14.Hydroxyl Compounds Lecture Notes

14

Hydroxyl Compounds Alcohols & Phenols

CONTENT 1

Introduction 1.1 Classification of alcohols 1.2 General formula 1.3 Nomenclature

2

Physical Properties of Alcohols and Phenols 2.1 Nature of the Hydroxyl Group 2.2 Boiling Points 2.3 Solubility in Water 2.4 Acidity of Alcohols and Phenols

3

Preparation of Alcohols 3.1 From Alkenes through Hydration 3.2 From Alkyl Halides through Nucleophilic Substitution 3.3 From Aldehydes and Ketones through Reduction 3.4 From Carboxylic Acids through Reduction

4

Chemical Reactions of Alcohols 4.1 Combustion 4.2 Halogenation 4.3 Dehydration 4.4 Reaction with Sodium Metal 4.5 Esterification 4.6 Oxidation 4.7 Tri-iodomethane (Iodoform) Formation

5

Chemical Reactions of Phenols 5.1 Reactions involving the breaking of O-H bond 5.2 Reactions involving the Electrophilic Substitution on the Benzene Ring 5.3 Complex Formation with neutral FeCl3 (aq)

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Independent Learning subtopic

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LEARNING OUTCOMES Candidates should be able to: (a)

recall the chemistry of alcohols, exemplified by ethanol: (i) (ii) (iii) (iv) (v)

combustion substitution to give halogenoalkanes reaction with sodium oxidation to carbonyl compounds and carboxylic acids dehydration to alkenes

(b)

classify hydroxy compounds into primary, secondary and tertiary alcohols

(c)

suggest characteristic distinguishing reactions, e.g., mild oxidation

(d)

deduce the presence of a CH3CH(OH)– group in an alcohol from its reaction with alkaline aqueous iodine to form tri-iodomethane

(e)

recall the chemistry of phenol, as exemplified by the following reactions:

(f)

(i)

with bases

(ii)

with sodium

(iii)

nitration of, and bromination of, the aromatic ring

explain the relative acidities of water, phenol and ethanol

REFERENCES 1.

Peter Cann & Peter Hughes (2002). Chemistry for Advanced Level. Chapter 27.

2.

John McMurry (2004). Organic Chemistry. Brooks/Cole. Chapters 15-16.

3.

Philip S. Bailey, Jr. & Christina A. Bailey (2000). Organic Chemistry – A Brief Survey of Concepts & Applications. Prentice Hall. Chapter 6.

1 Introduction Alcohols are compounds of the general formula ROH, where R is any alkyl or substituted alkyl group. All alcohols contain the hydroxyl (−OH) group, which, as the functional group, determines the properties characteristic of this family of compounds.

CH3 OH

CH3 H

CH3CH2OH

H

HO CH(CH3)2

Ethanol Often found in alcoholic beverages and in modern thermometers.

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Menthol

Occurs naturally in peppermint oil. It is well known for the cooling sensation that it provokes when inhaled, eaten or applied to the skin.

H

HO Estradiol

It is the predominant sex hormone present in females. It is an important hormone not only on reproductive and sexual functioning, but also affects other organs including the bones.

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Phenols are compounds in which the the hydroxyl (−OH) group, is directly attached to an aromatic ring. Phenols differ markedly from alcohols in both physical and chemical properties.

OH

1.1

Classification of alcohols

Alcohols can be subdivided into 3 different classes (similar to halogenoalkanes). An alcohol is classified as a primary, secondary, or tertiary alcohol according to the kind of carbon that bears the −OH group:

R = alkyl or aryl

Methanol, CH3OH, is classified as a primary alcohol. 1.2

General formula

Saturated aliphatic alcohols have the general formula CnH2n+1OH, where n is the number of carbon atoms.

n

Name

Structural formula

1

methanol

CH3OH

2

ethanol

CH3CH2OH

propan-1-ol

CH3CH2CH2OH

propan-2-ol

CH3CH(OH)CH3

Displayed formula

3

1.3 Nomenclature Alcohols are named by replacing the terminal e of the corresponding alkane name with ol.

H H C H H

H H C OH H

Methane

Methanol

The longest carbon chain containing the hydroxyl group is considered the parent structure.

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Number the alkane chain beginning at the end nearer the hydroxyl group, and use the appropriate number to indicate the position of the –OH group.

