new way chemistry for hong kong a-level book 3b1 carboxylic acids and their derivatives...

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

1

Carboxylic Acidsand their Derivatives

35.135.1 IntroductionIntroduction

35.235.2 Nomenclature of Carboxylic Acids and their Nomenclature of Carboxylic Acids and their DeriDerivativesvatives

35.335.3 Physical Properties of Carboxylic AcidsPhysical Properties of Carboxylic Acids

35.435.4 Preparation of Carboxylic AcidsPreparation of Carboxylic Acids

35.535.5 Reactions of Carboxylic AcidsReactions of Carboxylic Acids

35.635.6 Reactions of the Derivatives of Carboxylic AcidsReactions of the Derivatives of Carboxylic Acids

35.735.7 Uses of Carboxylic Acids and their DerivativesUses of Carboxylic Acids and their Derivatives

Chapter 35Chapter 35


2

35.1 Introduction (SB p.29)

Carboxylic acids refer to the class of organic compounds

with the carboxyl group


3


General formula of carboxylic acid: RCOOH

Examples:


4


Name Structure

Acyl chlorides

Acid anhydrides

Amides

Esters

Carboxylic acid derivatives


5

35.2 Nomenclature of Carboxylic Acids and their Derivatives (SB p.30)

Carboxylic acids are named by replacing the final “-e” of the name of the corresponding alkane with “-oic acid”

When other substituents are present, the carboxyl carbon is assigned position 1

Examples:

Carboxylic AcidsCarboxylic Acids


6


Acyl chlorides are named by replacing the final “-ic acid” o

f the name of the parent carboxylic acid with “-yl chlorid

e”

Examples:

Acyl ChloridesAcyl Chlorides


7


Acid anhydrides are named by dropping the word “acid” from the name of the parent carboxylic acid and then adding the word “anhydride”

Examples:

Acid AnhydridesAcid Anhydrides


8


• Amides that have no substituents on the nitrogen atom are named by replacing “-oic acid” from the parent carboxylic acid with “amide”

• Substituents on the nitrogen atom of amides are named and the named substituent is preceded by N-, or N,N-

Examples:

AmidesAmides


9


• The names of esters are derived from the names of the alcohol (with the ending “-yl”) and the carboxylic acid (with the ending “-oate”)

• The portion of the name derived from the alcohol comes first

Examples:

EstersEsters


10

Example 35-1Example 35-1Give the IUPAC names of the following compounds:

(a)

(b)

(c)

(d)

Answer


Solution:

(a) 3-Methylbutanoic acid

(b) N-methylethanamide

(c) Ethyl benzoate

(d) Benzoic anhydride


11

Example 35-2Example 35-2An ester is formed by reacting an alcohol with a carboxylic acid. Draw the structural formula of the following ester molecule and give the name of the alcohol and the carboxylic acid that form the ester.

(a) Methyl ethanoate

Answer


Solution:

(a) The structural formula of methyl ethanoate is:

It is formed from the reaction of ethanoic acid and methanol.


12

Example 35-2Example 35-2An ester is formed by reacting an alcohol with a carboxylic acid. Draw the structural formula of the following ester molecule and give the name of the alcohol and the carboxylic acid that form the ester.

(b) Ethyl methanoateAnswer


Solution:

(b) The structural formula of ethyl methanoate is:

It is formed from the reaction of methanoic acid and ethanol.


13

Check Point 35-1 Check Point 35-1

Complete the following table by filling in the molecular formulae, structural formulae or IUPAC names of the carboxylic acids.

