Benzene Flashcards
What is the molecular formula of benzene
C6H6
What are the characteristics of benzene
- colourless
- sweet smelling
- highly flammable liquid
Where can benzene be found
- naturally in crude oil
-petrol (benzene is a component) - found in cigarette smoke
What is benzene classified as
- a carcinogen
What can benzene cause
Cancer
What is an aromatic compound
Compounds that contain a benzene ring
Explain The Kekulé model of benzene
- he suggested that benzene was based on a 6 carbon ring, and that each carbon was joined by alternating single double bonds
- cyclohexa-1,3,5-triene
What are the 3 pieces of evidence that disprove Kekulé’s model
1) THE LACK OF REACTIVITY OF BENZENE
- if benzene contained the C=C bond it should decolourise bromine in an electrophilic addition reaction. BUT benzene doesn’t undergo electrophilic addition reactions and it doesn’t decolourise bromine
*so benzene cannot have any C=C bonds
2) THE LENGTHS OF CARBON-CARBON BONDS IN BENZENE
- when’s-ray diffraction is used they found all the bond lengths were the same (0.139nm) which is between the length of a single bond (0.153nm) and a double bond (0.134nm)
3) HYDROGENATION ENTHALPIES
- the enthalpy change of hydrogenation of cyclohexene is -120kJmol-1 so if benzene had Kekulé’s structure then it would be expected to have an enthalpy change of hydrogenation of -360kJmol-1 but the actual enthalpy change of hydrogenation of benzen is -208kJmol-1. Which means the actual structure of benzene is more stable than the Kekulé’s model of benzene
Explain the main features of delocalised model of Benzene
- planar (120 degrees), cyclic, hexagonal hydrocarbon containing 6 carbon atoms and six hydrogen atoms
- each carbon atoms uses three of its available four electrons in bonding (to two other carbon atoms and one hydrogen atom)
- each carbon atom has one electron in a p-orbital at right angles
- adjacent p-orbital electrons overlap sideways, above and below the plane of carbon to form a ring of electron density
- the overlapping of the p-orbital creates a system of pi-bonds
- the 6 electrons are said to be delocalised
What does the term monosubstituted mean
Aromatic compounds with one substituent group
What is the benzene ring considered to be when naming aromatic compounds
The parent chain
When a benzene ring is attached to an alkyl chain with a functional group or to an alkyl chin with scene or more carbon atoms, what is benzene considered to be, what happens
- A substituent
- so instead of benzene the prefix phenyl is used
An NH2 attached to a benzene ring is called what
Phenylamine
A COOH attached to a benzene ring is called what
Benzoic acid (benzenecarboxylic acid)
A CHO group attached to benzene ring is called what
Benzaldehyde (benzenecarbaldehyde)
What are disubstituted compounds
When a benzene ring has two substituent groups attached to it
What are two rules when naming disubstituted compounds
- has to be listed in alphabetical order
- have to use the smallest numbers possible
What reactions do benzene molecules undergo
- Electrophilic substitution
What is an electrophile
An electron pair acceptor to form a new covalent bond
Describe the nitration of benzene
- reacts slowly with nitric acid (HNO3) to form nitrobenzene and water
- reaction is catalysed by sulfuric acid (H2SO4)
-heated to 50 degrees celsius
Equation:
H2SO4
C6H6 + HNO3 ———> nitrobenzene + H2O
50 degrees celsius
What happens if the temperature rises above 50 degrees celsius
- further substitution
- produces dinitrobenzene
Equation:
H2SO4
Benzene + 2HNO3 ——> 1,3-dinitrobenzene + 2H2O
70 degrees celsius
What is nitrobrenzene used in
important starting material in:
- the preparation of dye
- pharmaceuticals
- pesticides
- paracetamol
What do halogens need to react with benzene
- A catalyst called a halogen carrier
What are common halogen carriers
- AlCl3
- FeCl3
- AlBr3
- FeBr3
How are halogen carriers generated
- in situ (in the reactions vessel) from the metal and the halogen
Explain the bromination of benzene
- happens at room temperature and pressure
-electrophilic substitution reaction
-either FeBr3 or AlBr3 is used as the halogen carrier
Overall equation:
FeBr3/ AlBr3
Benzene + Br2 ——> bromobenzene + HBr
Explain the chlorination of benzene
- reacts in the same way as bromine
- happens at room temperature and pressure
- halogen carriers= FeCl3 or AlCl3
Overall equation:
FeCl3/ AlCl3
Benzene + Cl2 ——> chlorobenzene + HCl
