aromatic chemistry (3.1.10)
No description yet. This curated deck is ready for daily study.
Card preview (24)
which theoretical molecule did Kekule suggest as the structure of benzene?
cyclohexa-1,3,5-triene
which pieces of evidence show that benzene has a different structure to cyclohexa-1,3,5-triene?
• reactivity • bond lengths between carbon atoms • enthalpies of hydrogenation
how does reactivity show that benzene has a different structure to cyclohexa-1,3,5-triene?
• Kekule’s structure suggests benzene should be highly reactive (3 double bonds which are areas of high electron density) • benzene is actually unreactive
how do bond lengths between carbon atoms show that benzene has a different structure to cyclohexa-1,3,5-triene?
• in cyclohexa-1,3,5-triene, the carbon to carbon bond lengths should alternate between 0.154nm (alkane/C-C bond length) and 0.134nm (alkene/C=C bond length) • the carbon to carbon bond lengths in benzene are all a value between alkane and alkene (0.140nm)
how do enthalpies of hydrogenation show that benzene has a different structure to cyclohexa-1,3,5-triene?
• benzene has a less negative enthalpy of hydrogenation (-152 kJ mol^-1 difference) than cyclohexa-1,3,5-triene (-360 kJ mol^-1) • this suggests that benzene is more stable (for molecules, lower energy = more stable)
what key points can be made about the structure and bonding in benzene?
• planar • bond length is an intermediate between single and double - C-C bond lengths are the same • bond angles are the same (120 degrees) • it is a 6C cyclic compound with each C atom bonded to a H atom • each C atom uses 3 outer electrons to form 3 covalent bonds • one electron in the p orbital of each C atom forms a delocalised ring of electrons • contains a delocalised pi system
why is benzene more stable than cyclohexa-1,3,5-triene?
• there are 6 delocalised p electrons across the planar ring of carbon atoms • the 2 electrons from the p orbitals of C atoms which are side by side are in close proximity so they overlap • this creates a delocalised pi bond which minimises repulsion between the electrons this makes the molecule lower in energy
how do the structures of benzene and cyclohexa-1,3,5-triene compare?
they are both planar and hexagonal
what do electrophilic attacks on benzene rings result in?
substitution
what is nitration?
an important step in synthesis, including the manufacture of explosives and formation of amines
what is another important step in synthesis?
Friedel-Crafts acylation reactions
what is the name of the mechanism that benzene rings undergo?
electrophilic substitution
why does benzene undergo substitution instead of addition reactions?
an addition reaction would break the highly stable delocalised pi system and require a large energy input, while substitution maintains the stability
what happens during the electrophilic substitution of a benzene ring?
• the positively charged electrophile accepts a pair of electrons from the delocalised ring of electrons • the delocalised ring of electrons breaks and becomes positively charged • the electrons from a C-H bond are attracted to the positive charge • the delocalised ring of electrons reforms
which two reactions does benzene undergo?
• nitration • Friedel-Crafts acylation
nitration - which reagents are needed to form the electrophile?
concentrated nitric acid and concentrated sulfuric acid
nitration - what is the equation for the formation of the electrophile?
HNO3 + H2SO4 -> +^NO2 + HSO4^- + H2O
nitration - what is the equation for the regeneration of the catalyst?
HSO4^- + H^+ -> H2SO4
nitration - what is the overall equation for the reaction?
C6H6 + HNO3 -> C6H5NO2 + H2O
Friedel-Crafts acylation - which reagents are needed to form the electrophile?
an acyl chloride and aluminium chloride
Friedel-Crafts acylation - what is the equation for the formation of the electrophile? (with ethanoyl chloride)
CH3COCl + AlCl3 -> CH3CO^+ + -^AlCl4
Friedel-Crafts acylation - what is the equation for the regeneration of the catalyst?
-^AlCl4 + H^+ -> AlCl3 + HCl
Friedel-Crafts acylation - what is the overall equation for the reaction? (with ethanoyl chloride)
C6H6 + CH3COCl -> C6H5COCH3 + HCl
what does the prefix phenyl refer to?
a group derived from benzene by removing a H atom (used in IUPAC names where benzene is part of compound)