Answer


Molecular formula Structural formula IUPAC name

C3H7COOH (a) (b)

(c) (d)

(e) (f)

(g) (h) Trichloroethanoic acid

(a)

(b) Butanoic acid (c) CH3CH(CH3)CH2COOH

(d) 3-Methylbutanoic acid

(e) C6H4ClCOOH (f) 2-Chlorobenzoic acid

(g) CCl3COOH

(h)


14

35.3 Physical Properties of Carboxylic Acids (SB p.34)

• Carboxylic acids are colourless liquids at room conditions

• They have characteristic pungent smell and sour taste

Carboxylic acid FormulaBoiling point (°C)

Melting point (°C)

Density at 20°C (g cm–

3)

Methanoic acid

Ethanoic acid

Propanoic acid

Butanoic acid

Pentanoic acid

Hexanoic acid

Benzoic acid

Phenylethanoic acid

2-Methylpropanoic acid

Chloroethanoic acid

HCO2H

CH3CO2H

CH3CH2CO2H

CH3(CH2)2CO2H

CH3 (CH2)3CO2H

CH3 (CH2)4CO2H

C6H5CO2H

C6H5CH2CO2H

(CH3)2CHCO2H

CH2ClCO2H

101

118

141

164

186

205

249

266

154

189

8.4

16.6

–20.8

–6.5

–34.5

–1.5

122

76

–47

63

1.220

1.049

0.992

0.964

0.939

0.927

—

—

0.950

—


15


Carboxylic acid FormulaBoiling point (°C)

Melting point (°C)

Density at 20°C (g cm–3)

Dichloroethanoic acid

Trichloroethanoic acid

Hydroxyethanoic acid

2-Hydroxypropanoic acid

Cis-Butenedioic acid

Trans-Butenedioic acid

Ethanedioic acid

Propandioic acid

Butanedioic acid

Benzene-1,2-dicarboxylic acid

CHCl2CO2H

CCl3CO2H

HOCH2CO2H

CH3CHOHCO2H

HO2CCH = CHCO2H

HO2CCH = CHCO2H

HO2CCO2H

HO2CCH2CO2H

HO2C(CH2)2CO2H

C6H4(CO2H)2

194

198

dec.

dec.

dec.

dec.

—

dec.

235 (dec.)

dec.

10.8

57.7

79

16.8

130 – 131

268 (dec.)

190 (dec.)

136 (dec.)

182

206 (dec.)

1.566

—

—

1.249

1.590

1.635

—

—

—

—


16

Due to formation of intermolecular hydrogen bonds high b.p. and m.p.

The hydrogen bonds are more extensive than those in alcohols higher b.p. and m.p. than

alcohols

Boiling Point and Melting PointBoiling Point and Melting Point



17

• The densities of carboxylic acids decrease with inc

reasing relative molecular masses

• Only methanoic acid and ethanoic acid are denser

than water at 20°C

∵ closer packing of the smaller molecules

in the liquid phase

DensityDensity



18

• Carboxylic acids of low molecular masses show appreciable solubilities in water∵ carboxylic acids are polar and can form strong hydrogen bonds with water molecules

SolubilitySolubility


• First four carboxylic acids are miscible with water

• The length of the hydrocarbon portion solubilities in water


19

Example 35-3Example 35-3(a) Propanoic acid has a boiling point of 141°C which is

considerably higher than that of butan-1-ol (117°C), although they have the same molecular mass. Explain why.

Answer


Solution:

(a) Each propanoic acid molecule forms two intermolecular hydrogen bonds with another propanoic acid molecule. However, two butan-1-ol molecules form only one hydrogen bond. As molecules of propanoic acid form more extensive intermolecular hydrogen bonds than those of butan-1-ol, the boiling point of propanoic acid is higher than that of butan-1-ol.


20

Example 35-3Example 35-3(b) Rank the following compounds in decreasing order

of solubility in water:

CH3CH2CH2COOH, CH3CH2COOCH3, CH3COOH

Answer


Solution:

(b) The solubility of the compounds in water decreases in the order:

CH3COOH > CH3CH2CH2COOH > CH3CH2COOCH3


21

Example 35-3Example 35-3(c) Propanedioic acid forms intramolecular hydrogen bo

nds. Draw its structural formula showing clearly the formation of intramolecular hydrogen bonds.

Answer


Solution:

(c)


22

• 2-hydroxyalkanenitriles is formed by nucleophilic addition reaction of aldehydes or ketones with hydrogen cyanide

• Acid hydrolysis or alkaline hydrolysis of 2-hydroxyalkanenitriles produce carboxylic acids or carboxylate ions respectively

Hydrolysis of NitrilesHydrolysis of Nitriles

35.4 Preparation of Carboxylic Acids (SB p.37)


23

Nitriles can be formed by nucleophilic substitution reaction of haloalkanes with sodium cyanide


e.g.

