CBSE Class-12 Chemistry Quick Revision Notes Chapter-11: Alcohols, henols an! Ethers •
Structure o" alcohols:
•
reparation o" alcohols: a) From alkene Acid catalysed hydration (H 2 O , H + ) Mark, Addition Or Hydroboration - oxidation B2 H 6 , H 2 O2 / OH − Mark, addition Product is anti mark
Alkene → Alcohol
b) From esters catalyst
H 2 → Alcohol Esters Esters Alcohol c) From aldehydes and ketones H 2 / Pd or NaBH 4 Or LiAlH 4 Or Grignard ' s reag reagen entt
Alcoho l ← ← Aldehyde and ketone
d) From carboxylic acids LiAlH H O
4 2 Alcoho l ← Carboxylic acids
•
Structure o" phenols:
•
reparation o" phenols: a) From benzene
Oleum
→
NaOH / H +
→
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b) From chlorobenzene
NaOH at 623K,320 atm.pres
HCl
→
→
c) From cumene
H 2 O,H+
→
O2
→
d) From aniline
N aN O2 , HC l
→
•
H 2 O ,w ar m
→
hysical properties o" alcohols an! phenols: a) Boiling points Boiling points of alcohols and phenols are higher in comparison to other classes of compounds. This is because the –OH group in alcohols and phenols is inoled in intermolecular hydrogen bonding.
The boiling points of alcohols and phenols increase !ith increase in the number of carbon atoms. This is because of increase in an der "aals forces !ith increase in surface area. #n alcohols$ the boiling points decrease !ith increase of branching in carbon chain. This is because of decrease in an der "aals forces !ith decrease in surface area. b) %olubility
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%olubility of alcohols and phenols are soluble in !ater due to their ability to form hydrogen bonds !ith !ater molecules. The solubility of alcohols decreases !ith increase in size of alkyl&aryl 'hydrophobic) groups. •
#.
Chemical properties o" alcohols: (eactions inoling cleaage of O–H bond *lcohols react as nucleophiles a) (eaction !ith metals 2 R − O − H
+
2 Na → 2 R − O − Na + H 2 Sodium alkoxide
b) +sterification reaction H +
RO − H + R '− COOH Alcohol
'
⇔ ROCOR + H 2O
H +
RO − H + ( R ' CO2 )O ⇔ ROCOR '+ R 'COOH Alcohol
Pyridine
RO − H + R ' COCl → R − OCOR '+ HCl Alcohol
##.
(eactions of alcohols inoling cleaage of carbon – oxygen ',–O) bond a) (eaction !ith hydrogen halides ROH
+
HX
conc. HCl + ZnCl2 / Lucas reagent
→
RX
+
H 2O
b) (eaction !ith phosphorus trihalides 3 ROH
+
PX 3
→
3R − X
+
H 3 PO3 ( X
= Cl
, Br )
c) -ehydration reaction Pr otic acids(conc.H2 SO4 orH 3 PO 4 ) Or Catalysts ( anhyd . ZnCl2 or alumina)
Alcohol → C
=
C + H 2O
d) Oxidation reaction Acidified
potassium permanganat Alcohol → Carboxylic acid
i) ii) iii) •
CU ,573k Or CrO3 Or PCC
→ Aldehyde Primary Alcohol CU ,573k Or CrO3
Secondary Alcohol → Ketone CU ,573k Or KMnO4
→ Alkene Tertiary Alcohol
,hemical properties of phenols #. (eactions inoling cleaage of O–H bond *lcohols react as nucleophiles a) (eaction !ith metals
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b) +sterification reaction +
H
Ar − OH + R '− COOH ⇔ Ar − OCOR '+ H 2O Phenol
H +
Ar − OH + ( R ' CO )2 O ⇔ Ar − OCOR '+ R 'COOH Phenol
Pyridine
Ar − OH + R ' COCl → ArOCOR '+ HCl Phenol
##.