OH Heptan-3-ol

Number the substituents according to their positions on the chain, and write the name listing the substituents in alphabetical order.

The –OH functional group is named as a hydroxy substituent when it appears on a structure with a higher priority functional group or when the structure is too difficult to name as a simple alcohol.

CH3

OH

O

CH

CH2 C

OH

3-hydroxybutanoic acid Compounds that have a hydroxyl group directly attached to a benzene ring are called phenols.

Exercise 1 Name and classify the following alcohols or phenols. CH3 OH

(i) CH3

C

CH

(ii) CH2 Br

OH

(iii)

CH2OH Br

CH3

CH3

1-bromo-3,3-dimethylbutan-2ol o (2 alcohol)

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phenylmethanol o (1 alcohol)

5-bromo-2-methylphenol

(phenol)

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2

Physical Properties of Alcohols and Phenols

2.1

Nature of the Hydroxyl Group (*INDEPENDENT LEARNING SUBTOPIC*)



Alcohols and phenols have nearly the same geometry around the oxygen atom as water.



The oxygen atom has two lone pairs of electrons.



Oxygen is more electronegative than carbon and hydrogen



Hence, alcohols have polar C–O and O–H bonds.

2.2

Boiling Points (*INDEPENDENT LEARNING SUBTOPIC*)

Alcohols and phenols have significantly higher boiling points than those of the hydrocarbons with similar relative molecular masses. o

o

Compound

Mr

b.p. / C

Compound

Mr

b.p. / C

C2H6

30

-88

CH3OH

32

65

C3H8

44

-42

C2H5OH

46

78

C4H10

58

0

C3H7OH

60

97

C5H12

72

36

C4H9OH

74

118

C6H5CH3

92

111

C6H5OH, phenol

94

182

Comparison between alkanes and alcohols 

The boiling point of an alcohol is higher than that of an alkane of similar relative molecular mass.



Reason: More energy is required to overcome the intermolecular hydrogen bonding in alcohols which is stronger than the dispersion forces in alkanes.

+

Comparison within the homologous series  Boiling point of alcohols increases as the number of carbon atoms increases.  Hydrogen bonding is the main type of intermolecular forces between the –OH groups. There are also dispersion forces between the alkyl chains.  Hydrogen bonding will be similar for all the alcohols, while the strength of dispersion forces increases as the alkyl chains get longer. These forces get stronger as longer molecules have more electrons and are more polarisable.  Thus, more energy is required to overcome the dispersion forces, and so the boiling point increases.

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Solubility in Water (*INDEPENDENT LEARNING SUBTOPIC*) 

Alcohols are more soluble in water compared to their corresponding alkanes due to their ability to form hydrogen bonds with water molecules.

+



Solubility of alcohols in water decreases with increasing carbon number due to the increasing length of the hydrophobic non-polar hydrocarbon chain. CH3CH2CH2CH2CH2CH2 −O−H non-polar



2.4

polar

Phenols are moderately soluble in water due to their large hydrophobic non-polar phenyl group; but dissolve completely when warmed.

Acidity of Alcohols and Phenols +

Alcohols and phenols dissociate to slight extent by donating a proton each to water, generating a H 3O and an − − alkoxide ion, RO , or a phenoxide ion, C6H5O , respectively. R-O-H + H2O  R-O + H3O −

+

Compound

CH3CH2OH

H2O

C6H5OH

pKa

16

15.7

10

As can be seen from the pKa values, CH3CH2OH is a weaker acid as compared to water, while phenol is a stronger acid as compared to water. 2.4.1 Alcohols 

+

H

O

CH3CH2



−

In alcohols, the O−H bond can break to give H and RO (alkoxide ion). The extent of dissociation is lower than that of water. Therefore alcohols do not turn litmus red. H O + H+ hydroxide ion

H

O

H

CH3CH2 O ethoxide ion

+

pKa = 15.7

H+

pKa = 16

Why do alcohols have lower acidity than water? 

Alkyl groups are electron-donating and will intensify the negative charge on the alkoxide ion.



Thus the alkoxide ion is destabilized. Deprotonation does not take place as easily. electron donating effect

H CH3

C H

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O

inductive effect of ethyl of ethyl group increases group increases the electron density on theon the electron density oxygen anion, the oxygen anion, destabilizing the anion

destabilizing the anion.

