Chapter 25: questions Flashcards
Explain relative resistance to chlorination of an alkene with benzene
In alkenes, the pi-bond is localised whilst in benzene, the pi-system is delocalised.
Therefore, electron density is higher in alkenes, making them more susceptible to electrophilic attack // Alkenes polarise electrophiles more
Similarities/ differences between bonding in kekule’s model vs delocalised model in terms of orbital overlap
Both contain pi bonds which sit at 90 degrees above and below the carbon plane, where there is an overlap of p orbitals.
Difference:
Kekule model: contains 3 alternating pi bonds with a localised electron density
Delocalised model: 6 pi orbitals which delocalise the electron density, forming rings of electron density above and below the entire ring
Why does phenol nitrate more readily than benzene
(In phenol) a (lone) pair of electrons on O is(partially)
delocalised/donated into the π-system / ring
–> electron density is greater than benzene
–> this makes phenol more susceptible to electrophilic attack
Describe a method to obtain a pure sample of 3-nitrobenzoic acid from impure solid and to check its purity
Recrystallisation
* Dissolve impure solid in minimum
volume of hot water/solvent (heated using a hotplate)
* Cool solution and filter solid
* Wash with cold water/solvent and dry
- Checking purity
* Obtain melting point
* Compare to known values
* Pure sample will have a (sharp)
melting point very close to data
book value
OR
3. Run a TLC
* Compare (Rf value) to known
data
* Pure sample will have a very
similar Rf
Trend in relative ease of nitration of phenol, benzene and benzoic acid + explain why this is the case
Phenol= most reactive due to the lone pair of electrons on oxygen being partially delocalised into the pi-system
Benzoic acid= least reactive due to the COOH group being an electron withdrawing group. This withdraws electrons from the ring, reducing the electron density of benzoic acid, making the ring less susceptible to attack
