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ORGANIC CHEMISTRY II CHM 301
CHAPTER 1
ALCOHOLS
ALCOHOLS Alcohols: Organic compounds containing
hydroxyl (-OH) functional groups.
R OH
Phenols: Compounds with hydroxyl group bonded directly to an aromatic (benzene) ring.
OH
NOMENCLATURE OF ALCOHOLS
NOMENCLATURE OF ALCOHOLS
IUPAC RULES1. Select the longest continuous chain of carbon atoms
containing the hydroxyl group.2. Number the carbon atoms in this chain so that the
one bonded to the –OH group has the lowest possible number.
3. Form the parent alcohol name by replacing the final –e of the corresponding alkane name by –ol. When isomers are possible, locate the position of the –OH by placing the number (hyphenated) of the carbon atom to which the –OH is bonded immediately before the parent alcohol name.
4. Name each alkyl branch chain (or other group) and designate its position by number.
Select this chain as the parent compound.
This is the longest continuous chain that contains an hydroxyl group.
43
2 1
This end of the chain is closest to the OH. Begin numbering here.
43
2 1
IUPAC name: 3-methyl-2-butanol
New IUPAC name: 3-methylbutan-2-ol
Select this chain as the parent compound.
This is the longest continuous chain that contains an hydroxyl group.
Example:
4 3
2 1
5
This end of the chain is closest to the OH. Begin numbering here.
IUPAC name: 3-methyl-2-pentanol
New IUPAC name: 3-methylpentan-2-ol
4 3
2 1
5 3
2
NOMENCLATURE OF CYCLIC ALCOHOLS
Using the prefix cyclo- The hydroxyl group is assumed to be on C1.
IUPAC name:
new IUPAC name:
trans-2-bromocyclohexanol
trans-2-bromocyclohexan-1-ol
H
Br
OH
H
12
345
6 HO CH2CH3
1-ethylcyclopropanol
1-ethylcyclopropan-1-ol
1
23
NOMENCLATURE OF ALCOHOLS CONTAINING TWO DIFFERENT
FUNCTIONAL GROUPS Alcohol containing double and triple bonds:
- use the –ol suffix after the alkene or alkyne name.
The alcohol functional group takes precedence over double and triple bonds, so the chain is numbered in order to give the lowest possible number to the carbon atom bonded to the hydroxyl group.
The position of the –OH group is given by putting its number before the –ol suffix.
Numbers for the multiple bonds were once given early in the name.
1234CH2 CH CH2 CH CH3
OH
5
1) Longest carbon chain that contains –OH group- 5 carbon
2) Position of –OH group - Carbon-2
3) Position of C=C - Carbon-4
COMPLETE NAME = 4-penten-2-ol
EXAMPLE
Some consideration:
- OH functional group is named as a hydroxy substituent when it appears on a structure with a higher priority functional group such as acids, esters, aldehydes and ketones.
- Examples:
1234O6
OH
CH3 CH
OH
CH2 C
O
OH
3-hydroxybutanoic acid 2-hydroxycyclohexanone
1234
5
MAIN GROUPS
AcidsEsters
AldehydesKetonesAlcoholsAminesAlkenesAlkynesAlkanesEthersHalides
decreasing priority
Alcohols with two –OH groups diols or lycols.
Naming of diols is like other alcohols except that the suffix diol is used and two numbers are needed to tell where the two hydroxyl groups are located.
NOMENCLATURE OF DIOLS
123CH3 CH
OH
CH2 OH
propane-1,2-diol trans-cyclopentane-1,2-diol
OH
OH
IUPAC name
1
23
5
4
NOMENCLATURE OF PHENOLS
The terms ortho (1,2-disubstituted), meta (1,3-disubstituted) and para (1,4-disubstituted) are often used in the common names.
