This, again, is very similar to what we saw in the Grignard reaction of esters.
![why can you use h2 pdc to remove benzyl ester why can you use h2 pdc to remove benzyl ester](https://slideplayer.com/slide/13600060/83/images/6/group+abbreviation+addition+structure+removal.jpg)
The newly formed carbonyl group is an aldehyde and it is more reactive than the ester, thus is attacked one more time by LiAlH 4: 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:
![why can you use h2 pdc to remove benzyl ester why can you use h2 pdc to remove benzyl ester](https://www.mdpi.com/catalysts/catalysts-07-00089/article_deploy/html/images/catalysts-07-00089-sch012.png)
The reduction of an ester to an alcohol requires two hydride additions to the carbonyl group and therefore an excess of LiAlH 4 is used: The Mechanism of LiAlH 4 Reduction of Esters Let’s now see how the reduction of esters by LiAlH 4 works. That is about the relationship between NaBH 4 and ester. Aldehydes and ketones are more reactive than esters since the electrophilicity of the carbon atom of the ester is partially suppressed by the lone pair of the oxygen through resonance stabilization:īecause of the resonance stabilization, the C=O carbon atom in the esters is not electrophilic and NaBH 4 being not very reactive is unable to attack it. We have also seen this in the Grignard reaction. 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). And this also has to do with the reactivity of the ester as well. The solvent has two functions here:ġ) It serves as the source of a proton (H +) once the reduction is completeĢ) The sodium ion is a weaker Lewis acid than the lithium ion and, in this case, the hydrogen bonding between the alcohol and the carbonyl group serves as a catalysis to activate the carbonyl group:īecause NaBH 4 is not very reactive, it is not strong enough to react with esters. Sodium borohydride reduces aldehydes and ketones by a similar mechanism with some important differences that we need to mention.įirst, NaBH 4 is not so reactive and the reaction is usually carried out in protic solvents such as ethanol or methanol. NaBH 4 Reduction of Aldehydes and Ketones – The Mechanism 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. The resulting alkoxide salt can react with the AlH 3 and convert it to another source of hydride. This decreases the electron density on the oxygen thus making the C=O bond more susceptible to a nucleophilic attack. The hydride addition to the carbonyl is also catalyzed by the lithium ion which serves as a Lewis acid by coordinating to the carbonyl oxygen. There are, however, some differences depending on the reagent and to address those, let’s start with the mechanism of LiAlH 4 Reduction: LiAlH 4 Reduction of Aldehydes and Ketones – The MechanismĪs mentioned earlier, both reagents function as a source of hydride (H −) which acts as a nucleophile attacking the carbon of the carbonyl C=O bond and in the second step the resulting alkoxide ion is protonated to form an alcohol.
![why can you use h2 pdc to remove benzyl ester why can you use h2 pdc to remove benzyl ester](https://europepmc.org/articles/PMC6211199/bin/nihms-993873-f0044.jpg)
Esters, on the other hand, are converted to primary alcohols by LiALH 4. Notice that LiALH 4 and NaBH 4 reduce aldehydes and ketones to primary and secondary alcohols respectively. NaBH 4, 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: For example, it reacts violently with water and therefore, LiAlH 4 reductions are carried out in d ry solvents such as anhydrous ether and THF. This high reactivity of the hydride ion in LiAlH 4 makes it incompatible with protic solvents. LiALH 4 is one of them most powerful reducing agents efficiently working for any carbonyl and some other functional groups as well. The hydride ion reacts with the carbonyl group which, in turn, is also a polar covalent bond and the presence of the π bond makes the H – addition possible: Because of higher electronegativity, the hydrogen bears higher electron density which eventually makes it react as a hydride ion:įor example, the Al-H bond in LiAlH 4 is so polar that it has nearly ionic character leaving the hydrogen as a hydride ion which is a very reactive both as a base and a nucleophile: The principle behind the hydride reducing agents is the presence of a polar covalent bond between a metal and hydrogen. The most common hydride reducing agents are the lithium aluminum hydride ( LiALH 4) also abbreviated as LAH and sodium borohydride ( NaBH 4): These reductions are a result of a net addition of two hydrogen atoms to the C=O bond: Alcohols can be prepared from carbonyl compounds such as aldehydes, ketones, esters, acid chlorides and even carboxylic acids by hydride reductions.