Posted by: Mark Foreman | April 20, 2020

Clean university life

The second sign on the fridge read

“This fridge is like a joke, it helps if you keep it clean !”

Also you should think of your university as being like “a joke”, it helps if you keep it clean. Keeping it clean includes not making a mess in the lab, returning your library books on time, not trashing the hall of residence, not having food fights in the dining hall and a series of other horrors. When I was a PhD student I had my evening meals in an undergraduate hall in my final year, one day at the end of term I sat down and some young men near me were acting strangely. I inquired of them if something was about to happen. One of them said yes. I got up went with my tray to the other side of the room. Shortly afterwards they started throwing food at each other and creating a vast mess. Shortly afterwards the person in charge of the kitchen came out and told them to clean up their mess. They laughed at this person and ran off. A short time later a cleaner came out and started to clean the mess. I felt very sorry for the cleaner having to deal with this wanton mess, I later discovered that the hooligan boys were issued with a painful fine which included the salary costs for the cleanup operation. I find the wanton destruction of food to be offensive in a world where we have not yet put an end to malnutrition and hunger.

My grandfather on my father’s side was a POW locked up in Poland by the Germans for most of world war two. Once while larking about he threw the lid of a cheese box at another British man. Just as he threw it a German guard opened the door and the box lid almost hit the guard. My grandfather was taken in front of the British officer who was in charge of day to day discipline amount British POWs. He was sent to the glasshouse (Guardhouse) for three days as a punishment. He meet a pair of men who had wasted food, they had filled another mans boots with treacle as a joke. The British commanding officer was incensed when he heard what these two men had done, he commented “Men have risked their lives to bring us this food so who are you to waste it !”. The treacle hooligans were given at least twice as long as my Grandfather in the glasshouse.

Keeping the university clean also includes being clean in your behavior as students. Please do not do stuff like singing offensive sounds as you walk home through some quiet neighborhood full of families close to the university.

Also also please do not do anything like the “horrid riots” of Aberdeen when the students from one university in 1659 fought in the street with the students from another university.

Being clean does not mean being judgmental about someone who is different to you. In fact being bigoted or discriminating against people is deeply morally unclean. The most important steps I think you can take towards being morally clean are to obey the university rules, treat all other people in a decent way, never cheat and never doing anything to assist another person cheating.

Cheating at university makes me both angry and sad at the same time. Once I had two students who were guilty of plagiarism when I was an academic at the University of Reading. I had an interesting discussion with the head of department (Matthew Almond) about what to do with them. Me and Matthew choose to exercise mercy and not drag them over red hot coals and up in front of a dean or some other similar figure. Other academics might not have so much mercy as me and Matthew.

I know a young lady who had a boy friend who cheated at university by copying essays which he stole by sneaking into someone else’s computer when they were out of the house. He was banned for several months from all university life, this young lady is now working as a school teacher in Sweden. As a result of having a cheating ex-boyfriend she has a very unsympathetic attitude to plagiarism.

The last thing I have to say about cheating at university is do not expect good value for money from sites which claim that they will write bespoke essays or reports for you in exchange for vast sums of money. There was a UK academic (Charles Oppenheim, Loughborough’s professor of information science) who did an interesting experiment. The academic set a question for an essay “Does UK copyright law provide an adequate balance between the needs of rightholders and users?”

He then told the students to buy a 1500 word essay on the subject and also write one of their own. The essays were all blind marked by Charles Oppenheim. He then found out from the students which were essays which were written by the students and which were purchased from the pay to cheat sites.

The results were interesting, he discovered the lowest marked essay (which cost about 2000 SEK) barely scrapped a pass (it only got 42 %) and it was riddled with basic errors and suffered from “appalling” English. Another pair of essays came from companies which claimed that they would get normally above 60 %, but they only scrapped in at about 55 and 57 %. Not good value for money !

I think that the UK students were getting better grades than the essay mills, it is typical in the UK to mark such that the average student gets a 2.1

Here is a rough conversion chart between the UK and Swedish systems

UK Percentage mark UK grade Swedish grade
Below 40 % Fail Fail
40 to 50 % Third Bare pass (G)
50 to 60 % 2.2 (Lower second) VG
60 to 70 % 2.1 (Upper second) Either VG or MVG
Above 70 % First MVG

The majority of students at the end of their UK first degree get either a 2.2, 2.1 or a first. Thankfully few students get thirds or fails right at the end of their BA or BSc.