OH
Br
OHO2NOH
CH3CH2
IUPAC name:
common name:
2-bromophenol
ortho-bromophenol
3-nitrophenol
meta-nitrophenol
4-ethylphenol
para-ethylphenol
Phenols may be monohydric, dihydric or trihydric - (number of hydroxyl groups) in the benzene ring.
benzene-1,3-diol benzene-1,4-diol benzene-1,2,3-triol
OH
OH
OH
OH
OHOH
OH
COMMON NAMES Derived from the common name of the alkyl group
and the word alcohol. For examples:
H3C CCH3
CH3
OH
IUPAC name: 2-methyl-2-propanolCommon name: tert-butyl alcohol
CH3CH2OH
IUPAC name: ethanolCommon name: ethyl alcohol
CH2CHCH3
OHIUPAC name: 2-propanolCommon name: isopropyl alcohol
CH3OH
IUPAC name: methanolCommon name:methyl alcohol
CLASSIFICATION OF ALCOHOLS
CLASSIFICATION OF ALCOHOLS
According to the type of carbinol carbon atom (C bonded to the –OH group).
C OH
Classes:
i) Primary alcohol
- -OH group attached to a primary carbon atom
- one alkyl group attached
ii) Secondary alcohol
- -OH group attached to a secondary carbon atom
- two alkyl group attached
iii) Tertiary alcohol
- -OH group attached to a tertiary carbon atom
- three alkyl group attached
CLASSIFICATION
TYPE STRUCTURE EXAMPLES
i) Primary (1°)
ii) Secondary (2°)
iii) Tertiary (3°)
CRH
OHH
CRH
OHR'
CRR''
OHR'
CH3CH2-OH CH3CHCH2
CH3
OH
ethanol 2-methyl-1-propanol
H3C CH
OH
CH2CH3OH
2-butanol cyclohexanol
2-methyl-2-propanol
C
CH3
OH
CH3
H3C
• Alcohols that contain more than one OH group - polyhydroxy alcohols.
• Monohydroxy: one OH group.
• Dihydroxy: two OH groups.
• Trihydroxy: three OH groups.
Polyhydroxy Alcohols
PHYSICAL PROPERTIES OF ALCOHOLS
PHYSICAL PROPERTIES OF ALCOHOLS
PHYSICAL PROPERTIES
PHYSICAL STATES OF ALCOHOLS
- aliphatic alcohols and lower aromatic alcohols liquids at room temperature.
- highly branched alcohols and alcohols with twelve or more carbon atoms solids.
BOILING POINTS
i) Boiling points of alcohols are higher > alkanes and chloroalkanes of similar relative molecular mass.- For example:
C2H5OH CH3CH2CH3 CH3ClRelative molecular mass: 46 44 50.5Boiling point: 78°C -42°C -24°C
- Reason: * intermolecular hydrogen bonds
RO
HO
H RAr
O
HO
H Ar
hydrogen bondinghydrogen bonding
δ+
δ-
δ+δ-
δ-
δ-
SOLUBILITY OF ALCOHOLS IN WATER
i) Alcohols with short carbon chains (i.e. methanol, ethanol) -dissolve in water. - dissolve in water (hydrogen bonds are formed).
ii) Solubility decreases sharply with the increasing length of the carbon chain.
iii) Higher alcohols are insoluble in water. - alcohol contains a polar end (-OH group) called ‘hydrophilic’ and a non-polar end (the alkyl group) called ‘hydrophobic’.
iii) Polyhydroxy alcohols are more soluble than monohydroxy form more hydrogen bonds with water molecule.
iv) Branched hydrocarbon increases the solubility of alcohol in water branched hydrocarbon cause the hydrophobic region becomes compact.
Alcohol weakly acidic. In aqueous solution, alcohol will donated its proton to
water molecule to give an alkoxide ion (R-O-).
ACIDITY OF ALCOHOLS AND PHENOLS
R-OH + H2O R-O- + H3O+ Ka = ~ 10-16 to 10-18
alkoxide ion
Example
CH3CH2-OH + H2O CH3CH2-O- + H3O+
The acid-dissociation constant, Ka, of an alcohol is defined by the equilibrium
R-OH + H2O R-O- + H3O+Ka
Ka = [H3O+] [RO-]
[ROH]
pKa = - log (Ka)
* More smaller the pKa value, the alcohol is more acidic
Acidity OF PHENOLS
Phenol is a stronger acid than alcohols and water.
