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Pyridine

Now pyridine is an important molecule, I happen to rather like pyridine and its derivatives. At several points in my life as a chemist I have used pyridine derivatives to do things.

While in John Plater’s lab in Aberdeen one of my favorite reagents for restricting the cross linking in a coordination polymer was to use 2,2′-bipyridine while when working in the group of the late Mike Hudson I used 2,2′-bipyridine as the core of the BTBP solvent extraction reagents. The synthesis for the BTBPs starts with 2,2′-bipyridine.

Now pyridine can be regarded as azabenzene, it is a benzene molecule in which one of the CH units has been replaced with a nitrogen atom.

The nitrogen lone pair points out into space at 120 degrees away from the two carbon nitrogen bonds in pyridine, it is on the same plane as all the atoms in the pyridine molecule are.

Pyridine is aromatic as it has 4n+2 pi electrons in a planar (flat) ring. As nitrogen is more electronegative than carbon the pi system of pyridine is more electron poor than that of benzene. The effect of the pyridine nitrogen is about the same as a nitro group attached to a benzene ring.

Much of the chemistry of pyridine is dominated by the lone pair on the nitrogen, this enables the pyridine to act as a base and to bind to metals. I am sure however with the d-block metals that pyridine is able to act as a pi-acid which will increase its ability to bind to metals. Some of the oligopyridines such as 2,2′-bipyridine are able to chelate to metals, it is interesting that Constable has made very large oligopyridines and done some interesting chemistry with them.

With iron(II) the chelating oligopyridines are able to form very dark red complexes, I have heard horror stories from people in nuclear reprocessing research community about the use of BTP in machines made from stainless steel. We will get onto that in a moment. First it is interesting that if a person holds a gun in their hand then they can deposit iron onto the skin of their hands. Depending on good your skin is at adsorbing iron from a gun, how long you hold the gun and a range of other things it can be possible to see shape of the steel parts of the gun which was held in the hand if the hand is treated with 3-(2-pyridyl)-5,6-diphenyl-1,2,4-triazine. While funny shapes caused by holding iron does not mean beyond all reasonable doubt that a person has held a gun recently, it may help the investigation of crimes which involve guns.

Many of the BTPs have such a strong ability to form iron(II) complexes that their solutions can strip iron from stainless steels such as SS316L. In case you do not know what BTP is here is a diagram of it.

Here is a picture of the iron(II) complex of the parent BTP (R = H), you can see how the iron is binding to six nitrogen atoms.

BTP is an example of a molecule which has both azabenzene (pyridine) and triazabenzene (1,2,4-triazine) rings. The BTPs are able to extract americium(III) from nitric acid. The americium is extracted as a tris-BTP complex with nitrate anions as counterions which enables it to enter a solvent such as octanol. The steric demands of the BTP when it binds to the americium are smaller than those of a terpyridine also the BTP is a weaker Bronsted base. This reduction in Bronsted base strength enables the BTP to extract from rather strong nitric acid solutions. The parent terpyridine is a far weaker extraction agent which requires a carboxylic acid such as 2-bromodecanoic acid to be added and this system only will function at very low nitric acid concentrations. Here is 2,2′:6′,2”-terpyridne.

An example of its use as a base is in the synthesis of O-phenyl phenylphosphonochloridothioate. When I was working in Josef Novosad’s lab in the Czech Republic I wanted to make a phosphorus compound which was chiral at the a phosphorus atom. While I had originally had the idea of reacting phenylphosphonothioic dichloride with an excess of methyl magnesium iodide to form 1,2-dimethyl-1,2-diphenyldiphosphane 1,2-disulfide which could be cleaved with bromine to form methyl(phenyl)phosphinothioic bromide we found it was more convenient to react a mixture of phenol and pyridine with phenylphosphonothioic dichloride in diethyl ether. The pyridine reacted with the hydrogen chloride formed by the reaction of the phenol with the phenylphosphonothioic dichloride.

