Aromatic Chemistry Flashcards
What is a aromatic compound?
a compound containing a benzene ring
What is an arene?
an aromatic hydrocarbon
What proves the kekule formula of benzene to be wrong?
- benzene doesnt easily undergo addition reactions of alkenes
- all the bond lengths between C atoms are the same length
- hydrogenation of benzene is less exothermic than expected
What happens during a reaction between benzene and Br2?
doesnt declourise bromine
reacts with a presence of a catalyst by substitution
What does hydrogenation of benzene being less exothermic suggest?
the pi bonds in benzene are more stable than expected
What shape molecule is benzene?
a planar molecule
How is a delocalised pi system formed in benzene?
each C atom has 1 electron in a p orbital - these orbitals overlap above and below the carbon ring
Why does benzene have a low reactivity?
delocalisation spreads out the pi-electrons so there is a lower electron density making them less attractive to electrophiles
Why are benzene’s bond lengths all equal?
the pi-electrons are distributed evenly around the ring
What is nitration?
the introduction of a nitro group - NO2
What are the conditions of nitration with benzene?
conc nitric acid and conc sulfuric acid
How is an electrophile of NO2 + formed by sulfuric acid? (equation)
HNO3 + H2SO4 <—–> NO2+ + H2O + HSO4-
What is eliminated from a electrophilic subsititution with benzene?
H+
What must be present for halogenation of benzene to take place?
a halogen carrier eg: AlCl3
What is the kekule model?
it demonstrates the double bonds present in benzene but doesnt consider the delocalised pi system
Why are aromatic compounds less reactive than alkenes?
delocalisation - there is a lower pi electron density in the aromatic ring so electrophiles are less strongly attracted
How do aromatic compunds react differently with electrophiles than alkenes with electrophiles?
aromatics react by substitution whereas alkenes react by addition
What is electrophilic substitution?
a chemical reaction in which the functional group attacged to a compound is replaced by an electrophile
What is the equation of the regeneration of the catalyst H2SO4?
(HSO4 -) + (H+) —–> H2SO4
What is observed when reacting benzene with bromine?
the bromine decoloursies - orange to colourless and steamy fumes of HBr are produced
What is the reversible reaction between Br2 and its halogen carrier?
Br2 + AlBr3 <—–> (Br+) + (AlBr4-)
What is the equation of the regeneration of the catalyst AlBr3?
(H+) + (AlBr4-) —–> AlBr3 + HBr
What happens in terms of bonding during an addition reaction?
replaces the pi bonding with new sigma bonds
How can an alkyl group be attached to a benzene?
electrophilic substitution with a halogen carrier forming an alkyl electrophile
What is the reversible reaction between CH3CH2Cl and AlCl3 to form the alkyl electrophile?
CH3CH2Cl + AlCl3 <—–> (CH3CH2+) + (AlCl4-)
Why are reactions with benzene and an alkyl group important?
important in organic synthesis to form C-C bonds and allows an organic molecule to be extended
What is the name for a benzene ring attached to another molecule?
a phenyl group
What is a aliphatic molecule?
any compound that doesnt contain a benzene ring
What does alicyclic mean?
any aliphatic (non-aromatic) compound that contains a ring
What is protonation?
adding of a proton ususally donated by H+ to an atom, molecule of ion
What is alkylation?
adding an alkyl group to a benzene
What is acylation?
adding an acyl group to a benzene
(carbonyl)
What is an acyl group?
a carboxylic acid functional group with a Cl instead of OH = a carbonyl - aldehyde or ketone
How many sigma bonds are present in benzene?
12
each C atom is bonded to another C and a hydrogen
How many delocalised electrons are in the delocalised pi system of benzene?
6
What is the molecular formula of benzene in its simplist form?
CH
How can you form cyclohexane from benzene?
hydrogenation - H2 and Ni catalyst
What state is bromine in to react with benzene?
liquid
Why is hydrogenation of benzene less exothermic than expected?
the pi system makes benzene more stable = more energy needed to break bonds