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2.4.2 Effect of Substituents on Acidity of Alcohols The presence of electron-withdrawing groups (e.g., –NO2, –F, –Cl, –Br, –I, –COCH3, –CO2H, –CN, –CO2R, –NH2, –OH, – OCH3) help to stabilize the alkoxide ion formed by dispersing the negative charge on the alkoxide ion, thus promoting the ionisation process. However, the presence of electron-releasing groups (e.g., –CH3) decreases the acidic strength of alcohols by intensifying the negative charge on the alkoxide ion, thus destabilising it. CF3 CF3 C OH CF3 nonafluoro-tert-butyl nonafluoro-tert-butylalcohol pK pKaa==5.4 5.4

H H C OH H methanol pKa = 15.5

CH3 CH3 C OH CH3 2-methyl-propan-2-ol pKa = 18

Exercise 2 Arrange the alcohols in order of acid strength, giving your reasoning. CH3CH2OH In order of increasing acid strength: (CH3)3COH <

CH2ClCH2OH

(CH3)3COH

(CH3)2CHOH

(CH3)2CHOH < CH3CH2OH < CH2ClCH2OH

Reasons: 

The more stable the alkoxide formed, the stronger is the alcohol as an acid.



Alkyl groups are electron-donating, intensifying the negative charge on the alkoxide ion and thus destabilizing it.



(CH3)3COH with three methyl groups, followed by (CH 3)2CHOH which has two methyl groups and CH3CH2OH which has one methyl group.



Electron-donating effect on the alkoxide is most in (CH3)3COH followed by (CH3)2CHOH and then CH3CH2OH.



CH2ClCH2OH has an electron-withdrawing group (chloro group) thus it disperses the negative charge on alkoxide ion and stabilizes it.

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2.4.3 Phenol −3

Phenols are more acidic than alcohols. The pH of a 0.1 mol dm phenol solution in water is 5.4 so it will turn universal indicator solution yellow. CH3CH2

O

H

CH3CH2 O

O H

O

+

+

Ka = 1.0 x 10-16 mol dm-3

H+

Ka = 1.3 x 10-10 mol dm-3

H+

The negative charge on the phenoxide ion can be delocalised over the benzene ring as shown in the diagrams below. This stabilises the phenoxide ion.

C

C

C

C

C

C

O

O

O

O

O

electron delocalization in phenoxide is represented by resonance among the structures

In the phenoxide ion, a p-orbital of the oxygen is able to overlap with the π-orbital of the carbon atom in the ring. The negative charge is able to spread around the benzene ring to some extent, thus the phenoxide anion is stabilised by resonance. The phenoxide anion is less likely to accept a proton to form phenol. This makes phenols slightly stronger acids than aliphatic alcohols. 2.4.4 Effect of Ring Substituents on Acidity of Phenols Similar to alcohols, the presence of electron-withdrawing groups (e.g., –NO2, –F, –Cl, –Br, –I, –COCH3, –CO2H, –CN, – CO2R), in the ring enables the ring in turn to withdraw more electron density from the oxygen, thus stabilising the phenoxide ion further and promoting the ionisation process. OH Cl

OH

OH

Cl

Cl

Cl

2,4,6-trichlorophenol pKa = 7.6

4-chlorophenol pKa = 9.4

phenol pKa = 10

The presence of electron-releasing groups (e.g., –CH3, –NH2, –OH, –OCH3) decreases the acidic strength of phenols by reducing the delocalisation of negative charge on oxygen into ring, and destabilising the conjugate base. OH

OH

OH

CH3

benzene-1,4-diol pKa = 10.35 Crystal Cheong / Low YZ / Jolin Lim / Grace Chua

4-methylphenol pKa = 10.3

OH

phenol pKa = 10 8

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Conclusions 

Phenols are more acidic than water as the phenoxide ion can be stabilized by the delocalization of the negative charge over the benzene ring.



Alcohols are weaker acids than water as alcohols have electron donating alkyl groups which intensify the negative charge on the alkoxide ion, thus destabilizing it.



Electron-withdrawing groups stabilize the conjugate bases (i.e., alkoxide or phenoxide anions) formed by dispersing the negative charge on the alkoxide or phenoxide ion, resulting in increased acidity.



Electron-donating groups destabilise the conjugate bases (i.e., alkoxide or phenoxide anions) formed by intensifying the negative charge on the alkoxide or phenoxide ion, resulting in decreased acidity.

Exercise 3 Arrange the following phenols in order of acidity (1 for the most acidic, 5 for the least acidic).

