Alkyl Halides: Reactions and Applications

• Module 6 covers Alkyl Halides

Learning Objectives

• Recognize the importance and disadvantages of common alkyl halides.
• Classify the different halogenated organic compounds
• Name common alkyl and aryl halides.
• Explain the basic physical properties of alkyl halides.
• Predict products of reactions involving alkyl halides.
• Identify the sources and the practical and medical applications/significance of alkyl halides.
• Manifest honesty in performing activities in the lecture and laboratory.

Discussion on Alkyl Halides

• Reactions of alkanes with halogens produce halogenated hydrocarbons

• Halogenated hydrocarbons are compounds where one or more hydrogen atoms of a hydrocarbon have been replaced by halogen atoms.

• Replacement of one hydrogen atom gives an alkyl halide (or haloalkane).

  • Examples: 1-chloroethane, 1,2-dichloropropane, 1-chloro-2-iodopropane

• Common names of alkyl halides include the alkyl group name plus the halogen stem with the -ide ending.

• The IUPAC system names the parent alkane with a prefix for the halogen substituents and a number for the substituent's location

  • Prefixes used are fluoro-, chloro-, bromo-, and iodo-

• Halogenated products can be derived from methane and ethane.

• Examples of halogenated hydrocarbons derived from methane:

  • Methyl chloride (chloromethane): used as a refrigerant and in manufacturing silicones, methyl cellulose, and synthetic rubber.
  • Methylene chloride (dichloromethane): used as a laboratory and industrial solvent.
  • Chloroform (trichloromethane): used as an industrial solvent.
  • Carbon tetrachloride (tetrachloromethane): formerly used as a dry-cleaning solvent and fire extinguisher
  • Halon-1301 (bromotrifluoromethane): used in fire extinguisher systems.
  • Chlorofluorocarbon-11 (trichlorofluoromethane): used in foaming plastics.
  • Chlorofluorocarbon-12 (dichlorodifluoromethane): used as a refrigerant.

• Examples of halogenated hydrocarbons derived from ethane:

  • Ethyl chloride (chloroethane): used as a local anesthetic.
  • Ethylene dichloride (1,2-dichloroethane): used as a solvent for rubber.
  • Methylchloroform (1,1,1-trichloroethane): used to clean computer chips and molds for shaping plastics.

Physical Properties of Haloalkanes

• Melting and Boiling Point

  • Halogens and carbon electronegativity differences cause highly polarized molecules.
  • Stronger intermolecular forces in halogen derivatives occur due to higher molecular mass and polarity compared to the parent hydrocarbon
  • Boiling Point depends on intermolecular force, chlorides, bromides and iodides points are higher than hydrocarbons

• Boiling point rises in homologous series due to molecule size and mass, but branching reduces it.

  • Methyl chloride, methyl bromide, ethyl chloride, and some chlorofluoromethanes are gases at room temperature
  • Higher members are liquids or solids.

• Melting point follows same trend as boiling point, but exception is para-isomers.

  • Para-isomers have higher melting points than ortho- and meta-isomers due to crystal lattice symmetry

• Density

  • Density is proportional to compound mass; density increases down homologous series.
  • Fluoro derivatives are less dense than chloro derivatives, and chloro derivatives are less dense than bromo derivatives

• Solubility

  • Haloalkanes are slightly soluble in water due to the energy required to break the bond between halogen and carbon and the smaller energy released when the bond is formed

Chemical Properties of Haloalkanes

• Haloalkanes are reactive compounds, undergoing substitution, elimination, and reduction reactions.

• They react with metals to form organometallic compounds.

• Reactivity is due to the polar nature of carbon-halogen bond

• Alkyl halide stability decreases with the C-X bond strength.

• Iodides liberate iodine due to low stability, acquiring brown or violet color

Halogenated Hydrocarbons and Health

• Many chlorinated hydrocarbons, once used in consumer products, are suspected carcinogens and cause liver damage

• Carbon Tetrachloride (CCl4)

  • Previously used as a dry-cleaning solvent and in fire extinguishers, it's no longer recommended.
  • Even small amounts of vapor can cause serious illness if exposure is prolonged.
  • Reaction with water at high temperatures forms deadly phosgene (COCl2) gas

• Ethyl Chloride

  • Used as an external local anesthetic; it quickly evaporates when sprayed on the skin, cooling the area to make it insensitive to pain.
  • It can also be used as an emergency general anesthetic

