A watched pot never boils… how to monitor reactions the easy way!

A watched pot never boils… how to monitor reactions the easy way!

When monitoring reaction progress for determination of reaction kinetic parameters, NMR spectroscopy has increasingly become the method of choice. The ease in which one can calculate the concentration changes of a substrate being consumed or a product being formed over time, directly from peak integration are the reason behind this.

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Life is sweet…maybe too sweet!

Life is sweet…maybe too sweet!

Sugar substitutes are gaining more and more relevance due to the heath problems associated with the consumption of high amounts of sugar. Specifically, excess sugar consumption has been associated with obesity, type II diabetes, cardiovascular disease, certain cancers, and tooth decay. In the United States, it is estimated that the average person consumes more than 126 grams of sugar per day, in Canada around 89 grams per day.

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The Dangers of Making Too Many Assumptions. Electronegativity, Acidity, and Chemical Shift

The Dangers of Making Too Many Assumptions. Electronegativity, Acidity, and Chemical Shift

Last month (which you can see here), we learned about how an acidic proton behaves in a 1H NMR experiment, particularly when it’s surrounded by D2O. For example, when an H+ leaves CH3COOH to join an accommodating D2O molecule, the resulting acetate (H3CCOO–) segment is reasonably comfortable bearing that negative charge. This phenomenon is the reason the solution is “acidic” in the first place. But why is acetate so capable of dealing with this negative electronic charge?

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To Decouple or Not to Decouple, That is the Question. Also….Acids.

To Decouple or Not to Decouple, That is the Question.  Also….Acids.

Acidity is something that you encounter on a daily basis, probably without even realizing it. The tangy taste of an orange (citric acid), that Vitamin C tablet you took this morning (ascorbic acid), those terrible jeans from the 80’s that you still wear (acid wash). My favourite acid is acetic acid.

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Electronegativity and Chemical Shift

Electronegativity is a concept that we have to study and understand early in our science or engineering degrees. In chemistry this concept stays with us and we commonly invoke it in order to explain multiple phenomena what we observe or study…and of course, NMR is not the exception!

Electronegativity is defined as the ability of an atom in a molecule to attract electrons to itself.[1] The first, and probably most commonly used, scale to measure electronegativity was developed by Linus Pauling. In this scale fluorine is the most electronegative element with an electronegativity of 4.0 and cesium is the least electronegative element with an electronegativity of 0.7. In the periodic table the electronegativity generally increases from left to right across a period and decreases from top to bottom down a group (Figure 1).

Figure 1 [ref 2]

Figure 1 [ref 2]

In proton NMR spectroscopy (this also applies for other nuclei) the chemical shift of a particular C-H system depends on the electronic environment around the proton. Electrons in the neighboring atoms could reduce the field experienced by the proton, shielding it from the external magnetic field and moving the signal to lower ppm (or upfield). On the contrary, electronegative groups close to the C-H system reduce the surrounding electron density, deshielding the proton from the external magnetic field and moving the signal to higher ppm (downfield). Do you get confused when somebody uses the downfield and upfield terminology?...you are not alone! They are confusing terms because they seem opposite of what we would expect. This terminology is not strictly correct in pulse Fourier transform (FT) NMR instruments because the magnetic field remains unchanged, but somehow it has been passed from the old days of NMR. When Fred Flintstone and Barney Rubble used the original NMR instruments (continuous wave), the magnetic field strength was increased from left to right at constant frequency. Thus, the resonances on the right were referred as upfield signals and on the left as downfield signals. Technically both terms are obsolete and have been replaced by shielding and deshielding respectively.

Hahahaha that was me trying to make a joke….Use this sentence “In the continuous wave (CW) method of measuring NMR spectra, the magnetic field strength was increased from left to right at constant frequency” instead of the Fred Flintstone one.

In a very easy to do experiment reported by Everest and Vargason[3] we can experimentally observe these electronegativity trends. In Figure 2A we see how the protons closer to X (in red) are sequentially shielded as we move down the group due to the decrease in electronegativity from fluorine to iodine. On the other hand, as we move across the period from carbon to fluorine, the protons in red are deshielded (Figure 2B) due to the increase in electronegativity. Keep in mind that the signal of interest in 1-fluoropentane appears a distorted doublet of triplets centered at 4.4 ppm due to coupling with the fluorine center. As you can see in these spectra, at 60 MHz the resolution of the spectra is lower compared to a high-field instrument. However, you can also see that the results that we got in this experiments are perfectly fine at 60 MHz. If you navigate through our website you will discover that in most cases the NMReady will give you the information that you are looking for and there is no need to need to use an expensive high-field instrument!

Figure 2: Stacked 1H NMR spectra showing change in chemical shift of alpha methylene.

Figure 2: Stacked 1H NMR spectra showing change in chemical shift of alpha methylene.

The beauty of this experiment is that it can be conducted in the first semester of General Chemistry or in second year Organic Chemistry. In the first case, the experiment can be used to emphasize the concepts of electronegativity and electron clouds in a guided-inquiry approach without the need of previous knowledge about NMR spectroscopy. Furthermore, the use of the NMReady allows the students to just touch the screen to get the spectra without having to worry about selecting parameters or shimming samples. In the second case, this experiment can be used as one of the first experiments in an NMR spectroscopy lab in order to experimentally discover and reinforce some of the important concepts in NMR spectroscopy.

References
1 - Modern Physical Organic Chemistry University Science Books, 2007.
2 - Carr, D. (2016, January 26). Periodicity (1) Ionisation energy and electronegativity of the elements. Retrieved from https://derekcarrsavvy-chemist.blogspot.ca/2016/01/periodicity-1-ionisation-energy-and.html
3 - Everest, M. A.; Vargason J. M. Journal of Chemical Education 2013, 90, 926.

Boron NMR Spectroscopy

Boron NMR Spectroscopy

For my first blog I have decided to talk a little bit about boron because it’s a fascinating element that imparts remarkable properties to its compounds. And believe me, there are hundred of different compounds with very different properties containing boron centers. We find boron in compounds like boronic acids (A), boronic esters (B), and MIDA boronates (C), all of which are very popular in Suzuki coupling reactions to make new C–C bonds.  Another popular family of compounds containing boron are the BODIPY dyes (D). These derivatives, composed of a dipyrromethene ligand supporting a BF2 unit, are used as stable functional dyes in several fields, but their application as biomolecular labels is the most investigated by far.

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#Worldbookday

I don’t know if you check out Compound Interest (@compoundchem) or not, but if you don’t you should probably consider remedying that IMMEDIATELY!  Pretty much every single time that I read it I have a “HMMMM, OHHHH, Cool! I had no idea!” moment.  Today, was no exception. They have interesting real world chemistry examples of …

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