Reduction of Carboxylic Acids to Primary Alcohols by LiAlH4
Carboxylic acids (RCOOH) can be reduced to primary alcohols by lithium aluminum hydride (LiAlH4). This reaction is a two stages reaction. LiAlH4 is a strong reducing agent used in organic chemistry for reducing several organic compounds.
RCOOH + LiAlH4 → RCH2OH
R can be a carbon group or hydrogen atom. General equation for reduction of carboxylic acid is written above.
Lithium aluminum hydride as reducing agent
Lithium aluminum hydride (LiAlH4) is strong reducing agent used in organic chemistry. It can be used to reduce carbonyl groups (aldehyde and ketone), carboxylic acids, esters, amides etc.
Products after the reaction of carboxylic acid reduction by LiAlH4
- Carboxylic acid is reduced to a primary alcohol.
- LiAlH4 is oxidized and hydrogen gas is produced.
Oxidation number change in reduction of carboxylic acid
Due to reduction, oxidation number of carbon atom in the carboxylic group should be reduced.
- Except formic acid, oxidation number of carbon atom in the carboxylic group is +3. In formic acid, it is +2.
- When carboxylic acid (except formic acid) is reduced to a primary alcohol by LiAlH4, oxidation number of carbon atom is reduced to -1. When formic acid is reduced to methanol, its oxidation number of carbon is -2.
- Now, you see, in each case, oxidation number is decreased by 4 (a big reduction) in the reduction of carboxylic acid to primary alcohol by LiAlH4.
Reaction conditions
- After adding LiAlH4, dilute sulfuric acid should be added to release the primary alcohol from aluminum salt.
- LiAlH4 reacts with water. Therefore, LiAlH4 cannot meet water because LiAlH4 is destroyed when it reacts with water. So, LiAlH4 is dissolved in dry ethoxyethane (ether compound) to keep in dry medium
- Room temperature is enough to proceed the reaction.
Mechanism of reduction reaction
First, carboxylic acid is reduced to an aldehyde compound. But, formed aldehyde is readily reduced to a primary alcohol by LiAlH4. So. We cannot stop at intermediate stage to get the aldehyde.
Examples of carboxylic acid reduction
Formic acid (methanoic acid) reduction by LiAlH4
Methanol is formed when formic acid is reduced by LiAlH4
Reduction of acetic acid (ethanoic acid) by LiAlH4
When acetic acid reacts with LiAlH4, ethanol is formed.
Sodium borohydride or sodium /ethanol or hydrogen/Pt in reduction of carboxylic acids
Carboxylic acids cannot be reduced by any of the Sodium borohydride (NaBH4) or sodium / ethanol or hydrogen gas with platinum catalyst. Their reducing power is not enough to reduce carboxylic acid.
Questions asked by students
Ask your question and find the answer free.
Is it similar carboxylic acid chloride reduction and carboxylic acid chloride reduction by LiAlH4?
When carboxylic acid chloride is reduced by LiAlH4, a primary amine compound is given as the product. Therefore, carboxylic acid chloride reduction and carboxylic acid chloride reduction by LiAlH4 is not similar due to giving different products.
RCOOH + LiAlH4, is this a reaction of reduction of carboxylic acid?
Yes. RCOOH represents a carboxylic acid. R can be a carbon group or hydrogen. Equation can be written like this.
RCOOH + LiAlH4 → RCH2OH
What will happen to LiAlH4, when carboxylic acid is being reduced to primary alcohol?
If carboxylic acid is reduced, something should be oxidized. Therefore, LiAlH4 is oxidized. When you study the structure of LiAlH4, you will know there are hydrogen atoms at -1 oxidation state. These hydrogen atoms can be oxidized to hydrogen gas easily than Li+ and Al3+ ions.