Chapter 25 - Benzene Flashcards
Properties of benzene
Colourless with a sweet smell
Found naturally in crude oil
Carcinogenic
Describe the Kekule Model of benzene:
6 membered rings of carbon joined together by alternating single and double bonds
What is the evidence to disprove the Kekule Model?
Benzene is unreactive - it does not decolourise bromine water or undergo electrophilic addition reactions
In benzene, all the bonds are the same length - single bonds are typically longer than double bonds
The hydrogenation enthalpy of benzene is less exothermic than expected
Which model of benzene is now more widely accepted?
The delocalised model
Describe the delocalised model of benzene
The delocalised structure of benzene forms as a result of the sideways overlap of p orbitals above and below the plane of carbon atoms, forming a pi-electron cloud
What are the 6 main features of the delocalised model for benzene?
It is a planar, cyclic hexagonal hydrocarbon with 6 carbons and 6 hydrogens
Each carbon uses 3 electrons in bonding: 2 to other carbons and 1 to hydrogen
Each carbon has one electron in a p orbital at a right angle to the plane of the bonded C and H atoms
Adjacent p orbitals overlap sideways in both directions (above and below the plane) to form a ring of electron density
Creates a system of pi bonds spread over the carbon atoms
All 6 electrons in this system are delocalised
When naming benzene, when is it used as a parent group
When bonded to:
Alkyl groups with chains less than 7 carbons
Halogens
Nitro groups (NO2)
When naming benzene, when is the prefix phenyl-used
When bonded to:
An alkyl chain with a functional group
An alkyl chain with 7 or more carbons
What are the exceptions when naming benzene compounds?
