25-Aromatic Compounds Flashcards
Benzene properties
• Colourless with a sweet smell
• Found naturally in crude oil
• Carcinogenic
Kekule model
Evidence to disprove kekule
- lack of reactivity
- length of c-c bonds
- hydrogenation enthalpies
• Lack of reactivity
o Double bonds should decolourise bromine water
o Benzene does not discolourise bromine water
o Therefore doesn’t undergo electrophilic addition reactions
o Suggests benzene cant have any double bonds
• Length of C-C bonds
o Bond lengths can be measured using x-ray diffraction
o The bond length of a single bond = 0.153 nm
o Bond length of double bond = 0.134 nm
o In benzene, all the bonds were the same length (0.139 nm)
• Hydrogenation enthalpies
o The hydrogenation enthalpy of benzene is less exothermic than expected.
o The structure is more stable than the Kekulé model of benzene suggests.
Shape of benzene
trigonal planar
How is benzene formed
• The p-orbitals are above and below the plane of the benzene ring
• Sideways overlap of p-orbitals creates a delocalised ring of electron density above and below the plane of the benzene ring
Features of new benzene model
- Planar, cyclic, hexagonal hydrocarbon with 6 Cs and 6 Hs.
Main difference between kekules and new model
• In kekule or any double bond // electrons in pi bond are localized + in benzene they are delocalized
When is benzene a parent chain
Prefixes to benzene:
o Alkyl groups (any hydrocarbon chain with less than 7 C’s) – E.g. CH3, C2H5
o Halogens - F, Cl, Br
o Nitro groups - NO2
Name
propylbenzene
Name
bromobenzene
When is benzene considered a substituent chain
• An alkyl chain with a functional group
• An alkyl chain with seven or more carbon atoms
What to do when Benzene is a substituent chain
• In these cases, the prefix phenyl- is used
Name
2-phenyldecane
Name
3-phenylpropanoic acid
What to do with Compounds with more then 1 substituent on con benzene ring
• Need to number the carbon ring
• Numbering begins with one of the substituent groups
• Working out where the number starts – look to see if it is based on methylbenzene, chlorobenzene or nitrobenzene etc.
• The substituent groups are listed in alphabetical order using the smallest numbers possible
Practise
Explain the difference in reactivity between arenes and alkenes
Electrophilic substitution
• When benzene reacts with an electrophile, a hydrogen atom is replaced by another atom or group of atoms.
• The electrophile is attracted to the electron rich benzene ring.
What is the electrophile attracted to
electron rich benzene ring.
Electrophilic substitution of benzene and chloride ion
Electrophilic substitution with benzene - four types
- Nitration of Benzene
- Halogenation of Benzene (bromination and chlorination)
- Alkylation Reactions
- Acylation Reactions
