Lecture 4 Flashcards
What is neccessary for a hydrocarbon fire to be sustained?
- Molecular fragments with unpaired electrons, free radicals, are necessary for a hydrocarbon fire to be sustained.
- f our initial ignition source is removed, then the heat generated by the chemical reaction must be enough to sustain the fire.
Fire is a ?
Fire is a radical chain reaction
What is a radical?
- An atomic or molecular species that possesses unpaired electrons in an otherwise open-shell configuration (open shell = un-filled valence shell).
- Paramagnetic (unpaired electrons are attracted to a magnetic field).
- High reactivity, but are also isolable.
- Important to combustion and atmospheric chemistry, but also in synthesis and biological processes.
- Attracted to magnetic field as they have a magnetic componenet.
- Very reactive
Chlorine radical
- Bonding orbital of Cl2 gas, has a pair of electrons, spins parallel.
- If we break the bond evenly, we make two Chlorine radicals.
- ## Must be spin spin parallel not in the same direction.
Heterolysis
Movement of charge from one place to another
Homolysis
- Equal distribution of electrons between atoms
- One electron goes to each species across the bond so we get two radical species
LUMO
Lowest unoccupied MO, lowest energy without an electrons
HOMO
Highest occupied MO, highest energy that contains an electron
SOMO
- Singly occupied by one electron.
- In a Radical this is the one that will react with one of the three types of orbitals!
- Goes on to further react
Carbocations and carbanions
Carbocations = positive
Carbanions = negative
Forms of radicals
- Planar (carbocations)
- Tetrahedral (Carbanions)
- Hybrid orbital
- P orbital
What influence the stability of a radical?
- Sterics, pi sysem, and conjugation
- Electron donating or withdrawing groups
How can you stabilise radicals?
- Radicals can be stabilised using either electron donating or withdrawing groups.
- Basically, reactivity can be “tuned”
- Stability is inveresly correlated to homolytic bond strength
A conjugated benezene ring
Less stable
Less steric bulk
more distribution of charge so more reactive
Double bonded species
- More steric bulk
- Less distribution of charge so more reactive
How do radical reactions work?
- Initiation
- Propagation
- Termination
Initiation
Generation of reactive intermediate
Propagation
The generated reactive intermediate attacks a stable chemical species to generate another reactive intermediate.
Termination
- Two radicals combine to quench the unpaired electrons, halting the reaction. This is often a by-product.
- Two radicals join together to make a new bond.
- The concentration can become too high so it’s more favourable for termination to occur.
- If you end with less radicals that you started with then this is termination.
Most likely source for initiation
Bond that isn’t carbon
Example of compounds useful for initiation
- Peroxides as they decompose with heat exposure
- Dihalogens which split when exposed to UV light
- This generates two radicals per molecule and is used a lot in polymer chemistry
Bromines importance in radicals
Bromine is stored to reduce the amount of light getting into the bottle otherwise bromine radicals would form inside the bottle
Radical reactions utilise
- Radical reactions utilise highly reactive intermediates.
- Typically those with unpaired electrons (to overcome activation barriers).
- However, in order to create unpaired electrons, we need high energy and a suitable initiator.
What influences the nature of the products in radical chemistry
The ratio of the reagents (chlorine, methane) in the mixture, as well as reaction time, will influence the nature of the products.
For a combustion to occur, we need to be able to break x bond?
For combustion to occur, we need to be able to break the O=O bond so we can make oxygen free radicals (but this bond is relatively strong).
A materials flammability relates to?
- When considering a materials flammability, it relates to the concentration of free radicals the material needs to be subjected to before both initiation and propagation reactions become the dominant reactions.
- Once they do, the material combusts.
What happens once a material is consumed by a fire?
- Once a material is consumed by fire, there is a shift between the frequency of initiation/propagation reactions towards termination reactions, the fire goes out.
