of lactic acid. The "(III)" shows the oxidation state of the aluminium or boron, and is often left out because these elements only ever show the +3 oxidation state in their compounds. Reduction of aldehydes [LiAlH 4] Explained: Reduction of carbonyl compounds can be carried out by addition of molecular hydrogen or the action of hydride reagents. If conditions are anaerobic, pyruvic acid is converted into lactic Solid sodium tetrahydridoborate is added to a solution of the aldehyde or ketone in an alcohol such as methanol, ethanol or propan-2-ol. atoms or ions from NADH are involved as a reduction of of the The definitions are When the acid is added, the negative ion formed picks up a hydrogen ion to give an alcohol. (Remember that the initial reaction is carried out in carefully dried ethoxyethane.) For example, with ethanal you get ethanol: Notice that this is a simplified equation - perfectly acceptable to UK A level examiners. organic chemistry, although changes in electrons are important, oxidation numbers are not very useful in organic chemistry. It can be used in solution in alcohols or even solution in water - provided the solution is alkaline. The solvent has two functions here: 1) It serves as the source of a proton (H+) once the reduction is complete 2) 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 a… Despite the fearsome names, the structures of the two reducing agents are very simple. as follows: OXIDATION: loss of hydrogen, gain in oxygen, or loss Sodium borohydride reduces aldehydes and ketones by a similar mechanism with some important differences that we need to mention. In practice, there is an additional step for safety reasons. The oxidation of an alcohol to form an aldehyde or ketone is very important in synthesis. Use the BACK button on your browser to return to this page. In the second We also acknowledge previous National Science Foundation support under grant numbers 1246120, 1525057, and 1413739. They all contain the grouping -CH2OH. I have included this detail (even though UK A level students don't need it) because I object to giving simplifications that you might have to completely unlearn in the future if you do a Chemistry degree. may also be used in this reaction. In the first stage, a salt is formed containing a complex aluminium ion. In metal hydrides reductions the resulting alkoxide salts are insoluble and need to be hydrolyzed (with care) before the alcohol product can be isolated. The reduction of an aldehyde. The NADH supplies the hydrogen for the reduction of are: This time, the negative ion takes a hydrogen ion from a water molecule. oxidation and reduction is more readily observed from changes oxidizing agent and hydrogen gas in the presence of a metal catalyst The H in square brackets means "hydrogen from a reducing agent". As before, the reaction starts with a nucleophilic attack by the hydride ion on the slightly positive carbon atom. first to ethanal (acetaldehyde) and then to ethanol. Again the product is the same whichever of the two reducing agents you use. Depending on which recipe you read, it is either heated under reflux or left for some time around room temperature. The carbon-oxygen bond is polarized, the carbon of carbonyl group is electrophilic and the oxygen is nucleophilic. Reduction of aldehydes and ketones lead to two different sorts of alcohol. ketone + H22 ---> secondary alcohol, The reduction process is merely the reverse of oxidation. Unless otherwise noted, LibreTexts content is licensed by CC BY-NC-SA 3.0. This occurs under strenuous exercise conditions [H] means "hydrogen from a reducing agent". Sodium borohydride functions as a source of hydride. In the second stage of the reaction, water is added and the mixture is boiled to release the alcohol from the complex. I have chosen this one largely because I think I understand what is going on! You will find a simplified (for UK A level purposes) mechanism for the reaction by following this link. © Jim Clark 2004 (last modified November 2015), co-ordinate covalent (dative covalent) bonding. The simplification used is to write H- instead of BH4-. For example, with propanone you get propan-2-ol: Reduction of a ketone leads to a secondary alcohol. [H] means "hydrogen from a reducing agent". Again, the alcohol formed can be recovered from the mixture by fractional distillation. This page gives you the facts and mechanisms for the reduction of carbonyl compounds (specifically aldehydes and ketones) using sodium tetrahydridoborate (sodium borohydride) as the reducing agent. should be added during reduction. The practical details above are simplified to what you are likely to need in a theory exam. In this process, the hydroxy hydrogen of the alcohol is replaced by a leaving group (X in the figure below). You will need to use the BACK BUTTON on your browser to come back here afterwards. . of electrons. For example, with ethanal you get ethanol: Notice that this is a simplified equation - perfectly acceptable to UK A level examiners. Reduction of Aldehydes and Ketones with Complex Metal Hydrides Complex hydrides like lithium aluminum hydride or sodium borohydride are a perfect choice for the reduction of polar π-bonds like C=O. This mechanism is simplified to the point of being wrong, so if you are working outside of the UK A level system, don't bother to look at it! On the other hand, you may well be expected to know that the reaction is done initially in solution in ethoxyethane followed by treatment with acid. What happens now depends on whether you add an acid or water to complete the reaction. up to the production of pyruvic acid. Legal. ketone group on pyruvic acid. It reacts