R-OH + H2O R-O- + H3O+ Ka = ~ 10-16 to 10-18
alcohol alkoxide ion
OH H2O O- H3O+
phenol phenoxide ion
Ka = 1.2 x 10-10
H2O + H2O HO- + H3O+ Ka = 1.8 x 10-16
hydroxide ion
Phenol is more acidic than alcohols by considering the resonance effect.
i) The alkoxide ion (RO-)- the negative charge is confined to the oxygen and is not spread over the alkyl group. - this makes the RO- ion less stable and more susceptible to attack by positive ions such as H+ ions.
ii) The phenoxide ion- one of the lone pairs of electrons on the oxygen atom is delocalised into the benzene ring.
- the phenoxide ion is more stable because the negative charge is not confined to the oxygen atom but delocalised into the benzene ring.
- the phenoxide ion is resonance stabilised by the benzene ring and this decreases the tendency for the phenoxide ion to react with H3O+.
O O O O
The acidity decreases as the substitution on the alkyl group increase.
- Ethyl group is an electron-donating group strengthens the –O-H bond harder to release a proton.
- i.e: methanol is more acidic than t-butyl alcohol.
The present of electron-withdrawing atoms enhances the acidity of alcohols.
- The electron withdrawing atom helps to stabilize the alkoxide ion.
- i.e: 2-chloroethanol is more acidic than ethanol because the electron-withdrawing chlorine atom helps to stabilize the 2-chloroethoxide ion.
- Alcohol with more than one electron withdrawing atoms are more acidic. i.e. 2,2,-dichloroethanol is more acidic than 2-chloroethanol.
- Example of electron-withdrawing atom/groups:
Halogen atoms and NO2.
EFFECTS OF Acidity
IMPORTANT OF ALCOHOL
Ethanol - solvent for varnishes, perfumes and flavorings, medium for chemical reactions and in recrystallization. Also is an important raw material for synthesis.
Medically, ethanol is classified as a hypnotic (sleep producer), it is less toxic than other alcohol.
Ethanol is prepared both by hydration of ethylene and by fermentation of sugars. It is the alcohol of alcoholic beverages.
CH2 =CH2 + H2O -----------> CH3CH2OH
C6H12O6 -----------> 2CH3CH2OH + 2CO2
acid
yeast
Grignard synthesis Hydrolysis of alkyl halides Industrial and laboratory preparations of
ethanol
PREPARATION OF ALCOHOLS
GRIGNARD SYNTHESIS
The grignard reagent (RMgX) is prepared by the reaction of metallic magnesium with the appropriate organic halide. This reaction is always carried out in an ether solvent, which is needed to solvate and stabilize the Grignard reagent as it forms.
R-X + Mg R-Mg-X
(X = Cl, Br or I) organomagnesium halide
(Grignard reagent)
CH3CH2OCH2CH3
Grignard reagents may be made from primary, secondary, and tertiary alkyl halides, as well as from vinyl and aryl halides.
Alkyl iodides are the most reactive halides, followed by bromides and chlorides. Alkyl fluorides generally do not react.
CH3I Mg
CH3CH2Br Mg
Br Mg
CH3MgI
MgBr
CH3CH2MgBr
EXAMPLES
ether
ether
ether
Grignard reactions of carbonyl compounds
Formaldehyde (H2C=O) reacts with Grignard reagents giving primary alcohol.
R-MgX + C OH
HC OH
HR MgX R CH2 OH
ether H3O+
or
R-MgX + C OH
H
i) etherR CH2 OH
ii) H3O+
Example:
CH3CH2CH2CH2-MgBr + C OH
H
i) ether
ii) H3O+CH3CH2CH2CH2-C
H
H
OH
butylmagnesium bromide 1-pentanol (92%)
Aldehydes reacts with Grignard reagents giving secondary alcohols.
R-MgX + C OR'
HC OR'
HR MgX R C OH
ether H3O+
or
R-MgX + C OR'
H
i) ether
ii) H3O+
Example:
CH3CH2-MgBr + C OH3C
H
i) ether
ii) H3O+CH3CH2-C
R'
H
R C OHR'
H
CH3
HOH
Ketones reacts with Grignard reagents giving tertiary alcohols.