Acid hydrolysis of the nitrile produces a carboxylic acid with one more carbon atom


24

• Hydrolysis of esters give parent carboxylic acids and alcohols by boiling under reflux with the dilute HCl or dilute H2SO4

Hydrolysis of EstersHydrolysis of Esters


• Acyl chlorides, acid anhydrides, esters and amides can be hydrolyzed to give the corresponding carboxylic acids

• Reactive derivatives react vigorously with water, e.g. acyl chlorides

• Less reactive derivatives require more severe conditions to bring about the reaction


25

Aldehydes and 1° alcohols can be oxidized to carboxylic a

cids by KMnO4/H+

Oxidation of Alcohols and AldehydesOxidation of Alcohols and Aldehydes



26

Examples:



27

1° and 2° alkyl groups (but not 3°) directly attached to a benzene ring are oxidized by hot KMnO4/OH– to carboxyl group

e.g.

Oxidation of AlkybenzenesOxidation of Alkybenzenes


This oxidation takes place initially at the benzylic carbon alkyl groups longer than methyl group are ultimately degraded to benzoic acid


28


Write the equations for the acid-catalyzed and base-catalyzed hydrolyses of each of the following substances:

(a) Ethyl butanoateAnswer


(a)


29



(b) Propanamide

Answer


(b)


30



(c) Benzoyl chlorideAnswer


(c)


31

• Carboxylic acids are weak acids

• Their acidic properties are due to the presence of io

nizable hydrogen atom

Acidity of Carboxylic AcidsAcidity of Carboxylic Acids

35.5 Reactions of Carboxylic Acids (SB p.39)


32

The acid strength of ethanoic acid is shown by the value of its

acid dissociation constant (Ka)

]COOHCH[

]O][HCOO[CHK

3

33a

-

pKa is used for convenience as Ka is small for weak acids

pKa = –log Ka , the smaller the pKa value, the stronger is the acid


e.g. ethanoic acid


33

• Unsubstituted carboxylic acids have pKa values in the r

ange of 3 to 5

• Alcohols have pKa values in 15 to 18

carboxylic acids are much more acidic than alcohols


Name Formula pKa

Methanoic acid

Ethanoic acid

Propanoic acid

Butanoic acid

Benzoic acid

HCOOH

CH3COOH

CH3CH2COOH

CH3CH2CH2COOH

C6H5COOH

3.75

4.76

4.87

4.82

4.20


34

The greater stability of carboxylic acids is attributed to resonance stabilization of the carboxylate ions

Example:


Resonance Effect


35

• By the resonance effect, the ethanoate ion can be stabilized by spreading the negative charge over two oxygen atoms


• No stabilizing resonance structures for alkoxide ions e.g.

• Carboxylic acids are much acidic than alcohols ∵ formation of more stable conjugate base


36

• Both compounds contain the highly polarized O – H bond due to the difference in electronegativity

• The greater acidity of ethanoic acid is due to the presence of the powerful electron-withdrawing carbonyl group


Inductive Effect


37

• The carbonyl atom of ethanoic acid bears a large partial positive charge and exerts a powerful electron-withdrawing inductive effect to hydroxyl oxygen atom, making the hydroxyl hydrogen atom much more positive proton dissociate more readily

• The electron-withdrawing inductive effect of the carbonyl group stabilizes the carboxylate ion carboxylic acids are stronger acid than alcohols


Two resonance structures for the carbonyl group:


38

1. Electron-withdrawing Groups

• The presence of electron-withdrawing groups increase the acidity of a carboxylic acide.g.


