12

Chapter 12: Alcohols, Phenols, Thiols, and Ethers

12.1 Alcohols: Structure and Physical Properties

  • Definition of Alcohol: Organic compound containing a hydroxyl group (-OH) attached to an alkyl group. The general formula is represented as R-OH, where R can be a hydrogen, alkyl, or aryl group.

12.1.1 Carbinol Carbon and Classification

  • Carbinol Carbon: The carbon atom that carries the hydroxyl group.
  • Classification: Alcohols are classified based on the number of alkyl groups attached to the carbinol carbon:
    • Primary Alcohols: One alkyl group attached to the carbinol carbon.
    • Secondary Alcohols: Two alkyl groups attached to the carbinol carbon.
    • Tertiary Alcohols: Three alkyl groups attached to the carbinol carbon.

12.1.2 Physical Properties of Alcohols

  • The structure of alcohols is similar to that of water, which contributes to their unique properties.
  • Polarity: The hydroxyl group is highly polar, enabling the formation of hydrogen bonds.
  • Boiling Points: Alcohols exhibit higher boiling points than hydrocarbons of similar molar mass due to the presence of hydrogen bonding.
  • Trends in Boiling Points:
    • Example Comparison:
    • Propane (CH3CH2CH3): Molar Mass = 44 g/mol; Boiling Point = -42°C
    • Ethanol (CH3CH2OH): Molar Mass = 46 g/mol; Boiling Point = 78°C

12.1.3 Solubility of Alcohols

  • Alcohols with Low Molar Mass (1 to 4 carbons) are soluble in water due to their polarity and ability to form hydrogen bonds. They are classified as hydrophilic (water-loving).
  • As the number of hydroxyl (-OH) groups in an alcohol increases, both polarity and water solubility increase.
    • Example: HOCH2CH2CH2CH2OH is very soluble in water.
  • Alcohols with High Molar Mass: As molar mass increases, solubility in water decreases despite their polar nature.
  • Influence of Hydroxyl Groups on Solubility: Diols and triols (two or three -OH groups) are more soluble in water than those containing a single -OH group.

12.2 Alcohols: Nomenclature

  • IUPAC Naming: Based on the longest carbon chain containing the -OH group.
  • Suffix: The -e of the corresponding alkane name is replaced with -ol.
  • Numbering: Number the chain from the end nearest to the -OH group, placing the position number directly before the -ol suffix.
    • In cyclic alcohols, the -OH is assigned to C-1.
Example 1: Naming a Linear Alcohol
  • For the structure CH3-CH(CH3)-CH2-CH(OH)-CH3:
    1. Identify the parent compound: butane (4 carbons).
    2. Replace -e with -ol: butanol.
    3. Number from the end to minimize the number of carbons with the -OH group: right to left.
    4. Identify substituents: methyl on C-3.
    • IUPAC Name: 3-methylbutan-2-ol.
Example 2: Naming a Cyclic Alcohol
  • For CH3-CH2-CH2-CH(OH)-CH3:
    1. Identify the parent compound: cyclohexane (6 carbons, cyclic).
    2. Replace -e with -ol: cyclohexanol.
    3. Number the ring to give the -OH group the lowest number: counterclockwise.
    4. Identify substituents: methyl on C-3.
    • IUPAC Name: 3-methylcyclohexanol.
Common Names of Alcohols
  • Common names take the alkyl group name followed by the word “alcohol”.
  • Example:
    • tert-butyl alcohol (t-BuOH)
    • isopropyl alcohol (also known as rubbing alcohol).

12.3 Important Alcohols

  • Methanol (CH3OH):

    • Characteristics: Colorless and odorless liquid, used as a solvent and toxic (can cause blindness and death).
    • Applications: Possible fuel source.

  • Ethanol (C2H5OH):

    • Characteristics: Colorless and odorless liquid, a widely-used solvent.
    • Source: Derived from fermentation of carbohydrates; types of beverages vary based on starting material and fermentation process.

  • Propan-2-ol (Isopropyl alcohol):

    • Characteristics: Colorless with a slight odor; commonly known as rubbing alcohol.
    • Uses: Disinfectant, astringent, industrial solvent; toxic when ingested.

  • Ethane-1,2-diol (Ethylene glycol):

    • Characteristics: Used in automobile antifreeze; sweet taste but extremely poisonous.
    • Applications: Lowers freezing point and raises boiling point of water.

  • Propane-1,2,3-triol (Glycerol):

    • Characteristics: Very viscous and thick with a sweet taste; non-toxic and highly soluble in water.
    • Uses: Cosmetics, pharmaceuticals, lubricants; obtained as a by-product of fat hydrolysis.

12.4 Reactions Involving Alcohols

  • Preparation of Alcohols via Hydration:

    • Involves adding water to a carbon-carbon double bond of an alkene; requires trace acid as a catalyst.

  • Preparation of Alcohols via Hydrogenation:

    • Addition of hydrogen to the carbon-oxygen double bond in aldehydes or ketones to produce alcohols; this is considered a reduction reaction using catalysts such as platinum (Pt), palladium (Pd), or nickel (Ni).