By the way pyridine has a horrible smell and there are wild stories about the effects of pyridine on male sexual health. I think that the stories about pyridine are a bunch of “urban myths“. But when I was an undergraduate I heard jokes about the subject. When I first used it there was a strange smell in the lab, I asked the PhD student (It was a lady) who was supervising us in the second year inorganic lab course what the smell was. She replied with “It will do more to you than it will do to me, and then stated it was pyridine”.

All very funny, but it is purely a myth. I have seen a review of the data from an experiment in which rats were fed pyridine in their drinking water. In rats pyridine does not have a harmful effect on the sexual health of the rats. There is a slight effect in female rats but it appears to be an artifact.

Pyridine will not make men go sterile. However if you contaminate yourself with pyridine it is likely to impede the process of someone else falling in love with you. I think that the stench of pyridine would discourage most people. If your clothing is contaminated with pyridine, I suggest you remove it and wash any contaminated parts of your body.

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Mechanistic eyes

Many things in the world which might at first appear to be unrelated can suddenly become related when you consider them in a mechanistic manner. For example alpha decay can be regarded as a helium nuclei escaping from an atomic nucleus by quantum tunneling. It is possible to observe other clusters of nucleons being emitted from some radioactive nuclei. This is known as “cluster emission”, I would argue that all cluster emission could be the same reaction in mechanistic terms.

If we consider the following two nuclear reactions, if we can imagine the carbon-12 nuclei knocking around inside the barium-112 nuclei trying to escape in the same way as alpha particles are thought to form inside the nuclei of alpha emitters before knocking around for a while. They either escape by quantum tunneling or the fall apart and the quarks rejoin the mass of quarks in the alpha active nuclei.

112Ba → 12C + 100Sn

224Ra → 4He + 220Rn

Equally often in organic chemistry we get the same mechanism dressed up differently, consider for a moment the reaction of benzyne, Lawesson’s reagent and maleic anhydride with a 1,3-diene. In all these cases this can be explained as a Diels-Alder reaction.

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Gas chromatography III

OK you have either had a good laugh at the idea of doing the waltz in a day glow rave suit or you were not up for such an idea. But now we have to come back to earth. We need to consider the question of what alters K in gas chromatography.

While do some molecules travel more quickly than others along the column. In general the bigger a molecule the slower it travels in the column, this is due to the fact that it has more surface area for the London forces to bind it weakly to the column coating.

There are other things to consider,

If we choose a column coating such as ZB-5MS column from Zebron, this has a 95 % dimethylpolysiloxane coating where the remaining 5 % of the polymer is aromatic groups.

This is a short section of a dimethylpolysiloxane chain.

One option for a 5 type column which has 5 % aromatic groups is to include some diphenylsiloxane units in the polymer chain. A pure polydiphenylsiloxane would look like this.

While a ZB-1 column has a pure dimethylpolysiloxane coating. This difference will greatly alter the behavior of aromatic compounds in the column. The difference is that the aromatics like benzene and the larger hydrocarbons such as naphthalene and anthracene interact with the benzene rings in the coating. Through attractive pi-pi interactions they bind more strongly than a molecule made of CH2 units (such as an alkane) would.

There are other column coatings such as the 35 % -Phenyl- 65 %-Dimethylpolysiloxane used in the ZB-35 columns.

There are more polar columns such as the ZB-23 which has a coating which is 50 % cyanopropyl methyl, 50 % dimethyl polysiloxane. This one is designed to bind to polar molecules. One of the most polar column coatings which is in regular use is the wax (Carbowax) formed by the polymerization of ethylene oxide. This is a long polyether, here is a short part of the polymer chain.

There is a general rule, “like dissolves like”. A polar solvent is more able to dissolve polar compounds than non polar compounds. So if we use a carbowax column then will give longer retention times for the polar compounds. It will also be better for this. For example there is a method for the measurement of ethyl acetate, methanol, propanol and isobutanol in alcoholic drinks. In this method a filtered sample of drink is injected into a GC fitted with a flame detector.

