Chapter 6 hydroxyl compounds

transcript

ORGANIC CHEMISTRY CHM 207

CHAPTER 6:HYDROXYL COMPOUNDS (ALCOHOLS AND PHENOL)

SUBTOPICS Nomenclature of alcohols, phenols. Classification of alcohols. Physical properties of alcohols:

- Physical state

- Boiling points

- Solubility of alcohols in water Acidity of alcohols and phenols Reactions of alcohols:

- Reaction with sodium- Oxidation- Esterification- Halogenation and haloform reactions- Dehydration- Formation of ether (Williamson ether synthesis)

Reactions of phenols:- Reaction with sodium- Esterification- Halogenation of the ring- Nitration of the ring

Tests to distinguish classes of alcohols: i) Lucas test ii) Oxidation

Haloform test to identify methyl alcohol group - Iodoform - Bromoform

Uses of alcohols and phenols.

ALCOHOLS Alcohols: Organic compounds containing

hydroxyl (-OH) functional groups.

Phenols: Compounds with hydroxyl group bonded directly to an aromatic (benzene) ring.

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.

This end of the chain is closest to the OH. Begin numbering here.

3-methyl-2-butanol

Select this chain as the parent compound.

This is the longest continuous chain that contains an hydroxyl group.

This end of the chain is closest to the OH. Begin numbering here.

3-methyl-2-pentanol

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

6 HO CH2CH3

1-ethylcyclopropanol

1-ethylcyclopropan-1-ol

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

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

CH3 CH

3-hydroxybutanoic acid 2-hydroxycyclohexanone

MAIN GROUPS

AcidsEsters

AldehydesKetonesAlcoholsAminesAlkenesAlkynesAlkanesEthersHalides

decreasing priority

Alcohols with two –OH groups are called diols or glycols.

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

CH2 OH

propane-1,2-diol trans-cyclopentane-1,2-diol

IUPAC name

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.

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 according to the number of hydroxyl groups present in the benzene ring.

benzene-1,3-diol benzene-1,4-diol benzene-1,2,3-triol

According to the type of carbinol carbon atom (C bonded to the –OH group).

CLASSIFICATION

Classes:

i) Primary alcohol

- -OH group attached to a primary carbon atom

ii) Secondary alcohol

- -OH group attached to a secondary carbon atom

iii) Tertiary alcohol

- -OH group attached to a tertiary carbon atom

TYPE STRUCTURE EXAMPLESTYPE STRUCTURE EXAMPLES

i) Primary (1°)

ii) Secondary (2°)

iii) Tertiary (3°)

CH3CH2-OH CH3CHCH2

ethanol 2-methyl-1-propanol

H3C CH

CH2CH3OH

2-butanol cyclohexanol

2-methyl-2-propanol

• Alcohols that contain more than one OH group attached to different carbons are called polyhydroxy alcohols.

• Monohydroxy: one OH group per molecule.

• Dihydroxy: two OH groups per molecule.

• Trihydroxy: three OH groups per molecule.

Polyhydroxy Alcohols

PHYSICAL PROPERTIESPHYSICAL PROPERTIES

PHYSICAL STATES OF ALCOHOLS

- simple aliphatic alcohols and lower aromatic alcohols (such as phenylmethanol, C6H5CH2OH) → liquids at room temperature.

- highly branched alcohols and alcohols with twelve or more carbon atoms → solids.

BOILING POINTS

- The boiling points of alcohols are higher than those of 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 exist between the –OH

groups in the alcohol molecules. R

hydrogen bondinghydrogen bonding

δ+δ-

- Branched chain alcohols boils at a lower temperature (more volatile) than the straight chain alcohols with the same number of carbon atoms.

SOLUBILITY OF ALCOHOLS IN WATER

i) alcohols with short carbon chains (such as methanol, ethanol, and propanol) dissolve in water. - when alcohols dissolve in water, hydrogen bonds are formed between the –OH group of the alcohol molecule and the –OH group of the water molecule.

ii) the solubility of alcohols in water decreases sharply with the increasing length of the carbon chain. 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’.- the water solubility decreases as the alkyl group becomes larger.

iii) alcohols with more than one hydroxyl group (polyhydroxy alcohols) are more soluble than monohydroxy alcohols with the same number of carbon atoms. This is because they can form more hydrogen bonds with water molecule.

iv) branched hydrocarbon increases the solubility of alcohol in water. - reason: branched hydrocarbon cause the hydrophobic region becomes compact so decrease surface area. As the surface area of the non-polar part in the molecule decreases, the solubility increases.

