Comprehensive Study Guide on Alcohols: Structure, Synthesis, and Reactions

General Chemistry II: Alcohols Course Overview

• Course Codes: $CHM 102$ / $CHM 121$
• Institution: University of Lagos
• Lecturer: Dr. J. Izunobi
• Core Topics Covered:
  • Alcohol Structure and Properties
  • Alcohol Nomenclature (IUPAC and Common Names)
  • Synthesis of Alcohols
  • Classification of Alcohols
  • Reactions of Alcohols

Alcohol Nomenclature and IUPAC Rules

• General IUPAC Naming Convention:

  • The hydroxyl ($-OH$) group is the primary functional group.
  • Identify the parent hydrocarbon name, remove the final $-e$, and replace it with the suffix $-ol$.
  • The carbon chain must be numbered so that the hydroxyl group receives the lowest possible number.
  • Indicate the numerical position of the hydroxyl group on the parent chain.
  • Name and number all side chains or other functional groups as substituents.

• Step-by-Step Example: Naming 5-methyl-3-hexanol

  • Structure: $CH_3CH(CH_3)CH_2CH(OH)CH_2CH_3$
  • Step [1]: Find the longest carbon chain containing the $-OH$ group. In this structure, there are $6$ carbons in the longest chain.
  • Step [2]: Change the $-e$ ending of the parent alkane ($\text{hexane}$) to the suffix $-ol$ ($\text{hexanol}$).
  • Step [3]: Number the chain to give the $-OH$ group the lower number. Numbering from right-to-left puts $-OH$ at $C3$. (Numbering left-to-right would put it at $C4$).
  • Step [4]: Name and number substituents. There is a methyl group at $C5$.
  • Final Name: $5\text{-methyl-3-hexanol}$.

• Additional IUPAC Examples:

  • $C-C-C-C-OH$: Base is butane, becomes $1\text{-butanol}$.
  • $C-C-C-C-C-OH$: $1\text{-pentanol}$.
  • $C-C(C)-C-C-C-C-C(OH)-C$: $6\text{-methyl-2-octanol}$.
  • $Cl-C-C-C(OH)-C-C$: $5\text{-chloro-3-hexanol}$.
  • $2\text{-methyl-1-propanol}$: $CH_3CH(CH_3)CH_2OH$.
  • $2\text{-methyl-2-propanol}$: $CH_3C(CH_3)(OH)CH_3$.
  • $2\text{-butanol}$: $CH_3CH(OH)CH_2CH_3$.
  • $3\text{-bromo-3-methylcyclohexanol}$: A substituted cyclic alcohol.

• Naming Priority and Unsaturated Alcohols:

  • If an alcohol is present, it takes naming/numbering priority over alkenes.
  • Example 1: $CH_2=CHCH_2OH$ is named $\text{prop-2-en-1-ol}$ or $2\text{-propen-1-ol}$. Its common name is allyl alcohol.
  • Example 2: A cyclohexene ring with an $-OH$ group is named $3\text{-cyclohexen-1-ol}$ or $\text{cyclohex-3-en-1-ol}$. Numbering prioritizes the hydroxyl group ($C1$).
  • Example 3: $3\text{-phenylbutan-2-ol}$ or $3\text{-phenyl-2-butanol}$.
  • Example 4 (Stereochemistry): $\text{trans-1-penten-1-ol}$. This is a five-member carbon chain with a double bond (pentene), where the $-e$ is dropped for $-ol$, and the trans configuration is noted.

• Polyhydric Alcohols (Multiple OH Groups):

  • Use the full name of the alkane with a modified ending based on the number of hydroxyl groups.
  • 2 OH groups: Ending is $-diol$. Example: $1,2\text{-ethanediol}$ (Commonly known as antifreeze).
  • 3 OH groups: Ending is $-triol$. Example: $1,2,3\text{-propanetriol}$ (Commonly known as glycerol).

