Synthesis and Reactions of Alcohols & Phenols | General Organic Chemistry

Introduction to Alcohols and Phenols

• Alcohols are organic compounds characterized by the hydroxyl (-OH) group as their primary functional group.
• Phenols are described as "alcohol-like" compounds where the hydroxyl group is bound directly to a benzene ring.
• Classification is determined by the number of carbon atoms attached to the carbon bearing the -OH group: Primary (1°), Secondary (2°), and Tertiary (3°).

Nomenclature Rules

• IUPAC System: The parent alkane name is modified by removing the final -e and replacing it with the suffix -ol.
• Numbering: The carbon chain must be numbered starting from the end that gives the hydroxyl group the lowest possible number.
• Priority: The hydroxyl group takes precedence over alkenes and other substituents in both naming and numbering priority.
• Polyhydric Alcohols: If a molecule contains two or three -OH groups, the suffixes -diol (e.g., 1,2-ethanediol) or -triol (e.g., 1,2,3-propanetriol) are used.
• Common Names: These are formed by naming the alkyl group followed by the word "alcohol," such as methyl alcohol for methanol.

Physical Properties

• Polarity: Alcohols are the first organic compounds in this study that are polar, unlike non-polar hydrocarbons like alkanes and alkenes.
• Boiling Points and Solubility: Due to their ability to form hydrogen bonds, alcohols have significantly higher boiling points and higher water solubility than hydrocarbons of similar mass.
• Chain Length Effect: Solubility in water decreases as the non-polar carbon chain length increases.

Synthesis and Preparation

• Hydration of Alkenes: Water is added to an alkene in the presence of an acid catalyst, following Markovnikov’s Rule (the hydrogen attaches to the carbon with more hydrogens).
• Grignard Reagents: A versatile method where a Grignard reagent reacts with different carbonyl compounds:
  • Formaldehyde yields a primary (1°) alcohol.
  • Aldehydes yield a secondary (2°) alcohol.
  • Ketones yield a tertiary (3°) alcohol.
• Reduction Reactions:
  • Sodium Borohydride (NaBH₄): Reduces aldehydes and ketones to alcohols but does not react with esters or carboxylic acids.
  • Catalytic Hydrogenation: Uses H₂ and Raney nickel to reduce carbonyls; note that this will also reduce any carbon-carbon double bonds present.

Key Chemical Reactions

• Oxidation:
  • Primary (1°) alcohols can be oxidized to aldehydes using PCC (pyridinium chlorochromate), or further to carboxylic acids using chromic acid.
  • Secondary (2°) alcohols are oxidized to ketones.
  • Tertiary (3°) alcohols do not oxidize because they lack the necessary hydrogen atoms on the carbonyl carbon.
• The Lucas Test: Uses ZnCl₂ in concentrated HCl to distinguish between alcohol classes based on the speed of an SN1 reaction:
  • 3° alcohols react in less than 1 minute.
  • 2° alcohols react in 1–5 minutes.
  • 1° alcohols react very slowly or not at all.
• Dehydration: Alcohols can lose a molecule of water to form an alkene when treated with acid and heat.
• Esterification: Alcohols react with carboxylic acids to produce sweet-smelling esters.
• Reaction with Active Metals: Metals like Na, Li, and K act as strong bases to deprotonate alcohols, forming alkoxides and liberating hydrogen gas.