18A.1 Flashcards
In every heart of aromatic compound lies
benzene
what are the physical properties of benzene
- it has boiling point of 80 c
- colours liquid
- insoluble in water
- toxic nature - a carcinogen (cancer causing)
what are the aliphatic compounds
all compounds that aren’t aromatic
benzene is described as an
arene with the ene indicating C=c but sometimes their is no C=C
what does the Molecular formula for benzene (C6H6) say
that it is very unsaturated since alkane with 6 carbons has 14 hydrogens
benzene is found and obtained from
crude oil and the fuels from crude oil
what is a aromatic
the original meaning was a description of the smell of certain organic compounds. the new meaning is a description of the bonding in a compound - delocalized electrons forming a pi bonding in a hydrocarbon ring
how does the kekule structure of benzene look like
page 222
what are the 5 problems of the kekule structure
- if benzene contains a C=C then it should readily decolorize bromine water in an addition reaction, however it doesn’t and when they don’t react a substitution reaction occurs and this suggest that there is non C=C
- after benzene was discovered, many compounds containing the benzene ring were discovered, along those was dibromobenzene (C2H4Br2), if the kekule structure was correct then there should be 4 isomers of this compound, (page 223) but their are only 3 known to exist with the 2 isomers where the Br is attached to adjacent carbons are identical, but in one isomer the Br are attached with two carbons that have the bond of C–C and the other isomer the Br are attached to the carbons with a C=C bond in-between them, yet there structure is the same this suggest that the bonding between the carbons of the benzene ring are the same. (which goes against the kekule structure)
- data from X-ray diffraction about the lengths of covalent bonds in cyclohexene compared with the data from the covalent bonds of benzene suggest that the carbon-carbon bonds in benzene are all the same and perhaps an intermediate in character between C—C and C=C this goes against the Kekule structure of C-C and C=C being different
- once thermochemical data about enthalpy changes of hydrogenation becomes available another problem occurs. following the data the amount of energy required to add 1 mole of H2 to 1 mol of C=C bonds in cyclohexene(has only one C=C) bond is about 120 Kj mol^-1, and the value for cyclohexa-1,4-diene is double of that since there is two (3ani twice as many) C=C bonds present in Cyclohexa-1,4-dienen. now benzene following the kekule structure can be named as Cyclohexa-1,3,5-triene so following that trend it should have an enthalpy change of hydrogenation triple that of cyclohexene since it has triple the amount of C=C bonds but in reality Benzene has a much lower enthalpy change going against the kekule structure
- the infrared structure of cyclohexene and benzene show that cyclohexene has a absorption around 1650 which is typical to that off a alkene C=C stretch but Benzene has a stretch around 1500, 1580, 1450 which is typical of a aromatic showing that the carbon bonds of Benzene don’t fit the trend
what’s the enthalpy level diagram that shows the difference between Cyclohexa-1,3,5-triene (theoretical) and benzene
page 223
what is the new suggested model form benzene
so after all the sigma bonds in the carbon has formed and one electron in each P orbital is left imagine instead of the formation of 3 single pi bonds with the delocalized electrons, there would be one large Pi bond made up of all 6 electrons, where it’ll be 2 donut shapes one above the molecule and one under it with 3 electrons in each donut, the 6 electrons would then form a delocalized pi bond
does the new model of benzene solve all the problems mentioned earlier and if yes how
yes,
- there are no induvial C=C bonds, so there is no addition reaction with Bromine
- there are three isomers of Dibromobenzene as when Br atoms are on adjacent carbon atoms their is no difference in the arrangement of their electrons these atoms
- The carbon-carbon bonds are all the same length because they are identical, and are not individual C-C and C=C
- When a charge is spread around a species there is increased stability which explains why Benzene has greater stability than Cyclohexa-1,3,5-Triene
- The infrared spectrum of benzene has very strong absorption around 1500 which is typical for a aromatic compound, while absorption of C=C in non-aerometric compounds (aliphatic) is around 1650
why does substitution with Br atoms occur instead of addition when reacting with benzene
because the substitution of Br preserves the stability of the delocalized electrons in the pi bond, when the addition would produce a product with two C=C bonds and that will be less stable than the product formed in substitution
what is the skeletal formula for the new benzene model
page 225