Posted by: Mark Foreman | April 20, 2020

IUPAC names of organic molecules

Now I know that not all of you like the system of naming things, but you will have to get used to it if you want to study organic chemistry.

Before we go any further it is noteworthy that Shakespeare is often thought to have written the phrase

A rose by any other name would smell as sweet

This is a paraphrase of part of the text of the play Romeo and Juliet where one of the young lovers (Juliet) compares Romeo with a rose. The phrase is understood to mean that the names of things do not affect what they. If I give you a series of names of people unless you know your history it is impossible to tell which were good people and which were bad people.

Here are six men who I have randomly selected, all of them have either done something exceptional, half of them were exceptionally bad people while the other half did something exceptionally good to help others.

  • Alexander Fleming
  • Colin Ireland
  • Ronnie Biggs
  • Leopold Socha
  • James Braidwood
  • William Burke

In the list we have two serial killers, a cowardly criminal who was part of a gang who crippled a traindriver during a robbery, you have a heroic firefighter who displayed great leadership, a sewage worker who protected jews from the nazis during world war two and the person who discovered penicillin. Most of them are British people so as part of my general knowledge I happen to know who most of them are (both the good and the bad ones), but I imagine that many of you would have to look them up.

People get given their names before they have a chance to grow up and distinguish themselves as either pillars of society, just ordinary people or as the “worst of the worst” (dregs of society, that person who you would be deeply disappointed with if they were engaged to your daughter / son or various other ways of expressing your negative thoughts about them). We have no choice in the names we are given as small children but we have to choose how we will live our lives.

Equally organic chemistry is littered with names which were almost randomly assigned to compounds, the names were chosen by the discoverers and were references often to the thing or place where the substance was isolated. We have the following trivial names and even more names.

  • Toluene
  • Phenol
  • Anisole
  • Acetopheone
  • Morphine
  • Lawessons’s reagent
  • Diazald
  • Orotic acid
  • Benzidine
  • Benzedrine
  • Adamantane
  • Housane
  • Churchane
  • Basketane

Those of you who have studied organic chemistry for a long time will know all or maybe most of the compounds I have listed, but for many people they are random words. In organic chemistry as a response to this problem of names which give no indication as to the nature of the substance we have the IUPAC names.

Rather than naming a molecule with a random name, it is better if the name tells the reader what it is.

We should regard the IUPAC names as being like a plant, imagine we have a plant.

flower

In this plant we have a stem which has group on the end, it also has two side groups. The stems in organic chemistry are named after the alkanes, if we assume that we have a butane (4 carbon chain) we could name our plant

1-pretty yellow/black flower-2,3-dileaf butane.

The 1-pretty yellow/black flower part of the name indicates that the yellow / black flower goes on the chain at the 1 position while the two leaves go on at the 2 and 3 positions. We can change the nature of the groups. If we have other plants with other bits we could imagine other names which would paint a picture of the plant using words.

Back to the chemistry, consider 2-methylbutane, here is a picture of the molecule.

2methylbutane

the but part of our molecule’s name indicates that we have a four carbon long chain.

The an part of the name indicates that the chain is saturated, it does not have any pi bonds in it.

The e ending indicates that the molecule does not have any extra groups at the end. Be careful as in some parts of the world the chemists do not use the e at the end of a hydrocarbon. For example in the Czech Republic they have methan instead of methane.

The 2-methyl part of the name indicates that we have a methyl group which has replaced a hydrogen which would have been attached to carbon 2 of the main chain of the molecule.

Thus if you know the rules you can very quickly understand the structure of the molecule from the name.

Posted by: Mark Foreman | April 20, 2020

Radicals the worksheet I

Dear Reader,

Some of you might have heard of a man named Stuart Warren, he is an organic chemist from the UK. What made him famous in my mind is his book on organic chemistry. He wrote a book designed to teach you how to design organic synthesis route. Now his workbooks on the disconnection method work like this. He presents a problem.

You then think about it, you then try to answer the problem. His book then shows you the answer.

Now I have chosen with the radical chemistry at Chalmers to use the same method. I will present you with some idea or problem.

You think about it, then you have a go at answering it and finally I show you the answer and have a go at explaining it to you. Sometimes you might even get a short lecture from me about the chemistry where I explain things to you.

To get the most of this on line tutorial it is important that you do not just flip to the end to see the answers. Instead you need to look at the problem, and have a go.

