Reduction Reactions: show all work Reduction of aldehydes & ketones using H2/Pd, NaBH4/H2O, ROH, or LiAlH4, Et2O, then H3O+. Reduction of RCOOH to primary alcohols (Ch. 19 A3). Reduction of amides, nitriles, and nitro compounds to amines. Reduction of esters or nitriles to aldehydes using DIBAL. Reduction of esters to alcohols using LiAlH4. Reduction of acid chlorides to aldehydes using LiAlH(OC(CH3)3)
The Correct Answer and Explanation is:
Reduction reactions are a class of chemical processes in which a molecule gains electrons, typically accompanied by the addition of hydrogen or the removal of oxygen. In the context of organic chemistry, reducing agents are often used to reduce carbonyl compounds, nitriles, amides, and other functional groups.
1. Reduction of Aldehydes and Ketones
- Using H2/Pd (Catalytic Hydrogenation): Both aldehydes and ketones can be reduced to their corresponding alcohols using hydrogen gas in the presence of palladium (Pd) as a catalyst. The reaction proceeds via the addition of hydrogen to the carbonyl carbon, resulting in the formation of a hydroxyl group.
RCHO+H2→PdRCH2OH(Aldehyde to Primary Alcohol)\text{RCHO} + H_2 \xrightarrow{Pd} \text{RCH}_2\text{OH} \quad \text{(Aldehyde to Primary Alcohol)}RCHO+H2PdRCH2OH(Aldehyde to Primary Alcohol) R2C=O+H2→PdR2CHOH(Ketone to Secondary Alcohol)\text{R}_2\text{C=O} + H_2 \xrightarrow{Pd} \text{R}_2\text{CHOH} \quad \text{(Ketone to Secondary Alcohol)}R2C=O+H2PdR2CHOH(Ketone to Secondary Alcohol)
- Using NaBH4/H2O (Sodium Borohydride): NaBH4 is a selective reducing agent that can reduce aldehydes and ketones to alcohols without affecting other functional groups like esters or carboxylic acids.
RCHO+NaBH4→H2ORCH2OH\text{RCHO} + \text{NaBH}_4 \xrightarrow{H_2O} \text{RCH}_2\text{OH}RCHO+NaBH4H2ORCH2OH R2C=O+NaBH4→H2OR2CHOH\text{R}_2\text{C=O} + \text{NaBH}_4 \xrightarrow{H_2O} \text{R}_2\text{CHOH}R2C=O+NaBH4H2OR2CHOH
- Using ROH (Alcohol as Solvent): In some cases, alcohols (ROH) can act as both the solvent and the reducing agent, especially in reductions that involve less reactive carbonyl compounds. The alcohol itself donates hydrogen to the carbonyl group, forming an alcohol.
- Using LiAlH4 (Lithium Aluminum Hydride): LiAlH4 is a strong reducing agent that reduces aldehydes and ketones to alcohols under anhydrous conditions. The reduction mechanism involves hydride transfer from LiAlH4 to the carbonyl carbon.
RCHO+LiAlH4→Et2ORCH2OH\text{RCHO} + \text{LiAlH}_4 \xrightarrow{\text{Et}_2\text{O}} \text{RCH}_2\text{OH}RCHO+LiAlH4Et2ORCH2OH R2C=O+LiAlH4→Et2OR2CHOH\text{R}_2\text{C=O} + \text{LiAlH}_4 \xrightarrow{\text{Et}_2\text{O}} \text{R}_2\text{CHOH}R2C=O+LiAlH4Et2OR2CHOH
After the reaction, acidic work-up with H3O+ is typically used to neutralize the reaction and isolate the alcohol.
2. Reduction of Carboxylic Acids (RCOOH) to Primary Alcohols
Carboxylic acids can be reduced to primary alcohols using strong reducing agents like LiAlH4. RCOOH+LiAlH4→H2ORCH2OH\text{RCOOH} + \text{LiAlH}_4 \xrightarrow{\text{H}_2\text{O}} \text{RCH}_2\text{OH}RCOOH+LiAlH4H2ORCH2OH
This reaction involves the complete reduction of the carboxyl group (-COOH) to a primary alcohol (-CH2OH).
3. Reduction of Amides, Nitriles, and Nitro Compounds to Amines
- Amides and Nitriles: These can be reduced to amines using LiAlH4 or other powerful reducing agents.
RCONH2+LiAlH4→H2ORCH2NH2\text{RCONH}_2 + \text{LiAlH}_4 \xrightarrow{\text{H}_2\text{O}} \text{RCH}_2\text{NH}_2RCONH2+LiAlH4H2ORCH2NH2 RCN+LiAlH4→H2ORCH2NH2\text{RCN} + \text{LiAlH}_4 \xrightarrow{\text{H}_2\text{O}} \text{RCH}_2\text{NH}_2RCN+LiAlH4H2ORCH2NH2
- Nitro Compounds: Nitro compounds can also be reduced to amines with agents like tin (Sn) in the presence of HCl or using other reducing agents.
RNO2+3H2→PtRNH2\text{RNO}_2 + 3 \text{H}_2 \xrightarrow{\text{Pt}} \text{RNH}_2RNO2+3H2PtRNH2
4. Reduction of Esters to Aldehydes Using DIBAL (Diisobutylaluminum Hydride)
DIBAL is often used to reduce esters to aldehydes. The reaction proceeds via selective reduction of the ester, where the carbonyl group is partially reduced to form an aldehyde rather than a primary alcohol. RCOOR’+DIBAL-H→H2ORCHO+R’OH\text{RCOOR’} + \text{DIBAL-H} \xrightarrow{\text{H}_2\text{O}} \text{RCHO} + \text{R’OH}RCOOR’+DIBAL-HH2ORCHO+R’OH
5. Reduction of Esters to Alcohols Using LiAlH4
LiAlH4 is used to reduce esters to primary alcohols in a two-step process: RCOOR’+LiAlH4→H2ORCH2OH+R’OH\text{RCOOR’} + \text{LiAlH}_4 \xrightarrow{\text{H}_2\text{O}} \text{RCH}_2\text{OH} + \text{R’OH}RCOOR’+LiAlH4H2ORCH2OH+R’OH
6. Reduction of Acid Chlorides to Aldehydes Using LiAlH(OC(CH3)3)
A specialized reducing agent, LiAlH(OC(CH3)3), reduces acid chlorides to aldehydes selectively, without further reducing the aldehyde to an alcohol. RCOCl+LiAlH(OC(CH3)3)→H2ORCHO\text{RCOCl} + \text{LiAlH(OC(CH}_3)_3) \xrightarrow{\text{H}_2\text{O}} \text{RCHO}RCOCl+LiAlH(OC(CH3)3)H2ORCHO
These reductions are crucial in organic synthesis, allowing chemists to selectively reduce various functional groups to desired products with high precision. Each reducing agent offers different selectivity, making them useful for specific reactions depending on the functional groups involved.