* Phenol is sparingly soluble (9.3%) because of its compact shape and the particularly strong hydrogen bonds formed between phenolic –OH groups and water molecules.

Alcohol is weakly acidic. In aqueous solution, alcohol will donated its proton to

water molecule to give an alkoxide ion (R-O-).

ACIDITY OF ALCOHOLS AND PHENOLSACIDITY 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-]

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 than the alkoxide ion 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.

- Reason: a more highly substituted alkyl group inhibits solvation of the alkoxide ion and drives the dissociation equilibrium to the left.

- For example: methanol is more acidic than t-butyl alcohol.

The present of electron-withdrawing atoms enhances the acidity of alcohols.

- Reason: the electron withdrawing atom helps to stabilize the alkoxide ion.

- For example: 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. For example, 2,2,-dichloroethanol is more acidic than 2-chloroethanol.

- Example of electron-withdrawing atom/groups:

Halogen atoms and NO2.

EFFECTS OF Acidity

Reaction with sodium Oxidation Esterification Halogenation and haloform reactions Dehydration Formation of ether (Williamson ether

synthesis)

REACTIONS OF ALCOHOLSREACTIONS 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

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

or CrO3/H+CH3(CH2)4-CH2-OH CH3(CH2)4-C-OH

CH3(CH2)4-CH2-OH CH3(CH2)4-C-H

1-hexanol hexanal

1-hexanol hexanoic acid

Examples:

1° alcohol

R C OH

R-C-R'

2o alcohol ketone

or CrO3/H+

R C OH

3o alcohol

or CrO3/H+no reaction

2° alcohol

3° alcohol

CH3 CH

CH2CH3 CH3 C

CH2CH3

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

O H O R'HH+

CH3CH2-O-H CH3 C

CH3-O-H C

OCH2CH3

carboxylic acid alcohol ester

EXAMPLES

ethanol ethanoic acid ethyl ethanoate

methanol benzoic acid methyl benzoate

H+ = catalyst

CH3-O-H CCH3

Cl CCH3

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°

2) Phosphorus trihalides, PX3

3R-OH + PX3 3R-X + H3PO3

(PX3 = PCl3 or PBr3 or PI3)

Example:(CH3)2CHCH2-OH + PBr3 → (CH3)2CHCH2-Brisobutyl alcohol isobutyl bromide

3) Phosphorus pentahalides (PX5)

R-OH + PX5 → R-Cl + POCl3 + HCl phosphorus trichloride oxide (phosphorus oxychloride)

Example:

CH3CH2-OH + PCl5 → CH3CH2-Cl + POCl3 + HCl ethanol chloroethane (white fumes of HCl)

4) 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

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

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

CH3CH2-OTs

CH3CH2-O

CH3CH2-O-CH3

OCH2CH3

EXAMPLES

cyclohexanol ethoxycyclohexane

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

name: butan-2-ol

b) i) compound M ii) compound N iii) Compound O

c) i) Step 1: Oxidation

ii) Step 2: Dehydration (of alcohol)

iii) Reagent A: Na Metal

Reaction with sodium Esterification Halogenation of the ring Nitration of the ring

REACTIONS OF PHENOLSREACTIONS OF PHENOLS

REACTION WITH SODIUM

OH Na O- Na+ 1/2 H2(g)

sodium phenoxide

OH NaOH O- Na+

sodium phenoxide

REACTION WITH AQUEOUS SODIUM HYDROXIDE

ROH + NaOH no reaction

ESTERIFICATION

ONa CH3CCl

sodium phenoxide

phenyl benzoate

EXAMPLES

More reactive towards electrophilic substitution than benzene. ortho-para director.

1) Halogenation of phenol: If liquid bromine or bromine water is added to an aqueous solution of phenol at room temperature,

decolorisation occurs and a white precipitate of 2,4,6-tribromophenol is formed (bromination). No catalyst (halogen carrier) is needed. This reaction is used to test the presence of phenol and determine the mass of phenol in an

aqueous solution.