The boiling point of methanol is 65 oC while the boiling point of ethyl acetate is 77 oC. While in general we expect the more volatile things to come out first in with the carbowax column the ethyl acetate comes out before the methanol. This is clear evidence that the polar group in the methanol (hydroxyl group) is attracted to the polar carbowax. It is also worth noting that the boiling point of ethanol is 78 oC, the retention time of the ethanol in that column is about 3.8 minutes while the ethyl acetate has a retention time of about 3.1 minutes. The retention time of the methanol in that column was 3.2 minutes. So it is clear that the polar compounds are being retained.

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Gas Chromatography II

OK welcome back, we need to think of what can alter beta and K. Beta is easy to change if you are willing to buy a new column, it is a parameter of the column, by making the coating on the inside of the column thicker or thinner it can be changed.

The value of K is a bit more complex, K is a measure of how the thing distributes between the gas and the coating. One way to alter K is to change the temperature. In general raise the temperature and K becomes smaller. This makes things move more quickly along the column, a wide range of things can be made to travel down a GC column.

If you want to cut to the chase then click here, otherwise for some fun continue to read.

One of the ways which the UN uses to monitor for clandestine nuclear bomb tests is to take large air samples from which the xenon is separated. There are automated radiochemical labs which suck vast air samples from which are separated the noble gases, one method for doing the measurement is to separate the radon, xenon and krypton from the air. By using gas chromatography the xenon in the sample can be separated from the radon and krypton before being measured in a ultra low background radiation detector. If some short-lived isotopes of xenon appear in the air sample it suggests that someone has performed a fission event and shortly afterwards allowed the noble gases to escape into the air. In the event of a bomb detonation in the air the xenon enters the air instantly while if it is underground the xenon will diffuse out through the cracks in the ground. If you compare the ratio of the different radioactive noble gases then it is possible to make an estimate of the time of the fission event. This would help the people with seismographs. Rather than telling then to review two weeks worth of data you could narrow down the event to a few hours. This would then allow them to look in a more narrow time window for a suspicious seismic event.

Keep in mind that seismic events include earth quakes, minor tremors, conventional explosive detonations (like those used in oil / gas prospecting plus mining), traffic whizzing around near the seismograph and also that late night folk dancing party in a nearby building. I would love to know how far away the test ban seismograph could detect me doing the St Bernard’s Waltz. I suspect that of the standard English and Scottish Ceilidh dances I have done that this will have one of the greater seismic significances as at one moment in the dance cycle you stamp your feet twice.

For those of you who do not know it, here it is. I think that for a greatest effect you would have to do it in Stonehaven town hall or some other dance venue which has a solid stone floor. This would give the optimum coupling to the rocks below the building.

I have joked that it would be funny to write rather slow trace music in 3/4 time rather than the 2/4 time which it is normally written in. If someone was to do this then it would be funny to see what would happen in a trace nightclub when the people were suddenly exposed to 3/4 time. What would they do with the extra beat in a bar ? Would they stop dancing confused, would they lose synchronization with the music (if they were the sort of person who keeps a constant phase relationship between the bars of music and the movements of their body). One suggestion would be to get a man and a woman dressed in the most extreme dayglow clothing and film them doing the waltz to the trace 3/4 music and then show the clubbers this film while playing 3/4 trance.

A raver I know has suggested using dayglow rave suits from Cyberdog, but that suggestion came about 15 years ago and I have no idea what the trace crowd view as the correct attire to wear while dancing.

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Gas Chromatography

One of the great tools of modern chemistry is chromatography as a means of following a reaction. One can think of a gas chromatography machine as a long tube with a coating on the inside. Different molecules have different affinities to the coating, the greater the affinity the slower the molecule moves in the gas flowing through the tube.

We have a thermodynamic partition coefficient (K) which when divided by the phase ratio of the column (Beta) gives us k which is the retention factor for the column.

According to one paper (T.M. McGinitie and J.J. Harynuk, Journal of Chromatography A, 2012, 1255, 184-189.) K can be predicted using an equation with A, B and C in it.