violently with water and alcohols, and so any reaction must exclude these common solvents. to make wine, beer, and other alcoholic products from grains This page looks at the reduction of aldehydes and ketones by two similar reducing agents - lithium tetrahydridoaluminate(III) (also known as lithium aluminium hydride) and sodium tetrahydridoborate(III) (sodium borohydride). The carbon-oxygen double bond is highly polar, and the slightly positive carbon atom is attacked by the hydride ion acting as a nucleophile. The product is then treated with a dilute acid (such as dilute sulphuric acid or dilute hydrochloric acid) to release the alcohol from the complex ion. Other reducing agents which give up hydrogen The BH4- ion is essentially a source of hydride ions, H-. Using lithium tetrahydridoaluminate (lithium aluminium hydride). A hydride ion is a hydrogen atom with an extra electron - hence the lone pair. You get exactly the same organic product whether you use lithium tetrahydridoaluminate or sodium tetrahydridoborate. Traces of water in this react with the excess LiAlH4. Reduction of aldehydes [NaBH 4] Explained: By reducing the carbonyl group alcohols can be obtained. Carbonyl compounds act as the in the muscles. At the end of this time, a complex similar to the previous one is formed. 3) The two electrons which were in the double bond are used in The complication doesn't actually stop here! Sodium tetrahydridoborate (previously known as sodium borohydride) has the formula NaBH4, and contains the BH4- ion. In each case, there are four hydrogens ("tetrahydido") around either aluminium or boron in a negative ion (shown by the "ate" ending). is lactic acid with a secondary alcohol group on the second carbon 5 back into NAD+. 1) One hydrogen adds to the carbon with the double bond oxygen. Some un-dried ethoxyethane is added first before you add the acid. The negative ion reacts with a hydrogen ion from the acid added in the second stage of the reaction. If this is the first set of questions you have done, please read the introductory page before you start. The lone pair of electrons on the hydride ion forms a bond with the carbon, and the electrons in one of the carbon-oxygen bonds are repelled entirely onto the oxygen, giving it a negative charge. REDUCTION: gain of hydrogen, loss of oxygen, or gain In each of the negative ions, one of the bonds is a co-ordinate covalent (dative covalent) bond using the lone pair on a hydride ion (H-) to form a bond with an empty orbital on the aluminium or boron. At least a part of the organic bits attached to the boron will probably come from whatever alcohol molecules are used as the solvent for the reaction. and fruits. Sodium tetrahydridoborate (previously known as sodium borohydride) has the formula NaBH 4, and contains the BH 4-ion. First, NaBH4 is not so reactive and the reaction is usually carried out in protic solvents such as ethanol or methanol. There are several quite different ways of carrying out this reaction. The LibreTexts libraries are Powered by MindTouch® and are supported by the Department of Education Open Textbook Pilot Project, the UC Davis Office of the Provost, the UC Davis Library, the California State University Affordable Learning Solutions Program, and Merlot. In what follows, I am choosing one out of many different methods. 5) The final product is a primary alcohol. That ion acts as the reducing agent. or fermentation to produce ethanol from sugars. R and R' can be any combination of hydrogen or alkyl groups. The alcohol formed can be recovered from the mixture by fractional distillation. of electrons. This produces an intermediate which can be converted into the final product by boiling it with water. To make the names shorter, that's what I shall do for the rest of this page. This almost certainly varies depending on the nature of the aldehyde or ketone. Missed the LibreFest? Use the BACK button on your browser to return to this page if you want to continue to explore aldehyde and ketone reactions. GENERAL EQUATION: REDUCTION OF ALDEHYDES AND KETONES, aldehyde + H2 ---> primary alcohol In fermentation, pyruvic acid is converted This is the process In You must be careful though, as NaBH 4 and LiAlH 4 are quite different in their strength, so they are not 100% interchangeable. Sodium tetrahydridoborate is a more gentle (and therefore safer) reagent than lithium tetrahydridoaluminate. Primary alcohols are obtained from aldehydes, and secondary from ketones. In the sodium borohydride reduction the methanol solvent system achieves this … acid. For more information contact us at info@libretexts.org or check out our status page at https://status.libretexts.org. They all contain the grouping -CHOH. The reduction is an example of nucleophilic addition. The product of the reaction The difference is in the In general terms, reduction of an aldehyde leads to a primary alcohol. fate of pyruvic acid. A primary alcohol is one which only has one alkyl group attached to the carbon with the -OH group on it. Two possible variants (there are several others!) are: In each case, reduction essentially involves the addition of a hydrogen atom to each end of the carbon-oxygen double bond to form an alcohol. Watch the recordings here on Youtube! Two possible variants (there are several others!) The formulae of the two compounds are LiAlH4 and NaBH4. The reaction happens at room temperature, and takes place in two separate stages. The reactions of glycolysis are identical to those described Read More on This Topic A secondary alcohol is one which has two alkyl groups attached to the carbon with the -OH group on it. The reaction sequence is shown on the left.