Common Names of Alcohols

• Naming Rule: Name the carbon chain as an alkyl group (ending in $-yl$) and add the word "alcohol."
• IUPAC vs. Common Name Examples:
  • $\text{methanol}$ = methyl alcohol
  • $\text{ethanol}$ = ethyl alcohol
  • $2\text{-propanol}$ = isopropyl alcohol
  • $2\text{-methyl-2-propanol}$ = tert-butyl alcohol

Important Industrial Alcohols

• Methanol ($CH_3OH$):

  • Synonyms: Methyl alcohol, wood alcohol.
  • Uses: Common solvent, used in perfumes, industrial starting material.
  • Toxicity: Highly dangerous if ingested; can cause blindness and death.
  • Physical Properties: Colorless, odorless liquid.
  • Production Methods:
    1. Destructive Distillation: Heating wood in the absence of air.
    2. Chemical Synthesis: From carbon monoxide and hydrogen gas at $400^{\circ}C$ and $200\text{ atm}$ using metal catalysts: $CO + 2H_2 \rightarrow CH_3OH$.

• Ethanol ($CH_3CH_2OH$):

  • Synonyms: Ethyl alcohol, grain alcohol.
  • Uses: Solvent in flavors and medicines, industrial starting material, found in alcoholic beverages.
  • Toxicity: Can be ingested at low levels; metabolic depressant.
  • Physical Properties: Colorless, odorless liquid.
  • Production Methods:
    1. Fermentation: Action of yeast on sugars; ethanol is a waste product of yeast metabolism.
    2. Hydration of Ethene: Reaction of ethene ($CH_2=CH_2$) with steam at $325^{\circ}C$ using a catalyst: $CH_2=CH_2 + H_2O \rightarrow CH_3CH_2OH$.

• Ethylene Glycol and Glycerol:

  • 1,2-ethanediol (Ethylene glycol): A dihydric alcohol ($HO-CH_2-CH_2-OH$). Used in antifreeze/coolant and as a starting material for polyester.
  • 1,2,3-propanetriol (Glycerol): A polyhydric alcohol ($HO-CH_2-CH(OH)-CH_2-OH$). Used in hand lotions, cosmetics, and serves as the backbone of fats.

Classification of Alcohols

• Alcohols are classified based on the number of alkyl groups attached to the carbon bearing the hydroxyl group ($C-OH$):
  • Methyl Alcohol: No alkyl groups attached to the central carbon ($CH_3OH$).
  • Primary ($1^{\circ}$): The carbon with the $-OH$ is bonded to one other carbon ($R-CH_2OH$).
  • Secondary ($2^{\circ}$): The carbon with the $-OH$ is bonded to two other carbons ($R_2CHOH$).
  • Tertiary ($3^{\circ}$): The carbon with the $-OH$ is bonded to three other carbons ($R_3COH$).

Methods of Alcohol Preparation

• More than $7$ different functional groups can be converted into alcohols, including alkyl halides, alkenes, ethers, ketones, aldehydes, epoxides, carboxylic acids, acid chlorides, and esters.

• Hydration of Alkenes:

  • Addition of water ($H_2O$) across a double bond.
  • Requires a small amount of acid catalyst ($H^+$).
  • Markovnikov's Rule: The hydrogen atom adds to the carbon with the most hydrogens already attached; the hydroxyl group adds to the more substituted carbon.
  • $\text{Alkene} + H_2O \rightarrow \text{Alcohol}$.

• Halide Exchange (From Alkyl Halides):

  • Substitution reaction where a halide is replaced by a hydroxyl group (using $OH^-$).
  • Note: This reaction does not occur with tertiary alkyl halides.

• Grignard Reagent Synthesis:

  • Allows for the creation of new carbon-carbon bonds and specific alcohol types:
    1. Grignard + Formaldehyde: Yields a primary ($1^{\circ}$) alcohol with one additional carbon.
    2. Grignard + Aldehyde: Yields a secondary ($2^{\circ}$) alcohol.
    3. Grignard + Ketone: Yields a tertiary ($3^{\circ}$) alcohol.
  • The process involves the attack of the Grignard reagent on the carbonyl, followed by protonation of the resulting alkoxide ion with dilute acid ($HOH$).

