Chemistry of Group 13 and Group 14 Elements - Preparation and reactions of lithium aluminium hydride

  • The Group 13 elements in the periodic table are boron (B), aluminum (Al), gallium (Ga), indium (In), and thallium (Tl).
  • The Group 14 elements in the periodic table are carbon (C), silicon (Si), germanium (Ge), tin (Sn), and lead (Pb).
  • Lithium aluminum hydride (LiAlH4) is a powerful reducing agent commonly used in organic synthesis.
  • It is prepared by the reaction of lithium hydride (LiH) with aluminum chloride (AlCl3).
  • The reaction is as follows: LiH + AlCl3 -> LiAlH4 + LiCl.

Preparation of Lithium Aluminium Hydride

  • Lithium aluminum hydride can also be prepared by the reaction of lithium hydride (LiH) with aluminum powder (Al).
  • The reaction is as follows: 3LiH + Al -> LiAlH4.

Properties of Lithium Aluminium Hydride

  • Lithium aluminum hydride is a white crystalline solid.
  • It is highly reactive and moisture-sensitive.
  • The compound has a high hydrogen content, making it an excellent reducing agent.

Reactions of Lithium Aluminium Hydride

  • Lithium aluminum hydride reacts vigorously with water, releasing hydrogen gas.
  • The reaction is as follows: LiAlH4 + 4H2O -> LiOH + Al(OH)3 + 4H2.

Reduction of Carbonyl Compounds

  • Lithium aluminum hydride is commonly used as a reducing agent for carbonyl compounds.
  • It can reduce aldehydes and ketones to alcohols.
  • The reaction is as follows: RCHO + 4LiAlH4 -> RCH2OH + 4LiAl + H2.

Reduction of Carboxylic Acids and Esters

  • Lithium aluminum hydride can also reduce carboxylic acids and esters to alcohols.
  • The reaction is as follows: RCOOH + 4LiAlH4 -> RCH2OH + 4LiAl + H2.

Reduction of Amides and Nitriles

  • Lithium aluminum hydride can reduce amides and nitriles to primary amines.
  • The reaction is as follows: RCONH2 + 4LiAlH4 -> RCH2NH2 + 4LiAl + H2.

Reduction of Nitro Compounds

  • Lithium aluminum hydride can reduce nitro compounds to primary amines.
  • The reaction is as follows: RNO2 + 4LiAlH4 -> RNH2 + 4LiAl + 2H2O.

Reduction of Epoxides

  • Lithium aluminum hydride can reduce epoxides to alcohols.
  • The reaction is as follows: RCH2OCH2R’ + LiAlH4 -> RCH2OH + R’CH2OH + LiAlH4.

