Colour By Design Flashcards
Aromatic compounds are?
Arenes/aromatic compounds contain benzene rings.
Carbon structure?
Carbon has 4 valent electrons.
Each carbon is bonded to 2 other carbons and 1 hydrogen.
Lone electron is on p-orbital which sticks out above and below the planar ring.
Drawing on notes.
Different benzene structure?
Can be shown using the:
- Kehule structure (single and double bonds alternating).
- delocalised structure (a circle in the middle of the 6 sided shape).
C6H6 formula.
It’s a cyclic, planar molecule.
Why was the Kehule structure proven wrong using carbon length?
Benzene couldn’t have been a simple carbon chain because there aren’t enough hydrogens.
August Kehule solved this, proposing the carbons were arranged in a planar ring with single and double alternating bonds.
However, the X-ray diffraction studies showed that all carbon-carbon bonds were the same length (Kehule proposed that 3 would be shorter and 3 would be longer because of the double and single bonds).
Kekules structure is called cyclohaxa-1,3,5-triene.
The bond length for a single bond is 154pm.
For double bond is 134pm.
Benzene’s c-c- lengths are right in the middle.
The delocalised model of benzene?
All bonds are the same length - between a double and single.
Each carbon donated an electron from its p-orbital. The p-orbitals therefore combine to form a ring of delocalised electrons.
Draw circle in middle.
Why does benzene react via substitution?
You would expect from Kekules structure that benzene would react via electrophilic addiction because of the double c-c bonds (act as alkenes). It was expected that a quick reaction would occur.
This did not happen. Benzene doesn’t react via addition. And it reacts very slowly, needing a catalyst begin to react at all.
Benzene reacts via electrophilic substitution instead.
The electrons in the delocalised ring have more room than if they were squeezed into localised double bonds. They can get further away from eachother, spreading out the negative charge so that the molecule is more stable.
An addition reaction would need to take electrons from the stable delocalised ring to form new bonds. Substitution reactions don’t do this - a hydrogen atom just gets swapped for something else and the stability of the delocalised electrons is preserved.
Enthalpy changes of benzene?
We work out the enthalpy change by undergoing hydrogenation of benzene.
Cyclohexane has one double bond and the enthalpy change of that bond is -120kj mol-1.
In kekules model, you would expect the enthalpy change to be -360 (3x that^) because it has 3 double bonds.
The enthalpy change was actually -208kj mol-1 - far less exothermic than expected. This is the experimental value.
Energy is put in to break bonds and that energy is released again when bonds are made. So more energy must have been put in to break the bonds in benzene than would be needed to break the bonds in the Kehule structure.
This difference indicated that benzene is more stable than the kekule structure. This is because of the delocalised ring of electrons.
The symbol of hydrogenation is triangle, H then a little circle at top.
What is an electrophile?
The compound that accepts the electrons from the delocalised ring.
They need to be strongly positively charged to be able to attack the stable ring of benzene.
Only strongly polarised compounds or positive ions can do this.
Nitration of benzene?
Arenes/aromatic compounds including benzene undergo electrophilic substitutions with nitronium ions as the electrophile.
If you warm benzene with concentrated nitric and sulfuric acids, you get nitrobenzene.
Sulfuric acid is the catalyst - it helps make the nitronium ion (NO2^+) which is the electrophile.
Mechanism for this is on paper.
- Nitronoum ion attacks the benzene ring.
- An unstable intermediate forms.
- The H+ ion is lost.
The catalyst - sulfuric acid - is then reformed:
HSO4^- + H+ —> H2SO4
If you only want one substitution (mononitration) do it below 55 degrees. If you want lots of substitutions (lots of NO2 groups added), do it above 55 degrees.
Nitration of benzene equation?
- HNO3 + H2SO4 —> H2NO3^+ + HSP4^-
- H2NO3^+ —> NO2^+ + H2O
Overall:
C6H6 + HNO3 (concentrated) —(conc H2SO4)—> C6H5NO2 + H2O
Conc H2SO4 is sulfuric acid as catalyst.
Sulfonation of benzene?
Sulfur trioxide ions are SO3. They are the electrophile (the thing that attacks the benzene ring).
If you wanted to make benzenesulfonic acid:
1. Boil benzene with concentrated sulfuric acid for several hours.
Or,
2. Warm benzene to 40 degrees with fuming sulfuric acid for half and hour.
The electrophile in these reactions is SO3 because concentrated sulfuric acid breaks like this:
H2SO4 —> H2O of SO3
And fuming sulfuric acid is lots of SO3 molecules dissolved in sulfuric acid. It’s richer in SO3 than conc. This is why it needs less heat and it’s quicker.
The mechanism is on paper.
- The SO3 attacks the benzene, drawing a pair of electrons from the delocalised ring.
- The -ve O atom on the SO3^- takes an H atom from the benzene. The pair of electrons in the C-H bond move to the delocalised ring.
- Benzenesulfornic acid is formed.
How do halogen carriers help make good electrophiles?
An electrophile has to have a strong positive charge to be able to attack the stable benzene ring (only positive ions or strongly polar compounds will attack the benzene ring).
Most compounds are not polarised enough - but some can be made into stronger electrophiles using a catalyst called a ‘halogen carrier’.
A halogen carrier accepts a lone pair of electrons from the polar molecule containing a halogen - the electrophile. As the lone pair of electrons is pulled away, the polarisation in the electrophile increases and sometimes a carbocation forms.