HALOGENATION

3X2 (aq)

3Br2(aq)

temperature

EXAMPLE

temperature

2,4,6-tribromophenol (white precipitate)

3HClroom

temperature

2,4,6-trichlorophenol (white precipitate)

Phenol reacts similarly with chlorine or chlorine water to form white precipitate of 2,4,6-trichlorophenol.

This reaction is called chlorination of phenol.

2Br2 (CCl4)

Monobromophenols are obtained if the bromine is dissolved in a non-polar solvent such as CCl4.

NITRATION

Dilute nitric (v) acids reacts with phenol at room temperature to give a mixture of 2- and 4-nitrophenols.

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.

O2N3H2O

2,4,6-trinitrophenol(picric acid)

Aqueous iron (III) chloride and aqueous bromine are used to test phenols.

i) Complex formation with iron (III) chloride

- when two or three drops of iron (III) chloride solution is added to a very dilute solution of phenol, a violet-blue coloration is produced.

- methylphenol (toluene) produce a blue colour.

CHEMICAL TESTS FOR PHENOLSCHEMICAL TESTS FOR PHENOLS

FeCl3 (neutral)6 H3[Fe3+ ( )6]

violet complexphenoxide ligand

Phenol compound

Colour of complex with

FeCl3 (aq)

Phenol compound

Colour of complex with

FeCl3 (aq)

Violet Green

Violet Red

Blue Violet

Violet

OHCOOH

ii) Bromine water

- when bromine water is added gradually to a concentrated solution of phenol, the bromine water is decolorised.

- when excess bromine water is added, a white precipitate of 2,4,6-tribromophenol is obtained.

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 TESTS TO DISTINGUISH CLASSES OF ALCOHOLSALCOHOLS

C CH3CH3

CH2CH3

CH3CH2CH2CH2OH

C CH3CH3

CH2CH3

HCl/ZnCl2room temperature

3o alcohol (cloudy solution almost immediately)

2o alcohol (cloudy solution within 5 minutes)

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.

OH R C

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 HALOFORM TEST TO IDENTIFY METHYL ALCOHOL GROUPALCOHOL 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.

(methyl alcohol group)

C RCH3

+ 4I2 + 6NaOH CHI3 (s) + RCOONa + 5NaI + 5H2Otriiodomethane(iodoform)yellow precipitate

where R = hydrogen, alkyl or aryl group

C HCH3

+ 4I2 + 6OH CHI3 (s) + 5I- + 5H2O

iodoform

ethanol

methanoate

• The iodoform test can distinguish ethanol from methanol

+ 4I2 + 6OH

methanol

no reaction

positive iodoform test

negative iodoform test

C HCH3

+ 4I2 + 6OH CHI3 (s) + 5I- + 5H2O

iodoform

C OCH3

2-propanol

ethanoate

• The iodoform test can distinguish 2-propanol from 1-propanol

positive iodoform test

+ 4I2 + 6OH

1-propanol

no reactionCH

HH negative iodoform test

* TERTIARY ALCOHOLS DO NOT GIVE POSITIVE IODOFORM TEST

C RCH3

+ 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

As solvents:- examples: the lower alcohols such as methanol, ethanol and propanol.- methanol is used as a solvent for varnish and paints.

As fuels:- biofuel (fuel derived from a biological source).- ethanol can be produced from sugars such as sucrose from sugar cane, through fermentation and distillation. It can be blended with petrol and used as fuel in motor vehicles.- methylated spirit is ethanol made undrinkable by the addition of a little methanol. It is used as a fuel in camping stoves.

USES OF ALCOHOLSUSES OF ALCOHOLS

In alcoholic drinks:- ethanol is used for making wine, beer and etc.

As intermediates:- methanol can be oxidised to methanal (HCHO), a chemical feedstock (starting material) for the manufacture of thermosetting plastics such as bakelite.- methanol is used to make methyl methacrylate which is used in the manufacture of another plastic called perspex.

In cosmetics:- ethanol is used as solvent for fragrances in perfumes and after-shave lotions.- polyhydroxyl alcohols (for example, glycerol) are used in moisturising creams.

Making plastics such as bakelite (phenol-methanal plastic).

The synthesis of cyclohexanol and hexanedioic acid to make nylon 6,6.

Making dyes. Making antiseptics such as 4-chloro-3,5-

dimethylphenol which is active ingredient in ‘Dettol’.

USES OF PHENOLSUSES OF PHENOLS

Chapter 6 hydroxyl compounds

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