Alcohols

• Alcohols


  Organic compounds with the general formula ROH, where R is an alkyl or aryl group
• Alcohols


  • They belong to the homologous series CnH2n + 2
  • An example of an aliphatic alcohol is CH3CH2OH
  • An example of an aromatic alcohol is phenol, C6H5OH
• Classification of alcohols


  • Primary
  • Secondary
  • Tertiary
• Primary alcohols


  Have the general formula RCH2OH
• Secondary alcohols


  Have the general formula RCHR'OH
• Tertiary alcohols


  Have the general formula RCR'R"OH
• Alcohols


  • Like water, they are V-shaped
  • The bond angle in alcohols is larger than in water due to greater Van-der Waals repulsion between the OH and OR bonds
  • The OH bond length is about the same in each case
• Alcohols vary in physical state depending on the number of carbon atoms
• Alcohols


  • Most of the alcohols, C1 to C12 are liquids at room temperature
  • Beyond that, alcohols are solid
  • As the number of carbon atoms increases, the molecules becomes more lipophilic and hydrophobic
  • On descending the alcohol group, hydrophobicity increases
• Boiling points of alcohols

  Alcohols usually boil at higher temperature than expected due to intramolecular and intermolecular hydrogen bonding
• Alcohols


  • They are polar molecules by virtue of the fact that there exists a net dipole moment
  • There is charge separation
• Solubility of alcohols


  • Alcohols are soluble in polar solvents such as water
  • Alcohols are miscible in water and acetone, but not in hexane
  • As one descends the alcohol homologous series, the aliphatic portion of the molecule increase in chain length
  • Hence, hydrophobicity is greater than hydrophilicity, and the alcohol becomes less miscible
• Preparation of alcohols


  1. Acid catalysed hydration of an alkene

  2. Grignard reaction with aldehydes and ketones

  3. Hydroboration/oxidation

  4. Oxymercuration/Demercuration

  5. Nucleophilic attack on Epoxides
• Acid catalysed hydration of an alkene


  The reaction is Markovnikov's in nature, whereby the hydrogen attaches itself to the carbon bearing the more number of hydrogens
• Grignard reagent


  • It has the general formula: RMgX, where R is an alkyl or aryl group and X = halogen
  • The R group of the Grignard reagent is nucleophilic, since it has an overall negative charge and can attack electrophilic (E+) centres or electron deficient centres
• Grignard reaction with aldehydes and ketones

  Grignard reagent + HCHO -> Primary alcohol

  Grignard reagent + RCHO -> Secondary alcohol

  Grignard reagent + RCOR' -> Tertiary alcohol
• Hydroboration/oxidation


  • It converts an alkene to an Anti-Markovnikov's alcohol
  • The first step involves the formation of the alkylborane via the reaction of an alkene with diborane (BH3)2 across a carbon carbon double bond
  • The second step involves the oxidation of the alkylborane with hydrogen peroxide, H2O2 in an alkaline medium to yield the corresponding Anti-Markovnikov's alcohol
• Oxymercuration/Demercuration


  1. It converts an alkene to a Markovnikov's alcohol

  2. It involves the conversion of an alkene to a mercurial alcohol via the addition of mercuric (II) acetate dihydrate to the carbon-carbon double bond, to form a mercurial alcohol

  3. Reduction of the mercurial alcohol with NaBH4 in an acidic medium leads to the formation of the Markovnikov's alcohol
• Nucleophilic attack on Epoxides

  It involves the nucleophilic attack of a nucleophilic carbon such as that of a Grignard Reagent with that of the Epoxide, which leads to an alkoxymagnesium halide salt, which on protonation leads to the alcohol
• Anti-Markovnikov's alcohol


  Alcohol formed by a reaction that does not follow Markovnikov's rule
• Reagents


  • BH3
  • BH2
  • H2O2/-OH
  • OH
• Oxymercuration/Demercuration


  1. Convert alkene to Markovnikov's alcohol

  2. Add mercuric (II) acetate dihydrate to alkene to form mercurial alcohol

  3. Reduce mercurial alcohol with NaBH4 in acidic medium to form Markovnikov's alcohol
• Mercurial alcohol


  Alcohol formed by addition of mercuric (II) acetate dihydrate to alkene
• Markovnikov's alcohol


  Alcohol formed by a reaction that follows Markovnikov's rule
• Mechanism of Oxymercuration/Demercuration


  1. Hg(OAc)2.2H2O forms mercurial alcohol

  2. NaBH4/H+ reduces mercurial alcohol to Markovnikov's alcohol
• Nucleophilic attack on Epoxides


  1. Nucleophilic carbon of Grignard Reagent attacks epoxide

  2. Forms alkoxymagnesium halide salt

  3. Protonation forms alcohol
• Grignard Reagent


  Nucleophilic carbon source
• Epoxide


  Highly strained 3-membered ring
• Mechanism of nucleophilic attack on epoxides


  1. Grignard Reagent attacks epoxide

  2. Forms alkoxymagnesium halide salt

  3. Protonation forms alcohol
• Reactions of Alcohols


  • Reaction with HX
  • Dehydration reactions
  • Esterification reactions
  • Reactions with electropositive metals
  • Oxidation reactions
• Reaction of alcohols with HX

  1. Protonation of alcohol

  2. Nucleophilic substitution

  3. Formation of alkyl halide
• SN2 mechanism


  Nucleophilic substitution with inversion of configuration
• SN1 mechanism


  Nucleophilic substitution with formation of carbocation intermediate
• Lucas reagent


  HCl/ZnCl2 used to halogenate alcohols
• Mechanism of alcohol halogenation with Lucas reagent


  1. Lewis acid ZnCl2 assists chloride substitution

  2. SN1 mechanism for tertiary alcohols

  3. SN2 mechanism for primary alcohols
• Dehydration of alcohols


  Treat alcohol with H2SO4 and heat to form alkene
• Mechanism of alcohol dehydration

  1. Protonation of alcohol

  2. Elimination of water to form carbocation

  3. Deprotonation to form alkene
• Esterification of alcohols


  Alcohol reacts with carboxylic acid in presence of mineral acid to form ester
• Reaction of alcohols with electropositive metals

  Alcohol reacts with Group 1 metal to form alkoxide
• Alkoxide


  Salt formed from reaction of alcohol and electropositive metal