R-MgX + C OR'
R''C OR'
R''R MgX R C OH
ether H3O+
or
R-MgX + C OR'
R''
i) ether
ii) H3O+
Example:
CH3CH2-MgBr +i) ether
ii) H3O+
R'
R''
R C OHR'
R''
H3C C CH2CH2CH3
O
CH3CH2-C CH2CH2CH3
OH
CH3
Hydrolysis of alkyl halides is severely limited as a method of synthesizing alcohol, since alcohol are usually more available than the corresponding halides;indeed, the best general preparation of halides is from alcohols.
For those halides that can undergo elimination, the formation of alkene must always be considered a possible side reaction.
HYDROLYSIS OF ALKYL HALIDES
Example:
1) Second-order substitution: primary (and some secondary halides)
(CH3)2CHCH2CH2-Br (CH3)2CHCH2CH2-OH
2) First-order substitution: tertiary (and some secondary) halides
KOH
H2O
H3C CCH3
ClCH3
acetone/water
heatH3C C
CH3
OHCH3 H2C C
CH3
CH3
There are three principle ways to get the simple alcohols that are the backbone of aliphatic organic synthesis. These methods are:
a) hydration of alkenes obtained from the cracking of petroleum
b) the oxo process from alkenes, carbon monoxide and hydrogen
c) fermentation of carbohydrate
INDUSTRIAL AND LABORATORY PREPARATION OF ETHANOL
Aldehydes and ketones can be reduced to alcohols using:
a) lithium aluminium hydride (LiAlH4)
b) sodium borohydride (NaBH4)
c) catalytic hydrogenation
REDUCTION OF ALDEHYDES, KETONES AND CARBOXYLIC ACIDS
H+ = diluted acid such as H2SO4
R C H
O
LiAlH4 or NaBH4 or H2, Ni R C H
O-
H
R C H
OH
H
H+
aldehyde
1o alcohol
R C R'
O-
H
R C R'
OH
H
H+
2o alcohol
R C R'
O
LiAlH4 or NaBH4 or H2, Niketone
Examples:
CH3 C H
O
LiAlH4 CH3 C H
O-
H
CH3 C H
OH
H
H+
ethanal
ethanol
CH3 C CH3
O-
H
H+
2-propanol
CH3 C CH3
O
H2/Nipropanone
CH3 C CH3
OH
H
Carboxylic acids primary alcohols
Reducing agents: LiAlH4 in dry ether
R C OH
O
R C OH
H
Hprimary alcohols
carboxylic acids
examples:
CH3 C OH
O(1) LiAlH4 / ether
(2) H2OCH3 C OH
H
Hethanol
ethanoic acid
(1) LiAlH4 / ether
(2) H2O
- Benzoic acid can be reduced to phenylmethanol by using LiAlH4 in ether at low temperatures.- An alkoxide intermediate is formed first.- On adding water, hydrolysis of the intermediate yields the primary alcohols.
(1) LiAlH4 / ether
(2) H2OC
O
OH C
H
OH
Hphenylmethanol
benzoic acid
reduced
Benzoic acid can be reduced to phenylmethanol by using LiAlH4 in ether at low temperatures. An alkoxide intermediate is formed first. On adding water, hydrolysis of the intermediate yields the primary alcohols.
R C OH
O
R C OH
H
Hprimary alcohols
carboxylic acids
examples:
CH3 C OH
O(1) LiAlH4 / ether
(2) H2OCH3 C OH
H
Hethanol
ethanoic acid
(1) LiAlH4 / ether
(2) H2O
- Benzoic acid can be reduced to phenylmethanol by using LiAlH4 in ether at low temperatures.- An alkoxide intermediate is formed first.- On adding water, hydrolysis of the intermediate yields the primary alcohols.
(1) LiAlH4 / ether
(2) H2OC
O
OH C
H
OH
Hphenylmethanol
benzoic acid
LiAlH4 has no effect on the benzene ring or the double bond. -COOH is reduced to –CH2OH but the C=C bonds remains
unchanged.
CH3CH2CH=CHCOOH CH3CH2CH=CHCH2OH
1) LiAlH4
2) H2O
Question:
i) Give the structural formulae of L, M and N
OHM
PBr3 Mg
etherN
i) Lii) H3O+
ii) How to prepare alcohol A from the reduction process?
