HYDROXY Flashcards
Why cant phenols undergo Nu Sub unlike alcohols(2 reasons)
Partial double bond char: The substitution involves the breaking of the C-O bond. The p-orbital of the O atom overlaps w/ the π e- cloud of the BZ ring, resulting in C-O bond having a partial double bond character, thus the amt of energy req to break the C-O bond is higher than a normal C-O bond, thus Nu sub does not happen.
Inter e- repulsion: Nucleophiles are e-rich species which will face inter-electronic repulsion with the π e- system of the BZ ring, BZ ring also provides high steric hindrance, hindering the approach of the nucleophile
Why are phenols more reactive towards E-Sub compared to alcohols/benzene
The -OH group bonded to benzene is an activating EDG as the O atom carries a LP of e- which can be delocalised into the BZ ring and increases the e- density of the ring, which allows the benzene ring to attract E+ more easily, thus making the benzene ring in phenol much more susceptible towards Esub
DT using alcohol(Na(s))
Effervescence of H2 gas that extinguishes lighted splint with a pop sound
DT using PCl5 + eqn
All Alcohols: White fumes of HCl formed
eqn: ROH + PCl5 -> RCl + POCl3 + HCl
phenol: no white fumes
List all DT for alcohol
Na(s), PCl5 rtp, K2Cr2O7(primary,secondary)/KMnO4(all) H2SO4(aq) heat, I2 NaOH heat
List all DT for phenol
Neutral FeCl3(aq), Br2(aq) rtp
DT using K2Cr2O7, H2SO4(aq), heat
1°, 2°: Orange K2Cr2O7 turns green
3°,phenol: Orange K2Cr2O7 remains orange
DT using KMnO4, H2SO4(aq),warm using a water bath
1°, 2°, phenol: purple KMnO4 decolourises
3°: Purple KMnO4 remains purple
DT using iodoform(I2(aq), NaOH, warm using water bath) + eqn
2°: Yellow ppt of ChI3 formed, decolourisation of brown iodine solution seen
eqn: RCH(OH)(CH3) + 4I2 + 6OH- -> RCOO- + CHI3 + 5I- + 5H2O
the rest: no ppt formed
DT using Neutral FeCl3(aq)
Phenol: A violet complex observed
the rest: no colouration observed
DT using Br2(aq), rtp
Phenol: Orange aqeuous Bromine soln rapidly decolourises, white ppt of 2,4,6-tribromophenol will be observed
eqn: C6H5OH + 3Br2(aq) -> C6H5OHBr3 +3HBr
Compare the acidity of alcohols with water and explain why
The conjugate base of alcohols(RO-) are less stable than that of water(OH-) as the presence of e- donating alkyl groups increase the intensity of the negative charge on the O atom in the alkoxide RO- ion. RO- is less stable than OH-, thus ROH has a smaller extent extent of dissociation, and is a weaker acid than water.
Compare the acidity of phenols with water and explain why
Phenols are a stronger acid than water. The p orbital of the O atom overlaps w/ the π e- cloud of the BZ ring, resulting in deloc of the -ve charge on oxygen into the ring, reducing the intensity of the -ve charge. Thus the CB of phenol(C6H5O-) is more stable than the CB of water(OH-) due to charge dissociation. Hence C6H5O- will have a greater extent of dissociation and is a stronger acid.
List all synthesis of alcohols
1) EAdd of Alkene w/ H2O
2) Nu sub of RX
3) Reduction of Aldehyde to primary alc
4) Reduction of ketone to secondary alc
5) Reduction of carboxylic acid to primary alcohol
Why must LiAlH4 be in dry ether
LiAlH4 is able to react with water which destroys the RA, thus reduction must be done in anhydrous condition where LIAlH4 is in dry ether
What is the role of H2SO4 in condensation rxn
Catalyst(increase rate of rxn)
Dehydrating agent(removes water, by LCP, POE shift right to favour formation of ester, increase yield of ester)
Why do condensation reactions of alcohol w/ acyl chloride require much milder conditions compared to condensation of alcohols with carboxylic acid?
There is a much higher EN difference between C-Cl compared to C-O therefore the C atom on COCl is more electron deficient which allows the nucleophile to attack the carbon atom more easily, thus the reaction does not require H2SO4 or heat
Why do phenols require NaOH(aq) in condensation rxn with acyl chloride while alcohols do not?
Phenol is a weaker Nu than alcohol as its LP of e- on O has delocalised into the BZ ring, hence the LP is less avail for condensation reaction, therefore the phenoxide ion must first be produced via an acid base rxn as it is a stronger Nu
