PCHEM2

• Alkyl halides

  Derivatives of alkanes in which hydrogen atom is replaced by a halogen atom F, Cl, Br or I
• Alkyl halides


  Represented by R-X, R-alkyl group, X-halogen like chloro, Fluoro, Bromo, and Iodo group
• Alkyl halides are compounds with a halogen atom bonded to a saturated, sp3-hybridized carbon atom
• Methyl halides


  • Halide is attached to a methyl group
• Primary alkyl halide

  • Carbon to which halogen is bonded is attached to only one other carbon
• Secondary alkyl halide

  • Carbon to which halogen is bonded is attached to two other carbons
• Tertiary alkyl halide


  • Carbon to which halogen is bonded is attached to three other carbons
• Boiling points and melting points increase as the size of the alkyl group present increases
• Boiling points and melting points increase as the size of the halogen atom increases from fluorine (F) to iodine (I)
• Solubility of alkyl halides

  Slightly soluble in water, but more soluble in organic solvents
• Acidity of alkyl halides

  As the amount of alkyl group increases, the acidity decreases
• The bond strength of C-X decreases as the size of X increases, iodine is a better leaving group than fluorine
• The order of reactivity of alkyl halides is: Iodide > Bromide > Chloride (Nature of the halogen atom), Tertiary > Secondary > Primary (Type of the halogen atom)
• The high reactivity of alkyl halides can be explained in terms of the nature of C-X bond which is a highly polarized covalent bond due to the large difference in the electronegativities of carbon and halogen atoms
• Uses of alkyl halides

  • Solvents
  • Refrigerants
  • Propellants
  • Fire retardants
  • Alkylation reactions
  • Radical cascades
  • Alkyl cross-coupling chemistry
  • Precursor for treatment of typhoid fever
  • Potential blood substitute
• Alkyl halides can be used as industrial and household solvents, e.g. carbon tetrachloride and methylene chloride
• Alkyl halides can be used as reagents as starting materials for making complex molecules, e.g. conversion to organometallic reagents
• Alkyl halides like CFCs have been used as propellants in aerosol inhalers for drugs used to treat asthma, but are being replaced by halocarbons that do not have the same effect on the ozone layer
• Alkyl halides like chloroform (CHCI3) and halothane (CF3CHCIBr) have been used as anesthetics, but they are toxic and carcinogenic
• Alkyl halides like freons have been used as refrigerants and foaming agents, but they can harm the ozone layer so they have been replaced by low-boiling hydrocarbons or carbon dioxide
• Pesticides like DDT, which are alkyl halides, are extremely toxic to insects but not as toxic to mammals
• Haloalkanes cannot be destroyed by bacteria so they accumulate in the soil to a level which can be toxic to mammals, especially humans
• Haloalkanes


  • Highly toxic to insects but not as toxic to mammals
  • Cannot be destroyed by bacteria so they accumulate in the soil to a toxic level for mammals, especially humans
• Chloromethane


  • Colorless, extremely flammable gas with a mildly sweet odor
  • Present in volcanic gases; produced by algae and giant kelp; used as an industrial solvent; once used widely as a refrigerant; no longer found in consumer products because of toxicity concerns
• Dichloromethane Methylene chloride


  • Colorless, volatile liquid with a mildly sweet odor
  • Chemical intermediate in production of silicone polymers; used as a paint stripper and degreaser; once used to decaffeinate coffee but has been replaced by liquid carbon dioxide due to concern about trace amounts remaining in the coffee
• Trichloromethane Chloroform


  • Colorless, very dense sweet-smelling liquid
  • Used in manufacture of teflon; good industrial solvent; was an early popular anesthetic but discontinued because of toxicity effects
• Tetrachloromethane Carbon tetrachloride Carbon tet


  
  

  • Colorless, nonflammable sweet-smelling liquid
  • Good solvent for fats, oils, and greases; once widely used in dry cleaning of clothing but has been replaced by tetrachloroethylene, which is more stable and less toxic
• Alpha-carbon

  Carbon bonded to a halide
• Beta-carbons

  Carbons bonded to the alpha-carbon
• Leaving Group


  The halogen that must leave since carbon can only create four bonds
• Alkyl halide reactions with nucleophiles/bases

  1. Substitution of the X group by the nucleophile

  2. Elimination of HX to yield an alkene
• Nucleophilic substitution reaction

  • A nucleophile (Nu:) reacts with a substrate ROX and substitutes for a leaving group X: to yield the product RONu
  • If the nucleophile is neutral (Nu:), the product is positively charged to maintain charge conservation
  • If the nucleophile is negatively charged (Nu:), the product is neutral
• SN1 reaction mechanism


  1. 2 steps

  2. Rate = k [RX] (first-order kinetics)

  3. Planar intermediate carbocation leads to racemization at a single stereocenter

  4. The more stable the carbocation, the faster the reaction (SN1 faster with tertiary haloalkanes)

  5. The nature of the nucleophile has no importance, favored by polar protic solvents
• SN2 reaction mechanism


  1. 1 step

  2. Rate = k [RX] [Nu-] (second-order kinetics)

  3. Backside attack of the nucleophile leads to inversion of configuration

  4. Inhibited by steric hindrance (SN2 faster with primary haloalkanes)

  5. Favored by stronger nucleophiles and polar aprotic solvents
• E2 elimination


  1. Concerted mechanism

  2. A base (B:) attacks a neighboring hydrogen and begins to remove it at the same time as the alkene double bond starts to form and the X group starts to leave

  3. Neutral alkene produced when the C-H bond is fully broken and the X group has departed with the C-X bond effect pair
• E1 elimination


  1. Carbocation mechanism

  2. Spontaneous dissociation of the tertiary alkyl chloride yields a carbocation in a slow, rate-limiting step

  3. The base then removes a hydrogen from the adjacent carbon rather than by attacking the carbocation directly
• Common elimination reactions

  • Dehydrohalogenation (loss of HX from an alkyl halide)
  • Dehydration (loss of water from an alcohol)