Phenols Flashcards
What is a phenol structure?
an aromatic benzene ring with an -OH group attached directly
What state is phenol at room temp?
solid
Why is phenol solid at room temp unlike liquid alcohols?
phenol is a bigger molecule with a bigger contact surface area so it has more london forces that need to be broken to melt it
Why is phenol only slightly soluble?
has an -OH group which can form H bonds with water
but it has a much larger non-polar hydrocarbon part which cannot form H bonds
the H bonding effect is outweighed by the hydrophobic aromatic ring
What is the acidity of phenol like?
a very weak acidity - only a small proportion is ionised
What is phenols molecular formula?
C6H5OH
why is phenol more acidic than water and ethanol?
when H+ dissociates from an acid, it leaves behind a negative ion = the more stable this ion is the more favourable the dissociation will be (more acidic)
the (-) of the phenoxide ion is spread out in the pi system making the ion more stable
Do phenols react better with strong or weak bases?
strong bases like -OH
What is the reaction of phenol with NaOH?
C6H5OH + NaOH —-> C6H5O(-) Na(+) + H2O
what type of structure does C6H5O(-) Na(+) have?
ionic - simple molecular
What 2 weak base ions does phenol NOT react with?
carbonate - CO3 (2-)
hydrogen carbonate ions - HCO3 (-)
What test is used to distinguish between a phenol or carboxylic acid?
reacting with carbonate or hydrogen carbonate ions
phenol = will not react
carboxylic acid = fizzing
Why does phenol undergo electrophilic substitution more easily than benzene?
the ring in phenol is said to be activated by the -OH group because:
a lone pair of electrons on oxygen is delocalised into the pi system of the ring = increases electron density in the aromatic ring, making it more attractive to an electrophile
The OH is said to be an electron donating group
Where is electron density increased the most on phenol?
the 2, 4 and 6 positions
What is observed when phenol reacts with bromine?
bromine is decolourised
white precipitate - 2,4,6 - tribromophenol
What are the 2 electron-donating groups?
-OH and -NH2
How do electron-donating groups affect electrophilic substitution?
increase reactivity of the ring = higher electron density so ring is more attractive to an electrophile
they direct substitution reactions to take place at the 2,4 and 6 positions
What is an electron-withdrawing group?
a group of atoms attached to the benzene ring which decreases the electron density in an aromatic ring to which it is bonded
What is an example of an electron-withdrawing group?
-NO2
What effects do electron-withdrawing groups have on electrophilic substitution?
decrease the reactivity of the ring = lower electron density = less atrracted to electrophile
if strongly deactivating like -NO2, they direct substitution reactions to take place at the 3 position
reaction must take place with a halogen carrier catalyst
What is the pH of phenol roughly?
pH 5-6 = weak acid
What is deduced from the reaction of phenol with NaOH?
phenol acts as an acid by donating a H+ to the NaOH therefore it is a weak acid
What is deduced about phenol from reacting it with DILUTE nitric acid?
dilute = more reactive as benzene, does not require a fume cupboard like benzene
doesnt require an acid catalyst for nitration so phenol is more reactive
How would you test if an alkene or phenol is present?
add HNO3 as alkenes do not react and phenols form an orange precipitate
Why is phenol more reactive than benzene?
the p-orbitals on the oxygen atom in the phenoxide ion become delocalised in the pi-electron system in the benzene ring. This increases electron density of the delocalised pi-system making it more attractive to electrophiles and polarising electrophiles
What are all the reactions with phenol you need to know with naming the products?
Br2 (aq) = 2,4,6-tribromophenol
NaOH = sodium phenoxide and water
dilute nitric acid = nitrophenol (2 products 1st or 4th position = 1st position is major product)
strong base reactions
carbonates (weak bases = no reaction
Why is the OH in phenol called an activating group?
it donates electrons into the benzene ring = increases reactivity of phenol
What is the directing effect?
electron donating groups like OH and NH2 direct substituents towards the 2,4 and 6 posititon on the benzene ring
What is an example of electron withdrawing groups?
NO2
what is the directing effect of electron withdrawing groups?
carbon 3
