Chapter 25 - Aromatic compounds Flashcards
whats the bonding in benzene like?
- each carbon atom uses 3 of its avaliable 4 electrons to bond to 2 carbons and a hydrogen these are sigma bonds
- each carbon atom has one electron in a p-orbital at right angles to the plane of the bonded c&h atoms
- adjacent p-orbitals directly overlap sideways in both directions above and below the plane of C atoms to from a ring of electron density
- this overlapping of the p-orbitals creates a system of pi bonds which spread over all 6 caron atoms in the ring structure these 6 electrons are delocalised
what are the three pieces of evidence to disprove kekules benzene structure
- The lack of reactivity of benzene- if it contained c=c bonds then it would decololurise bromine water in electrophilic addition reactions but it doesnt it undergoes electrophilic substitution reactions instead
- The lengths of bonds in benzene - all of the bonds in benzene are the same length 0.139nm but double bonds are short than single bonds at 0.134nm to 0.153nm
- The hydrogenation enthalpy of benzene- if benzene had kekules structure it would have a enthalpy change of 3x cyclohexenes so -360 buts its actually -208 so less exothermic/ more stable than expected
what are the 2&4- directing groups
-NH2
-OH
-C6H5
-F, -Cl , -Br, -I
what are the 3- directing groups
-CHO
-COOH
-NO2
Where does the -NH2 group direct the second substituent to
position 2 or 4
where does the -NO2 group direct the second substituent to
position 3
what causes the need for a halogen carrier catalyst and a high temperature when bromine reacts with nitrobenzene
the -NO2 group deactivated the aromatic ring so the ring now reacts less readily with electrophiles
what causes bromine to react rapidly with phenylamine
the -NH2 group activates the aromatic ring so the ring reacts more readily with electrophiles
whys phenol more reactive than benzene
the increase in reactivity in phenol is caused by a lone pair of electrons from the oxygen p-orbital of the -OH group bring donated into the pi-system of phenol so the electron density of the benzene ring in phenol is increased this attracts electrophiles more strongly than with benzene
Why does the bromination of phenol not require a halogen carrier
there’s a lone pair of electrons being donated from the -OH to the pi-system of phenol this means the electron density in the phenol ring structure is sufficient enough to polarise bromine molecules so no halogen carriers required
What are the conditions & reagent needed for the nitration of benzene
Conc HNO3
conc H2SO4
50 degrees
How can common halogen carriers be generated
In Situ (in the reaction vessel) from the metal & the halogen
Reagents for bromination of benzene
Br2
AlBr3 halogen carrier
Room temp
What types of reactions does benzene undergo
Electrophilic substitution
What do directing groups help you tell on a molecule with two groups attached
Which group was substituted first
What are position 2 3 & 4 called
2- ortho
3- meta
4- para
What reactions do phenols undergo& under what conditions compared to benzene and how readily
Electrophilic substitution
Milder conditions and more readily than benzene
What’s phenols solubility like compared to alcohols
It’s less soluble in water than alcohols
What’s a phenol
An organic chemical containing an -OH group bonded directly to an aromatic ring
What happens if temp rises about 50 degrees in the nitration of benzene
Further substitution may occur leading to dinitrobenzene
How can nitration of benzene be altered to form dinitobenzene
Above 50 degrees
Excess HNO3 eg 2HNO3
Why does benzene need a halogen carrier to react with bromide
Because benzene has delocalised pi-electrons spread above and below the plane of carbon atoms in the ring structure the electron density around any 2 carbon atoms in a benzene ring is less than c=c in alkanes. When a non polar molecule eg Br approaches the benzene there’s insufficient pi-electron density around any 2 C to polarise the Br this prevents any reaction taking place
What’s the acidity of phenol like
It’s a weak acid
Why is phenol classified as a weak acid
When dissolved in water phenol partially dissociates forming a phenoxide ion and a proton
What reactions show that phenol is more acidic than alcohols but less acidic that czrnoxylic acids
- éthanol doesn’t react with NaOH (a strong base) or NaCO3 (a weak acid)
- phenols can’t react with solutions of strong bases like NaOH (aq) but not weak bases eg. NaCO3
- carboxylic acids phenolscan react with solutions of strong bases like NaOH (aq) and weak bases eg. NaCO3
3 steps to nitration of benzene
HNO3 + H2SO4 —> NO2+ + HSO4- + H2O
smiley face mechanism
H+ + HSO4- —-> H2SO4
3 steps of bromination of benzene
Br2 + AlBr3 —-> Br+ + FeBr4-
smiley face mechanism
H+ + FeBr4- —-> FeBr3 + HBr
what type of reaction is NaOH plus phenol and what is formed
a salt sodium phenoxide and water
it’s a neutralisation reaction
what colour precipitate is tribromophenol
white
reagents for bromination of phenol
3Br2
reagents for nitration of phenol
dilute HNO3
nitration of phenol what’s formed
H2O
2-nitrophenol or 4-nitrophenol