5

3

2

3

4

1

Preparation of Alcohols

Alcohols occupy a central position in organic chemistry. They can be prepared from many other organic compounds (alkenes, alkyl halides, aldehydes, etc.) and they can be transformed into an equally wide assortment of organic compounds.

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From Alkenes through Hydration

There are two methods commonly used for the hydration of alkenes to form alcohols. Industrial method

Laboratory method 1. Conc. H2SO4 2. H2O, warm

3.2

From Alkyl Halides through Nucleophilic Substitution (or Alkaline Hydrolysis)

Nucleophilic substitution of alkyl halides produces the corresponding alcohols in high yield. RCH2 X

3.3

+ OH heat under reflux

RCH2 OH + X

From Aldehydes and Ketones through Reduction

LiAlH4 in dry ether

LiAlH4 in dry ether 

Reduction of aldehydes yield primary alcohols.



Reduction of ketones yield secondary alcohols.



Tertiary alcohols cannot be formed with this method.



For the reduction of aldehydes and ketones, the reducing agents used may be: o Lithium aluminium hydride in dry ether o hydrogen with nickel catalyst, high T & high P o sodium borohydride (NaBH4) in methanol

3.4

From Carboxylic Acids through Reduction LiAlH4 in dry ether



Reduction of carboxylic acids yield primary alcohols.



For the reduction of carboxylic acids, the only reducing agent used is: o Lithium aluminium hydride in dry ether

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+ H2O

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Chemical Reactions of Alcohols

Alcohols are versatile starting materials for the preparation of a variety of organic compounds. Alcohols undergo reactions involving various combinations of the cleavage of the C–O and O–H bonds of the hydroxyl group.

This bond is able to break releasing H+ ions and the alcohols are converted to alkoxides. This bond is broken when alcohols are subjected to acid-catalysed dehydration or converted to alkyl halides. 4.1

Combustion

Ethanol can be used as a fuel. It burns in air to form carbon dioxide and water. CH3CH2OH(l) + 3O2(g) → 2CO2(g) + 3H2O(l) 4.2

∆Hc = –1367 kJ mol

–1

Halogenation

Alcohols can be converted to alkyl halides using many different methods. However, all these methods involve the substitution of the –OH group with the halogen atom via the breaking of the C–O bond.

4.2.1 Using hydrogen halides Tertiary alcohols are readily converted into alkyl chlorides by treatment of HCl. Primary and secondary alcohols are much more resistant to the acid thus require a different treatment. For tertiary alcohols only,

conc HCl

For primary and secondary alcohols, For chlorination, anhydrous ZnCl2 is added as a catalyst and heating is required. conc HCl / ZnCl2 heat

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For bromination, HBr is produced by heating solid NaBr in the presence of concentrated H2SO4. The acid serves as both a catalyst and as a dehydrating agent, removing water as it is formed. NaBr / conc H2SO4 heat

4.2.2 Using phosphorus halides Phosphorus pentahalide: Phosphorus pentachloride, PCl5, is a good chlorinating agent that reacts with alcohols to produce the respective chloro-containing compound.

Reagents & Conditions: PCl5(s), room temperature Observations: Dense white fumes of HCl (*good distinguishing test for the presence of the –OH group*) Phosphorus trihalides:

2

Reagents & Conditions:

Comments:

PCl3 (phosphorus trichloride), room temperature or PBr3 (phosphorus tribromide), room temperature or P, I2, heat Phosphorus triiodide, PI3, is prepared in situ by heating red phosphorus with iodine. Phosphorus tribromide can also be prepared in this manner. 2P + 3X2  2PX3

X = Br, I

4.2.3 Using Sulfur Oxide Dichloride, SOCl2 (Thionyl Chloride) Another good chlorinating agent is thionyl chloride, SOCl2. When the alcohol is reacted with thionyl chloride the corresponding chloro-compound is formed together with SO2 and HCl gases.

Reagents & Conditions: SOCl2(l), warm Observation: SO2(g) and white fumes of HCl(g) Comments: both gaseous by-products are easy to separate from the halogenoalkanes formed.

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Dehydration

+ H2O Reagents & Conditions:

o

excess concentrated H2SO4, 170 C

Dehydrating agents and conditions: o o excess concentrated H2SO4, heated to 170 C o Aluminium oxide, Al2O3, heat Dehydration is only possible on molecules containing at least 1 hydrogen atom on the carbon atom adjacent to the carbon atom with the hydroxyl group.

