Alcohols-Phenols-Thiols-and-Ethers

Thu Chapter 12: Alcohols, Phenols, Thiols, and Ethers

12.1 Alcohols: Structure and Physical Properties

• Definition:

  • Organic compounds with a hydroxyl group (-OH) attached to an alkyl group.

  • General formula: R-OH

• Physical Properties:

  • Structure of R-O-H is similar to water.

  • Hydroxyl group is highly polar, enabling hydrogen bond formation.

Boiling Points

• Alcohols exhibit abnormally high boiling points due to hydrogen bonding:

  • Example boiling points:

    • CH₂CH₂CH₃ (Alkane) - bp -42 °C

    • CH₂O-CH₃ - bp -23 °C

    • CH₂CH₂OH (Alkyl alcohol) - bp +78.5 °C

• Hydrogen Bonding:

  • The presence of hydrogen bonds in alcohols like Ethane (CH₃CH₂OH) contributes to their elevated boiling points compared to their molecular weights.

Solubility

• Low molecular weight alcohols (5-6 carbons) are generally soluble in water due to polarity and ability to hydrogen bond:

  • Example:

    • CH₃CH₂OH (Ethanol) is very soluble.

    • CH₃OCH₃ (Dimethyl ether) is barely soluble.

12.2 Alcohols: Nomenclature

• IUPAC Naming:

  • Bases on the longest carbon chain containing the -OH group.

  • Replace the alkane suffix -e with -ol.

  • Number the chain to give the -OH carbon the lowest number.

  • Prefix with numbers indicating the position of the -OH group.

Example of Naming an Alcohol:

• For CH₃CH₂CH-CH₃ (Butane with an OH):

  1. Parent compound: Butane.

  2. Replace -e with -ol: Butanol.

  3. Number from right to left to minimize the number with -OH: 3-methyl-2-butanol.

12.3 Medically Important Alcohols

• Methanol:

  • Colorless, odorless, used as a solvent; toxic and can cause blindness.

• Ethanol:

  • Odorless and colorless; prevalent in alcoholic beverages, derived from carbohydrate fermentation.

• 2-Propanol (Rubbing Alcohol):

  • Used as a disinfectant; toxic when ingested.

• 1,2-Ethanediol (Ethylene Glycol):

  • Used in antifreeze; sweet taste but extremely poisonous.

• 1,2,3-Propanetriol (Glycerol):

  • Non-toxic, used in cosmetics and pharmaceuticals, by-product of fat hydrolysis.

12.4 Classification of Alcohols

• Alcohols Classification:

  • Based on the number of alkyl groups attached to the carbinol carbon:

    • Primary (1°) Alcohol: One alkyl group attached.

    • Secondary (2°) Alcohol: Two alkyl groups attached.

    • Tertiary (3°) Alcohol: Three alkyl groups attached.

12.5 Reactions Involving Alcohols

Preparation of Alcohols

• Hydration:

  • Adding water to cc double bond of alkenes.

  • Requires acid as a catalyst.

• Hydrogenation:

  • Adding water to cc double bond of aldehydes or ketones.

  • Also a reduction reaction requiring a Pt, Pd, or Ni catalyst.

Dehydration of Alcohols

• Alcohols can dehydrate to form alkenes when heated in strong acid.

• Elimination of -OH and -H from adjacent carbons.

Oxidation Reactions

• Primary Alcohols: Oxidize to carboxylic acids or can yield aldehydes with care.

• Secondary Alcohols: Oxidize to ketones.

• Tertiary Alcohols: Do not oxidize (no hydrogen on carbonyl carbon).

12.6 Oxidation and Reduction in Living Systems

• Oxidation: Loss of electrons or gain of oxygen.

• Reduction: Gain of electrons or loss of oxygen.

• Biological redox reactions are catalyzed by oxidoreductases, with coenzymes like NAD involved.

12.7 Phenols

• Definition: Compounds with hydroxyl group attached to a benzene ring (formula: Ar-OH).

• Polar due to the hydroxyl group. Used in flavorings, fragrances, and healthcare (germicides, antiseptics).

12.8 Ethers

• Definition: Ethers have formula R-O-R; slightly polar.

• Boiling points are lower than alcohols due to lack of hydrogen bonding.

• Naming: Combination of the two alkyl groups with "ether" suffix.

12.9 Thiols

• Definition: Compounds with formula R-SH, similar to alcohols but with sulfur.

• Naming based on longest alkane chain and suffix -thiol.

• Important in forming protein structures via disulfide bonds between cysteine residues.

Summary of Reactions Preparation of Alcohols

1. Hydration of Alkenes.

2. Reduction of Aldehydes or Ketones.

Reactions of Alcohols

1. Dehydration: Forms alkenes.

2. Oxidation:

   • Primary to aldehyde.

   • Secondary to ketone.

   • No reaction for tertiary.

3. Dehydration Synthesis of Ethers.