Inductive Effects of Other Groups


39

• The chlorine atom withdraws electron from the carbonyl group and oxygen making the hydroxyl hydrogen more positive than that of ethanoic acid


• The chlorine atom also stabilizes the chloroethanoate ion to greater extent


40


CH3COOH < < < <

Ethanoic Iodoethanoic Bromoethanoic Chloroethanoic Fluoroethanoicacid acid acid acid acid4.76 3.17 2.90 2.86 2.66

Acidity

pKa

CH3COOH < < < CCl3COOH

Ethanoic Chloroethanoic Dichloroethanoic Trichloroethanoic acid acid acid acid4.76 2.86 1.29 0.65

Acidity

pKa

Effect of electron-withdrawing substituents on the acidity of carboxylic acids

• The greater the number of electron-withdrawing groups, the stronger is the acid

• The more electronegative of halogen substituents (F > Cl > Br > I), the stronger is the acid


41


• The further away of the substituents from the carboxyl

group, the weaker is the acid

< <

4-Chlorobutanoic 3-Chlorobutanoic 2-Chlorobutanoicacid acid acid

4.52 4.06 2.84

Acidity

pKa


42

2. Electron-releasing Groups

• The presence of electron-releasing groups reduce the acidity of a carboxylic acide.g.



43


• Electron-releasing alkyl groups release electrons towards the electron-deficient carbonyl carbon, thus reducing its charge

• Reduce the positive character of the hydroxyl hydrogen atom

dissociate less readily

• Intensify the negative charge on the carboxyl group destablizes the ethanoate ion


44


(a) Match the Ka values: 2.19 10–3 M and 1.26 10–3 M, to the carboxylic acids: CH2FCOOH and

CH2BrCOOH. Explain your answer briefly.

Answer


(a) The greater the Ka value, the stronger is the acid.

CH2FCOOH is a stronger acid than CH2BrCOOH.

This –F substituent exerts a stronger inductive effect than the –Br substituent, as fluorine is more electronegative than bromine. The –F substituent stabilizes the conjugate base (i.e. CH2FCOOH–) to a

greater extent than the –Br substituent. Therefore, CH2FCOOH has a Ka value of 2.19 10–3 M, and

CH2BrCOOH has a Ka value of 1.26 10–3 M.


45


(b) Arrange the following acids in ascending order of acidity:

CHCl2COOH, CCl3COOH, CH2ClCOOH, CH3COOH

Explain the order briefly. Answer


(b) CH3COOH < CH2ClCOOH < CHCl2COOH

< CCl3COOH

The –Cl substituent is an electron-withdrawing group. An increasing number of the –Cl substituent brings about a greater negative inductive effect. Thus, the conjugate base will be stabilized more, and the acidity of the acid increases.


46


(c) Arrange the following aryl carboxylic acids in descending order of acidity:

Explain the order briefly.



47


(c)

is more acidic than ,because contains

the –Cl substituent which is an electron-withdrawing group. Thus, it exerts a negative inductive effect to the conjugate base, and hence stab

ilizes the carboxylate ion formed.

is less acidic than , because contains

a methyl group which is an electron-releasing group. Thus, it exerts a positive inductive effect to the conjugate base, and hence destabilizes the carboxylate ion formed.


48


(d) Which is a stronger acid, CH2ClCOOH or CH3CHClCOOH? Explain your answer briefly.

Answer


(d) CH2ClCOOH is a stronger acid as CH3CHClCOOH

contains a methyl group which is an electron-releasing group. It exhibits a positive inductive effect and thus destabilizes the conjugate base of the acid. Therefore, CH3CHClCOOH is less acidic.


49

• Carboxylic acids react with reactive metals such as Na o

r Mg to give the corresponding metal carboxylates and

hydrogen gas

Formation of SaltsFormation of Salts


Reaction with Active Metals


50

Example:


51

Carboxylic acids react with strong alkalis such as NaOH to form sodium carboxylates and water


Reaction with Bases

Examples:


52

Carboxylic acids also react weak alkalis such as Na2CO3 or

NaHCO3 to form sodium carboxylates, carbon dioxide and

water



53

• This reaction serves as a test to distinguish carboxylic acids and other acidic organic compounds

Examples:



54


NaOH and Na2C

O3 can be used to

separate a mixture of alcohols, phenols and carboxylic acids


55


(a) Describe how to separate a mixture of benzoic acid and phenol in the laboratory.

