Hydroxy Compounds

New Way Chemistry for Hong Kong A-Level Book 3A

1

Hydroxy Compounds33.133.1 IntroductionIntroduction

33.233.2 Nomenclature of Hydroxy CompoundsNomenclature of Hydroxy Compounds

33.333.3 Physical Properties of Hydroxy CompoundsPhysical Properties of Hydroxy Compounds

33.433.4 Acidic Properties of Hydroxy CompoundsAcidic Properties of Hydroxy Compounds

33.533.5 Preparation of Hydroxy CompoundsPreparation of Hydroxy Compounds

33.633.6 Reactions of AlcoholsReactions of Alcohols

33.733.7 Reactions of PhenolReactions of Phenol

33.833.8 Uses of AlcoholsUses of Alcohols

Chapter 33Chapter 33


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33.1 Introduction (SB p.206)

Hydroxy compounds: Organic compounds with the hydroxy group(s) (–OH) attached to an alkyl group or aromatic ring

1. Alcohols

• Compounds containing one or more –OH attached to an alkyl group

• General formula for alcohols with –OH group: CnH2n+1OH

e.g.


3


• Depending on the number of alkyl groups attached to the

carbon which is bonded to the hydroxy group

• Alcohols can be classified as:

Primary alcohol Secondary alcohol Tertiary alcohol


4


2. Phenols

• Compounds that have a – OH group directly attached t

o a benzene ring

• General formula for phenols: Ar – OH

e.g.


5

33.2 Nomenclature of Hydroxy Compounds (SB p.207)

1. Select the longest possible straight chain to which the h

ydroxyl group is directly attached. Change the name of t

he alkane correspending to this chain by dropping the

final ‘-e’ and adding the suffix ‘-ol’.

AlcoholsAlcohols


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2. Number the longest possible straight chain in such a

way so as to give the carbon atom bearing the hydroxyl

group the lower number.

3. Designate the position of the hydroxyl group by using

this number, and also the positions of other substituents

by using the numbers corresponding to their positions

along the carbon chain.


7


Examples:


8


• Phenol will be the parent name when a benzene ring containing a –OH group

• When substituents are present, the –OH group is assumed to be in position 1, and numbers are assigned to the substituents according to their positions in the benzene ring

e.g.

PhenolsPhenols


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Example 33-1Example 33-1For each of the following hydroxy compounds:

(i) Draw the complete structural formula.

(ii) Classify them as primary, secondary or tertiary alcohols.

(a) Hexan-3-ol

(b) 3-Methylbutan-1-ol

(c) 2-MethylcyclopentanolAnswer


Solution:

(a) (i)

(ii) A secondary alcohol

(b) (i)

(ii) A primary alcohol

(c) (i)

(ii) A secondary alcohol


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Check Point 33-1 Check Point 33-1

(a) Draw the structural formulae of all isomers of alcohols having the molecular formula C4H10O. Give their

IUPAC names. Answer


(a)


11


(b) Draw the structural formulae of three isomeric bromophenols with the molecular formula C6H4BrOH. Give their IUPAC names.

Answer


(b)

4-Bromophenol


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33.3 Physical Properties of Hydroxy Compounds (SB p.209)

Hydroxy compound

Formula Boiling

point (°C)

Melting point (°C)

Density at 20°C (g cm–

3)

Alcohols:

Methanol

Ethanol

Propan-1-ol

Butan-1-ol

Pentan-1-ol

Hexan-1-ol

Propan-2-ol

Methylpropan-1-ol

Butan-2-ol

Methylpropan-2-ol

CH3OH

CH3CH2OH

CH3(CH2)2OH

CH3(CH2)3OH

CH3(CH2)4OH

CH3(CH2)5OH

(CH3)2CHOH

(CH3)2CHCH2OH

CH3CH2CHOHCH3

(CH3)3COH

64.5

78.5

97.2

117

138

157

82.4

108

99.5

82.5

–97.7

–117

–127

–89.5

–78.8

–51.6

–89.5

–108

–115

25.5

0.791

0.789

0.803

0.810

0.814

0.814

0.785

0.802

0.808

0.786

Phenols:

Phenol

2-Methylphenol

C6H5OH

CH3C6H4OH

182

191

40.9

31.0

1.073

1.046


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Boiling Point and Melting PointBoiling Point and Melting Point

Intermolecular hydrogen bonds between alcohol molecules is stronger than van der Waals’ forces between alkane molecules

alcohols have higher b.p. and m.p.


