1.10 Aromatic Chemistry Flashcards
Who proposed the first structure for benzene?
Kekulé
Describe Kekulé’s structure for benzene
three C=C and three C-C bonds
give evidence the Kekulé structure was incorrect
- X ray studies revealed all 6 of he carbon-carbon bond lengths in benzene are the same length (0.139nm), and intermediate between C-C (0.153nm) and C=C (0.134nm)
- Energy required to hydrogenate 1 mole benzene should be 3 times the energy required to hydrogenate 1 mole of cyclohexene to cyclohexane. If so, the energy change should be -360kJmol-1, however it is actually -208kJmol-1. The extra energy is the stability that is conferred by the π-system of electrons.
Describe the structure of benzene
- Consists of six carbon atoms in a hexagonal planar ring arrangement (each carbon is bonded in a trigonal planar arrangement with a 120º bond angle)
- Each carbon is bonded to two other carbons and a hydrogen. As carbon has 4 electrons in its outer shell, each carbon has an unbounded p orbital electron.
- The p orbitals containing the free electrons can overlap to form a π-delocalised system of electrons above and below the plane of the main ring.
what is the name for C6H5OH?
phenol
what is the name for C6H5CH3?
methylbenzene/tolune
what is the name for C6H5NH2?
phenylamine
aniline
what is the name for C6H5I?
iodobenzene
what is the name for C6H5Br?
bromobenzene
what is the name for C6H5COOH?
benzoic acid
describe the reactivity of benzene
- Arenes are very stable due to the stability of the delocalised π system of electrons in the six membered rings. Therefore it doesn’t undergo addition reactions easily, so the majority of reactions involve substitution to ensure the delocalised π system remains intact.
- Benzene attracts electrophiles due to the region of high electron density above and below the plane of carbon atoms.
describe the mechanism for electrophilic addition of bromine to cyclohexene
- electrons move from double bond in cyclohexene to bromine atom in the bromine molecule. Electrons move to the other bromine atom
- intermediate carbocation of the double bond going, one bromine bonded to the cyclohexene, and carbon 2 being positive, with electrons from bromine anion moving to carbon 2
- final molecule formed - 1,2-dibromocyclohexane
Describe the conditions required for the monohalogenation of benzene
Takes place in the presence of a halogen carrier catalyst - such as iron or aluminium chlorides or bromides (FeCl3, FeBr3, AlCl3, AlBr3)
why is a halogen carrier catalyst used for monohalogenation of benzene?
Halogen carriers make the halogen strongly electrophilic - cause heterolytic fission of the X-X bond, electrophilic X+ then attacks the ring.
What are Lewis acids and why are they used as halogen carriers in benzene reactions?
the metal doesn’t have full octet and therefore has a vacant orbital to attract electrons to (eg. FeBr3, AlCl3, metallic iron (forms FeBr3 in presence of bromine))
Give the mechanism and equation for the generation of the electrophile in the monobromination of benzene
electrons in bromine molecule move to one bromine atom, and then to the aluminium in the AlBr3 halogen carrier
Cr2 + AlBr3 -> Br+ + AlBr4-
describe the mechanism for the monobromination of benzene
- electrons move from the pi bond system in benzene to the Br+ electrophile
- carbocation intermediate where the pi bond is broken, the molecule is positively charged, and hydrogen and bromine are bonded to the same carbon. The electrons in the hydrogen move into the pi system
- hydrogen leaves as a H+ ion, and bromobenzene is formed as the pi bond system is reformed.
Give the mechanism and equation for the regeneration of the catalyst in the monobromination of benzene
electrons in AlBr4- move to H+, bromine molecule breaks off and forms a coordinate bond with the hydrogen
AlBr4- + H+ -> AlBr3 + HBr
describe the appearance of nitrobenzene
a yellowish oily liquid
describe the uses for nitrobenzene
the starting material for a variety of substances including dyes and explosives
name the electrophile ion used in the nitration of benzene and how it is formed
NO2+ - nitronium ion
obtained from nitrating mixture of concentrated sulfuric and concentrated nitric acid
describe the conditions necessary for the nitration of benzene, and why they are necessary
Temperature must be kept below 10ºC to prevent di- and tri-substituted nitrobenzene rings, which are explosive compounds
give the equation for the generation of the electrophile in nitration of benzene
2H2SO4 + HNO3 _> NO2+ + H3O+ + 2HSO4-
Describe the mechanism involved in the nitration of benzene
- Nitronium ion is attracted to the high electron density of the π system in the benzene ring, substituting one of the H atoms in electrophilic substitution.
- electrons in pi system move to NO2+
- carbocation intermediate where H and NO2 are bonded to the same carbon, pi bond system is broken, and molecule is positive. electrons in H move into pi bond system
- H breaks off as H+ and nitrobenzene is formed as the pi bond system is restored