Answer


(a) Sodium hydrogencarbonate, water and diethyl ether are added to the mixture. Benzoic acid reacts with sodium hydrogencarbonate to form a water-soluble salt which dissolves in the aqueous layer. After separation by the use of a separating funnel, phenol can be recovered by distilling off the ether. To the aqueous layer, hydrochloric acid is added to generate the benzoic acid which can be extracted by diethyl ether. After separation from the aqueous

layer, benzoic acid is obtained by distilling off the ether.


56


(b) Outline how a mixture of butanone and ethanoic acid can be separated in the laboratory.

Answer



57


(b)


58

• Acyl chlorides are the most reactive amongst the carboxylic acid derivatives

• They can be prepared by the use of SOCl2, PCl3 or P

Cl5 in good yields

Formation of Acyl ChloridesFormation of Acyl Chlorides



59

• Acyl chlorides can be used to prepare aldehydes, ketones,

esters, amides and acid anhydrides in organic synthesis


• Acyl chlorides are extremely sensitive to moisture, t

herefore, it must be stored in anhydrous conditions

∵ hydrolyze rapidly to form carboxylic acids


60

• Acid anhydrides can be prepared by reacting acyl chlorides with carboxylic acids in the presence of pyridine

• This can be used to prepare mixed anhydrides (R R´) or simple anhydrides (R = R´)

Formation of Acid AnhydridesFormation of Acid Anhydrides



61

• Acyl chlorides react with sodium salts of carboxylic

acids to give acid anhydrides



62

• Ammonia reacts with carboxylic acids to form ammonium salts. The dry salts are heated to dehydrate to give amide

Formation of AmidesFormation of Amides


• The better way to prepare amides is reacting ammonia or amines with acyl chlorides


63

• Carboxylic acids react with alcohols to form esters through condensation reactions

• Esterification reactions are acid-catalyzed and reversible

Formation of EstersFormation of Esters



64

LiAlH4 is a powerful reducing agent which reduces

carboxylic acids to primary alcohols in good yields

Reduction with Lithium TetrahydridoaluminateReduction with Lithium Tetrahydridoaluminate


Example:


65

Example 35-4Example 35-4(a) Complete and balance the following acid-base equatio

ns:

(i)

(ii)

Answer


Solution:

(a) (i)

(ii)


66

Example 35-4Example 35-4(b) Draw the structural formulae for the products formed

from the following compounds when they are reduced with lithium tetrahydridoaluminate.

(i)

(ii)

Answer


Solution:

(b) (i)

(ii) HOCH2CH2CH2CH2OH


67

Example 35-4Example 35-4(c) Complete the following equations:

(i)

(ii)

(iii)

Answer


Solution:

(c) (i)

(ii)

(iii)


68

• Both chlorine and oxygen atoms are electron-withdrawi

ng groups, thus making the acyl carbon much electron-d

eficient

acyl carbon is a good nucleophilic site

Reactions of Acyl ChloridesReactions of Acyl Chlorides

35.6 Reactions of the Derivatives of Carboxylic Acids (SB p.50)

• Chloride ion is a good leaving group which is substituted by other atoms or groups easily acyl chlorides are very reactive


69

• Benzoyl chloride is much less reactive than aliphatic acyl chlorides∵ reduction of electron-deficiency on the acyl carbon

due to delocalization of electrons (resonance effect)



70

Acyl chlorides are hydrolyzed by water to form the parent c

arboxylic acids and hydrogen chloride

Reaction with Water

Examples:



71

Acyl chlorides react with alcohols to give esters and HCl

Reaction with Alcohols

Examples:



72

Acyl chlorides react with phenols to give esters in the presence of base as catalyst∵ an alkaline medium converts phenol to a powerful nucleophile, phenoxide ion

Example:



73

Acyl chlorides react with ammonia to form amides rapidly

Reaction with Ammonia and Amines

Example:



74

Acyl chlorides react rapidly with 1° and 2° amines to form N-, N,N- substituted amides respectively


• The reaction takes place at room temperature and produces amides in high yield

• This method is widely used in laboratory for amides synthesis


75


Explain why ethanoyl chloride must be protected from atmospheric moisture during storage.

Answer


This is because ethanoyl chloride reacts readily with water (from atmospheric moisture) to form ethanoic acid.