K = exp (A-{B(1/T)}+ {C ln T})

Where

A = {Delta S – (Delta Cp ln To) – Delta Cp}/R

B = {Delta H To – Delta Cp To}/R

C = Delta Cp / R

Where To is a reference temperature, this is 90 oC.

The speed at which the analyte moves along the column is given by

dx/dt = ux {1/(1+k)}

Where ux is the speed at which the carrier gas moves along the column. If we keep this constant we can consider what happens in a column if we have several things in it.

If Delta H, Delta S and Delta Cp are -36.7 kJ mol-1, -56.7 JK-1 mol-1 and 75.9 JK-1 mol-1 for undecane and these values for tetradecane are -47.8 kJ mol-1, -72.3 JK-1 mol-1 and 92.8 JK-1 mol-1. Then at 50 oC we can calculate their speeds through the column if the carrier gas moves at 20 ms-1. We assume that beta is equal to 1000000 and that the column is 20 meters long.

Then the the undecane will come out after 5.35 minutes and the tetradecane after 132.5 minutes. This would require us to run less than one GC sample every two hours. If we were to heat up the column to 100 oC then the udecane will come out in less than one minute while the tetrdecane will require 11.3 minutes to come out. We would be stuck with a GC which is only able to measure one of the two alkanes if we insist on using it in the isothermal mode.

But there is hope, as I listen to Eyna (So I could find my way) I know how the GC can produce a result as beautiful as her angelic voice. The answer is to use a temperature ramp. If we were to inject the alkanes with the column at 40 oC and hold it there for four minutes then the acetone we dissolved our sample in should fly through the column at a great speed and disappear. After the four minutes we switch on the electron gun on the mass spectrometer, by keeping it off while the solvent is clearing out of the column we help to reduce the wear on the mass spectrometer.

By my calculations if we heat at 5 oC min-1 then after 7.2 minutes the undecane will come out the end of our column. At this moment the column will be at 56 oC. The tetradecane will have only travelled about 74 cm along the column at this point. I predict that 20.4 minutes after starting the experiment the tetradecane will be eluted from the column. While we will need about 30 minutes per sample it is a great improvement in about two hours per sample.

If we ramp the column at 10 oC min-1, then we can shorten the analysis time. Here I predict the undecane will come out at 6.4 minutes while the tetradecane will come out at 14 minutes. We could now run the GC much faster in terms of the number of samples per day which we examine.

Now while this is all very well we might ask ourselves the question of what alters K and beta. To consider this question click here.

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Working out a chemical structure part one

A common problem in chemistry is to try to work out the structure of a molecule. If we are lucky and are able to either use mass spectroscopy or wet chemical measurements to determine the molecular formula then one thing we can do is to work out possible structures which match the formula.

If we take as an example C7H10O then we can go through the process. The first thing to do is to consider the number of hydrogen atoms that the straight chain alkane with the same number of carbons would have.

For an alkane with n carbons, it will have 2n+2 hydrogens. Thus for heptane we will have 16 hydrogen atoms.

If we had a heptene then the molecule would have one double bond in it and a total of 14 hydrogens. So for our molecule C7H10O we can understand that three double bond equivalents (DBE). A double bond equivalent is an accounting term in chemistry for a structural feature which reduces the number of hydrogens in a molecule by two.

A ring with no double bonds in it is worth 1 DBE

An acetylene triple bond is worth 2 DBE

A benzene ring is worth a staggering 4 DBE

An ether is worth zero (0) DBEs

An alcohol is worth zero (0) DBEs

A ketone or aldehyde is worth one DBE

So when we do the math it is clear that our molecule has three double bond equivalents. We have been give a clue that it has no carbon-carbon double or triple bonds. We have too few DBE for a benzene ring.

If we assume that the oxygen is part of an alcohol then we can dream up a series of tricyclic structures using our seven carbons. I can come up with some possible alcohols based on two tricyclic hydrocarbons.

There is another hydrocarbon which is so super strained that I think it is a crazy molecule. It would be tricyclo[2.1.1.11,3]heptane. This is would have such high ring strain that I discounted it.

