Chemical Society Reviews Blog

Chemical reactions often have reaction barriers that must be overcome in order for reactants to become products. Appreciating the origins of these barriers and more importantly quantifying their heights from raw data is of significant use to the Chemist. Therefore, the Chemist would like to have these features in the general model of reactivity which they use. A model that can predict barriers from raw data is the Valence Bond model, the focus of this quality Tutorial Review.

Sason Shaik from The Hebrew University of Jerusalem and colleagues share with the reader their insight from the development of the Valence Bond model. They focus on hydrogen atom transfer, the step most chemical oxidations begin with and which is therefore immensely important. They begin from the simplest hydrogen exchange reaction and work up to the more complex hydroxylation by Cyctochrome P450.

Valence Bond Models and the effect of different intermediates on the energy profile (bold line).

The authors take the reader through the preparation and use of valence bond diagrams and thus equip the reader with the tools required to understand mechanisms and predict chemical reactivity patterns. The authors have taken their role as tutors seriously and have provided the reader with supplementary data which they can use to work through problems and reconstruct results on their own. This focus on the reader, as a student, is very welcome and will ensure the interested reader appreciates the quality of the Valence Bond model as a useful interface between experiment and theory and between computations and understanding.

Read the Chem Soc Rev Tutorial Review in full now – for free*

A Tutorial for Understanding Chemical Reactivity Through The Valence Bond Approach
Dandamudi Usharani, Wenzhen Lai, Chunsen Li, Hui Chen, David Danovich and Sason Shaik
Chem. Soc. Rev., 2014, advance article
DOI: 10.1039/C4CS00043A

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Organic polymers with metal-like properties have been an active area of chemical research for nearly seventy years. In this HOT Chem Soc Rev review, Adam Pron, Piotr Bujak, and colleagues from the Warsaw University of Technology and the Joseph Fourier University in Grenoble give an accessible and detailed account of the current state of the art in delocalised polymeric organic materials, focussing on neutral polymers which function as semiconductors. The application of semiconducting organic materials in devices such as FETs, LEDs, photovoltaic cells, photodiodes and magnetic photoswitches is described.

The synthesis, by Grignard reaction, transition metal or boron catalysed coupling reactions, and basic chemical characterisation of the major classes of organic semiconducting polymer are summarised.  This is followed by a detailed review of the chemistry and application of 4 classes of polymer in various electronic devices over the last three years.  The classes described are: polymeric semiconductors with n-type and with p-type conductivity, low-band gap semiconductors and also high-spin macromolecules.

Possible packing patterns in poly-3-hexylthiophene polymers

Methods of delivering highly ordered phases of polymers via solution processing are given, along with relevant electrochemical and magnetic information such as electron affinity, ionization potentials and magnetic moments. High-spin materials are of interest in spintronics, due to their potential for lower power consumption and faster information transfer than classical electronic components.

This is a very detailed review that successfully gives a comprehensive picture of the history, synthesis, characterisation, development and current state of the art of neutral semiconducting and high-spin polymers, and their application in modern electronics.

Read this HOT Chem Soc Rev article today!

You wouldn’t think it, but mid-infrared (IR) optical sensing technologies and I have a lot in common. I know. Unlikely as it seems that I – a small, Scottish, Beyoncé-wannabe – can overlap with a spectral regime known for its ability to directly characterise the structure of molecular species with finger print specificity, it is true. We are both children of the 80s and we are both suffering from ‘middle (child) syndrome’. As such, our potential often goes unnoticed and we are often overlooked by our bookends (visible and near IR in mid IR’s case; the 25-year-old ‘baby’ of the family and the first-born in my case). In this recent Chem Soc Rev article, Boris Mizaikoff at the University of Ulm in Germany champions the case for mid-IR sensors.

Miziakoff discusses how and why the field of mid-IR sensing has matured more slowly than visible and near IR technologies, as it requires a variety of optical components and materials compatible with  its spectral range, ultimately making it more costly than near IR. However, mid-IR offers unrivalled advantages in the level of molecular specificity, and discrimination and quantification of the components of complex mixtures.

Leading us through the developments in mid-IR sensing devices, Mizaikoff demonstrates that widespread use of these technologies is not only possible, but could enhance many emerging scientific concepts, together with practical applications such as security and surveillance, the monitoring of water quality, as well as use in the medical field.

As Mizaikoff writes, the future is promising for this sensing concept with its potential impact on the fields of environmental analysis and bioanalytics.

Read this HOT Chem Soc Rev article now in full:

Waveguide-enhanced mid-infrared chem/bio sensors
Boris Mizaikoff
Chem. Soc. Rev., 2013, Advance Article
DOI: 10.1039/C3CS60173K, Review Article

Sarah Brown is a guest web-writer for Chem Soc Rev.  Sarah hung up her lab coat after finishing her PhD and post-doctorate in nanotechnology for diagnostics and therapeutics, and now works in scientific publishing. When not trying to explain science through ridiculous analogies, you can often find her crocheting, baking or climbing, but not all at once. All views are her own.

Some people complain that modern life is too focussed on looks; while this may be true we can not ignore the fact that we are visual creatures. However, we have four other senses (sound, touch, smell and taste) which we can exploit. This Chem Soc Rev review by Kenneth Suslick and colleagues from the University of Illinois at Urbana-Champaign and Tehran University of Medical Sciences investigates how the sense of smell has led to developments in optical sensor arrays.