• Reduction of Carbonyl Compounds:

  • Sodium Borohydride ($NaBH_4$): The hydride ion ($H^-$) attacks the carbonyl carbon to form an alkoxide ion, which is then protonated by dilute acid ($H_3O^+$). $NaBH_4$ only reacts with aldehydes or ketones, not esters or carboxylic acids.
  • Catalytic Hydrogenation: Uses hydrogen gas ($H_2$) with a Raney nickel catalyst. This method reduces carbonyls but will also reduce any carbon-carbon double bonds ($C=C$) present in the molecule.

Chemical Reactions of Alcohols

• Dehydration:

  • Removal of water from an alcohol to produce an alkene.
  • Requires acid catalyst ($H^+$) and heat.
  • $\text{Alcohol} \rightarrow \text{Alkene} + H_2O$.

• Oxidation:

  • Conversion of an alcohol ($-OH$) to a carbonyl group ($=O$).
  • Primary ($1^{\circ}$) Alcohols: Can be oxidized to aldehydes, and then further to carboxylic acids. To stop at the aldehyde, Pyridinium chlorochromate (PCC) must be used. Stronger reagents like chromic acid ($H_2CrO_4$) drive the reaction to the carboxylic acid.
  • Secondary ($2^{\circ}$) Alcohols: Oxidized to ketones. Reagents include $Na_2Cr_2O_7 / H_2SO_4$ (active reagent is $H_2CrO_4$) or PCC.
    • Chromic Acid Test: Observation of a color change from orange to greenish-blue indicates oxidation occurred.
  • Tertiary ($3^{\circ}$) Alcohols: Do not oxidize because they cannot lose two hydrogen atoms from the central carbon.

• Reaction with HCl (The Lucas Test):

  • Tests for the classification of alcohols using $ZnCl_2$ in concentrated $HCl$.
  • $ZnCl_2$ is used because Chloride is a weaker nucleophile than Bromide; it bonds to the $-OH$ to promote the reaction.
  • The resulting chloride product is insoluble (appears as a cloudy precipitate).
  • Reaction Rates:
    1. Primary ($1^{\circ}$) alcohols: React slowly or not at all.
    2. Secondary ($2^{\circ}$) alcohols: React in $1-5 \text{ minutes}$.
    3. Tertiary ($3^{\circ}$) alcohols: React in less than $1 \text{ minute}$.
  • Secondary and tertiary alcohols follow the $S_N1$ mechanism with the Lucas reagent.

• Formation of Esters:

  • Reaction between a carboxylic acid and an alcohol.
  • Products include an ester and water.
  • Esters are characterized by their sweet smells.
  • Example 1: Ethanoic acid + Ethanol $\rightarrow$ ethyl ethanoate.
  • Example 2: Propanoic acid + Methanol $\rightarrow$ methyl propanoate.

• Reaction with Active Metals (Reaction with Base):

  • Metals such as $Na$, $Li$, $K$, and $Ca$ act as very strong bases.
  • They deprotonate the alcohol to form an alkoxide and liberate hydrogen gas ($H_2$).
  • Example 1: $2 \text{ Ethanol} + 2 Na \rightarrow 2 \text{ Sodium ethoxide} (CH_3CH_2O^-Na^+) + H_2$.
  • Example 2: $2 \text{ Isopropyl alcohol} + 2 K \rightarrow 2 \text{ Potassium isopropoxide} + H_2$.

Solubility and Physical Properties

• Polarity: Alcohols are polar. In contrast, alkanes, alkenes, alkynes, and arenes are non-polar hydrocarbons. Alkyl halides are only weakly polar.
• Boiling Points: Hydrocarbons and alkyl halides exhibit characteristically low boiling points and are insoluble in polar solvents like water.
• Water Solubility Data ($g/100\text{ mL}$ at $20-25^{\circ}C$):
  • n-butyl alcohol: $9$
  • sec-butyl alcohol: $12$
  • isobutyl alcohol: $10$
  • tert-butyl alcohol: Miscible
  • n-pentyl alcohol: $2.7$
  • n-hexyl alcohol: $0.6$
  • 1-heptanol: $0.2$
  • 1-octanol: $0.05$
  • 1,4-butanediol: Miscible
  • Phenol: $6.7$
• Comparison (Non-polar compounds):
  • n-pentane: $0.05$
  • carbon tetrachloride: $0.08$
  • n-butyl bromide: $0.06$