3.3.10 Aromatic Chemistry Flashcards
what is an arene
hydrocarbon baced of benzene
benzene molecular formular
C6H6
hexagon with circle in middle
where did arenes first come from
sweet smelling oils that is why they are called aromatic compounds
what is a phenyl
a benzene that has lost a hydrogen
C6H5
Benzene important structure and bonding point s
- flat benzene ring
- unsaturated - but doesnt undergo addition reactions
- bond length in-between c-c and c=c
cyclohexane
C6H12
has 3 double bonds
benzenes delocalization and stability explained
- each carbon creates 3 covalent bonds
- carbon has the capacity for 4 bonds means leaves a delocalized electron
- 6 delocalized electrons per benzene
- the 4th electron is in the p orbital - as the p orbitals overlap leads to the areas of delocalized electrons
- means there is an equal region of electron density above and below the carbon ring
what is benzenes stability sometimes called
benzenal stability
thermochemical evidence for the stability of benzene
- cyclohexene to cyclohexane’s enthalpy change of hydration is - 120 kjmol-1
- means you would assume theoretically if - benzene has 3 double bonds like in cyclohexa-1,3,5 triene that bond enthalpy = -360kjmol-1
- benzene actually is -208kjmol-1
reasoning behind the -152kjmol-1 difference in enthalpy
- less exothermic than expected
- means takes more energy to break bonds in molecule
- because it is more stable from the delocalized electrons
what is bonds breaking and making endo or exo
bond breaking - endothermic - pos
bond making - exothermic - neg
models for benzene
- kekule model - original - with alternating double and single bonds
- Pauling’s delocalized model - overcomes all problems and explains non polarity
evidence against the first benzene model explained
- thermodynamics
- less exothermic than expected as takes more energy to break bonds as more stable - reactivity
- should undergo electrophilic addition but actually doesnt as to stable needs a catalyst
- does undergo substitution - bonds length
- kekule c-c = 0.154 , c=c = -o.34
- real
0.140 not single or double - isomers
- is kekule should have 4 isomers but only has 3 that actually exist
reactivity of benzene
- ring is an area of high electron density - means it is attacked by electrophiles
- ring is very stable - means energy has to be put in to break ring - ring remains intact in arenes
physical properties of benzene
- colorless liquid at room temperature
- boils at 353K freezes 297K
- bp is similar to hexanes
- mp is not similar to hexanes its higher - because benzene is flat so packs together well in solid state
- non polar
