3.3.10 Aromatic Chemistry! Flashcards
define aliphatic
straight or unbranched chain organic substances
define aromatic/arenes
includes one or more benzene rings
what is the molecular formula of benzene
C6H6
describe the structure of benzene
6 C atoms arranged in a planar ring
each C bonds to 1 H atom
4 valence electrons on carbon, 3 in sigma bonds and 1 lone electron in a p-orbital
these p-orbitals overlap sideways forming a delocalised ring of electrons
each C-C bond is the same length
what is the IUPAC name for kekule’s strcuture of benzene
cyclohexa-1,3,5-triene
suggest why kekule’s structure is wrong
the enathlpy of hydogenation of cyclohexene is -120kJmol-1 so you would expect cyclohexa-1,3,5-triene to be -360 (x3) but in reality the enthalpy of hydration of benzene is less at -208kJmol-1
why is benzene’s enthalpy of hydrogenation lower than Kekule’s structure
benzene is more stable due to delocalisation energy (caused by the ring of delocalised electrons)
what is another reason why Kekule’s structure is incorrect
in Kekule’s model, benzene should have alternating single and double bonds of different lengths
BUT each C-C bond has equal bond length-0.139 nm
which is between length of single (0.154 nm) and double (0.134nm) bonds
what is the name used of a benzene ring if it is a substituent side group
phenyl group
what does benzene attract and why
nucleophiles
benzene has a high electron density
name the reaction that benzene undergoes
electrophilic substitution
why does benzene not undergo addition reactions
it would involve breaking up the delocalised system, which requires a lot of energy and is not favoured
what is less toxic benzene or methylbenzene
methylbenzene
why does methylbenzene react more readily than benzene
the methyl side groups releases electrons into the delocalised system
making it more attractive to electrophiles
name the mechanism, reagents and conditions for the nitration of benzene
electrophilic substitution
concentrated nitric acid
concentrated H₂SO₄ (catalyst)
60°C
what is the elctrophile in the nitration benzene
NO₂⁺
outline the mechanism for the nitration of benzene
benzene ring and NO₂⁺
1 arrow in initial step and 1 arrow in intermediate
curly arrow from ring to NO₂⁺
curly arrow form hydrogen to ring with broken positive charge
what is the overall equation for the formation of the electrophile, NO₂⁺
HNO₃+2H₂SO₄
=NO₂⁺ + 2HSO₄⁻ + H₃O⁺
outline the mechanism for the formation of the electrophile in two steps
STEP 1:
2 curly arrows in
curly arrow from O-H bond in H₂SO₄ to the O atom
curly arrow from lone pair on oxygen in HNO₃ to hydogen in same O-H bond
HSO₄⁻ and H₃O⁺ formed
STEP 2:
1 curly arrow from O-NO2 bond to positive oxygen atom
water and NO₂⁺ formed
what does HNO₃ acts as in the first step in the formation of the nitronium ion
base
proton acceptor
what useful compounds can be synthesised by the nitration of benzene
TNT and formation of amine which dyestuffs are manufactured
