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