• Bromine-containing compounds

  • Widely used fire extinguishers and retardants on clothing/materials.
  • They are toxic and have adverse environmental effects

• Chlorofluorocarbons (CFCs) and the Ozone Layer

  • Alkanes substituted with fluorine (F) and chlorine (Cl) atoms are used as dispersing gases in aerosol cans, foaming agents for plastics, and refrigerants

• Chlorofluorocarbons:

  • Contribute to the greenhouse effect in the lower atmosphere.
  • Diffuse into the stratosphere, where UV radiation breaks them down, releasing Cl atoms - destroys O3 molecules, removes Earth's UV protection.
  • Reduced use of CFCs with more benign replacements.
  • Hydrofluorocarbons (HFCs) such as CH2FCF3, which have no Cl or Br to form radicals, are one alternative

Module 8: Ethers, Thiols, and Sulfides

Learning Objectives

• Identify the functional groups of Ethers, Thiols and Sulfides
• Name ethers, thiols, and sulfides using IUPAC rules and corresponding common names.
• Explain physical and chemical properties of ethers, thiols, and sulfides.
• Explain the preparation methods for ethers, thiols, and sulfides.
• Learn about the importance and uses of common ethers, thiols, and sulfides.
• Simulate reactions involving ethers in the laboratory and follow safety protocols.

Discussion - Ethers

• Ethers (ROR') are water derivatives where both hydrogen atoms are replaced by alkyl or aryl groups.
• Ethers can be viewed as alcohol (ROH) derivatives where the OH hydrogen atom is replaced by a second alkyl or aryl group

Common Names of Ethers

• Individual ethers have multiple names.
• Simple ethers have common names based on naming groups attached to the oxygen atom, followed by "ether."
• If both groups are the same, prefix "di-" added
  • Example: CH3-O-CH2CH2CH3 is methyl propyl ether; CH3-O-CH3 is dimethyl ether

IUPAC Nomenclature of Ethers

• Select the longest continuous carbon chain, name as the corresponding alkane.
• Change the other hydrocarbon group ending from "-yl" to "-oxy" to get the alkoxy group name; i.e. CH3O- is methoxy.
• Combine the alkoxy name, position, and alkane name.

Physical Properties of Ethers

• Boiling points are similar to alkanes with comparable mass, much lower than comparable alcohols.
• More soluble in water than alkanes.
• They are somewhat more polar than alkanes, less polar than alcohols.
• Ethers, especially diethyl ether, are good solvents for organic compounds.
• No hydrogen atom on the oxygen atom = no intermolecular hydrogen bonding = lower boiling points
• Boiling points are about the same as alkanes of comparable molar mass, lower than those of comparable alcohols

Williamson Synthesis of Ethers

• The alkyl halide (RX) can only be methyl or primary, but not secondary, tertiary alkyl, or aryl.
• The alkoxide (R'ONa) can be methyl, primary, secondary, tertiary, or aryl

Chemical Properties of Ethers

• Ethers react slowly with oxygen from the air, forming unstable hydroperoxides and peroxides

Ethers as General Anesthetics

• Diethyl ether (CH3CH2OCH2CH3)

  • First general anesthetic to induce unconsciousness/insensitivity through brain interaction
  • Relatively safe gap between anesthesia-producing dose and lethal dose

• Concerns:

  • It is highly flammable and causes nausea
  • Replaced by safer inhalants such as halothane, enflurane, and isoflurane
  • Halothane may increase miscarriage rates for operating room staff
  • However, the modern halogen-containing compounds are less flammable than diethyl ether

Thiols

• Thiols (mercaptans) are sulfur analogs, having the general formula RSH.
• The "SH" group is called sulfhydryl group
• Named in the same way alcohols
  • IUPAC system -ol becomes -thiol.
  • methanethiol (methyl mercaptan), CH3SH
  • ethanethiol (ethyl mercaptan) is the odorant for liquid propane (LP) gas
• Proteins that contain cysteine and methionine contain sulfur atoms.
  • Disulfide linkages are important protein structures.

Thioethers

• Are sulfur analogs of ethers, general formula RSR'.
• An example is dimethylsulfide (CH3SCH3), responsible for the odor of cooking cabbage and related vegetables.
  • Methionine has a thioether functional group

Chemical and Physical Properties of Thiols

• Properties:
  • Low boiling points
  • Strong odor
  • Thiols responsible for skunk odor
  • Oxidation of Thiols:
  • Form disulfides
  • Two thiol groups lose hydrogen atoms, the sulfur atoms are linked to the disulfide group.