When I worked as a factory laborer there was two slogans written on the door of the fridge in the factory. The first was “This Fridge is like life, you get out of it what you put into it”.You should think of your time learning chemistry as being like life, “you get out of it what you put into it”.

By putting in an effort and making yourself make an effort to learn organic chemistry you will learn a lot more than if you flip to the answers without thinking about the problems.

First question:

Calculate the reaction enthalpy for the following reactions

H2 + F2 → 2HF

CH4 + I2 → CH3I + HI

(CH3)3CH + Cl2 → (CH3)3CCl + HCl

When you have done the question please click here for the answer.

Here are links to the other questions two, three, four, five, six, seven and eight

The fridge at the factory had a second message on the door.

The second was “This Fridge is like a joke, it is best if you keep it clean”

Posted by: Mark Foreman | April 20, 2020

Answer to radicals I

OK , depending on the data source you might get slightly different answers. But the method of getting the answer is more important than the answer.

Sadly due to the corona virus I am separated from my favorite organic chemistry text book which has a table to bond energies. But we can deal with the problems.

I am hoping that for the first one you got an enthalpy of about -544 kJ per mole.

Now we need to see how to get the value.

I would suggest that you use Hess’s law, if you create a cycle similar to a Born-Harber cycle everything will become more clear. Now I am going to use the data from the paper by Stephen J. Blanksby and G. Barney Ellison along with another set of data. This can be summarized by the following table.

Now I would like to warn you that the table I cited uses kCal instead of kJ. Now it does not matter what energy unit you use in life, as long as you know how to convert it to one which someone else uses. Being a good SI man I tend to use the joule, but if I am working on the atomic or nuclear scale I tend to use the electron volt.

If you feel the urge to be different I suggest that you consider using the foot pound, this is the energy required to lift a British pound one foot upwards against gravity. I have an alternative joke energy unit which I call the elephant inch. It is the energy required to lift a male African elephant one inch above the ground. The measurement should be done in Regents Park in London where London Zoo is. Please note that anyone who submits their homework using elephant inches as the energy unit will be likely to irk their university teacher.

OK joke over lets get on with the chemistry

The energy required to break the hydrogen-hydrogen bonds in hydrogen molecules to form hydrogen atoms is 436 kJ mol-1 and the energy required to break the bond in a fluorine molecule to form fluorine atoms is 159 kJ mol-1. These values are expressed in terms of the energies of the bonds, thus breaking one mole of bonds will make two moles of atoms.

Now if we start with a mole of H2 and F2, if we break the bonds and create two moles of hydrogen atoms and two moles of fluorine atoms then we will need to put in 595 kJ of energy.

The atoms can then react with each other to form products, it might be possible for the atoms to reform the homonuclear (same nuclei) molecules. But as an alternative they can form hydrogen flouride (HF).

The homoyltic bond energy of HF is 570 kJ per mole, as two moles of HF is formed in our reaction then the total energy released by converting the atoms to molecules of HF will be 1141 kJ.

What we do is to regard the process of breaking the bonds to form the atoms as work done on the system, hence bond breaking always has a positive delta H (ΔH). While the formation of bonds is always an exothermic process with a negative ΔH.

So our first step in the reaction required 595 kJ per mole of hydrogen and fluorine molecules consumed, while the second step gave out 1141 kJ per mole of hydrogen consumed by the reaction. Look at the following diagram which has two steps in the mechanism.

energy diagram for HF formation

Hence the overall reaction enthapy is -546 kJ per mole of hydrogen molecules reacted. I hope that you can imagine that this reaction releases a lot of energy. It can been calculated that a combination of hydrogen and fluorine is the fuel/oxidant system which offers the highest energy for a given mass. In the following diagram I have added more details, there is a magneta arrow to show the overall reaction.

formation of HF overall

We can repeat the process for the formation of methyl iodide and hydrogen iodide from methane and elemental iodine.

Here we start by breaking the iodine-iodine bond to form two iodine atoms, this will require 213 kJ per mole of iodine molecules. We will also need to break the C-H bonds in a mole of methane to form methyl radicals and hydrogen atoms. This will require a lot of energy (439 kJ per mole of methane).

This is a total of 652 kJ for one mole of iodine and one mole of methane.