Dehydration cannot take place when there is no hydrogen atoms bonded to the carbon atom adjacent to the carbon atom with the hydroxyl group. Examples: H R H

C H

R C C H R OH

OH

or

Sometimes a mixture of alkenes is formed, depending on the position of the hydroxyl group. H H H H H H H H H CH3

C

C

C

H

OH H

H

- H2O

CH3 CH C3CH C2CH=CH C H2

+

Hbut-1-ene but-1-ene

+ 3 C CCHC3CH=CHCH CH H 3 cis-but-2-ene & H trans-but-2-ene but-2-ene

Exercise 4 Give structures of all products formed when 1-methylcyclohexanol reacts with each of the following reagents. (a) phosphorus pentachloride (b) NaBr, conc H2SO4, heat (c) hot concentrated sulfuric acid (excess)

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CH3

OH 1-methylcyclohexanol

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14 – Hydroxyl Compounds

Reaction with Sodium Metal

Alcohols react with reactive metals such as sodium and potassium to form alkoxides together with hydrogen gas. This reaction involves the breaking of the O–H bond.

Reagents & Conditions: Observation:

Na(s), room temperature Slow effervescence of hydrogen gas

In this reaction, the alcohols act as acids. The reaction is similar to that of sodium with water except that it is much + slower (ethanol is a much weaker acid than water). This reaction shows that alcohols have replaceable H . However, they are not acidic enough to either react with alkalis (e.g., NaOH) or liberate CO2 from carbonates. 4.5

Esterification

Alcohols react with carboxylic acids and acyl chlorides to form esters in a condensation process. 4.5.1 With carboxylic acids

heat under reflux

Reagents & Conditions: Carboxylic acid and alcohol with few drops of conc. H2SO4, heat under reflux Observation: Sweet smelling liquid formed

This reaction is slow and reversible. Concentrated sulfuric acid acts as both a catalyst as well as a dehydrating agent. 4.5.2 With acyl chlorides

Reagents & Conditions: Acyl chloride and alcohol, room temperature Observations: Sweet smelling liquid formed, white fumes of HCl observed

This reaction is very rapid due to the high reactivity of the acyl chloride and does not require heating.

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14 – Hydroxyl Compounds

Oxidation

For the oxidation of alcohols, the alcohols must have at least one hydrogen atom bonded to the carbon atom to which the hydroxyl group is attached.

Two common reagents and conditions used for the oxidation of alcohols: o

K2Cr2O7 / dilute H2SO4, heat

o

KMnO4 / dilute H2SO4, heat

The product of oxidation is governed largely by the type of the alcohol.

4.6.1 Oxidation of Primary Alcohols Primary alcohols are readily oxidised to aldehydes on heating. With excess oxidising agent, the reaction proceeds further, yielding the carboxylic acid.

Oxidation to aldehydes 

Controlled oxidation of primary alcohols yields aldehydes.



Once the aldehyde is formed in the reaction vessel, it will come into contact with more oxidant and this will lead to further oxidation to form carboxylic acid. This can be avoided by distilling the aldehyde away from the reaction mixture as soon as it forms. This is possible as the aldehyde lacks intermolecular hydrogen bonding and thus has a lower boiling point than the corresponding alcohol.

Figure 1. Experimental setup for the distillation of aldehydes by oxidising primary alcohols. (Diagram taken from Chemistry for Advanced Level by P. Cann and P. Hughes; Hodder Murray)

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K2Cr2O7 / dilute H2SO4 heat with immediate distillation

Reagents & Conditions: Observations: Comment:

K2Cr2O7, dilute H2SO4, heat with immediate distillation 2– 3+ Orange solution (Cr2O7 ) turns green (Cr ) Only K2Cr2O7 is selective enough to oxidise primary alcohols to the aldehydes, whereas KMnO4 will simply oxidise them to the carboxylic acids.

Oxidation to carboxylic acids To obtain carboxylic acids from primary alcohols, the reacting solution needs to be heated under reflux. This allows the alcohol to be converted to carboxylic acid.

K2Cr2O7 / dilute H2SO4 heat under reflux

Reagents & Conditions: Observations:

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K2Cr2O7/dil H2SO4 or KMnO4/dil H2SO4, heat under reflux 2– 3+ Orange solution (Cr2O7 ) turns green (Cr ) or – 2+ Purple solution (MnO4 ) turns colourless (Mn )

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4.6.2 Oxidation of Secondary Alcohols Secondary alcohols are oxidised to ketones which are resistant to oxidation.

