Phenols Flashcards
(17 cards)
What are phenols?
- AROMATIC compounds
- Hydroxyl group, -OH, is attached directly to the benzene ring
What happens when a PHENOL is dissolved in water?
- Loses the H+ from the -OH group
- The NEGATIVE CHARGE on the OXYGEN can be stabilised via its DELOCALISATION around the benzene ring
- REVERSIBLE reaction
- Aqueous phenol forms a WEAK acidic solution
- Position of equilibrium ~ LEFT
- Concentration of protons in the mixture ~ LOW
What happens when an ALCOHOL is added to water?
- Does NOT form an acidic solution
- The negative charge on the oxygen CANNOT be stabilised via delocalisation when the proton is lost
- The O- ion strongly attracts the H+ ion and the alcohol is REFORMED
Reaction of phenol with NaOH
acid + alkali —– salt + water
C6H5OH + NaOH —- C6H5O-Na+ + H2O
- An aqueous phenol contains aqueous phenoxide ions and protons
Salt ~ Sodium Phenoxide
Bonding ~ IONIC
- The H+ has been replaced by na+
Reaction of phenol with Na
acid + metal —- salt + hydrogen
C6H5OH + Na —- C6H5O-Na+ + 1/2H2
Salt ~ Sodium phenoxide
SODIUM:
- loses an electron (Na —- Na+ + e-)
- Reduces H+(aq) to produce 1/2H2(aq)
The Acidity of functional groups
- Strong base ~ NaOH
- Weak base ~ Na2CO3
Strong Base :
H+(aq) + OH-(aq) ——- H2O(l)
Alcohol ~ no reaction
Phenol ~ reaction takes place
Carboxylic acid ~ reaction takes place
Weak acid :
2H+(aq) + CO32-(aq) —— CO2(g) + H2O(l)
Alcohol ~ no reaction
Phenol ~ no reaction
Carboxylic acid ~ reaction takes place
How to distinguish between a carboxylic acid and a phenol?
UI INDICATOR ~ confirms their acidity
CARBONATE ~
Carboxylic acid ~ effervescence
Phenol ~ NO effervescence , not acidic to react with carbonates
Phenols & water
- observations
- inference
OBSERVATION:
- Orange with universal indicator ~ pH 3
- Crystals dissolve slowly
- Some crystals do not dissolve & sink to the bottom of the test tube
INFERENCE:
- Phenol is sparingly soluble in water
- Aqueous phenol is acidic
Phenol & sodium hydroxide
- Observations
- Inference
OBSERVATION:
- white emulsion forms when H2SO4 added
- Crystals dissolve more readily in NaOH(aq) than water
INFERENCE:
- Phenol is soluble in NaOH(aq)
- The reaction is reversible to produce solid
phenol
Phenol & bromine water
- Observations
- Inference
OBSERVATIONS:
- Colour change from orange to colourless
- White ppt
INFERENCE:
- Phenol can react with bromine directly without the need of a halogen carrier
Phenol & Sodium metal
- Observations
- Inference
OBSERVATIONS:
- effervescence
- solid dissolves
INFERENCE:
- Liquid phenol reacts with metals to produce H2 gas
- The H+ ion from phenol reacts with the metal
Why is phenol only slightly soluble in water?
- The -OH group forms hydrogen bonds with water
- However, the benzene ring can only form weak London forces with water
- The benzene rings of phenol form London forces with each other
- This causes layers to form
What is observed when Br2(aq) is added to phenol?
- Phenol DECOLOURISES aqueous bromine
- Produces a white precipitate of 2,4,6-tribromophenol
The reactivity of phenol in comparison with benzene, towards bromine
BENZENE:
To enable bromine to react with benzene , a halogen carrier catalyst is required
PHENOL:
- does not need a halogen carrier
- More reactive than benzene towards halogens
- The reaction between phenol and bromine water takes place with RELATIVE EASE
Which structural feature in phenol is responsible for its greater reactivity in comparison with benzene?
- The lone pairs of electrons on the oxygen in the phenol group
- Each lone pair is located in a separate p-orbital
The reactivity of phenol
- One of the lone pairs of electrons on the oxygen atom is DELOCALISED into the benzene ring via p-orbital overlap
- The electron density of the ring structure is INCREASED
- The 8 pi-electrons are delocalised up to the oxygen
BROMINE:
- sufficiently POLARISED
- Attracted more strongly to the ring in phenol as compared with benzene
- therefore a reaction occurs without halogen carrier
Electrophilic Substitution in phenol
- The -OH group in phenol directs SUBSTITUTIONS to the 2,4 and 6 positions in the benzene ring
- Occurs due to the DELOCALISATION of the oxygen lone pair into the benzene ring produces the highest electron density at these positions
