Chapter 25.3/4 Aromatic compounds Flashcards
25.3 Chemistry of phenol 25.4 Directing groups
What are phenols?
Organic chemicals that contain a hydroxyl OH group functional group directly bonded to an aromatic ring
C6H5OH = phenol
–> NOT alcohol
Phenol as a weak acid: solubility in water
Phenol is less soluble in water than alcohols due to the NON-POLAR benzene ring
–> partially dissociates in water, forming phenoxide ion and proton (C6H5O- + H+)
Phenol as a weak acid: acidity
phenol= more acidic than alcohols but less acidic than carboxylic acids
–> found by comparing Ka (acid dissociation constant)
Phenol as a weak acid: test for differentiating between phenol and carboxylic acid
Reaction with Sodium carbonate as:
- ethanol does not react with sodium hydroxide NaOH (strong base) or sodium carbonate Na2CO3 (weak base)
- Phenols and carboxylic acids react with strong bases
-ONLY carboxylic acids can react with weak bases like Na2CO3
Phenol reacting with NaOH
C6H5OH + NaOH –> C6H5Na+O- + H2O
neutralisation: forms salt ( sodium phenoxide) and water
Reactivity of phenol vs benzene
OH functional group makes phenols more reactive than benzene
- oxygen brings lone pair which can become PARTIALLY DELOCALISED into the ring, making it more electron dense
- increased electron density attracts electrophiles more strongly
=> aromatic ring of phenol is more susceptible to attack from electrophiles than benzene
Bromination of phenol: standard reaction
Phenol reacts with aqueous solution of bromine to form white precipitate of 2,4,6-tribbromophenol
- reaction decolorises bromine water (orange to colourless)
- halogen carrier is not needed (phenol more reactive than benzene) and room temperature
Bromination of phenol: equaiton
C6H5OH + 3Br2 –> C6H2Br3OH + 3HBr
nitration of phenol
Phenol reacts readily with DILUTE nitric acid at room temp (no catalyst)
- forms either 2-nitrophenol or 4-nitrophenol
What are activating groups? name all examples
Groups that increase the reactivity of benzene ring aka electron donating groups
Includes: NH2, NHR, OH, OR, R
What are deactivating groups? name all examples
Groups that decrease the reactivity of benzene ring aka electron withdrawing groups
Includes: COR (carbonyl) , COOR, SO3H, CHO (aldehyde), COOH, CN (nitrite), NO2, NR3+, Halogens
–> all of these have double/ triple bonds and also contain permanent dipoles due to large difference in electronegativity)
What happens due to activating groups?
Reactions will be faster / require lower temperatures/ need more mild conditions in relation to benzene itself
What happens due to deactivating groups?
Reactions will be slower/ require higher temperatures/ need more harsh conditions in relation to benzene
What are Ortho/para directing groups?
Groups force an additional group to substitute onto ortho (2nd carbon) and para (4 carbon) on benzene ring
includes: NH2, NHR,OH,OR,R or C6H5 or HALOGENS
How does ortho/para groups affect the benzene ring?
These groups donate electrons onto ring
- increases reactivity of ring (except when halogens attach)
- 2nd stabilisation at ortho or para position