If we assume that the oxygen is part of an ether group then we can consider some possible ethers, I was able to quickly come up with nine ethers. Here are these ethers.

We know that the compound is a ketone so we can discount all the alcohols and ethers which I have drawn so far. We have 1 DBE from the ketone group so we have two more and we have the constraint that we have no C-C multiple bonds present. I then came up with eight different ketones.

What we have to do next is to try to narrow down the selection of ketones. What I would do if it was a real life problem I would want to get hold of a sample of the ketone. The first thing I would do is to record an infra-red spectrum of the ketone. The reason is that ketones which have the carbonyl carbon in a small ring tend to have high carbonyl C=O stretching frequencies. (Cyclopropanones at about 1816 cm-1 and cyclobutanones at about 1780 cm-1). This would allow me to rule out many of the possible ketones.

While the first year Chalmers student does not need to know about infra red spectroscopy it is a very valuable method which the basics of which are explained here.

In our example we have a carbonyl C=O stretching frequency of 1750 cm-1 so the following two ketones are possible.

My next act would be to run a carbon-13 NMR spectrum of the ketone. the first thing I would do it to count the number of lines in the spectrum. One of the candidates will have only three different carbon environments while two will have four environments and the remaining five have seven carbon environments. This simple counting of carbon lines might resolve our problem. But be careful if you are unlucky you can have lines which overlap in the carbon spectrum. While carbon NMR spectra tend to be less congested than proton spectra it is still possible.

One of the two ketones which fit the conditions in our question would have the following carbon-13 spectrum.

While the other one would have this spectrum.

Now we need to consider if two or more of the carbons in the molecule are equivalent. We can consider the ketone for a moment. Lets look at the molecule. For clarity we will only look at the non hydrogen atoms.

Now with the hydrogen atoms it looks like this

You should be able to imagine how the mirror image of this ketone can be superimposed onto the original molecule. The two CH units are related by symmetry (a mirror plane going through the carbonyl carbon, the carbonyl oxygen and a point half way between the CH2 groups on the left along with the point half way between the CH2 groups on the right hand side of the molecule. As a result the molecule only contains three different carbon environments.

If we move the ketone group to the other possible site then we get a much less symmetric molecule. This molecule is chiral, it is impossible to superimpose the mirror image of it onto itself. I made a model of the molecule in which I represented the whole of the carbonyl group with a red atom. I made both stereoisomers. I photographed them in front of a mirror.

Molecule A is one of the two stereoisomers, its mirror image (B) can not be superimposed onto A no matter how much you rotate the molecule. But reflection B can be superimposed onto molecule C (The other stereoisomer). The reflection of C (which is D) can be superimposed onto A. This is a perfect example of what is sometimes described in text books.

As molecule A has no mirror plans or Cn axis of rotation all the carbons in it are unique. As a result the carbon-13 spectrum is far more complex.

At the same time as I would go for a carbon-13 NMR I would want to record the proton NMR spectrum of the compound. This is due to the fact that the hydrogens in a cyclopropane ring are very distinctive, they come at a very low chemical shirt relative to tetramethyl silane. Also the shift for a cyclobutane hydrogen in the proton NMR is unusually high for a cycloalkane. This proton NMR experiment would allow me to narrow the search to three or less ketones almost the instant I look at the spectrum.

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Attack on the carbonyl group (3.6b)

attack on acetone

Now there are two reactions which are related, there is the nucleophilic attack on the carbonyl group which is followed by the loss of a leaving group from the carbonyl group to form a different carbonyl. There is also the addition of a nucleophile to the carbonyl group to form a sp3 carbon. This sp3 carbon might or might not do something further.

Now to get our minds warmed up I want to set you a quick fun challenge.

Q: Devise molecules which fit the following descriptions and limitations.

a. A ketone with no carbon-carbon double or triple bonds with the formula C7H10O

b. A ketone (C16H32O2), with an alcohol group on the carbon next to the carbonyl which contains two butyl (CH2CH2CH2CH3) groups.

c. A ketone (C22H28O2), with a nonyl (C9H19) group and an alcohol group.

d. A ketone found in fruit (C10H12O2) which contains a hydroxyl group.