A
OH
Answers
L: OBr
MgBr
M:
N:
(1) LiAlH4 / ether
(2) H2O
O OH
i)
ii)
Reaction with sodium Oxidation Esterification Halogenation and haloform reactions Dehydration Formation of ether (Williamson ether
synthesis)
REACTIONS OF ALCOHOLS
Reaction with sodium
Alcohols reacts with Na at room temperature to form salts (sodium alkoxides) and hydrogen.
2R-O-H + 2Na → 2R-O- Na+ + H2
For example:
CH3CH2OH + Na → CH3CH2O-Na+ + 1/2H2
alcohol sodium ethoxide
Reactivity of alcohols towards the reactions with sodium:
CH3 > 1° > 2° > 3°
Oxidation
R C OH
H
H
R C OH
H
H
R C OH
H
H
H
H
R-C=O
R-C=O
O
R-C-OH
Pyridinium chlorochromate (PCC)
CH2Cl2, 25oC
1o alcohol aldehyde
Cu or Cr3O/pyridine
1o alcohol aldehyde
KMnO4/H+ or K2Cr2O7/H+
1o alcohol carboxylic acid
or CrO3/H+
Cr3O/pyridine = Collins reagent
1° alcohol
KMnO4/H+ or K2Cr2O7/H+
or CrO3/H+CH3(CH2)4-CH2-OH CH3(CH2)4-C-OH
O
CH3(CH2)4-CH2-OH CH3(CH2)4-C-H
O
1-hexanol hexanal
1-hexanol hexanoic acid
PCC
Examples:
1° alcohol
R C OH
H
R'
O
R-C-R'
KMnO4/H+ or K2Cr2O7/H+
2o alcohol ketone
or CrO3/H+
R C OH
R"
R'
KMnO4/H+ or K2Cr2O7/H+
3o alcohol
or CrO3/H+no reaction
2° alcohol
3° alcohol
CH3 CH
OH
CH2CH3 CH3 C
O
CH2CH3
KMnO4/H+ or K2Cr2O7/H+
or CrO3/H+
2-butanol 2-butanone
Example:
Esterification:
- the reaction between an alcohol and a carboxylic acid to form an ester and H2O.
Esterification
R C
O
O H O R'HH+
CH3CH2-O-H CH3 C
O
O H
CH3-O-H C
O
OHH+
H+
R C
O
O R'
C
O
OCH3
CH3 C
O
OCH2CH3
H2O
H2O
H2O
carboxylic acid alcohol ester
EXAMPLES
ethanol ethanoic acid ethyl ethanoate
methanol benzoic acid methyl benzoate
H+ = catalyst
CH3-O-H CCH3
O
Cl CCH3
O
OCH3HCl
methanol ethanoyl chloride methyl ethanoate
Esterification also occurs when alcohols react with derivatives of carboxylic acids such as acid chlorides
Halogenation and haloform reactions
1) Hydrogen halides (HBr or HCl or HI)
R-OH + H-X → R-X + H2O
Example:
C2H5-OH + H-Br C2H5-Br + H2O
• Reactivity of hydrogen halides decreases in order HI > HBr > HCl
• Reactivity of alcohols with hydrogen halides: 3° > 2° > 1°
H+
2) Phosphorus trihalides, PX3 or phosphorus pentahalides, PX5
3R-OH + PX3 3R-X + H3PO3
(PX3 = PCl3 or PBr3 or PI3)
Example:(CH3)2CHCH2-OH + PBr3 → (CH3)2CHCH2-Brisobutyl alcohol isobutyl bromide
3) Thionyl chloride (SOCl2)
R-OH + SOCl2 → R-Cl + SO2 + HCl
Example:
CH3(CH2)5CH2-OH + SOCl2 → CH3(CH2)5CH2-Cl + SO2 + HCl 1-heptanol 1-chloroheptane
Dehydration of alcohols will formed alkenes and the products will followed Saytzeff rules.
Dehydration
conc. H2SO4R-CH2-CH2-OH R-CH=CH2 + H2O
Saytzeff rule:
- A reaction that produces an alkene would favour the formation of an alkene that has the greatest number of substituents attached to the C=C group.
