25.1 Introducing benzene Flashcards
What is the molecular formula of benzene?
C6H6
What is the empirical formula of benzene?
CH
Properties of benzene and where it is found
-colourless
-sweet smelling
-high flammable liquid
-found naturally in crude oil, component in petrol and cigarette smoke
-classified as a carcinogen (causes cancer)
-naturally found in volcanoes and forest fires
-found in glues, cleaning products, paint stripper and gasoline
What is the structure of benzene?
-hexagonal ring of 6 carbon atoms, with each carbon joined to two carbon atoms and one hydrogen atom
-aromatic hydrocarbon or arene
What was Kekule’s suggested model of benzene?
Hexagonal ring with 6 carbon atoms joined by alternating double and single bonds between carbon atoms
What are the three pieces of evidence used to disprove Kekule’s structure?
- Lack of reactivity
- Length of the carbon-carbon bonds
- Hydrogenation enthalpies
How was lack of reactivity used to disprove Kekule’s structure of benzene?
-If benzene contained C=C bonds it should be highly reactive and should decolourise bromine water in an electrophilic addition reaction
-It did not discolour bromine water and therefore does not contain C=C bonds
How was the length of C-C bonds in benzene used to disprove Kekule’s structure of benzene?
-Using x-ray diffraction you can measure bond lengths in a molecule
-All the bond lengths in benzene were the same (0.140 nm, between the length of C-C single bond and a C=C)
-If Kekule was right, we would expect 3 longer C-C and three shorter C=C double bonds
How was hydrogenation enthalpies used to disprove Kekule’s structure of benzene?
-Kekle’s structure would be named cyclohexa-1,3,5-triene
-Benzene expected to have an enthalpy change of hydrogenation of 3 x cyclohexene as cyclohexane has 1 double bond
-When cyclohexene hydrogenated one double bond reacts with hydrogen to give the enthalpy change of hydrogenation -120 kJ/mol
-Kekles structure is predicted to have 3 double bonds
3 x -120 = -320 kJ/mol
-the actual enthaply change of hydrogenation of benzene is only -208 kJ/mol
What was Kekule’s equilibrium model?
-He tired to resolve problems by suggesting that there were two forms of benzene that rapidly interconverted (molecule flipped)
-He proposed that this model could explain the low reactivity and as the structure changed the location of double bonds changed before any attracted model had time to react
What is the delocalised model of benzene?
-Each carbon atom uses three of its four electrons in bonding to bond to two other carbon atoms and one hydrogen atom
-Each carbon atom has one electron in a p-orbital at right angles to the plane of the bonded carbon atom and hydrogen atom
-Adjacent p-orbitals overlap sideways in both directions above and below the plane of carbon atoms in the ring structure
-This overlapping of p-orbitals creates a system of pi-bonds which spread all over the 6 carbon atoms in the ring structure
-The six electrons occupying the system of pi-bonds are said to be delocalised
What type of compound is benzene?
Aromatic
How is a benzene with an alkyl group, halogen or nitro groups (NO2) named?
The alkyl group, halogen, nitro group (NO2) are considered prexies to benzene
Name this compound
ethylbenzene
Name this compound
chlorobenzene
Name this compound
nitrobenzene
When is benzene considered as a prefix?
When it is attached to an alkyl chain with a functional group or to an alkyl chain with seven or more carbon atoms, benzene is considered to be substituent and prefix phenyl is used
Name this compound
phenylethanone
Name this compound
2-phenyloctane
What are the exceptions for IUPAC rules?
Benzoic acid, phenylamine, benzaldehyde
Name this compound
benzoic acid
Name this compound
phenylamine
Name this compound
benzaldehyde
What is an arene?
A hydrocarbon containing one of more benzene rings
What reactions to Benzene undergo?
Electrophillic substitution