Now we need to consider the energy released by forming the bonds in the product. We will form one mole of methyl iodide. The energy of the carbon iodine bond there is 241 kJ mol-1 and the energy of the hydrogen iodine bond in hydrogen iodide is 298 kJ mol-1. So we will get a total of 539 kJ out for each mole of methyl iodide we form.

Hence overall the free radical reaction of elemental iodine and methane to form methyl iodide is endothermic by 113 kJ mol-1.

Here is the next question.

Obiter dictum (by the way)

This leads to an interesting trick in radiation chemistry, if you expose an organic material to radiation you can form free radicals. These free radicals have a very short lifetime, but if you mix elemental iodine with the substance of interest and then irradiate it then you can identify the original organic radicals by looking for the organic iodine products.

The nice thing is that if we break down elemental iodine into iodine atoms, then these iodine atoms have too little energy to react with most organic compounds. If we consider the reaction of a mole of iodine atoms with a mole of methane.

The formation energy for a mole of iodine atoms from iodine is 106.7 kJ per mole of iodine atoms. If one mole of these were to react with a mole of methane to form a mole of methyl radicals and a mole of hydrogen iodide then

We would release 298.3 kJ mol-1 for the formation of the hydrogen iodide, and the formation of the methyl radicals would require 438.9 kJ mol-1. Overall we would need 140.6 kJ to transform the iodine atoms and methane (methyl hydride) into hydrogen iodide and methyl radicals. This is a lot of energy to carry out a step in a chain reaction.

The next step in the chain reaction would be to react a methyl radical with a molecule of iodine to form an iodine atom and a molecule of methyl iodide. In this step we will break a iodine molecule into two iodine atoms (need 213 kJ per mole of iodine molecules) and we will make a C-I bond in methyl iodide (gives us 241 kJ per mole of methyl iodide formed). Thus this stage is exothermic, it will give out 27.6 kJ for each mole of methyl iodide formed.

If we combine the enthapy of the first stage with the second stage we will find that each cycle will require 113 kJ mol-1, this prevents us having a chain reaction forming methyl iodide.

If we repeat the calculation for the bromination, chlorination and fluorination of methane then we get energies of -5.4, -100.4 and -453.5 kJ per mole of the methyl halide formed.

It is interesting to note that when you have large amounts of radioactivity combined with elemental iodine, you tend to form methyl iodide. The reason for this is that air contains a trace of methane gas. The methane gas reacts with free radicals such as hydroxyl radicals (formed by the irradiation of air). Some of the resulting methyl radicals are then trapped as methyl iodide by the reaction of the methyl radicals with the elemental iodine.

Here is the next question.

 

Posted by: Mark Foreman | April 20, 2020

Question two

OK it is time to continue, we need to a series of radicals in order of descending energy.

four radicals

Please think about it and consider the order of the energies of the radicals.

When you are ready here is the answer.

Posted by: Mark Foreman | April 20, 2020

Answer to question two

Now before we get going, it is clear that the radicals do not have the same energy. They differ in energy.

Now if you cast your mind back the first year chemistry and consider the stability of carbo cations you will be aware that the methyl cation is the least stable of the common carbocations while things like tert-butyl cations are much more stable.

But why ?

What we need to do is to consider the hypoconjugation in the system. If we consider some of the orbitials in a ethyl radical. We should be able to imagine that the radical can be drawn as being localised to a single p orbital as shown below.

ethyl radical

We have some orbital overlap between the orbitals of one of the C-H bonds in the methyl group on the left.

ethyl radical with orbs of CH bond

Now to make it even more clear I will do some primary school art, I am unsure if the late would approve but here is my art. I have draw with the blue brush the orbitial overlap.

ethyl radical with orbs of CH bond with oribitial overlap included

Now it might be tempting to draw the next thing, but for goodness sake do not ! The increased distance between the red lobe of the sp3 orbitial on the left will reduce the degree of orbitial overlap between the red bits to something very small. Maybe about half of nothing ! Do not draw this, to make this clear I have crossed it out with purple crayon as seeing this would put me in a purple rage.

ethyl radical with orbs of CH bond with oribitial overlap included do not draw this

Now you might want to ask about the idea of getting one of the right hand C-H bonds to do the same thing. The geometry of the orbitals is wrong for there to be any net overlap. Now one of my passions in life is radio electronics. Now if I use 4NEC2 software to predict the polar plot for the field strength for a dipole aerial. The dipole is a wire which is at x = 0 and z = 0 which goes from y = -2 to y = 2. What you will see is when y = 0 is that the electric field of the dipole is zero. Here is a picture of the image which I saw in 4NEC2.

electric field for a dipole

Equally the C-H on the right will be in a nodal plane where the wavefunction of the p orbital of the radical is equal to zero. Hence there will be no orbital overlap. As a result the C-H bonds on the right can not use their orbitals to overlap with the radical.