- A shift where termination becomes more favourable which is when the fire will start to go out unless a new fuel is introduced
Branching reaction
- This is where two (or more) radicals are produced when a radical reacts with a molecule
- The generation of a flame is due to branching reactions dominating over termination reactions
- When branching reactions dominate over termination reactions, the fire will consume a lot of fuel.
- It is a different way of doing a propagation which increases the rate of reaction.
- This is what happens in flames
Why are flames so destructive?
- They have branching reactions
- They can accelerate and grow
- When you’ve got these branching reactions dominating over termination reactions, which is very common in a flame, you’re going to have a fire which can really consume a lot of fuel. In this case, more radicals are formed instead of quenched.
fission reactions
Breaking up reactions
O (2 radicals) + H (1 radical) –> OH (1 radical)
- Going from 3 radicals to 1 which is more reflective for termination but it is actually a initiation process as it is very energetically favoured .
- The end process is a really receive radical instead of two unreactive ones
- The generated radical is very reactive so it is an exception.
Oxygen
- The only abundant, paramagnetic molecule found in our atmosphere with a triplet ground state.
- Triplet oxygen contains two unpaired electrons.
- There is such a thing as singlet oxygen, which is the excited state of it!
- Oxygen is thus a di-radical species.
Hunds rule
When adding electron to an MO diagram, equivalent orbitals must be singly occupied before being doubly occupied, and electrons must have identical spin to minimise energy
Bond order calculation
Bond order = (no. electrons bonding – no. electrons anti-bonding) / 2
Dioxygen
- Has equivalent x and y orbitals, and two electrons to spare.
- These consequently don’t pair
- They also share an identical spin
- The only solution is for two radicals to exist, one located on the x orbital, the other on the y orbital.
Diradical oxygen
- Unusual properties
- Highly energetic so used as rocket fuel
- Is essential for combustion of organic matter
- Every single element reacts exothermically with oxygen, with the exception of gold
- At room temperature, O2 exists in a triplet state, which can only undergo a chemical reaction by making the forbidden transition into a singlet state.
Halon 1211
- Numbers relate to C, F, Cl, Br
- Boiling point = - 4 degrees celcius
- The benefit is that under mild pressure, Halon 1211 is liquified.
- It has a vapour pressure of 2.3 atm. at 20 °C, and a vapour density of 16.5 g/L-1
- This means a dense cloud forms over a fire that is difficult to disperse, aiding smothering.
- The canister containing Halon 1211 is under much less pressure than a CO2 extinguisher, meaning it wont disperse a fire when applied
- Fire retardant, the radical counter the fires
- Much denser than air so oxygen gets pushed up stopping the fire as oxygen can’t access it.
Halon Fire extinguishers
History
- Halon fire extinguishers target the chemical chain reaction in fires
- They were banned following the Montreal Protocol of 1987 owing to their use of CFCs
- Came into force in UK in 2003, but is not a complete ban – some critical uses are still allowed
- Bad for environment
- Sometimes still used in labs as they are unreactive with most species
Actions of Halon
- Halon is not an oxidant – it will naturally smother a fire by depriving it of oxygen
- Halon undergoes reaction to form free radicals but it requires energy to do this.
- It takes this energy from the fire, absorbing some energy of activation that would otherwise propagate the fire
- There is lots of initation but not propagation
- It starves the fire of oxygen and then it starves the fire of heat.
- We get increased radicals but halon radicals are more stable
- Radicals steal energy from the fire.
Chemistry of halons
- Halons are really stable not reactive so its happy to sit in the fire as a radical so it doesn’t propagate into products
- It waits to terminate with another radical, it prefers to do this over propagating
- It takes the termination step and we end up with less radicals in the fire
- It steals the more stable radicals and terminates them.
- The free radicals produced by Halon react with the free radicals produced by the fire, quenching them a termination reaction.
- These radicals would normally branch and add to the fire but instead they are quenched by the halons.
Why does O2 participate in branching reactions?
Because O2 has a triplet ground state, it participates in branching reactions which accelerate the overall reaction.
