Unit 3.4.6 - Aromatic Chemistry Flashcards
What are aromatic compounds?
Compounds that contain a benzene ring
What is another name for aromatic compounds?
Arenes
What is the molecular formula for benzene?
C₆H₆
What is the shape of the structure of benzene
Cyclic structure
The benzene ring is said to be what shape? And why?
Planar - all the hydrogens stick out in the same plane
How many models are there to explain the structure of benzene?
Two
What are the name of the two models to explain the structure of benzene?
Kekulé model and the delocalised model
Which is the accepted model for the structure of benzene?
The delocalised model
What is the Kekulé model?
There is a ring of carbon atoms with alternating single and double bonds
How was the Kekulé model later adapted
There are two isomers of the benzene and the benzene molecule constantly flips between them
What is the delocalised model?
There is a ring of carbon atoms and each carbon donates one electron from its p orbital to form a ring of delocalised electrons
What are the two main pieces of evidence to support the delocalised model?
1.) If the Kekulé model was correct you’d expect alternating short and long bonds however all the bonds are the same length 2.) The enthalpy of hydrogenation of benzene is 152 kjmol⁻¹ more than you’d expect it to be
Explain why the enthalpy of hydrogenation of benzene is 152 kjmol⁻¹ more than you’d expect it to be if it had the Kekulé structure.
The enthalpy of hydrogenation of one double bond takes -120 kjmol⁻¹, so if it has the Kekulé structure then it would require -360 kjmol⁻¹, however it only requires -208 kjmol⁻¹ and so it 152 kjmol⁻¹ more than you’d expect it to be - it is less exothermic than predicted
How do you name aromatic compounds if the benzene ring was the main functional group?
The suffix -benzene is used and then prefixes of any other functional group of the benzene ring
How do you name aromatic compounds if benzene isn’t the main functional group?
Use the prefix -phenyl and then the suffix comes from the other functional groups than the benzene group was attached to
What makes benzene more stable than the Kekulé model predicted?
The ring of delocalised electrons means the electron density is shared over more atoms, which means the energy of the molecule is lowered and so it becomes more stable
What does benzene attract? And why?
Electrophiles because it is a region of high electron density
What is an electrophile?
Electron pair acceptor
Name three common electrophiles?
H⁺, NO₂⁺ and polar molecules (like carbonyl compounds)
What type or mechanism does benzene most often go under and why?
Electrophilic substitution to preserve the delocalised ring
Give the five steps in the mechanism for the reaction between benzene and an electrophile.
1.) The ring of delocalised electrons attracts an electrophile 2.) The electrophile takes a pair of electrons from the centre of the benzene ring and forms a bond with one of the carbons 3.) This partially breaks the delocalised electron ring and gives the molecule a positive charge 4.) To regain stability of the benzene ring the carbon bonded to the electrophile loses a hydrogen 5.) A H⁺ had now been substituted for an electrophile
Name two examples of reactions where benzene cause under electrophilic substitution.
Nitration and Frieden-Crafts Acylation
What are the conditions for nitration of benzene?
You warm it with concentrated nitric acid and concentrated sulphuric acid
What is the reaction called that produces nitrobenzene from benzene?
Nitration
What acts as catalyst during nitration?
Sulphuric acid
How does sulphuric acid do during nitration?
It makes to make the NO₂⁺ which acts as an electrophile
What are the equations for how the NO₂⁺ is produced to use as an electrophile in nitration?
HNO₃ + H₂SO₄ → HSO₄⁻ + NO₂⁺ + H₂O HSO₄⁻ + H⁺ → H₂SO₄
What is mononitration?
When only one NO₂ group is added to benzene
How do you ensure you only carry out mononitration?
Temperature below 55ºC
Name one thing nitro benzene can be reduced to.
Aromatic amines
What are two uses of aromatic amines?
Making dyes and making pharmaceuticals
What s one use of nitro compounds?
As explosives like TNT
Why is Friedel-Crafts acylation used?
Because benzene is so stable it is fairly unreactive, and so it is used to add an acyl group to make it easier modify the side chains in further reactions
What is added to to benzene during Friedel-Crafts acylation?
An acyl group
How do you carry out Friedel-Crafts acylation?
Heat an acyl chloride and benzene under reflux in a non aqueous environment
What is produced at the end of Friedel-Crafts acylation?
HCl and a phenylketone
What is used to make an electrophile stronger for Friedel-Crafts acylation?
Halogen carrier
What electrophile and halogen carrier are use in Friedel-Crafts acylation?
Acyl chloride and AlCl₃
How is an acyl chloride made a stronger catalyst by AlCl₃
The AlCl₃ accepts a lone pair of electrons from the chlorine in the acyl chloride, the polarisation in the acyl chloride increases and forms a carbocation, which makes a much stronger electrophile
Give the seven steps in the mechanism for the reaction between benzene and AlCl₃ and an acyl chloride.
1.) The ring of delocalised electrons attracts the positively charged carbocation 2.) The carbocation takes a pair of electrons from the centre of the benzene ring and forms a bond with one of the carbons 3.) This partially breaks the delocalised ring and gives the molecule a positive charge 4.) The AlCl₄⁻ is attracted to the positively charged ring 5.) One chloride ion breaks away from the aluminium chloride ion and bonds with the hydrogen 6.) This removes hydrogen that is on the carbon bonded to the electrophile and HCl is produced 7.) A H⁺ has been substituted for an acyl group
Draw the Kekulé model.
Draw the delocalised model.
Draw the eqaution for the nitraion of benzene.
Draw the structure of TNT
Draw the equation for Freidel-Crafts acylation of benzene.
Draw the equation for the foramtion of the stronger electrophile used in Friedel-Crafts acylation.