14


Branching of the carbon chain reduces the surface area in contact with other molecules

Reduction in the extent of intermolecular hydrogen b

onds that can be formed between neighbouring molecules

Isomers of C4H9OH Boiling point (°C)

117

108

99.5

82.5


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DensityDensity

• Simple alcohols are less dense than water at 20°C

• Phenols are slightly denser than water at 20°C

• The densities of alcohols increase with increasing

relative molecular masses


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SolubilitySolubility

• Alcohols with a relatively short carbon chain (e.g. met

hanol, ethanol, propan-1-ol and propan-2-ol) are complet

ely miscible with water

• The solubility decreases as the hydrocarbon chain inc

reases

e.g. the solubility of butan-2-ol is 8 g per 100 g of water


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(a) Arrange the following compounds in order of increasing boiling points:


Answer

(a)


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(b) Which member in each of the following pairs is more soluble in water?

(i) CH3CH2OH or CH3CH2CH2CH2OH

(ii)

(iii)

Answer


(b) (i) CH3CH2OH

(ii)

(iii)


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33.4 Acidic Properties of Hydroxy Compounds (SB p.212)

• Alcohols are neutral

• Alcohols show acidic properties when reacting with strong bases (e.g. sodium metal)

• The acidic property depends on the stability of the alko

xide ion formed which is partly influenced by the struct

ure of the carbon chain


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• 3° alkoxide ions are the least stable

∵ 3 electron-releasing alkyl groups release electrons to the negatively charged oxygen atom

3° alcohol is the least acidic

Hydroxy compound pKa

Methanol

Ethanol

Methylpropan-2-ol

Phenol

2-Chlorophenol

2-Nitrophenol

2-Methylphenol

15.5

16

18

10.00

8.11

7.17

10.20

Phenols have smaller pKa

stronger acids than alcohols


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• Phenols are more acidic than alcohols

∵ phenoxide ion is more stable than alkoxide ion due to resonance stability (4 resonance structures can be drawn)


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The negative charge disperses over the entire benzene ring a

nd oxygen atom by extended delocalized electron cloud

phenoxide ion is stabilized


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The order of acidity of some organic compounds and water:

No reaction

No reaction

CO2 evolved

No reaction

React to give salts and water

React to give salts and water

Alcohols

Phenols

Carboxylic acids

Sodium hydrogencarbonateSodium hydroxide

Simple tests to distinguish alcohols, phenols, carboxylic acids

Phenol is acidic enough to react with NaOH but not Na2CO3


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Example 33-2Example 33-2Arrange the following compounds in order of increasing acidity.


Answer

Solution:

The acidity of the compounds increases in the order:


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Which of the following compounds is more acidic? Explain briefly.

(a) CF3CH2COH and CH3CH2OHAnswer


(a) CF3CH2OH is more acidic, because –CF3 is a strong elec

tron-withdrawing group. It exerts a negative inductive effect on t

he conjugate base of CF3CH2OH (i.e. CF3CH2O–) and thus stabili

zes the CF3CH2O– ion.


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Which of the following compounds is more acidic? Explain briefly.

(b)

Answer


(b) is more acidic. The reason is that the conjugate

base is stabilized by the negative inductive and resonance

effects of the –NO2 group.


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33.5 Preparation of Hydroxy Compounds (SB p.216)

Preparation of AlcoholsPreparation of Alcohols

Fermentation of Carbohydrates

2(C6H10O5)n + nH2O nC12H22O11

C12H22O11 + H2O 2C6H12O6

C6H12O6 2CH3CH2OH + 2CO2

The concentration of ethanol can be increased by

fractional distillation

amylase

60°Cmaltase

15°Czymase

15°C


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Alkaline Hydrolysis of Haloalkanes

• Nucleophilic substitution reaction of haloalkanes w

hereby 1°, 2° and 3° alcohols can be prepared

• The equilibrium lies towards the right

∵ OH– is a better nucleophile than halide ion


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Reduction of Aldehydes and ketones

• Reduction of aldehydes and ketones respectively by powerful reducing agents (e.g. LiAlH4 in dry ether)

produces 1° and 2° alcohols


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Preparation of PhenolPreparation of Phenol

Industrial Process

• Alkaline hydrolysis of chlorobenzene under severe conditions of high temperature and pressure produces phenol


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Laboratory Process

• By refluxing benzene and conc. sulphuric(VI) acid f

or 1 day to form benzenesulphonic acid and then subje

ct to alkaline hydrolysis


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• By hydrolysis of benzenediazonium salt



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Example 33-3Example 33-3Comment on the feasibility of the following preparation of hydroxy compounds in the school laboratory.