K2Cr2O7 / dilute H2SO4 heat under reflux

Reagents & Conditions: Observations:

K2Cr2O7/dil H2SO4 or KMnO4/dil H2SO4, heat under reflux 2– 3+ Orange solution (Cr2O7 ) turns green (Cr ) or – 2+ Purple solution (MnO4 ) turns colourless (Mn )

4.6.3 Oxidation of Tertiary Alcohols 

Tertiary alcohols have no hydrogen on their hydroxyl-bearing carbon and thus are not oxidized.

 However, in the presence of strong oxidising agents at elevated temperatures, the oxidation of tertiary alcohols leads to the cleavage of various carbon-carbon bonds at hydroxyl-bearing carbon atom, resulting in a complex mixture of products.

Exercise 5 Draw the structures of the organic compounds formed by oxidation of each of the following compounds. K2Cr2O7 / dilute H2SO4

O

(a) heat under reflux

KMnO4 / dilute H2SO4 heat under reflux

O O HO

O OH

CH3

(b)

K2Cr2O7 / dilute H2SO4

H

heat with distill

KMnO4 / dilute H2SO4

O

CO2H CO2H

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Tri-iodomethane (Iodoform) Formation

Reagents & Conditions: Observation:

I2 (aq), NaOH(aq), warm Yellow ppt (CHI3) formed

 This reaction can only take place for an alcohol containing a methyl group and a hydrogen atom attached to the carbon with the hydroxyl group.



“R” group can be a hydrogen atom or a hydrocarbon group (alkyl or aryl).

 Ethanol (R = H) is the only primary alcohol that gives positive tri-iodomethane test.  The tri-iodomethane reaction is also a method of breaking a C–C bond and removing a methyl, –CH3 group. It is therefore a useful method of shortening a carbon chain by a single carbon atom (step-down reaction). Examples: H

I2 (aq) / NaOH (aq) aq alkaline iodine CH3

C

H

H CHI3

warm

+ O

OH

CH2CH3 CH3

C

O

C

-

H

I2 (aq) / NaOH (aq) aq alkaline iodine

CH3CH2 CHI3

warm

+

C

-

O

O

OH

Exercise 6 Which of the following alcohols give positive tri-iodomethane reaction?

OH

OH

OH OH

A

B

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C

D

Answer: A & D

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Exercise 7 The video clip, (http://www.youtube.com/watch?v=W4gmxGHIwIs), shows the reactions of four organic compounds, labelled as A, B, C and D. The compounds are ethanol, propan-1-ol, 2-methylpropan-2-ol and cyclohexene but not necessarily in that order. Watch the video and identify the compounds from your knowledge of the reactions of organic compounds that you have learnt.

Reaction with Na Reaction with hot acidified K2Cr2O7 Tri-iodomethane test

5

A Effervescence Solution remains orange 2-methylpropan-2-ol x

B No effervescence cyclohexene x x

C Effervescence

D Effervescence

Orange solution turns green Yellow ppt ethanol

Orange solution turns green No ppt propan-1-ol

Chemical Reactions of Phenols

Phenols are compounds containing a hydroxyl group attached to an aromatic ring. o

Most phenols are colourless solids. The melting point of phenol is 42 C. The presence of hydrogen bonds makes the melting point of phenols higher than those of hydrocarbons of comparable relative molecular mass. There are two sites for reaction to take place in phenol:

O H A comparison with alcohols Phenols Breaking of C-O bond

More difficult than alcohols

Breaking of O-H bond

More readily than alcohols

Why is the breaking of C-O bond for phenol more difficult than alcohols? The C-O bond in phenol is very strong, due to the delocalisation of the lone pair of electrons on the oxygen over the benzene ring. This gives rise to the partial double bond character and thus strengthens the C-O bond. Hence, for phenols, there are no reactions in which the C-O bond breaks, unlike the case of alcohols. Thus, phenols do not react with PCl5 or SOCl2 or HCl or HBr. The absence of fumes of HCl on adding PCl5 to phenols clearly distinguishes a phenol from an alcohol.

Why does the breaking of O-H bond for phenol occur more readily than alcohols? See Section 2.4.3.

