4.2 aromaticity benzene rings Flashcards
structure and bonding in benzene
- C6H6
- in a hexagonal ring bonded to h atoms
- planar and the c-c bonds are the same length
- each c atom is bonded to two other c atoms and one h atom by single covalent bonds
- which leaves one unused electron on each c atom in a p orbital, perpendicular to the plane of the ring
- each p orbital overlaps with neighboring p orbitals to form pi bond
- electron cloud above and below plane of ring
- electrons in pi system free to move around pi system- delocalised
- benzene very stable
evidence for structure of benzene
bond lengths
c-c are all same length in benzene
partway between c=c and c-c
and in cyclo triene it’s 3 single and 3 double
evidence for structure of benzene
addition reactions
-benzene does not u sergio addition reactions but substitution instead so the ring remains intact
- in cyclo triene addition does take place(across double bond)
evidence for structure of benzene
enthalpy values
- in cyclotriene it was expected to be -360 which is -120x3 (single c=c)
- instead it is -208 in benzene ring more stable
what is delocalisation energy?
energy saved by benzene having delocalised ring rather than 3 double and 3 single bonds.
(360-208)
why does benzene undergo substitution reactions?
- cuz of delocalised electrons above and below plane of ring it is attracted to electrophiles- species that seek electron rich areas
- but the ring of delocalised electrons in benzene is very stable and resists addition reactions in order to remain intact
- instead undergoes substitution by replacing hydrogen as the hydrogens aren’t involved with the delocalised electrons
what happened if further substitutions during nitration occur in electrphillic substitution? (if temp and other conditions arent controlled)
- if you want a particular substance you do not want more than one nitro attached
- it will produce more harmful chemicals such as dinitro and trinitro benzene as they are explosive