By overlapping with the radical’s p orbital we can spread the radical character over a larger volume. This will lower the energy of the radical. In my experience anything which can enable a radical to be spread around by resonance will lower the energy of the radical.

The more carbon groups that are attached to the carbon where the radical is centred the more stable the radical is because the radical can be spread out more and more.

Here is the next question

Obiter dictum

In my youth when I was a postdoc at University of Bleep I heard a mighty tale of utter horror which is all about free radicals. Years ago a PhD student once said that I am full of stories. These stories are part of the way in which knowledge is passed from one generation to another. Consider it the oral traditions of chemistry.

What had happened was that someone had stashed a bottle of diisopropyl ether in a cupboard before forgetting it and leaving the university. This is a deeply foolish act as we will later discover.

The presence of a heteroatom on the carbon bearing a free radical will increase the stability of the free radical by lowering the energy of it. Equally an electron withdrawing group such as a carbonyl will do likewise.

isopropyl ether radical

The increase in the stability of the radical thus increases the ease of having a radical chain reaction in which the ether reacts with the oxygen of the air to form a crystalline and shock sensitive explosive. Now before we go any further I would like to warn you to have nothing to do with home made explosives. Of all the home made horrors which I am aware of one of the worst are the crystalline organic peroxides, I know someone who worked at a university where a foolish student stole chemicals from a lab and made a large batch of one such material which I am not naming for public safety reasons.

The student later had a fatal accident with the homemade explosive, the explosion was sufficient to destroy the whole of the kitchen of the hall of residence where the student was playing with this dangerous “toy”.

In the university of bleep case the reaction slowly occurred over many years until the liquid diisopropyl ether turned into a solid mass of the peroxide. It was discovered by a wiser chemist years later who immediately set in motion the emergency response. One of the people at the chemistry department had recently left the army so he politely asked his old army friends to get rid of it for them. A group of  soldiers arrived and took charge of the bottle. They thought that a lab chemical could not be that bad. They rode with it in a small van to the nearest rifle range.

They had the plan that they would lay the bottle on a pile of sand and then walk back a short distance before firing a rifle bullet into the bottle, they were then going to return and sweep up the broken glass. They did so and everything went according to plan until the moment the rifle bullet strikes the bottle. At this point the contents of the bottle detonated and a pile of sand the size of a house vanished in an instant. The men then had no broken glass to sweep up but I imagine the experience gave them food for thought.

The part which scares me is how blase the men were with the bottle and how so many of them needlessly traveled in the same van as the bottle. I would have wanted the bottle packed in a very careful and special way before the bottle was transported along a carefully chosen route with as few people close to it as possible. For this I imagine that expert advice would be needed.

To give you some idea of the true horror of the bottle of close to pure diisopropyl ether peroxide consider the following items of ordinance. Keep in mind that the energetic materials in these different objects (all of them can be lethal) differ a lot.

Object Energetic material filling
9 mm pistol cartridge 300 mg
44 magnum cartridge 580 mg
7.62 x 39 mm (AK-74) 1.6 grams
7.62 x 54 mm (Hunting rifle) 3.2 grams
UK 6 pound shell 110 grams
German WWII grenade 170 grams
The university of bleep ether bottle >1000 grams

To finish off this story, I once was instructed by the head of department at the bleep university to clear out an old lab which was no longer needed (A different university of Bleep to the one with the nasty bottle which blew up a pile of sand). The occupants had left behind lots of things years before. I found a bottle of an ether which was starting to go cloudy and contained plenty of peroxides. I then made arrangements for its safe disposal long before it became shock sensitive.

My advice is the following

  1. Never distill an ether to dryness unless you know it to be free of peroxides
  2. Never store ethers pure, always store them for long times with an inhibitor present
  3. Check all ether bottles at least once a year for signs of peroxide formation
  4. When you close down a lab, always make arrangements to dispose of ethers

Here is the next question

Posted by: Mark Foreman | April 20, 2020

Free radicals question three

During the free radical chlorination of methane, why does some chloroethane (ethyl chloride) form. Explain this with a mechanism.

methane to ethyl chloride

When you have an answer please click here for the answer.

