Isomers And Aromaticity Flashcards
Explain why E-Z isomerism occurs
One end of the double bond is unable to rotate relative to the other
Describe E isomerism
The two atoms with the highest Ar are attached to opposite sides of the double bond
Explain why EZ isomerism may not be able to occur even if there is a double bond present
Two of the same atom or group may be bonded to one carbon
Define an enantiomer
Non-superimposable mirror-image isomers
Define a racemic mixture
An equimolar mixture of both optical isomers
What is a chiral carbon centre?
A carbon atom with 4 different groups bonded directly to it. As a result any carbon atom that has a double bond to another cannot be a chiral centre.
Define delocalisation
Electron pairs are shared between three or more atoms (e.g. in benzene)
Five reasons against kekule’s structure
- Benzene does not decolourise bromine water (as the double bonds in kekule’s structure suggest)
- Benzene tends to react by substitution rather than addition
- All bond lengths were found to be the same
- The enthalpy of hydrogenation is lower and more stable
- Only one 1,2-disubstituted product was found
Explain why benzene tends to react by substitution rather than addition
If addition occurred then the stable pi-electron cloud of benzene would be lost.
The preferred way for benzene to react is by substitution as this retains that delocalised structure of the molecule
What is electrophilic substitution?
Replacement of ‘hydrogen atoms’ by an electron deficient group (NO2+)
Nitration of Benzene
Reagents and Conditions
> Conc. sulphuric and nitric acid (to produce nitryl cation)
Reflux below 50°C (if temp rises above this then dinitration can occur producing a pale yellow solid of 1,3-dinitrobenzene)
Bromonation of Benzene
Reagents and Conditions
> FeBr3 catalyst (halogen carrier)
Not in direct sunlight (may encourage addition reaction)
Reaction needs to be carried out in a fume cupboard due to hydrogen bromide gas being given off
Chlorination of Benzene
Reagents and Conditions
> FeBr3 or AlCl3 catalyst
Not in direct sunlight
Anhydrous
Carried out in a fume cupboard
Define polarisation
An unequal electron distribution in a covalent bond
Friedel-Crafts Alkylation
Reagents and Conditions
> Chloromethane
AlCl3 or FeCl3
Not in direct sunlight
What is meant by refluxing?
Evaporation and condensation occur at the same time, and the condensed material returns to the flask.
Production of phenol from chlorobenzene
Reagents and Conditions
> 300°C
Increased pressure
Aqueous sodium hydroxide
Define stereoisomerism
A form of isomerism where the compound has the same structural formula but that atoms take up a different spatial arrangement
Examples of benzene addition reactions
- Benzene and hydrogen [temp>300°C, pressure>30atm]
2. Benzene and chlorine [sunlight]