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Reactions involving the breaking of O-H bond

5.1.1 Reaction with Sodium Metal and Sodium Hydroxide Phenols reacts with reactive metals to yield the phenoxide ion and hydrogen gas. Phenols are also acidic enough to react with bases like NaOH to give salt and water. This is seen as a cloudy solution in water dissolving in aqueous NaOH to form a homogeneous colorless solution.

With metals

+ Na

With bases

+ NaOH

+ ½ H2

+ H2O

However, phenol is not acidic enough to liberate CO2 from carbonates. Comparison of reactions of Alcohols and Phenols with Na, NaOH and Na 2CO3 Reaction with

Alcohols

Phenols

Sodium metal





Alkali (e.g. NaOH)

X



Carbonates (e.g. Na2CO3(aq))

X

X

5.1.2 Esterification With carboxylic acids Phenols do not react with carboxylic acids to form esters as they are weaker nucleophiles than alcohols. This is due to the delocalization of the lone pair of electrons on the oxygen atom into the benzene ring. Hence, phenol is not nucleophilic enough to undergo esterification in the usual way (that is, by heating with a carboxylic acid in the presence of a few drops of concentrated H2SO4) With acyl chlorides Phenoxides can react with acyl chlorides to form esters. O

Cl +

O O

O propanoyl chloride

Reagents & Conditions:

Crystal Cheong / Low YZ / Jolin Lim / Grace Chua

+ Cl-

-

phenoxide phenoxide (form from Formed fromphenol phenolwith withNaOH) NaOH

phenyl propanoate

Acyl chloride and phenoxide, room temperature

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Hwa Chong Institution 2013

14 – Hydroxyl Compounds

Phenol is first converted to the phenoxide salt by reacting with Na(s) or NaOH(aq) to form phenoxide ion, a stronger nucleophile, before reacting with the acyl chloride to form the ester. This will give a high yield.

OH

+

-

OH -

O

+

-

O

O

O

+

R

C

H2O

O

C

R

+

Cl -

Cl

Reaction may also occur between phenol and acyl chlorides directly. However, this will give a lower yield.

5.1.3 Oxidation Phenols are not oxidized because they do not have a hydrogen atom on the hydroxyl-bearing carbon. 5.2

Reactions involving the Electrophilic Substitution on the Benzene Ring

Due to the delocalisation of the lone pair of electrons of oxygen into the benzene ring, the electron density in the ring is greatly increased, making phenol much more susceptible to electrophilic attack than benzene. Hence, the presence of the –OH group highly activates the benzene ring towards electrophilic substitution and no catalyst is required. The hydroxyl group on phenol is 2, 4 –directing. 5.2.1 Bromination With aqueous bromine (bromine water) Phenol reacts with aqueous bromine at room temperature without the need of a Lewis acid catalyst (e.g., FeBr 3) to produce a white precipitate of 2,4,6-tribromophenol.

Reagents & Conditions: Aqueous bromine (bromine water), room temperature Observation: Yellow-orange solution decolourised and white ppt formed.

Crystal Cheong / Low YZ / Jolin Lim / Grace Chua

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Hwa Chong Institution 2013

14 – Hydroxyl Compounds

With liquid bromine in organic solvent Monobromination of phenol can be achieved by carrying out the reaction in a non-polar organic solvent (such as CCl4) at low temperature, conditions that reduce the electrophilic reactivity of bromine.

Reagents & Conditions: Observation:

Liquid bromine in inert organic solvent, room temperature Reddish-brown solution decolourised

5.2.2 Nitration With dilute HNO3 Phenols react with dilute HNO3 at room temperature to yield mono-substituted nitro compounds. With concentrated HNO3 Phenols react with concentrated HNO3 (without concentrated H2SO4) at room temperature to yield a tri-substituted product.

Reagents & Conditions: Comment:

5.3

conc. HNO3 / dilute HNO3, room temperature To achieve di-substitution, moderately concentrated HNO3 may be used.

Complex Formation with neutral FeCl3 (aq)

Phenol reacts with neutral aqueous iron(III) chloride to form a violet complex.

Reagents & Conditions: neutral FeCl3(aq), room temperature Observation: Violet complex formed

This reaction is used as a characteristic test for the presence of phenolic group. The colour of the complexes may vary slightly if other substituents are attached to the ring. Crystal Cheong / Low YZ / Jolin Lim / Grace Chua

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