reactions • SN2 Mechanism for the reaction of methyl iodide and hydroxide ion:

  33. SN2 Reactions • Reaction Energy Diagram: • large Ea due to 5-coordinate carbon atom in transition state • no intermediates • exothermic

  34. SN2 Reactions • SN2 reactions occur with inversion of configuration at the electrophilic carbon. • The nucleophile attacks from the back side (the side opposite the leaving group). • Back-side attack turns the tetrahedron of the carbon atom inside out.

  35. SN2 Reactions • Inversion of configuration: • a process in which the groups bonded to a chiral carbon are changed to the opposite spatial configuration: • R S or S R

  36. SN2 Reactions Example: Predict the product formed by the SN2 reaction between (S)-2-bromobutane and sodium cyanide. Draw the mechanism for the reaction.

  37. SN2 Reactions • The SN2 displacement reaction is a stereospecific reaction • a reaction in which a specific stereoisomer reacts to give a specific diastereomer of the product

  38. SN2 Reactions • SN2 reactions occur under the following conditions • Nucleophile: • strong, unhindered nucleophile • OH- not H2O • CH3O- not CH3OH • CH3CH2O- not (CH3)3CO- • Substrate: • 1o or methyl alkyl halide (most favored) • 2o alkyl halide (sometimes) • 3o alkyl halides NEVER react via SN2

  39. SN2 Reactions • The relative rate of reactivity of simple alkyl halides in SN2 reactions is: methyl > 1o > 2o >>>3o • 3o alkyl halides do not react at all via an SN2 mechanism due to steric hinderance. • The back side of the electrophilic carbon becomes increasingly hindered as the number or size of its substituents increases

  40. SN2 Reactions • Steric hinderance at the electrophilic carbon:

  41. SN2 Reactions • SN2 reactions can be used to convert alkyl halides to other functional groups: • RX + I - R-I • RX + OH- R-OH • RX + R’O- R-OR’ • RX + NH3 R-NH3+ X- • RX + xs NH3 R-NH2 • RX + CN- R-CN • RX + HS- R-SH • RX + R’S- R-SR’ • RX + R’COO- R’CO2R KNOW THESE! Be able to apply these!

  42. SN2 Reactions Example: Predict the product of the following reactions:

  43. ? ? SN2 Reactions Example: What reagent would you use to do the following reactions:

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