Posted by: Mark Foreman | April 20, 2020

Answer to free radical question three

Now we need to consider the radical chain reaction mechanism for the reaction of chlorine with methane.

The normal method of starting the reaction is to split a chlorine molecule into two chlorine atoms using either ultraviolet light or heat. After we have formed a chlorine atom then the reaction can start. In the first step it will abstract (remove) a hydrogen atom from a methane molecule.

first stage in reaction of chlorine with methane

Next the methyl radical will then react with a chlorine molecule to form a chlorine atom and a molecule of methyl chloride.

second stage of the reaction of chlorine with methane

What you should notice is that both these reactions do not change the number of radicals in the reaction mixture, they merely transform one radical into another. Under the right conditions the radical reaction could have a very long chain reaction which forms methyl chloride.

We need to consider what other side reactions can occur. The chlorine atom could react with a methyl radical to form methyl chloride but such radical-radical reactions are not used much in synthesis. In most reactions the concentration of radicals is low, so as a result it is very unlikely that two radicals would randomly encounter each other.

Mark Foreman has on some occasions in his life run reactions where radical radical combination occurs. He can think of four examples. In two of them he was generating radicals at a metal surface, as the whole of the reaction occurred in a thin film on the surface of the metal rather than in the bulk of the liquid in the flask the local concentration of radicals was very very high.

Another time he managed to get radicals to react with each other he did it by making the concentration of the radicals very high by tipping everything into the reaction flask very quickly. This reaction was a very dirty reaction which gave a horrible low yield, I used to use kilos of starting material and end up with very little product.

The last time I managed to do a radical radical reaction was the time I irradiated butanol with a very high dose of gamma rays. I deoxygenated the butanol and then irradiated it with a very high dose in a very high dose rate gamma irradiation machine. I later found in the tube after breaking it open some octane-4,5-diol which was formed by the combination of the radicals. Another of my sealed tube organic radiation chemistry exploits was the time that I irradiated a deoxygenated sample of a long straight chain alkane. This caused the formation of larger molecules which were formed by the dimerization of radicals.

The synthetic examples which I mention are very much reactions which occur under unusual conditions. One of my mentors (Anthony Francis Hill) once discussed with me the chemistry which makes the clock strike 13, he was of the view it is more interesting than the normal chemistry. But there is a problem, in the same way a little spice on your food is nice but we can not live on a diet of nothing but spices. Equally as students it is my responsibility first to teach you the normal chemistry of things before maybe later I teach you some of the unusual chemistry.

The more common effect of radical-radical reactions is to slow down the chain reactions, it is important to notice that during free radical polymerisations that often the rate will suddenly accelerate as the viscosity of the mixture increases. There are various names for this effect such as the gel effect and more grandly the Trommsdorff or Norish-Smith effect. What happens in these polymerisations is that free radicals are being generated at a constant rate by the break down of a initiator.

The initial radical reacts with a momomer to form a slightly larger radical this then grows in size by reacting over and over again. As the mixture becomes more and more viscous the macromolecular radicals are less able to move around. One of the important termination processes which stops the growth of a chain are the radical-radical reactions where two radicals react to form non radical products. As it becomes harder for the radicals to diffuse towards each other then they have a longer life time. As a result of the longer life time the concentration of the radicals (in terms of radicals in a given volume) increases. As the higher the radical concentration the higher the rate of the reaction then the reaction can speed up. As free radical polymerization is exothermic this can result in a sudden increase in the reaction rate which under some conditions could result in a runaway reaction.

But back to the chemistry if a chlorine atom was to react with a molecule of hydrogen chloride then we could have two different products. Either another molecule of hydrogen chloride and a chlorine atom (no net reaction) or forming a chlorine molecule (dichlorine) and a hydrogen atom.

I estimate that the second of these options will  be an endothermic reaction so it is unlikely, but if it occurred then I would expect that the hydrogen atom would react with a molecule of chlorine to form a molecule of hydrogen chloride and a chlorine atom. So the reaction would continue.

While it is a rare reaction if two chlorine atoms combine then the reaction will stop, this is due to the fact that we decrease the number of free radicals in the mixture by two. This reaction will form chlorine molecules which were one of the reagents in our reaction so we are unlikely to notice it.

