What is the structure/ bonding of benzene?
- Cyclic, planar molecule with formula: C₆H₆.
- Each carbon has 3 covalent bonds (ie. bonded to 2C’s and 1H.)
- Final spare/ lone electrons in p orbital from each C overlap forming delocalised ring of electrons.
- Delocalised ring of electrons causes all C-C bonds to be same length, intermediate between single and double bond.
Describe C-C bonds in benzene?
- All C-C bonds are equal in length due to delocalised electron ring.
- Bond length is intermediate between single and double bond (ie. between the length of single/ double bond.)
What is the bond angle in benzene? Why?
- 120 degrees.
- Each C forms 3 covalent bonds, trigonal planar arrangement = 120.
What was Kekule’s structure/ named?
- Hexagonal shape of alternating single/ double bonds.
- DOESN’T EXIST!!
- Thereotically named: cyclohexa-1,3,5-triene.
Why is benzene more stable than theoretical molecule: cyclohexa-1,3,5-triene?
- Compare enthalpy change of hydrogenation in benzene and cyclohexa-1,3,5-triene.
- Enthalpy change of hydrogenation for cyclohexene: -120kJmol-1.
- Cyclohexa-1,3,5-triene: (-120x3 = -360kJmol-1.)
- ACTUAL: -208kJmol-1.
- Benzene = less exothermic than cyclohexa-1,3,5-triene by -152kJmol-1.
- Benzene is more stable than cyclohexa-1,3,5-triene due to delocalised electron ring.
What is hydrogenation?
- Addition of hydrogen to molecule.
What evidence would we describe enthalpies of hydrogenation?
- Thermochemical evidence.
How many H₂ molecules would be needed for hydrogenation of benzene/ theoretical cyclohexa-1,3,5-triene?
- 3.
True or False
C=C bond is longer than C-C bond.
- False.
- C-C is longer than C=C bond.
What are “arenes?”
- Molecules that contain a benzene ring.
How would we name:
- Benzene ring with Br on C 1.
- Benzene ring with methyl on C 1 and 2.
- Benzene ring with NO2 group on C1.
- Bromobenzene
- 1,2-dimethylbenzene
- Nitrobenzene
How would we name:
- Benzene ring with OH group on C1.
- Benzene group with NH2 group on C1.
- Phenol
- Phenylamine.
Why do arenes undergo electrophillic substitution instead of electrophillic addition?
- Benzene has high electron density/ delocalised ring of electrons.
- This is attracted to electrophiles.
- Benzene = stable/ not traditional alkene so doesn’t undergo electrophillic addition as this would distrupt the stable delocalised ring of electrons.
True or False
Only a H can be substituted from arene in electrophillic substitution mechanism.
- False.
- H OR functional group on benzene ring could be substituted.
What two types of electrophilic substitution mechanisms could arenes undergo?
- Friedel-Crafts acylation.
- Nitration
Uses of benzene. Issue? Solution?
- Pharmaceuticals/ dyes.
- Difficult to react due to stability.
- Solution: Friedel Crafts Acylation: makes benzene structure weaker/ easier to modify to make further products.
Benefits of friedel crafts acylation?
- Adds acyl group (RCO) to benzene molecule.
- Makes benzene structure weaker/ easier to modify to make further useful products.
What do we react an acyl chroride with to form a stronger electrophile? What does this act as?
- AlCl₃
- Acts as a catalyst
True or False
Acyl chlorides don’t contain a positive charge AT ALL on the C so cannot act as electrophiles.
- False.
- Acyl chlorides have positive charge but not STRONG ENOUGH to act as an electrophile.
- Reacting acyl chloride with AlCl3 creates strongly positive electrophile.
Write the equation for the formation of electrophile from ethanoyl chloride.
- CH₃COCl + AlCl₃–> CH₃C⁺O + AlCl₄⁻
- AlCl₃ gains Cl/ C becomes positively-charged.
Outline friedel-crafts acylation mechanism. Benzene/ CH₃C⁺O. What are the products?
Diagram in YR 2 mechanisms dox.
- Arrow from delocalised ring of electrons to C⁺ on CH₃C⁺O.
- Breaks ring/ positive charge in centre.
- AlCl₄⁻ is attracted to positively-charged ring/ arrow from Al-Cl bond to H. Bond forms between Cl and H.
- Arrow from H to + charge –> reform delocalised ring of electrons.
- Products: CH₃COC₆H₅, HCl, AlCl₃ catalyst = reformed.
True or False
It doesn’t matter how we draw the broken delocalised ring of electrons.
- False.
- The horseshoe/ gap must be in direction of where the electrophile is joining onto the benzene ring.
What are conditions for Friedel Crafts acylation?
- ANHYDROUS aluminium chloride.
- Al gains pair of electrons from C-Cl bond.
- AlCl3 must be anhydrous or it would lose its ability to accept a pair of electrons.
Why is nitration of benzenes useful?
- Important step in synthesis of manufacture of explosives ie. TNT/ formation of amines/ makes dyes.
