unit 8

• Organohalides

  Compounds that contain one or more halogen atoms
• Organohalides

  Found in algae and various other marine organisms
• Halogen-containing compounds

  Used as solvents, inhaled anesthetics, refrigerants, and pesticides
• Alkyl halides

  Comprise a halogen atom bonded to a saturated, sp3-hybridized carbon atom
• Boiling Points

  Three of these have boiling points below room temperature (taken as being about 20°C), so they will be gases at room temperature. All the others are likely to be liquids.
• Physical Properties

  • RI > RBr > RCl > RF
  • Slightly soluble in water
  • Tend to dissolve in organic solvents
  • Density is directly proportional to the mass of the compound
  • Electronegativities decrease in the order of: F > Cl > Br > I
  • Carbon-halogen bond lengths increase in the order of: C-F < C-Cl < C-Br < C-I
• Preparing Alkyl Halides

  1. Reactions with X2 and HX

  2. Alkyl halides from Alcohols: HX reaction works best with tertiary alcohol, R3COH. Primary and secondary alcohols react much more slowly, treated with either thionyl chloride (SOCl2) or phosphorous tribromide (PBr3).
• Grignard reagents (RMgX) were discovered by Victor Grignard
• Grignard reagents

  Alkyl halides, RX, react with magnesium metal in ether solvent to yield alkyl magnesium halides, RMgX. An organometallic compound since it contains carbon–metal bonds. Don't occur in living organisms, but they are useful carbon-based nucleophiles in important laboratory reactions.
• Gilman reagents

  Lithium organocuprates (R2CuLi) that react with organohalides to yield coupled hydrocarbon products
• Nucleophilic Substitution Reactions

  Alkyl halides, if it reacts with nucleuphile/electrophile (-OH), may undergo:

  A. Substitution of the X group by the nucleophile

  B. Elimination of HX to yield an alkene.
• SN2 Reaction

  Substitution, nucleophilic, bimolecular. A nucleophile (Nu:) reacts with a substrate R-X and substitutes for a leaving group X:- to yield the product R-Nu. If the nucleophile is neutral (Nu:), then the product is positively charged to maintain charge conservation. If the nucleophile is negatively charged (Nu:-), the product is neutral.
• SN1 Reaction

  Substitution, nucleophilic, unimolecular. Takes place only on tertiary substrates and only under neutral or acidic conditions in a hydroxylic solvent such as water or alcohol. The more stable the carbocation intermediate, the faster the SN1 reaction.
• Zaitsev's rule

  In the elimination of HX from an alkyl halide, the more highly substituted alkene product predominates.
• Types of Elimination Reaction

  E2, E1, E1cB
• Treatment of an alkyl halide with a strong base such as KOH yields an alkene.
• Summary of SN1, SN2, E1, E1cB, and E2

  • Primary alkyl halide: SN2 substitution, E2 elimination, E1cB elimination

  Secondary alkyl halide: SN2 substitution predominates if a weakly basic nucleophile is used, E2 elimination predominates if a strong base is used, E1cB elimination takes place if the leaving group is two carbons away from a carbonyl group

  Tertiary alkyl halide: E2 elimination occurs when a base is used, SN1 substitution and E1 elimination occur together under neutral or acidic conditions, E1cB elimination takes place if the leaving group is two carbons away from a carbonyl group
• Iodomethane

  liquid
• Chloroethane

  gas
• Tertiary alcohol

  R3COH
• Organometallic compound

  contains carbon-metal bond
• alkenyl

  vinylic
• aryl

  aromatic
• Zaitsev's Rule

  base-induced elimination reactions