(a) KOHCH3CH2CH2Cl CH3CH2CH2OH

(b)

Answer


Solution:

(a) The preparation of the alcohol is feasible in this way. It is a nucleophilic substitution reaction of haloalkanes, and the nucleophile is the hydroxide ion, OH–.

(b) The preparation of phenol is not feasible in this way. It is because halobenzenes do not undergo nucleophilic substitution reactions unless under severe reaction conditions.


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33.6 Reactions of Alcohols (SB p.218)

Reactions of alcohols can be classified into two main types:

1. Reactions involving the cleavage of the C – O bond:

2. Reactions involving the cleavage of the O – H bond:


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Reaction Involving Cleavage of the C – O bondReaction Involving Cleavage of the C – O bond

Formation of Haloalkanes

• Alcohols react with hydrogen halides by nucleophilic substitutions

• The order of reactivity of hydrogen halides:

HI > HBr > HCl

• The order of reactivity of alcohols: 3° > 2° > 1° < methyl


Reaction with Hydrogen Halides


37

1. Secondary and tertiary alcohols

• 2° and 3° alcohols react by SN1 mechanism

• e.g.


Reactions of Alcohols with Hydrogen Chloride


38

Step 1: The alcohol is protonated


Step 2: The protonated alcohol dissociates to give a carbocation and water


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Step 3: The carbocation reacts with a nucleophile to give the product



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2. Primary alcohols and methanol


• 1° alcohol and methanol react by SN2 mechanism

Step 1: The alcohol is protonated


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Step 2: The halide ion displaces a molecule of water from

the carbon to give the product



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• Lucas reagent: Solution of ZnCl2 in conc. HCl

• Used to to differentiate between simple primary, secondary and tertiary alcohols

• When an alcohol is treated with Lucas reagent, the corresponding chloroalkane is formed.


Lucas Test


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• 1°, 2°, 3° alcohols react with Lucas reagent at different rates



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• The order of the reactivity of alcohols towards Lucas reagent:

3° alcohol > 2° alcohol > 1° alcohol


• The reactions are believed to take place via the SN1 mecha

nism which involves the formation of carbocation in the rate-determining step


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The relative stability of carbocations:


tertiary carbocations are formed rapidly from tertiary alcohols primary carbocations are difficult to form, so primary alcohols react very slowly


46

• Alcohols react with hydrogen bromide to give bromoalkanes

• Hydrogen bromide is generated in situ by adding excess conc. H2SO4 to NaBr


Reactions of Alcohols with Hydrogen Bromide

• Iodoalkanes cannot be prepared by reacting HI with alco

hols as iodide will be oxidized by conc. H2SO4


47


Reaction with Phosphorus Halides

Laboratory set-up for the reaction of an alcohol with PBr3


48

• PI3 is used in iodination of alcohols


3R – OH + PI3 3R – I + H3PO3

• Chloroalkanes are formed readily from the reaction of alcohols with PCl5

R – OH + PCl5 R – Cl + POCl3 + HCl

• SOCl2 converts 1° and 2° alcohols to chloroalkanes

R – OH + SOCl2 R – Cl + SO2 + HCl

(1° or 2°)


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Dehydration of Alcohols

• Alkenes are formed when alcohols are heated with strong acids

• Elimination reactions are favoured at high temperatures


Formation of Alkenes


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Dehydration occurs when passing alcohol vapour over alu

minium oxide at 350°C


Laboratory set-up for dehydration of an alcohol


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• The experimental conditions required to bring the dehydration are related to the structure of alcohols

• 1° alcohols are the most difficult to dehydratee.g.


• 2° alcohols usually dehydrate under milder conditionse.g.


52


• 3° alcohols dehydrate at very mild conditionse.g.

The order of the relative ease of alcohols to undergo dehydration:


53


When 2° or 3° alcohols having more than 3 carbon atoms is dehydrated, two or more alkenes are formed.

But-2-ene is major product

more highly substituted ∵alkene (Saytzeff’s rule)


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• Dehydration of an alcohol to an ether takes place at lower temperature than that to an alkene


Formation of Ether


55


Give the structural formula and IUPAC name of the major product formed by reacting 3-methylpentan-2-ol with heated aluminium oxide.