If you are stuck then here is some advice for dealing with such problems. Please do not just go and look for the answers, you will learn very little. It is better than you make an effort to solve the problems yourself before looking at the answers.

There is a saying (the origin is unclear) which is

“You give a poor man a fish and you feed him for a day. You teach him to fish and you give him an occupation that will feed him for a lifetime.”

I would say

“Give the student the answer to a question and you keep them happy for a few minutes, teach them to solve that type of questions and you give them a skill which lasts a lifetime”

Q: You work at a chemical company, your supervisor slept through their organic chemistry lessons. They need you to suggest how to make ketones and aldehydes from other things.

a. How would you make menthone from menthol

Menthol on the left and on the right menthone

b. E isomer of 3,7-dimethyl-2,6-octadienal (Geranial) from Geranic acid. These two molecules are shown below.

Here is the answer.

There are other methods for making ketones and aldehydes, while an alcohol is an attractive starting material. There are times when other things such as alkenes (olefins), esters, phenols, esters and other things are attractive starting materials for ketones and aldehydes. The thing in life is that we cannot fix every problem with the same tool. In my tool store I have a range of tools, for example I used my crowbar for lifting a washing machine an inch to allow me to slide a strap under it. While I used a hammer to knock the nails into a plank needed to stop my Christmas lights from falling over (again).

Equally in organic chemistry we need different tools for different problems, sometimes what we need is a tool which only acts on part of a molecule. Now there is a nice way to make 2-hydroxyketones which uses an aromatic aldehyde as a starting material. One key area of organic chemistry is the synthesis of molecules, it is often best to be able to assemble a molecule with the fewest synthetic acts (reactions). I like the benzoin condensation as a means of making 1,2-diaryl 2-hydroxy-ethane-1-ones as we can assemble the molecule from two equal sized parts. It is a reaction where two molecules of the same aldehyde are used.

I boiled up water (50 ml), ethanol (50 ml) and benzaldehyde (50 ml) with potassium cyanide (2 grams). There was nothing special to look at until I allowed the mixture to cool and stand. While all three of the main items added to the flask are liquids the volume of two grams of potassium cyanide is small. When I allowed the mixture to cool and stand the flask was filled with a white solid. This was mixed with an orange liquid, after giving the flask a mighty shake I tipped out the slurry onto a filter. I then washed the solid with ethanol a few times on the filter to get an off white solid. Here is a photograph of the solid. Note that there are some parts of the solid which are more yellow than others.

While the change of the physical properties are very telling, the best evidence we can get for the change comes from gas chromatography mass spectroscopy. In the new year I will post the GCMS traces of the product and the starting material. For an explanation of GCMS please click here.

Now I know that cyanide has a dire sinister reputation but it is a useful reagent in the lab. I just pick and choose who I trust to use the stuff.

I blame the reputation on Agatha Christie and films like “Murder most Foul” where her detective investigated a series of murders. While sometimes the murderer uses items like hat pins (Murder at the Gallop) it seemed that more often than not it was poison. Also it has been used many times to commit suicide (For example Eva Braun, Göring and Alan Turing). Also the notorious poisoner (Graham Young) is reputed to have murdered another patient (John Berridge) while locked up in the high security psychiatric hospital using cyanide.

The cyanide acts as a catalyst, it reacts with the aldehyde group to form an addition product. By proton transfer we form a new nucleophile which then attacks another molecule of the aldehyde. Then after some more proton transfer the cyanide is lost, the key thing about this reaction is that the cyanide acts both as a nucleophile and a leaving group.

In the first plate we show the attack of the cyanide anion on the benzaldehyde molecule, followed by proton transfer to for the new nucleophile. This nucleophile is an anion which is stabilized by resonance. I have drawn only two resonance forms but I believe that the negative charge is partly accommodated inside the benzene ring as a result of resonance.

Next the new nucleophile attacks a second molecule of benzaldehyde to form the new C-C bond.