Other chemical reactions of phenols
###. *cidic nature of phenol and alcohol a) henol / H0O / rimary alcohol / %econdary alcohol / Tertiary alcohol The acidic character of alcohols is due to the polar nature of O–H bond. *lkyl group is an electron1releasing group '–,H2$ –,0H3) or it has electron releasing inductie effect '4# effect). -ue to 4# effect of alkyl groups$ the electron density on oxygen increases. This decreases the polarity of O1H bond. *nd hence the acid strength decreases. Material downloaded from http://myCBSEguide.com and http://onlineteachers.co.in Portal for CBSE Notes, Test Papers, Sample Papers, Tips and Tricks
b) henol is more acidic than alcohol #n phenol$ the hydroxyl group is directly attached to the sp2 hybridised carbon of benzene ring !hich acts as an electron !ithdra!ing group !hereas in alcohols$ the hydroxyl group is attached to the alkyl group !hich hae electron releasing inductie effect. #n phenol$ the hydroxyl group is directly attached to the sp2 hybridised carbon of benzene ring !hereas in alcohols$ the hydroxyl group is attached to the sp3 hybridised carbon of the alkyl group. The sp0 hybridised carbon has higher electronegatiity than sp3 hybridised carbon. Thus$ the polarity of O–H bond of phenols is higher than those of alcohols. Hence$ the ionisation of phenols is higher than that of alcohols. The ionisation of an alcohol and a phenol takes place as follo!s
#n alkoxide ion$ the negatie charge is localised on oxygen !hile in phenoxide ion$ the charge is delocalised.
The delocalisation of negatie charge makes phenoxide ion more stable and faours the ionisation of phenol. *lthough there is also charge delocalisation in phenol$ its resonance structures hae charge separation due to !hich the phenol molecule is less stable than phenoxide ion.
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c) #n substituted phenols$ the presence of electron !ithdra!ing groups such as nitro group enhances the acidic strength of phenol. On the other hand$ electron releasing groups$ such as alkyl groups$ in general$ decreases the acid strength. #t is because electron !ithdra!ing groups lead to effectie delocalisation of negatie charge in phenoxide ion. •
#i""erentiate $et%een or&anic compoun!s: a) *lcohols and phenols henol on reaction !ith neutral Fe,l2 gies purple colour !hereas alcohols do not gie purple colour. 6C6 H 5OH
+
Fe 3
+
→
3 [ Fe(OC 6 H 5 ) 6 ]
−
+
+
6H
Purple colour
b) rimary$ secondary and tertiary alcohols
5ucas reagent test ROH
+
HCl
conc. HCl + ZnCl2 / Lucas reagent
→
RCl
+
H 2O
#f it is a primary alcohol$ no turbidity appears at room temperature. Turbidity appears only on heating. #f it is a secondary alcohol$ turbidity appears in 3 minutes. #f it is a tertiary alcohol$ turbidity appears immediately. c) 6ethanol and ethanol #odoform test +thanol !hen reacted !ith '#0 and 7aOH) or 7aO# gies yello! ppt of iodoform since it has the presence of ,H21,H 'OH)1 group. C2 H 5OH + 4 I 2 + 6 NaOH → CHI 3 + 5 NaI + 5H 2O + HCOONa Yellow ppt
CH 3OH
+
I 2 + NaOH
•
Structure o" ethers:
•
reparation o" ethers: a) From alcohols
→
No yellow ppt
H SO or H PO at413K
2 4 3 4 → Ethers Alcohol
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b) From alkyl halide and sodium alkoxide Williamson s
' synthesis Ethers ← Alkyl halide and sodium alkoxide
Here$ the alkyl halide should be primary and alkoxide should be tertiary. #n case of aromatic ether$ the aromatic part should be !ith phenoxide ion. •
hysical properties o" ethers a) 6iscibility 6iscibility of ethers !ith !ater resembles those of alcohols of the same molecular mass. This is due to the fact that 8ust like alcohols$ oxygen of ether can also form hydrogen bonds !ith !ater molecule.
b) Boiling points +thers hae much lo!er boiling points than alcohols. This is due to the presence of hydrogen bonding in alcohols. Hydrogen bonding is absent in ethers. R |
R |
R − O− H − O− H − O− H − O− | H
•
| R
Chemical properties o" ethers: a) ,leaage of ,–O bond in ethers (1O1(9 4 H: ; (1: 4 (9OH +xcess The order of reactiity of hydrogen halides is as follo!s H# / HBr / H,l *lkyl halide formed is al!ays the lo!er alkyl group. But if a tertiary alkyl group is present$ the alkyl halide is al!ays tertiary. #n case of phenolic ethers$ the cleaage occurs !ith the formation of phenol and alkyl halide. b) +lectrophilic substitution reaction in aromatic ethers
The electrophilic substitution reaction of aromatic ether inoles the follo!ing reaction
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•
Other conersion reactions a) henol to salicyldehyde
b) henol to benzene diazonium chloride
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