But if two methyl radicals combine then we will form a molecule of ethane, ethane is a substance which we never added to the reaction pot in the first place. So the appearance of ethane is something which we might be able to notice.

formation of ethane

As ethane reacts with chlorine to form a different product (ethyl chloride) than methane, it is possible for us to find ethyl chloride in the products of the reaction.

Posted by: Mark Foreman | April 20, 2020

Radical chemistry question four

When ethane is reacted with chlorine why are two different dichloroethanes formed, draw mechanisms to explain this fact.

two dichloroethanes

When you have the answer please press here for the answer.

Posted by: Mark Foreman | April 20, 2020

Answer to question four on radicals

When we consider ethane then all the hydrogen atoms are identical, it does not matter which one you remove. You will still form the same ethyl radical. If a chlorine atom reacts with an ethane molecule it will form an ethyl radical as shown below.

stage one of ethane plus chlorine

The second stage of the reaction is the reaction of the ethyl radical with a molecule of chlorine to form a chlorine atom and a molecule of ethyl chloride.

second stage in the formation of ethyl chloride

What will happen next is that the ethyl chloride will be able to react with a chlorine atom to form a new radical. For this reaction we have two sites on the molecule where it can occur. This can be summed up in the following diagram.

branching of ethyl chloride to the two products

There is a paper by C. Cillien, P. Goldfinger, G. Huybrechts and G. Martens  from the 1960s in which the relative reaction rates of ethane, ethyl chloride and a series of other chlorinated ethanes with chlorine atoms are documented. (Trans. Faraday Soc., 1967,63, 1631-1635).

What was done was chlorine and the organic compounds were exposed to UV light, a standard compound was present in the vial. It was found that the two different sites in ethyl chloride do not have the same reactivity. The reactivity of the different hydrogen atoms in the molecule is given by the following Arrhenius equation.

k = A exp (-EA/RT)

At 298 K the rate constant for the reaction of each of the hydrogen atoms in ethyl chloride clearly differs between the methyl group and the chloromethyl group. The three hydrogen atoms on the methyl (CH3) have a reaction rate constant of 2.97 x 108. While the two hydrogens on the chloromethyl group (CH2Cl) each have a reaction rate constant of 1.409 x 109.

If we keep the chlorine atom concentration constant it is possible to work out the rates of the reactions. Using the Bateman equations it is possible to model the kinetics of the reaction of ethane with chlorine to form the various different products. If we were to start with a mixture of pure ethane and chlorine and keep a constant chlorine atom concentration then for the concentration of ethane we can write the following equation.

[Ethane]t = [Ethane]0 exp (-k1[Clatom]t)

The current concentration of ethane at time t is given by [Ethane]t and the original concentration of ethane is [Ethane]0. The value of k1 will be six times the reaction rate constant for each of the hydrogens in ethane. Using the same type of simulation we can predict how the mixture of organic compounds will change as a function of time if the chlorine atom concentration stays the same and if we assume that hydrogen abstraction from the organic molecule is the rate determining step.

ethane with chlorine

The first thing we should look at in the graph is the fact that the ethane (brown) is changed into ethyl chloride (1-chloroethane shown in gray) which in turn is converted into both ethylene dichloride (1,2-dichloroethane in yellow) and 1,1-dichloroethane (blue). The majority of the ethyl chloride which reacts forms 1,1-dichloroethane as the product.

This is likely to be due to the fact that the radical in which a chlorine is attached to the carbon where the radical is drawn as being increases the stability of the radical.

At the end of the simulation the majority of the organic matter is in the form of two isomers of trichloroethane. There is the 1,1,1-trichloroethane (methyl chloroform) which is shown in green and then there is the 1,1,2-trichloroethane. The first of these isomers is only formed from 1,1-dichloroethane while the second isomer is formed either from 1,2-dichloroethane or 1,1-dichloroethane.

In a real life industrial plant the products I would have been aiming for making years ago would have been 1,1,1-trichloroethane (methyl chloroform) and 1,1,1,2-tetrachloroethane. The 1,1,1-trichloroethane was used as a common solvent when I was a boy while the less symmetric tetrachloroethane was useful for the production of fluoro compounds such as 1,1,1,2-tetrafluoroethane. This tetrafluoro compound has been used as the working fluid in air conditioning machine and as the propellant in asthma inhalers. Due to concerns about global warming many asthma inhaler manufacturers have changed to new designs which do not use a fluorine containing gas.

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