Phenols Flashcards
Phenol + Na:-
Phenol salt (phenoxide) and H2 (g)
Phenol + NaOH:-
Phenoxide + H2O
Phenol + Na2CO3:-
Phenol too weak, no reactt
Phenol relative to alcohols and carboxylic acids:-
Less acidic than carboxylic acids, more than alcohols.
Phenol structure:-
Benzene ring w/ OH group
Why is phenol a weak acid?
The negative charge on the phenoxide ion is delocalised into the ring’s pi-system. Therefore, the phenoxide ion is stabilised. Consequently, phenol is a weak acid.
Phenoxide ion:-
Benzene ring with O^- branch.
Acid acidity level:-
Determined by stability of ion when H+ is lost. The more the negative charge can be delocalised, the more stable the ion.
Phenol use:-
Mainly used as disinfectants.
Phenol at room temp:-
Solid at room temp as is bigger molecule than ethanol w/ a bigger surface contact area = more induced dipole-dipole forces need to be broken to melt it.
Phenol water solubility:-
Only slightly soluble in water (compared to freely soluble ethanol). Phenol has a much larger non-polar hydrocarbon part which cannot form hydrogen bonds.
Why does phenol undergo electrophilic sub reactions much more easily than benzene?
The ring is “activated” by the -OH group, no catalyst needed.
Ring = activated:-
Lone pair of electrons delocalised into pi-system, increasing electron density and making it more attractive to an electrophile
The -OH is therefore said to be an electron donating group.
2,4,6 substitution:-
The electron density is increased most at these positions so this where substitution normally occurs e.g:-
Phenol + 3Br2 -> 2,4,6-tribromophenol + 3HBr.
White precipitate and bromine decolourised.
Benzene + Br vs Phenol+ Br:-
No solvent for Benz, water for Phenol.
Halogen carrier catalyst for Benz, none for Phenol.
1 H atom easily replaced for Benz, 3 for Phenol.
Blocking substitution:-
Only H on aromatic ring can normally be replaced i.e if another group is at 2,4 or 6, substitution will not happen there.
Electron-donating groups:-
-OH and -NH2 are edgs when bonded to aromatic ring bc the O in (in -OH) or N (in -NH2) has a lone pair which can delocalise into pi-system of ring, increasing electron density.
2 important effects of EDGs on electrophilic sub reactions:-
- increase ring reactivity bc higher electron density makes ring more attractive to an electrophile.
- direct sub reactions to take place at 2,4,6 positions unless blocking occurs.
Electron withdrawing groups:-
-NO2 is an EWG, which decreases electron density in aromatic ring it’s bonded to.
2 EWG effects on electrophilic sub reactions:-
- decrease ring reactivity as lower electron density = less reactive to an electrophile.
- if they are strongly deactivating, they direct sub reactions to take place at the 3 position, provided it isn’t blocked.