CH3CH2-CH-CH3OH
H+
H+
CH3CH=CH-CH3 + H2O
CH3CH2-CH=CH2 + H2O
2-butanol2-butenemajor product
1-butene
Reactivity of alcohols towards dehydration:
3° > 2° > 1° Reagents for dehydration:
i) Concentrated H2SO4
conc. H2SO4CH3-CH2-OH CH2=CH2 + H2O
ii) With phosphoric (v) acid
OH85% H3PO4, 165-170oC H2O
iii) Vapour phase dehydration of alcohols
CH3CH2OH CH2=CH2 + H2OAl2O3
heat
Involves the SN2 attack of an alkoxide ion on an unhindered primary alkyl halides.
The alkoxide is made by adding Na, K or NaH to the alcohol.
R-O- + R’-X → R-O-R’ + X-
alkoxide
(R’ must be primary)
Formation of ether (Williamson ether synthesis)
The alkyl halides (or tosylate) must be primary, so that a back-side attack is not hindered.
If the alkyl halides is not primary, elimination usually occurs to form alkenes.
CH3CH2-OH
CH3CH2-OH Na
CH3I
OH
CH3CH2-OTs
CH3CH2-O
CH3CH2-O-CH3
Na+
CH3CH2-O-CH3
OCH2CH3
NaI
CH3I
NaI
EXAMPLES
or
1) Na
2)
1) Na
2)
cyclohexanol ethoxycyclohexane
Reaction with sodium Esterification Halogenation of the ring Nitration of the ring
REACTIONS OF PHENOLS
REACTION WITH SODIUM
OH Na O- Na+ 1/2 H2(g)
sodium phenoxide
OH NaOH O- Na+
sodium phenoxide
H2O
REACTION WITH AQUEOUS SODIUM HYDROXIDE
ROH + NaOH no reaction
ESTERIFICATION
OH
OH
H2O
NaOH
C
O
OH
ONa CH3CCl
O
NaOH
OC
O
OCCH3
O
H2O
NaCl
sodium phenoxide
phenyl benzoate
EXAMPLES
H+
More reactive towards electrophilic substitution than benzene. ortho-para director.
HALOGENATION
OH
3X2
OH
3Br2
OH
3Cl2
OH
2Br2 (CCl4)
OHBr
OH
X
XX
OH
Cl
ClCl
OH
Br
BrBr
OH
Br
3HX
3HCl
3HBr
2HBr
room
temperature
EXAMPLES
room
temperature
2,4,6-tribromophenol (white precipitate)
room
temperature
2,4,6-trichlorophenol (white precipitate)
2
monobromophenols are obtained if the bromine is dissolved in a non-polar solvent such as CCl4
• Monobromophenols are obtained if the bromine is dissolved in a non-polar solvent such as CCl4.
OH
2Br2 (CCl4)
OHBr
OH
Br
2HBr2
NITRATION
Dilute nitric (v) acids reacts with phenol at room temperature to give a mixture of 2- and 4-nitrophenols.
OH
2HNO3
OHNO2
OH
NO2
2H2O2 < 20oC
2-nitrophenol 4-nitrophenol
By using concentrated nitric (v) acid, the nitration of phenol yields 2,4,6-trinitrophenol (picric acid).
Picric acid is a bright yellow crystalline solid. It is used in the dyeing industry and in manufacture of explosives.
OH
3HNO3
OHNO2
NO2
O2N3H2O
2,4,6-trinitrophenol(picric acid)
Question:Alcohol W is a secondary alcohol with a molecular formula of C4H10O.
Compound M C4H10OAlcohol W
Step 1CrO3 /pyrridine
Step 2
H+ / heat
Compound N (major) + Compound O (minor)
Reagent A
C4H10ONa
a) Draw and give the IUPAC name for alcohol W.b) Draw the structural formula for the following
compounds:i) Compound Mii)Compound Niii)Compound O
c) Give the correct name for the following:
i) Step 1ii) Step 2iii)Reagent A
Answers
a) Alcohol W
OH
name: butan-2-ol
b) i) compound M ii) compound N iii) Compound O
O
c) i) Step 1: Oxidation
ii) Step 2: Dehydration (of alcohol)
iii) Reagent A: Na Metal
1) Lucas Test
- The alcohol is shaken with Lucas reagent (a solution of ZnCl2 in concentrated HCl).
