The Haber process – well where do I start – it’s probably the most important industrial process of the last century and I could list what the syllabus requires us to impart to our students, that’ll be equations (don’t forget reversible arrows), conditions, uses of ammonia ….. but really how are we ever going to convert the masses when you literally watch them fall asleep before your eyes – so what’s a chemistry teacher to do?
Open the classroom door and bring in Fritz Haber – the man who got the Nobel prize in 1918 for the process. But hold on, why is there only fleeting acknowledgement to him on the Nobel website? Well, that’s because Fritz Haber was a very complicated individual. We’d like to think that Fritz spent his life pondering the importance of fixing nitrogen to allow us to grow more crops and feed the growing population, but no – Haber was too busy developing chemical weapons in WW1 including the use of the chemicals chlorine and bromine. After the war he tried to raise funds to pay off the German reparation payments by unsuccessfully attempting to extract gold from seawater.
Haber was a patriotic German, to the extent that his wife Clara’s opposition to his research into chemical weapons and her subsequent suicide did not sway his views. However, Fritz Haber was also a Jew and this began to play an important part in his destiny with the rise of Nazism in the 1930’s. Haber’s self confidence and ambition began to falter as he was singled out for his Jewish heritage. He had to leave Germany and was able to secure a post at Cambridge university but here he was given the cold shoulder by other eminent scientists unhappy with his role in Germany in WW1. Now it all could have ended there with Haber heading off to a research institute in Palestine where he passed away quietly en route in 1934. However, Fritz Haber’s legacy chillingly did not to end with his death. It turns out that some of the research he had carried out earlier in his career was developed by the Nazis to produce the deadly gas Zyklon B which was to be used in the gas chambers. It is thought that some of Haber’s relatives died in the concentration camps- possibly in gas chambers.
This is one of the posts that I had thought about when I initially decided to blog but I have put it on the back burner for a while. Why? Well, the story of Fritz Haber made me think of the role of scientists in today’s society and especially if in a country at war or considering it. Was Fritz Haber’s ambition and patriotism so unusual and are all scientists morally upstanding ? What happens if you pick the wrong side, are you consigned to a damning legacy ? I still don’t know the answer and maybe Fritz Haber’s legacy is that he makes scientists feel uncomfortable.
Well, what’s a hydrogen bond I hear you ask – who cares is probably what most people are secretly muttering under their breath. But yet again, it’s a fundamental chemical phenomenon that allows life on Earth as we know it – no hydrogen bonds, no liquid water, need I go on! Firstly let’s look at the definition and diagram of a hydrogen bond – a hydrogen bond is the electrostatic attraction between polar groups that occurs when a hydrogen (H) atom bound to a highly electronegative atom such as nitrogen (N), oxygen (O) or fluorine (F) experiences attraction to some other nearby highly electronegative atom.
Nowadays it seems like an obvious phenomenon when we look at the higher than predicted boiling points for certain hydrides but the acceptance of their presence seems to have been a slow process for the chemical community. Initial investigations seem to have taken place in the Lewis (that’s G N Lewis who defined an acid as an electron pair acceptor) research group and first publication by Latimer and Rodebush dates back to 1920. Of interest it was believed that Lewis had tried to encourage the scientists in his team to take the section on this hydrogen bond out of the paper before publication. I found an excellent paper about the history of the acceptance of the term hydrogen bond which ran for over twenty years and have put the link below. By the end of the 1930’s Linus Pauling mentioned the hydrogen bond in his seminal book ‘The Nature of the Chemical Bond’ and so began the journey identifying hydrogen bonds as the important interactions that give us the three dimensional biological molecules that are the building blocks of life.
History of Hydrogen Bond
Proof that these bonds exist has been confirmed by crystallographic studies and it was Pauling who was at the forefront of this work. Crystallography uses x-rays to study the structure of molecules and how atoms are bonded together. In 1948 Pauling was studying the three dimensional structure of proteins and he identified hydrogen bonds as holding the polypeptide chains in an alpha helix. This discovery was considered the dawn of molecular biology and is thought to have contributed to Watson and Crick’s discovery of the structure of DNA.
So where do we find hydrogen bonds? DNA, proteins, water, ethanol, polymers and the list goes on! And this week an entirely new class of hydrogen bond that forms between a boron–hydrogen group and the aromatic, π-electron system of a benzene ring has been reported. Interestingly this work has been completed by both theoretical and synthetic chemists and may have applications in boron based drugs. Recent publications show that we are still identifying the presence of traditional hydrogen bonds in molecule interactions and also discovering new types such as that just reported. The humble hydrogen bond is the glue that gives us Nature’s building blocks – it’s much more than just a chemical bond !
Anyone who has a drive to work knows the frustration of morning travel and wasted time but recently I’ve discovered a way to avoid the boredom – podcasts ! In particular the Radio 4 In Our Time Science series by Melvyn Bragg and it was the photosynthesis programme that got me thinking about chlorophyll. I’ve never really given the chemical process of photosynthesis much thought as it is firmly rooted in the biology curriculum but all that changed last week.
The combination of carbon dioxide and water to make glucose and oxygen is no mean feat and the elegant structure of the chlorophyll molecule allows for the absorption of light to drive the reaction. So what is chlorophyll – it has a porphyrin ring structure attached to a protein backbone. The porphyrin is built up of pyrrole molecules – 5 membered aromatic rings which are made of four carbons and one nitrogen atom. This ring system acts as a polydentate ligand and has a magnesium cation at its centre. It is delocalisation of electrons around this aromatic porphyrin system that allows the absorption of light that kick starts the photosynthetic process allowing the production of the energy-storage and transport molecules ATP and NADPH. Off interest is that chlorophyll absorbs light in the blue and red regions of the electromagnetic spectrum reflecting the mid region hence plants are green! You might also notice the similarities of the structure of haemaglobin and chlorophyll, both having porphyrin ring structures. You can not help but be transfixed by these amazing natural macromolecules which are not easy to make synthetically. A few months ago a guest post centred around the great organic chemist R B Woodward and one of the natural products he made was chlorophyll.
So probably the more important question is how do carbon dioxide and water react to produce an organic molecule? It’s obvious that it’s not a straightforward process and something that we don’t see happening without specific conditions. Water is in a sense acting as a fuel adding electrons to the carbon dioxide to allow it to grow into an organic molecule. The podcast mentioned that it might not have been water originally but iron or hydrogen sulfide acting as the electron donors. During this process water is oxidised forming oxygen, hydrogen ions and electrons 2H2O –> 4H+ + O2 + 4e- (photolysis). The conversion of carbon dioxide to glucose can be followed using the Calvin cycle. Carboxylate/oxygenase enzymes catalyse the ‘carbon dioxide fixing’ allowing it to combine with a five-carbon sugar, ribulose1,5-biphosphate. This intermediate quickly breaks down to give molecules with three carbons ( 3-phosphoglyceric acid ) which go on to build glucose.
Now anyone who knows me knows that this foray into biology is not a comfortable one (that’s my disclaimer!). It’s a pity that chlorophyll doesn’t get a mention on the A2 chemistry spec when we are studying polydentate ligand systems – surely this is the ultimate complex without which the World would be a truly different place! Oh and I loved the fact that when the historical biochemist talked about the evolutionary photosynthetic process, oxygen was originally considered a toxic (reactive) product – how things have changed !