Reduction of Acid Chlorides

  • Lithium aluminum hydride can reduce acid chlorides to primary alcohols.
  • The reaction is as follows: RCOCl + 4LiAlH4 -> RCH2OH + 4LiAlCl2 + H2.
  1. Properties of Lithium Aluminium Hydride
  • Lithium aluminum hydride is a strong reducing agent.
  • It can react violently with water, acids, and oxidizing agents.
  • The compound is usually handled and stored in a dry and inert atmosphere.
  • It has a high hydrogen content, making it a potential hydrogen source.
  • Lithium aluminum hydride is often used in organic synthesis as a versatile reducing agent.
  1. Reduction of Acyl Chlorides
  • Acyl chlorides can be reduced to primary alcohols using lithium aluminum hydride.
  • The reaction is as follows: RCOCl + 2LiAlH4 -> RCH2OH + LiAl(OC2H5)3 + AlCl3.
  • For example, the reduction of benzoyl chloride (C6H5COCl) with lithium aluminum hydride gives benzyl alcohol (C6H5CH2OH).
  1. Reduction of Carboxylic Acid Derivatives
  • Lithium aluminum hydride can reduce carboxylic acid derivatives to alcohols.
  • This includes esters, acid halides, acid anhydrides, and amides.
  • For example, the reduction of ethyl acetate (CH3COOC2H5) gives ethanol (CH3CH2OH): CH3COOC2H5 + 2LiAlH4 -> CH3CH2OH + LiAl(OC2H5)3.
  1. Reduction of Nitro Aromatics
  • Nitro aromatics can be reduced to corresponding amines using lithium aluminum hydride.
  • The reaction is as follows: ArNO2 + 3LiAlH4 -> ArNH2 + 3LiAl(O2)H + H2.
  • For example, the reduction of nitrobenzene (C6H5NO2) with lithium aluminum hydride gives aniline (C6H5NH2).
  1. Reduction of Epoxides
  • Epoxides can be reduced to alcohols by lithium aluminum hydride.
  • The reaction is as follows: RCH2OCH2R’ + 2LiAlH4 -> RCH2OH + R’CH2OH + 2LiAlH(OC2H5)2.
  • For example, the reduction of ethylene oxide (C2H4O) with lithium aluminum hydride gives ethanol (CH3CH2OH).
  1. Reduction of Carbon Dioxide
  • Lithium aluminum hydride can reduce carbon dioxide (CO2) to methanol (CH3OH).
  • The reaction is as follows: CO2 + 6LiAlH4 -> 2CH3OH + 6LiAl(O2)H + Al2O3.
  • This reaction has potential applications in carbon capture and utilization processes.
  1. Reduction of Esters to Aldehydes
  • Esters can be selectively reduced to aldehydes using lithium aluminum hydride.
  • The reaction is as follows: RCOOR’ + 2LiAlH4 -> RCHO + R’OH + 2LiAl(O2)H.
  • For example, the reduction of ethyl benzoate (C6H5COOC2H5) with lithium aluminum hydride gives benzaldehyde (C6H5CHO).
  1. Reduction of Ketones to Secondary Alcohols
  • Ketones can be reduced to secondary alcohols using lithium aluminum hydride.
  • The reaction is as follows: R2CO + 2LiAlH4 -> R2CHOH + LiAl(OC2H5)3.
  • For example, the reduction of acetone (CH3COCH3) with lithium aluminum hydride gives isopropanol (CH3CHOHCH3).
  1. Reduction of Aldehydes to Primary Alcohols
  • Aldehydes can be reduced to primary alcohols using lithium aluminum hydride.
  • The reaction is as follows: RCHO + 2LiAlH4 -> RCH2OH + LiAl(O2)H.
  • For example, the reduction of formaldehyde (HCHO) with lithium aluminum hydride gives methanol (CH3OH).
  1. Summary
  • Lithium aluminum hydride is a versatile and potent reducing agent.
  • It can selectively reduce a wide range of functional groups in organic molecules.
  • The reagent is moisture-sensitive and should be handled with care.
  • Its applications include the reduction of carbonyl compounds, carboxylic acids, esters, and other functional groups.
  • Lithium aluminum hydride plays a crucial role in organic synthesis and is widely used in laboratory and industrial settings.

Reduction of Acid Anhydrides

  • Acid anhydrides can be reduced to two alcohols using lithium aluminum hydride.
  • The reaction is as follows: (RCO)2O + 2LiAlH4 → 2RCH2OH + LiAl(OC2H5)3.

Reduction of Amides

  • Amides can be reduced to amines using lithium aluminum hydride.
  • The reaction is as follows: RCONH2 + 2LiAlH4 → RCH2NH2 + LiAl(OC2H5)3.

Reduction of Nitriles

  • Nitriles can be reduced to primary amines using lithium aluminum hydride.
  • The reaction is as follows: RCN + 4LiAlH4 → RCH2NH2 + 4LiAl(OC2H5)3.

Reduction of Azides

  • Azides can be reduced to amines using lithium aluminum hydride.
  • The reaction is as follows: RN3 + 3LiAlH4 → RCH2NH2 + 3LiAl(OC2H5)3.

Birch Reduction

  • Lithium aluminum hydride is used in the Birch reduction to reduce aromatic compounds.
  • The reaction is as follows: ArH + 2LiAlH4 → ArH2 + 2LiAl(OC2H5)3.
  • The Birch reduction is commonly used to reduce aromatic rings to cyclohexadienes.

Reduction of Sulfoxides

  • Sulfoxides can be reduced to sulfides using lithium aluminum hydride.
  • The reaction is as follows: RSO + 3LiAlH4 → RSH + 3LiAl(OC2H5)3.

Reduction of Sulfonyl Chlorides

  • Sulfonyl chlorides can be reduced to sulfides using lithium aluminum hydride.
  • The reaction is as follows: RSO2Cl + 2LiAlH4 → RSH + 2LiAl(OC2H5)3 + AlCl3.

Reduction of Epoxysilanes

  • Epoxysilanes can be reduced to corresponding silanols using lithium aluminum hydride.
  • The reaction is as follows: R2Si(OR’)2O + 2LiAlH4 → R2Si(OR’)2OH + LiAl(OC2H5)3.

Reduction of Ketones to Alkanes

  • Ketones can be reduced to alkanes using lithium aluminum hydride.
  • The reaction is as follows: R2CO + 2LiAlH4 → R2CH2 + 2LiAl(OC2H5)3.

Summary

  • Lithium aluminum hydride is a versatile reducing agent that can be used to reduce various functional groups.
  • Its applications include the reduction of carbonyl compounds, carboxylic acids, esters, amides, nitriles, and others.
  • The compound should be handled with care due to its reactivity and moisture sensitivity.
  • Lithium aluminum hydride is an essential tool in organic synthesis and plays a significant role in the production of various organic compounds.