- Tertiary alcohol - Immediate cloudiness (due to the formation of alkyl chloride).
- Secondary alcohol - Solution turns cloudy within about 5 minutes.
- Primary alcohol - No cloudiness at room temperature.
TESTS TO DISTINGUISH CLASSES OF ALCOHOLS
C CH3CH3
CH3
OH
CHCH3
OH
CH2CH3
CH3CH2CH2CH2OH
C CH3CH3
CH3
Cl
CHCH3
Cl
CH2CH3
HCl/ZnCl2room temperature
3o alcohol (cloudy solution almost immediately)
HCl/ZnCl2room temperature
2o alcohol (cloudy solution within 5 minutes)
HCl/ZnCl2room temperature
no reaction
1o alcohol
2) Oxidation of alcohols
- only primary and secondary alcohols are oxidised by hot acidified KMnO4 or hot acidified K2Cr2O7 solution.
- the alcohol is heated with KMnO4 or K2Cr2O7 in the presence of dilute H2SO4.- 1o or 2o alcohol:
→ the purple colour of KMnO4 solution disappears.
→ the colour of the K2Cr2O7 solution changes from orange to green.
- 3o alcohol do not react with KMnO4 or K2Cr2O7.
3RCHO
R CH
R'
OH R C
R'
O
3RCH2OH + Cr2O2-7 + 8H+
3RCHO + 2Cr3+ + 7H2O
1o alcohol (orange) aldehyde (green)
+ Cr2O2-7 + 8H+
aldehyde (orange)
3RCOOH + 2Cr3+ + 7H2O
carboxylic acid (green)
3 + Cr2O2-7 + 8H+
(orange)2o alcohol
3 + 2Cr3+ + 7H2O
(green)ketone
HALOFORM TEST TO IDENTIFY METHYL ALCOHOL GROUP
1) Iodoform: Ethanol and secondary alcohols containing the group
methyl alcohol group which react with alkaline solutions of iodine to form triiodomethane (iodoform, CHI3).
Triiodomethane – a pale yellow solid with a characteristic smell.
CCH3
H
OH
(methyl alcohol group)
C RCH3
H
OH
+ 4I2 + 6NaOH CHI3 (s) + RCOONa + 5NaI + 5H2Otriiodomethane(iodoform)yellow precipitate
where R = hydrogen, alkyl or aryl group
C HCH3
H
OH
+ 4I2 + 6OH CHI3 (s) + 5I- + 5H2O
iodoform
C OH
O
ethanol
methanoate
• The iodoform test can distinguish ethanol from methanol
C HH
H
OH
+ 4I2 + 6OH
methanol
no reaction
positive iodoform test
negative iodoform test
C HCH3
CH3
OH
+ 4I2 + 6OH CHI3 (s) + 5I- + 5H2O
iodoform
C OCH3
O
2-propanol
ethanoate
• The iodoform test can distinguish 2-propanol from 1-propanol
positive iodoform test
C HC
H
OH
+ 4I2 + 6OH
1-propanol
no reactionCH
H H
HH negative iodoform test
* TERTIARY ALCOHOLS DO NOT GIVE POSITIVE IODOFORM TEST
C RCH3
H
OH
+ 4Br2 + 6NaOH CHBr3 (s) + RCOONa + 5NaBr + 5H2Obromoform
where R = hydrogen, alkyl or aryl group
2) BROMOFORM
sample
iodoform
reagent
Question:
a) Classify each of the following alcohols as primary, secondary or tertiary.i) 2-Propanolii) 4-methylpentanoliii)2,3-dimethylbutan-2-ol
b) Name a simple test to distinguish 1°, 2°, 3° alcohol. State the reagents and conditions required for the test and write down the expected observations.
Answer:
a) i) 2° ii) 1° iii) 3°
b) Test: Lucas test Reagent and conditions : Lucas reagent / Mixture of HCl and ZnCl2 Observatios: - Clear homogenous solution change into 2 layers or cloudiness - Rate of reaction: 3° > 2° > 1° alcohol