Reduction of benzaldehyde with nabh4


  • Addition of NaBH4 to aldehydes to give primary alcohols
  • Alcohols can be prepared from carbonyl compounds such as aldehydes, ketones, esters, acid chlorides and even carboxylic acids by hydride reductions. This high reactivity of the hydride ion in LiAlH4 makes it incompatible with protic solvents. For example, it reacts violently with water and therefore, LiAlH4 reductions are carried out in dry solvents such as anhydrous ether and THF. NaBH4, on the other hand, is not so reactive and can be used, for example, in a selective reduction of aldehydes and ketones in presence of an ester: Notice that LiALH4 and NaBH4 reduce aldehydes and ketones to primary and secondary alcohols respectively.

    Esters, on the other hand, are converted to primary alcohols by LiALH4. The resulting alkoxide salt can react with the AlH3 and convert it to another source of hydride. However, for simplicity, most often we show only one addition to the carbonyl followed by a protonation of the alkoxide with water or aqueous acidic solutions which gives the final product alcohol. NaBH4 Reduction of Aldehydes and Ketones — The Mechanism Sodium borohydride reduces aldehydes and ketones by a similar mechanism with some important differences that we need to mention.

    First, NaBH4 is not so reactive and the reaction is usually carried out in protic solvents such as ethanol or methanol. And this also has to do with the reactivity of the ester as well. In general, aldehydes and ketones are the most reactive carbonyl compounds after acid chlorides which are only used as reagents and not final products because of their reactivity. We have also seen this in the Grignard reaction.

    That is about the relationship between NaBH4 and ester. The Mechanism of LiAlH4 Reduction of Esters The reduction of an ester to an alcohol requires two hydride additions to the carbonyl group and therefore an excess of LiAlH4 is used: This is because the tetrahedral intermediate formed after the first hydride addition contains a leaving group which is kicked out re-forming the carbonyl group: The newly formed carbonyl group is an aldehyde and it is more reactive than the ester, thus is attacked one more time by LiAlH4: This, again, is very similar to what we saw in the Grignard reaction of esters.

    Yes, the methoxide ion is not a great leaving group as we know from E2 or SN2 reactions. However, it is still a weaker base than the hydride ion and in addition, the tetrahedral intermediate with two oxygens and a negative charge is highly unstable and it is energetically favorable to expel the methoxide. One good alternative to this is the use of borane which is only efficient for the reduction of carboxylic acids and amides. Back to the LiAlH4. Despite the low reactivity of the carboxylate ion, the hydride addition does occur: The negatively-charged oxygen is then converted into a leaving group by coordinating to aluminum.

    The Stereochemistry of LiAlH4 and NaBH4 Reduction The reduction of unsymmetrical ketones with LiAlH4 or NaBH4 produces a pair of stereoisomers because the hydride ion can attack either face of the planar carbonyl group: If no other chiral center are present, the product is a racemic mixture of enantiomers.

    Experimental General All substrates and reagents were purchased from commercially sources with the best quality. The products were characterized by their 1H NMR or IR spectra and comparison with authentic samples melting or boiling points. Organic layers were dried over anhydrous sodium sulfate.

    All yields referred to isolated pure products. The purity of products was determinate by 1H NMR. Also, reactions are monitoring over silica gel 60 F aluminum sheet.

    The mixture was stirred and irradiated with ultrasound waves at room temperature for 60 minutes. Then, water 5 mL was added to the reaction mixture.

    Evaporation of the solvent afforded the pure benzyl alcohol 0. To this solution, NaBH4 0. TLC monitored the progress of reaction. After 60 min, the reaction mixture was quenched by addition of distilled water 5 ml and this mixture was then stirred for an additional 1 min. S reduces a variety of aldehydes vs. Reduction reactions were carried out with 1. In addition, regioselectivity of this system was also investigated with exclusive 1,2-reduction of conjugated enals to their corresponding allylic alcohols in excellent yields.

    All reductions were accomplished with high efficiency of the reductions, using the appropriate molar ratios of NaBH4 andPhCO2Na, convenient reaction times min under green chemistry protocol, and easy work-up procedure.

    Acknowledgments The authors gratefully appreciated the financial support of this work by the research council of Islamic Azad University branch of Mahabad. References Mason, T. Practical Sonochemistry, second ed.

    This high reactivity of the hydride ion in LiAlH4 makes it incompatible with protic solvents. For example, it reacts violently with water and therefore, LiAlH4 reductions are carried out in dry solvents such as anhydrous ether and THF. NaBH4, on the other hand, is not so reactive and can be used, for example, in a selective reduction of aldehydes and ketones in presence of an ester: Notice that LiALH4 and NaBH4 reduce aldehydes and ketones to primary and secondary alcohols respectively.

    Esters, on the other hand, are converted to primary alcohols by LiALH4.

    The resulting alkoxide salt can react with the AlH3 and convert it to another source of hydride. However, for simplicity, most often we show only one addition to the carbonyl followed by a protonation of the alkoxide with water or aqueous acidic solutions which gives the final product alcohol. Acknowlegment The authors gratefully appreciated the financial support of this work by the research council of Islamic Azad University branch of Mahabad. Refrences Cha. Korean Chem.

    Addition of NaBH4 to aldehydes to give primary alcohols

    Nutaitis, C. Ranu, B. Yumino, S. Chemistry Letters,doi: Ward, D. Maki, Y. Tetrahedron Lett. Also, reactions are monitoring over silica gel 60 F aluminum sheet.

    The mixture was stirred and irradiated with ultrasound waves at room temperature for 60 minutes. Then, water 5 mL was added to the reaction mixture.

    Evaporation of the solvent afforded the pure benzyl alcohol 0. To this solution, NaBH4 0. TLC monitored the progress of reaction.


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