chemistry chapter 25-aromatic compounds

Question 1

what is benzene?

Answer 1

-simplest aromatic hydrocarbon aka arene

-colourless, sweet smelling, highly flammable liquid

-found naturally in crude oil, is a component of petrol, and also found in cigarette smoke

-classified as a carcinogen

Question 2

What is an aromatic hydrocarbon?

Answer 2

Hydrocarbon that contains at least one benzene ring (can be called arene)

Question 3

What was Kekule’s model of benzene?

Answer 3

A six membered ring with alternating double and single bonds

Question 4

what evidence is there to disprove Kekule’s model?

Answer 4

1) The lack of reactivity of benzene
-benzene does not undergo electrophilic addition reactions
-benzene does not decolourise bromine under normal conditions
-so cannot have any c=c bond

2)The lengths of the carbon-carbon bonds in benzene
-using x-ray diffraction it is possible to measure bond lengths
-all bond lengths in benzene were the same 0.139nm which was between the length of a single and double bond

3)Hydrogenation enthalpies
-expected enthalpy change is -360 real is -208 so it is less exothermic and more stable than in Kekule’s model

Question 5

what is the delocalised model of benzene?

Answer 5

Planar, cyclic hexagonal hydrocarbon.

3 electrons bonded to adjacent carbons or hydrogen atoms.

4th electron is in a p-orbital right angle to the plane of the bonded carbon and hydrogen atoms

P orbital electrons overlap sideways in both directions above and below the plane. Forming a ring of electron density.

The overlapped P-orbitals creates a system of pi bonds spread over all 6 carbons in the ring.

The 6 electrons are delocalised

Question 6

what prefix to use when?

Answer 6

when compound with less than 7 carbon atoms added to structure use benzene after the group prefix
when a benzene ring is attached to an alkyl chain with a FUNCTIONAL GROUP or SEVEN OR MORE CARBON ATOMS, benzene is the substituent. The prefix phenol is used

Question 7

benzoic acid (benzenecarboxylica cid)

Question 8

phenylamine

Question 9

benzaldehyde (benzenecarbaldehyde)

Question 10

how do most reactions of benzene proceed?

Answer 10

electrophilic substitution

Question 11

nitration of benzene

Answer 11

-benzene reacts slowly with nitric acid to form nitrobenzene
-the reaction is catalysed by H2SO4 and heated to 50c for good rate of reaction
-water bath used to maintain steady temperature
-one hydrogen on the benzene rind is replaced by a nitro (NO2) group
-if temp > 50c further substitution reactions may occur leading to dinitrobenzene

Question 12

what is the electrophile in the nitration of benzene?

Answer 12

nitronium ion (NO2+) produced by the reaction of concentrated nitric and sulfuric acid

Question 13

mechanism for nitration of benzene?

Answer 13

1) HNO3 +H2SO4 ->NO2+ +HSO4- +H20
2)the electrophile accepts a pair of electrons from the benzene ring to form a dative covalent bond. The organic intermediate formed is unstable and breaks down to form the organic product by removing the hydrogen atom
3)H+ + HSO4- -> H2SO4

Question 14

halogenation of benzene?

Answer 14

halogens do not react with benzene unless a catalyst called a halogen carrier is present

1)benzene is too stable

Question 15

common halogen carriers

Answer 15

FeCl3, AlBr3, AlCl3, FeBr3, which can be generated in situ

Question 16

mechanism for halogenation of benzene

Answer 16

1) Br2 + FeBr3