A carbocation is an organic ion with a positively charged carbon atom.
This makes the electrophile a lot stronger.
Halogen carriers include alimony halides, iron halides and iron.
Mechanism is on paper.
Formation of benzenesulfornic acid overall equation?
C6H6 + H2SO4 —> C6H5SO3H + H2O
Why do we use acetylation and alkylation?
Aceyl group (R-CO) or alkyl group (R-) can be added to benzene via substitution reactions.
When it’s added, the benzene structure is weaker and so it’s easier to modify the benzene further and make useful products.
Friedel-Crafts reactions?
There’s two types:
Alkylation and acylation.
They follow the electrophilic substitution mechanism.
What does reflux mean?
Reflux means boiling reactants in a flak fitted with a condenser to stop them boiling away.
Alkylation of benzene?
This is where electrophilic substitution takes place to substitute a alkyl group onto a benzene so it becomes a ‘methylbenzene’ or something similar.
It involves reaction of benzene under reflux with a halogenalkane (which provides the alkyl group) and a halogen carrier as a catalyst.
The hydrogen carrier can be AlCl3 or FeCl3.
Since the halogen carriers don’t get used up, they’re sometimes called Friedel-Crafts catalysts.
This general reaction can be represented by equation:
C6H6 + RCl —(reflux and AlCl3^)—> C6H5R + HCl.
R is an alkyl group.
This is basically how an alkyl group can be added to a benzene ring.
The electrophile is R+ (the alkyl group).
Formation of electrophile:
R-Cl + AlCl3 —> R+ + AlCl4^-
The mechanism is on paper.
- The carbocation is the electrophile. It attracts t he electrons in the delocalised ring.
- An unstable intermediate forms.
- Methylbenzene is made an the H+ ion is lost.
AlCl3 is regenerated here.
Produces an alkylarene.
Acylation of benzene?
Used to substitute an acyl group onto a benzene ring producing a phenylketone.
Involves benzene under reflux with an acyl chloride (provides the acyl group) and a halogen carrier catalyst.
The halogen carrier catalyst can be AlCl3 or FeCl3.
The mechanism is on paper.
The electrophile is the R-C+—O.
Paper.
Formation of electrophile:
R-C—O-Cl + AlCl3 —> R-C+—O + AlCl4^-
On paper.
Overall:
On paper.
Phenylethanone is produced - this is when the acyl group is on the benzene ring.
AlCl3 is also regenerated here.
Regeneration of AlCl3?
Regeneration:
AlCl4- + H+ —> AlCl3 + HCl
Formation of a carboncation?
The carbocation if formed from the chloroalkane and AlCl3:
CH3Cl + AlCl3 —> CH3 + AlCl4^-
Chlorination/Bromination of benzene?
Mechanism is on paper.
How do halogen carriers help halogens substitute into the benzene ring?
Halogen carriers polarise halogen molecules, such as Br2 or Cl2.
The positively charged end of the halogen molecule then acts as an electrophile and reacts with the benzene ring in the usually electrophilic substitution reaction.
Mechanism on paper.
How does benzene react with bromine?
The electrophile is Br+.
C6H6 + Br —> C6H5Br + HBr.
This is an electrophilic substitution reaction.
It requires a halogen-carrier (catalyst), e.g. FeBr3.
It is less reactive than when alkenes react with bromine. Why?
1. Electrons in benzene are less reactive than alkenes. Therefore, the electron density is lower (because the electrons are more spaced out).
- This means that the Br-Br covalent bond (which is non-polar) cannot attract the pie electrons from the benzene because there is not enough repulsion from the pie electron pair and the benzene to generate a dipole which would split the Br-Br bond.
- A catalyst therefore needs to be used to generate an electrophile (Br+) which created a dipole between the Br-Br bond. This creates a sufficient attar film between the pie electrons and the Br-Br bond is split. This allows for the benzene to have a Br on it.
A catalyst is needed and therefore, it’s less reactive than alkenes which don’t need catalysts.
How do alkenes react with bromine?
Must know the mechanism for this reaction. Saved to notes.
Alkenes react with bromine water and a colour change occurs - brown-orange to colourless. Bromine (brown-orange) is the electrophile and adds to the alkene forming a dibromoalkane (colourless).
In this reaction, Br2 is polarised (a delta positive is formed) as the electrons in the double bond repels the electrons in the Br2.
An electron pair in the double bond is attracted to the delta positive bromine and forms a bond. This breaks the Br-Br bond.
A carbocation intermediate is formed and Br is attracted to C+.
Couolourless 1,2-dibromoethane is formed.
This is an electrophilic addition reaction.
Alkeke is C2H4. Double bond between C’s. On paper.
This reaction is more reactive than benzene.
Why?
1. Electrons in alkene between the C double bond C are localised. This means the electron density is higher (less spread out).
- The electrons between the Br-Br are repelled by pie electron in alkene, creating a dipole bond (slight positive and negative charge between atoms) between the Br-Br.
- This means the pie electrons in the alkene are attracted to the bromine and the Br-Br bond breaks.
- A carbocation intermediate is formed and Br- is attracted to C+.
- Colourless 1,2-bromoethane is formed.
There’s no need for a catalyst so this is more reactive.
This turns the brown-orange bromine to colourless when reacting with alkene.