Next after proton transfer the negative charge is moved to the top oxygen again, the molecule now reforms the C=O double bond and the cyanide anion is released. It will then go off and react with some other benzaldehyde molecules thus forming more protect.

Q: Acetone cyanohydrin was used in some plants for the synthesis of the methacrylic acid needed for plastics. Explain how 2-cyano-2-hydroxy propane is formed from acetone and hydrogen cyanide.

On the left acetone and on the right acetone cyanohydrin

Q: You have a supply of phenyl magnesium bromide in diethyl ether, choose reagents to react with this Grignard to form

a. Benzophenone (Diphenyl ketone)

Benzophenone

b. Triphenyl methyl alcohol

Triphenyl methanol also known as trityl alcohol

c. Diphenyl(p-tolyl)methanol

Diphenyl para-methylphenyl methanol
1-Phenyl-2,2-dimethyl propan-1-ol

d. 2,2-dimethyl-1-phenylpropan-1-ol

Q: You want to make a copper extraction agent similar to LIX63, you have a supply of a 2-hydroxyketone (Made by heating an ester with sodium metal in toluene). Explain how you will convert it into the oxime.

Final stage in the synthesis of LIX63

I want you to think about the problems and then we can go through the answers.

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Ethylene

The first thing we should look at is the sigma bonds, their arrangement is determined by electrostatic repulsion between the bonding pairs. Now in the COVID era I sincerely hope that you are keeping your distance from other people. The idea of social distancing will help you understand the shapes of molecules.

Keep in mind that COVID is no joke, it is a dangerous disease for which now (Dec 2020) there is no vaccine approved in Europe and there is no cure. I was about ten when HIV appeared in the world. I can recall when the “Monolith” advert appeared in the UK in 1987. It showed a man carving a tombstone. First he uses explosives, then a power drill and finally a hammer and chisel to finish it off. The message as I understood it was “Pay attention to the health protection advice about HIV / AIDS or else he will have to do a tombstone for you”.

Thankfully HIV cannot spread from one person to another by sneezing and coughing, COVID can.

Consider for a moment a square classroom with one person in it, it will not matter where the person sits. But if we put two people in the same classroom then they should sit at opposite corners to have the greatest distance between them.

If we add another person then the best arrangement to pack the people into the room is to put them like this to maximize the distance between them.

With four people I would go for this arrangement.

Now after using this “normal” classroom to understand the idea of how to put people in a room with the greatest distance between them. We should consider the first of our experimental classrooms. The 1970s was a time in the UK of educational experiments such as the school near Loughborough (Countesthorpe Communuity College) with no rules. But we are not here to discuss Summerhill and other “interesting” schools here.

Here is a 1970s education experiment classroom, the torus room. With two students we should put them 180 degrees apart (pi radians).

Now with three students I would go for this arrangement.

Now with four students then I would go for this arrangement.

If we move to our next 1970s educational experiment we have a classroom in space where the students are on the surface of a sphere. If we have two students then the ideal locations would be at the north and south poles of the sphere.

Now with three students we should put them 120 degrees (2pi/3 radians) apart on the equator of the sphere. To make it more easy to understand I have included lines from the balls to the centre of the sphere.

Now with four students we should put them 109.5 degrees apart, they will be arranged in a tetrahedral manner around the sphere.

In the same way that people should maximize the distance between them to reduce the probaility of one transmitting COVID to another the electron pairs in the sigma bonds and sigma lone pairs around an atom will repel each other. They are arranged in such a way to minimize the electrostatic repulsion between them. I have come to the view over the last thirty years that all chemistry is based on electrostatic effects.

We move onto the pi bonding in ethylene, we can estimate the strength of a pi bond using Hess’s law. But we will do that another day. First we need to see how the orbitals look. If we start with a pair of sp2 carbons next to each other. Each carbon bears two hydrogen atoms. It will also have a p orbital with a single electron in it.

Atomic p orbitals on the carbons in an ethylene molecule.

If we combine the two p orbitals we can form a pi bonding orbitial.