Greener route to carboxylic acids

Chemists in Israel have developed a new way to oxidise primary alcohols to carboxylic acids, using water as both the reaction solvent and the source of oxygen. The method could result in a cleaner and safer way to generate carboxylic acids and their derivatives, which are important compounds for the chemical industry.

alcohol oxidation

The new green chemistry approach to oxidising primary alcohols to carboxylic acids uses water as the oxidant © NPG

‘For the most part, carboxylic acids are made by using stoichiometric amounts of oxidising agents, such as permanganate, which give stoichiometric amounts of waste,’ says David Milstein of the Weizmann Institute of Science, who led the research team. ‘There are recent procedures using oxygen under pressure, but this introduces potential safety issues. We have developed something conceptually new, using water as the source of oxygen with the liberation of hydrogen gas.’
The key to the method is the use of a ruthenium catalyst, a so-called pincer complex in which a ruthenium centre is clamped by a bipyridine-based claw. In the presence of a base, the catalyst interacts with water and the alcohol substrate, transferring the water’s oxygen to the alcohol and releasing hydrogen. The oxidised alcohol becomes trapped by the base to form the carboxylic acid salt, which can be isolated and, if needed, simply converted into the acid.
green alcohol oxidation

A proposed mechanism for the reaction © NPG

The team succeeded in oxidising a wide range of primary alcohols in this way, with carboxylic acid yields ranging from around 60–90%. The catalyst loading used for the experiments was 0.2 mol%, so 500 molecules of product are obtained for every molecule of catalyst. ‘We think we can improve on this,’ says Milstein. ‘I think it is a procedure that can be upscaled and the hydrogen that is generated is a useful by-product.’ The cost of ruthenium is a potential issue, Milstein concedes, ‘although it is the least expensive of its class of noble metals’. The team is working to see if it might be possible to replicate the reaction with an iron-based catalyst.

Roger Sheldon of Delft University of Technology in the Netherlands, an expert on green catalytic oxidation, says that the use of water as both the solvent and the sole source of oxygen in the catalytic oxidation of primary alcohols to carboxylic acids and hydrogen is ‘an important and exciting discovery’. Sheldon adds: ‘The broad scope and relatively low catalyst loading coupled with good catalyst stability under the anaerobic conditions are additional advantages. The method could find industrial applications in fine chemicals and pharmaceuticals but in bulk chemicals the formation of one equivalent of salt as a co-product is an obvious drawback.’


Related Content

Chemistry World podcast - February 2013

4 February 2013 Podcast | Monthly

news image

Graham Richards discusses crowdsourcing, Eric Wolff talks about ice cores and the team cover the latest chemical news


26 January 2011 Podcast | Compounds

news image

This week's podcast is about ethanol

Most Read

Antimicrobial resistance will kill 300 million by 2050 without action

16 December 2014 News and Analysis

news image

UK report says resistance will cost global economy $100 trillion

Cutting edge chemistry in 2014

10 December 2014 Research

news image

We take a look back at the year's most interesting chemical science stories

Most Commented

A bad business

19 December 2014 Critical Point

news image

Targets and assessments can boost productivity at universities – but only if they do not stifle creativity and alienate the...

Unusual 2D silica allotrope predicted

25 June 2014 Research

news image

Simulations say graphene-like silica would become thicker when stretched