When you smell freshly mown grass, you are not identifying every chemical species, rather the smell originates from recognition of the pattern of responses of several hundred receptors. This response is what you ultimately recognise as freshly mown grass. Rather than using olfactory receptors, it is possible to use chemoresponsive colorants to generate unique optical fingerprints of any odorant or mixture of odorants. Such an approach combines the benefits of the olfactory sensing system with our preference for a visual display.

This very readable review introduces the concept and advances in the use of cross-reactive sensor arrays which allow molecular recognition via recording of unique patterns of response. Such arrays can probe a variety of chemical properties, such as: hydrophilicity, solubility, redox, hydrogen bonding, Lewis donor-acceptor and proton acidity and basicity. The variety of chemical properties that can be probed allows impressive discrimination between similar analytes.

The optoelectronic nose response to a range of volatile organic compounds.

Cross-reactive sensor array patterns can be very complex in nature. This complexity requires the use of more sophisticated statistical analysis than most chemists are comfortable with. However, an overview of the three most common statistical approaches is provided which is both informative and readily digestible by the non-expert.

Having reviewed the fundamentals of the optoelectronic nose the authors turn their attention to a myriad of applications, from the detection of toxic industrial chemicals and explosives to foods and beverages. The final section highlights how the primary limitation of sensor arrays is also their primary strength– but you will have to read the review to find out what that is!

I would strongly recommend this review on the statistical analysis section alone but I can confidently say that there will be something here for everyone.

Read this HOT Chem Soc Rev Review in full now:
Optical sensor arrays for chemical sensing: the optoelectronic nose
Jon Askim, Morteza Mahmoudi and Kenneth Suslick
Chem. Soc. Rev., 2013, Advance article
DOI: 10.1039/C3CS60179J

Iain Larmour is a guest web writer for Chem Soc Rev. He has researched a wide variety of topics during his years in the lab including nanostructured surfaces for water repellency and developing nanoparticle systems for bioanalysis by surface enhanced optical spectroscopies. He currently works in science management with a focus on responses to climate change.  In his spare time he enjoys reading, photography, art and inventing.

Friedrich Wöhler’s early syntheses of oxalic acid and urea heralded the age of synthetic organic chemistry. These reactions demonstrated the potential for man to generate compounds that had previously only been obtained from the extraction of biological substances. Remarkably, despite huge advances in chemical synthesis, almost all natural products synthesised to date have relied upon similar apparatus and techniques to those used by Wöhler in the 1820s. Steve Ley and his group are among the pioneers of the use of flow chemistry in synthesis, and have demonstrated the use of machines in place of the antiquated round-bottomed flasks still used in chemistry labs the world over.

The number of sequential operations required in traditional approaches to making molecules can make synthesis time-consuming. In particular, downstream processes such as purification of the desired compound from waste products can take much longer than the actual reaction. Importantly, flow chemistry can also offer significant improvements to work health and safety as hazardous chemicals can be manipulated in a closed system and therefore, risks associated with exposure are reduced.

In flow chemistry (at its most basic), a reaction is performed in a continuous flowing stream where substrates and reagents are combined inside inert tubing and pumped around a coil of tubing before being quenched or treated with the chemical required for the next stage of the transformation.

Ley and coworkers have recently published a review that presents some highlights from the use of flow chemistry in natural product synthesis. One of the notable examples featured in this review is the continuous flow, semi-synthesis of artemisinin by Seeberger and Lévesque. Artemisinin is a sesquiterpene that represents the frontline treatment for plasmodium falciparum malaria when used in combination with other therapeutics. The supply of artemisinin from natural sources is problematic as is the scalability of existing synthetic approaches.

Dihydroartemisinic acid 2, (derived from artemesinic acid 1) represents the starting point for this flow synthesis and first undergoes photooxidation to yield hydroperoxide 3. Subsequent treatment of 3 with strong acid, followed by oxidation provided hydroperoxide 5, which underwent a spontaneous cycloaddition sequence, leading to the generation of artemisinin 6.

The use of a continuous flow reactor particularly enhanced the challenging photochemical transformations associated with the synthesis. Issues such as low mass transfer of oxygen gas into solution and low penetration of light were resolved by coiling the reaction tubing around a lamp to enabled effective generation of the singlet-oxygen required for the reaction. Additionally, improved mixing and temperature control could also be achieved. Crucially, this synthesis provides a low cost method to meet the escalating demand for artemisinin at affordable prices for patients in the developing world.

The elegant syntheses described in this review span a range of natural product classes and highlight the advantages that mechanisation of chemical processes can offer. As chemists seek to address medicinal and environmental challenges, perhaps greater emphasis should be placed on rational design rather than labour-intensive and repetitive tasks. The effective implementation of flow systems and technology could revolutionise the chemical sciences, and this review provides some exciting food for thought.

For more, read this HOT Chem Soc Rev article in full:

Flow chemistry syntheses of natural products
Julio C. Pastre, Duncan L. Browne and Steven V. Ley
Chem. Soc. Rev., 2013, Advance Article
DOI: 10.1039/C3CS60246J

Alice Williamson is a guest web-writer for Chem Soc Rev.  She is currently a postdoc for the OSDDMalaria Project in Dr. Matthew H Todd’s group at the University of Sydney.