76

Acid anhydrides undergo hydrolysis to form carboxylic acids

Reactions of Acid AnhydridesReactions of Acid Anhydrides


Reaction with Water

Example:


77

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


Reaction with Alcohols

Example:


78

Acid anhydrides react with ammonia and with 1° and 2° amines in the ways similar to those of acyl chlorides


Reaction with Ammonia and Amines


79

Examples:



80

Amides are the least reactive among the carboxylic acid

derivatives towards nucleophilic substitution reactions

∵ NH2–, NHR– or NR2

– are strong bases and thus

poor leaving groups

Reactions of AmidesReactions of Amides



81

Amide undergo acid and alkaline hydrolyses to form carboxylic acids and carboxylates respectively

Acid hydrolysis:


Reaction with Water

Alkaline hydrolysis:


82

Amides are dehydrated by heating with P4O10 to form nitriles


Dehydration

• Useful synthetic method for preparing nitriles that are n

ot available by nucleophilic substitution reactions bet

ween haloalkanes and cyanide ions


83

• Amides react with a solution of Br2 in NaOH or a solution of C

l2 in NaOH to give amines through a reaction called Hofmann d

egradation

• The resulted amines have one carbon less than original amides


Hofmann Degradation

Example:


84

Amides are reduced by LiAlH4 in dry ether to

give 1° amines


Reduction with Lithium Tetrahydridoaluminate


85

Acid-catalyzed hydrolysis of esters is the reverse reaction of esterification

Reactions of EstersReactions of Esters


Acid-catalyzed Hydrolysis

Example:


86

• When esters are refluxed with an alkali such as NaOH, the corresponding alcohol and sodium salt of the carboxylic acid are produced

• Alkali-catalyzed hydrolysis are irreversible


Alkali-catalyzed Hydrolysis

Example:


87

• Soap is made by the alkaline hydrolysis of fats or oils (i.e. triesters) to produce sodium carboxylates (i.e. soap)

• The reaction is called saponification



88

Esters are reduced to alcohols by LiAlH4


Reduction with Lithium Tetrahydridoaluminate

Example:


89

Example 35-5Example 35-5Draw the structural formulae of the missing compounds A to M:

(a)

(b)

(c)


Answer

Solution:

(a)

(b)

(c)


90


(d)

(e)

(f)


Answer

Solution:

(d)

(e)

(f)


91


(g)

(h)

(i)


Answer

Solution:

(g)

(h)

(i)


92

• Benzoic acid and its sodium salt used as food preservatives

• Prevent the microbial growth and spoilage

• Used in oyster sauce, fish sauce, ketchup, non-alcoholic beverages, fruit juices, margarine, salad dressings, jams and pickled products

As Food PreservativesAs Food Preservatives

35.7 Uses of Carboxylic Acids and their Derivatives (SB p.59)


93

As Raw materials for Making PolymersAs Raw materials for Making Polymers


• Nylon-6,6, ,is a condensation polymer

• It is a polyamide made by adding NaOH containing hexanedioyl dichloride and hexane-1,6-diamine to CH3CCl3


94

Advantages of Nylon-6,6


• Drip dry easily

• Not attack by insects easily

• Resist creasing (no ironing required after washing)

• Used for making ropes, thread, cords, and various kinds of clothing from stockings to jackets


95

Terylene, , is a polyester m

ade from acid-catalyzed condensation polymerization of

benzene-1,4-dicarboxylic acid and ethane-1,2-diol


Terylene is used to make wash-and-wear garments


96

Liquid esters are widely used as solvents for all-purpose

adhesives, thinners for paints and nail varnish removers

As SolventsAs Solvents



97

• Volatile esters often have characteristic sweet and fruity smells

• They are used as artificial flavourings in food-processing industry

As FlavouringsAs Flavourings


An artificial flavouring made of esters


98


Structure of esterCharacteristic flavour

Banana

Apple

Apricot

Pineapple

Some esters and their characteristic flavours


99


Structure of ester Characteristic flavour

Rum

Grape

Wintergreen

Some esters and their characteristic flavours (cont’d)


100

The END

new way chemistry for hong kong a-level book 3b1 carboxylic acids and their derivatives...

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