Answer

The major product is 3-methylpent-2-ene. Its structur

al formula is: CH3CH = C(CH3)CH2CH3



56


• These reactions are less vigorous than the reaction of the metal with water ∵ alcohols are weaker acids than water

Reaction Involving Cleavage of the O – H bondReaction Involving Cleavage of the O – H bond

Formation of Alkoxides


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Example:


• This reaction provides a safe way of disposing sodium residues

• Addition of water regenerates the alcohol as the propoxide ion is a very strong baseCH3CH2CH2O–Na+ + H2O CH3CH2CH2OH + NaOH

• The evolution of hydrogen with sodium metal is a useful test for the presence of –OH group


58


Esterification: Alcohols react with carboxylic acids to form esters through a condensation reaction

Esterification

e.g.


59


• Esterification reactions are acid-catalyzed

• Conc. H2SO4 is used as catalyst

• The reactions proceed very slowly in the absence of catalyst but reach an equilibrium within few hours when a carboxylic acid and an alcohol are refluxed with a small amount of conc. H2SO4

• The equilibrium can be shifted to the product side with the use of an excess of either the carboxylic acid or the alcohol

• The yield of the reaction can be increased by removing water from the reaction mixture as it is formed


60


Esters can be synthesized by the reaction of alcohols with acyl chlorides in the absence of an acid catalyst

e.g.


61


Acid anhydrides react with alcohols to form esters in the absence of an acid catalyst

e.g.


62


• 1° alcohols are first oxidized to aldehydes and then to

carboxylic acids by oxidizing agents such as acidified

KMnO4 and acidified K2Cr2O7

Oxidation of Alcohols

Oxidation of Primary Alcohols


63


1. Oxidation of primary alcohols to aldehydes

• The oxidation is difficult to stop at the aldehyde stage because aldehydes are a reducing agent

• To obtain aldehydes, distill off the aldehydes from the reaction mixture as they are formed

• Oxidizing agent: acidified K2Cr2O7

e.g.


64


A laboratory set-up for the conversion of ethanol to ethanal


65


2. Oxidation of primary alcohols to carboxylic acids

• 1° alcohols can be oxidized to carboxylic acids by powerful oxidizing agents (e.g. acidified KMnO4)

• The oxidation of 1° alcohols by acidified KMnO4 will

not stop at the aldehydes

e.g.


66


(a) (b)

(a) A reflux apparatus for the oxidation of ethanol to ethanoic acid(b) A distillation apparatus for the separation of ethanoic acid from the reaction mixture after reflux


67


Breathalyser is used by police to rapidly estimate the ethanol content of the breath of suspected drunken drivers

• The breathalyser is based on the oxidation of ethanol by acidified K2Cr2O7

• As the driver blows into the bag, ethanol molecules reduce orange Cr2O7

2- to green Cr3+


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• 2° alcohols are oxidized to ketones by either acidified KMnO4 or acidified K2Cr2O7

• 2° alcohols cannot be oxidized to carboxylic acids because it would involve the breaking of strong carbon-carbon bond

Oxidation of Secondary Alcohols


69


Example:


70


• 3° alcohols are generally resistant to oxidation ∵ oxidation involves the breaking of carbon-

carbon bonds

• 3° alcohols can be oxidized by acidified KMnO4 to giv

e a mixture of ketones and carboxylic acids, both with fewer carbon atoms than the alcohol

Oxidation of Tertiary Alcohols

e.g.


71


• 1° alcohols are oxidized to aldehydes which can be detected by the reaction with 2,4-dinitrophenylhydrazine or Tollen’s reagent

• 2° alcohols are oxidized to ketones which can be detected by the reaction with 2,4-dinitrophenylhydrazine but not Tollen’s reagent

• The carboxylic acid formed from the oxidation of 3° alcohols can be detected by the reaction with sodium hydrogencarbonate or ester formation


72


Draw the structural formulae for the major organic products in the following reactions.

(a) PCl5CH3CH2OH

(b) P, Br2CH3CH2OH

reflux

(c) SOCl2CH3CH2CH2OH

(d) K2Cr2O7/H+

Propan-1-ol reflux

(e) K2Cr2O7/H+

Propan-2-ol reflux

Answer


(a) CH3CH2Cl

(b) CH3CH2Br

(c) CH3CH2CH2Cl

(d) CH3CH2COOH

(e)


73


Triiodomethane Formation (Iodoform Reaction)

Alcohols containing the group react with

iodine in sodium hydroxide (known as iodoform reagent) to give iodoform (CHI3)


74


• The iodoform formed is a yellow crystal with characteristic odour

• The iodoform test:

The iodoform reaction serves as a test for

alcohols containing the group


75

Example 33-3Example 33-3Write equations to show how each of the following transformations can be accomplished. Some conversions may require more than one step.