  • Dehydration of Alcohols:

    • Alcohols can dehydrate upon heating in the presence of strong acid to produce alkenes and water, a process categorized as an elimination reaction. This involves losing -OH and -H from adjacent carbon atoms.

  • Zaitsev’s Rule:

    • Indicates that in dehydration reactions, the most substituted alkene, i.e., the alkene with the greatest number of alkyl groups on the double-bonded carbon, will be the predominant product.
Example of Product Prediction during Dehydration

  • When dehydrating 3-methylbutan-2-ol, the predicted major product would follow Zaitsev’s rule indicating that the alkene with the highest substitution will be favored.

  • Oxidation of Primary Alcohols:

    • Typically oxidized to carboxylic acids. The oxidizing agents denoted as [O] (e.g., KMnO4/OH− or H2CrO4) convert primary alcohols to aldehydes. Example: oxidation of 2,2-dimethylpropane-1-ol to 2,2-dimethylpropanal.

  • Oxidation of Secondary Alcohols:

    • Secondary alcohols are oxidized to ketones, featuring the elimination of 2 hydrogen atoms.

  • Tertiary Alcohols:

    • Do not undergo oxidation reactions due to lacking hydrogens on the carbon that connects to the hydroxyl group.

12.5 Oxidation and Reduction in Living Systems

  • Definitions:
    • Oxidation: Loss of electrons.
    • Reduction: Gain of electrons.
  • These processes are often more subtle in organic systems and can be detected by changes not visible through charge indicators.

12.5.1 Tracking Organic Oxidation and Reduction

  • Oxidation:
    • In organic systems, oxidation can be characterized by the gain of oxygen or the loss of hydrogen.
  • Reduction:
    • Characterized by loss of oxygen or gain of hydrogen.
  • Biological Reactions:
    • NAD is a coenzyme commonly involved in biological oxidation and reduction reactions.

12.6 Phenols

  • Definition of Phenols: Compounds in which the hydroxyl group is attached to a benzene ring (Ar-OH).
  • Polarity: The presence of the hydroxyl group makes phenols polar, and simpler phenols exhibit limited water solubility.
  • Applications: Phenols are used in flavorings and fragrances.
Phenol Derivatives in Healthcare
  • Usage as:
    • Germicides.
    • Antiseptics.
    • Disinfectants.

12.7 Ethers

  • Definition of Ethers: Compounds that contain the ether functional group with the formula R-O-R, where R can be an aliphatic or aromatic group.
  • Polarity: Ethers are slightly polar due to the C-O bond but lack -OH groups, hence do not hydrogen bond with each other.
Physical Properties of Ethers
  • Ethers have lower boiling points than alcohols because they do not participate in hydrogen bonding.
    • Example Comparison:
    • Butane (M.M. = 58 g/mol; Boiling Point = -0.50°C)
    • Methoxyethane (ethyl methyl ether) (M.M. = 60 g/mol; Boiling Point = 7.9°C)
    • Propan-1-ol (propyl alcohol) (M.M. = 60 g/mol; Boiling Point = 97.2°C)
Common Names and IUPAC Nomenclature of Ethers
  • Common Naming: Constructed by combining the names of groups attached to the oxygen, listed in alphabetical order followed by 'ether'.
  • IUPAC Naming: Based on the longest alkane chain attached to the oxygen and substituents are named as alkoxy derivatives (where the alkane’s -ane ends with -oxy).
Reactivity of Ethers
  • Ethers are typically moderately inert, resisting reactions with reducing agents or bases. They are volatile and highly flammable, making them prone to oxidation in air.
  • Synthesis: Symmetrical ethers can be formed by the dehydration of two alcohol molecules, requiring heat and acid catalysis.
Medical Uses of Ethers
  • Ethers are historically important as anesthetics, accumulating in lipid components of nerve cells and disrupting nerve impulse transmission.
  • Currently, halogenated ethers are preferred due to their reduced flammability and safer handling properties.

12.8 Thiols

  • Definition of Thiols: Compounds with the formula R-SH, structurally related to alcohols where sulfur (S) replaces oxygen (O).
  • Nomenclature: Based on the longest alkane chain with the suffix -thiol indicating position, which must be specified by numbers.
Examples of Thiol Names
  • Parent compound: ethane, position of -SH: carbon-1.
    • Name: ethane-1-thiol (HS-CH2-CH3).
  • Parent compound: butane, position of -SH: carbon-1, substituent: 3-methyl.
    • Name: 3-methylbutane-1-thiol.
Scent Characteristics of Thiols
  • Some thiols possess unpleasant odors and are noted in natural contexts, like the defensive spray of striped skunks and the smells of onions and garlic.
Disulfide Formation
  • Thiols can form disulfide bridges (R-S-S-R) through oxidation, which plays a critical role in the structure and stability of proteins such as insulin.
  • The formation of disulfide bonds occurs when two cysteine molecules undergo an oxidation reaction, linking the two amino acids through the new bond structure.