Hydroxyl compounds (Alcohols) Flashcards
Physical properties of hydroxyl compounds
Structure: Simple molecular structure, polar molecule
1. Higher BP than alkanes (hydrogen bond > idid)
2. BP increases down the group
3. Alcohols and phenols soluble in water (hydrogen bond between water and alcohol/phenols > hydrogen bond between water molecules + hydrogen bond between alcohol/phenol molecules)
4. Alcohols and phenols soluble in organic solvents ( idid between alcohols/phenol molecule and organic molecules > idid between organic molecules + hydrogen bond between alcohol molecules)
Synthesis of alcohols
- electrophilic addition of alkenes
- nucleophilic substitution of RX
- Reduction of aldehydes and ketones
- Reduction of carboxylic acid
Electrophilic addition of alkenes
cold concentrated H2SO4, followed by H2O, heat OR
steam H2O(g), concentrated H3PO4, 300 degrees, 70 atm
Nucleophilic substitution of halogenoalkanes
NaOH/KOH(aq), heat under reflux
Reduction of aldehydes and ketones
LiAlH4 in dry ether OR H2 gas, Ni catalyst, heat
Reduction of carboxylic acids
LiAlH4 in dry ether
Reactions of alcohols
- combustion
- redox
- condensation ( Nucleophilic Sub)
- Nucleophilic Sub to form RX
- Elimination to form alkene
- Oxidation
Redox
Na(s) as Na is slightly acidic
OH–> O-Na+
Condensation to form ester
(alcohol)
- carboxylic acid, concentrated H2SO4, heat under reflux
- acyl chloride, room temperature
alcohol acts as nucleophile
concentrated H2SO4 acts as catalyst and dehydrating agent
Reaction (2) favoured as milder conditions needed and reaction goes to completion
Elimination to form alkene (Synthesis of alkene)
excess concentrated H2SO4, 170 degrees
Nucleophilic substitution to form RX
- PCl5, room temperature
- PCl3, heat under reflux
- PBr3, heat under reflux
- PI3, heat under reflux
- SOCl2, heat under reflux
…
Oxidation of primary alcohol
(i) Oxidation of primary alcohol to aldehyde
K2Cr2O7, H2SO4(aq), heat with immediate distillation
(ii) Oxidation of primary alcohol to carboxylic acid
K2Cr2O7, H2SO4(aq), heat under reflux OR KMnO4, H2SO4(aq), heat under reflux
Why heat with immediate distillation to produce aldehyde?
- Aldehyde could further oxidise to carboxylic acid
- Distillation required to separate aldehyde from oxidising agent to prevent further oxidation to carboxylic acid
- Aldehyde has lower BP than alcohol and vaporises faster
Oxidation of secondary alcohol
Oxidation of secondary alcohol to ketone
K2Cr2O7, H2SO4(aq), heat under reflux OR KMnO4, H2SO4(aq), heat under reflux
Iodoform test
I2, NaOH(aq), heat
Observation: Yellow ppt of CHI3 formed, brown I2 decolourises
–> Step-down reaction
Reactions of phenols
- electrophilic substitution
- redox
- condensation
Why do phenols not undergo nucleophilic substitution?
Partial double bond character of the C-O bond
- The p-orbital of the O atom can overlap with the pi electron cloud of the benzene ring.
- The lone pair of electrons on O can be delocalised into the benzene ring.
- This results in the C–O bond having a partial double bond character which makes the C–O bond stronger and harder to break.
Interelectronic repulsion of nucleophile
- Nucleophiles are electron-rich species which will face interelectronic repulsion with the pi electron cloud of the benzene ring.
- Benzene provides high steric hindrance.
Electrophilic substitution (phenols)
(i) electrophilic substitution of Br2 in CCl4
Br2 in CCl4, rtp
(ii) electrophilic substitution of Br2(aq)
Br2(aq), rtp
- forms 2,4,6 tribromophenol
(iii) electrophilic substitution of dilute HNO3
Dilute HNO3, rtp
(iv) electrophilic substitution of concentrated HNO3
concentrated HNO3, rtp
Redox
Na(s) or NaOH(aq)
Condensation
(phenol)
Na(s)/NaOH(aq), followed by acyl chloride, rtp to form ester
Why is phenol a poor nucleophile? What is its solution?
-Phenol is a poor nucleophile because the LP of electrons on O delocalises into the benzene ring and is less readily available for donation to the electron deficient C in acyl chloride
- Solution: Phenol reacts with Na/NaOH to convert it into a phenoxide ion, which is a stronger nucleophile than phenol
Why phenol form multi-sub product with Br2(aq) but forms mono-sub product with Br2 in CCl4?
- Bromine in aq state is more electrophilic because water is a polar solvent. Polar water molecule will polarise the electron cloud of bromine, giving rise to a partial positive charge on bromine atom. This makes the Br2(aq) more susceptible to nucleophilic attack of phenol molecule
Distinguishing tests for alcohols
- PCl5, rtp
Alcohols: White ppt of HCl
Phenols: No white ppt of HCl - Na(s)
Alcohols and phenols: Effervescence of H2 gas which extinguishes lighted splint with ‘pop’ sound - K2Cr2O7, H2SO4(aq), heat
Primary/secondary alcohol: Orange K2Cr2O7 decolourises
Phenols/tertiary alcohols: do not decolourise
Distinguishing test for phenols
1.Neutral FeCl3: violet colouration
2. Br2(aq), rtp : orange Br2 decolourises, white ppt of 2,4,6 tribromophenol
acidity of alcohols
- electron-donating alkyl groups bonded
- intensifies the negative charge of alkoxide ion
- destabilises conjugate base
- alcohols less acidic than water
acidity of phenols
- LP of electrons on O delocalises into benzene ring
- disperses the negative charge of the phenoxide ion
- conjugate base more stable
- phenols more acidic than water