(a)

(b)


Answer

Solution:

(a)

(b)


76


(c)

(d)


Answer

Solution:

(c)

(d)


77


(e)

(f)


Answer

Solution:

(e)

(f)


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Example 33-4Example 33-4Give a simple chemical test that can distinguish between the compounds in each of the following pairs:

(a) Ethanol and methoxymethane

(b) Propan-1-ol and propene

(c) Propan-1-ol and propan-2-ol

(d) Pentan-3-ol and pentan-2-ol Answer

Solution:

(a) Sodium reacts with ethanol to give hydrogen gas while methoxymethane does not.

(b) Propene decolourizes bromine in 1,1,1-trichloroethane while propan-1-ol does not.

(c) On the addition of Lucas reagent, propan-2-ol will give a cloudy appearance in a shorter time than propan-1-ol.

(d) On the addition of iodine in sodium hydroxide, pentan-2-ol will give a yellow precipitate (due to the fo

rmation of iodoform) while pentan-3-ol will not.



79


Draw the structural formulae for the missing products A, B and C in the reactions below.


Answer


80

33.7 Reactions of Phenols (SB p.233)

• The lone pair electrons on oxygen interact with the

delocalized electron cloud of benzene ring

• The resonance structures withdraw the electron from oxygen, making it slightly electron-deficient

• This strengthens the C – O bond but weakens the O – H bond reaction of phenols involve the cleavage of O – H bond


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Reaction with Sodium

Phenols react with sodium to form sodium phenoxide and

hydrogen gas



82

Reaction with Sodium Hydroxide

• Phenol is a weaker acid than carboxylic acids or mineral acids do not react with sodium hydrogencarbonate

• Phenol is a stronger acid than alcohols react with sodium hydroxide while alcohols do not



83

Esterification

• Phenol does not react with carboxylic acids to give e

sters

∵ the lone pair electrons on oxygen atom deloc

alized into the benzene ring, making it less nucle

ophilic and less likely to undergo reaction



84

• Esterification can be carried out by converting phenol

to sodium phenoxide firstly and then treating with ac

yl chlorides or acid anhydrides



85

Examples:



86

• Methanol and ethanol are good solvents in the labora

tory and industry

e.g. methylated spirit (which contains 95% etha

nol/water mixture and 5% methanol) is commonly

used as an industrial solvent

33.8 Uses of Alcohols (SB p.235)

As SolventsAs Solvents


87

• Alcohols usually burn with a clean, blue,

hot and non-luminous flame with a

very small amount of soot is formed

• As alcohols are too expensive, they are

not widely use as domestic fuel


As FuelsAs Fuels

Ethanol is used as a fuel


88

• Ethanol is the major component

in alcoholic beverages

• The ethanol is produced by the

fermentation of sugar or starch


As Alcoholic DrinksAs Alcoholic Drinks


89

• Ethanol (80%) is mixed with gasoline (20%) to produce a motor fuel, known as gasohol

• Gasohol is an alternative to gasoline and it produces less pollutants when burnt

• It is widely used in Brazil where sugar cane is abundant and fermented to ethanol


As a Motor Fuel Blending AgentAs a Motor Fuel Blending Agent


90

• Ethane-1,2-diol is formed in the laboratory by bubbling ethene into a cold and dilute alkaline solution of KMnO4


As an AntifreezeAs an Antifreeze

• Ethane-1,2-diol is miscible with water in all proportions and has a high boiling point

• Use as an anti-freeze in car radiators in cold countries


91

• Terylene (or Dacron) is a polyester which is formed by condensation polymerization of ethane-1,2-diol and benzene-1,4-dicarboxylic acid


As a Raw Material for Making TeryleneAs a Raw Material for Making Terylene


92

• Properties: strong, light and does not ‘wet’

• Uses: making clothing and sails



93

Phenol-methanal: by polymerization of phenol and methanal in acidic medium

Bakelite: using excess methanal, further reactions with phenol-methanal to form cross links


As a Raw Material for Making PlasticsAs a Raw Material for Making Plastics


94

• Bakelite is a rigid, three-dimensional, cross-linked polymer

• Phenol-methanal and bakelite are thermosetting plastics and are good insulators of heat and electricity

• Uses: making kitchenware (e.g. handles of saucepans) and electrical appliances (e.g. electric plugs)



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Hydroxy Compounds