new way chemistry for hong kong a-level book 3b1 nitrogen compounds 36.1introduction...

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

1

Nitrogen Compounds

36.136.1 IntroductionIntroduction

36.236.2 Nomenclature of AminesNomenclature of Amines

36.336.3 Physical Properties of AminesPhysical Properties of Amines

36.436.4 Preparation of AminesPreparation of Amines

36.536.5 Reactions of AminesReactions of Amines

36.636.6 Uses of Amines and their DerivativesUses of Amines and their Derivatives

36.736.7 Amino AcidsAmino Acids

Chapter 36Chapter 36


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36.1 Introduction (SB p.67)

Organic nitrogen compounds:

• amines, amino acids, amides and nitro compounds

• According to the number of organic groups (both

alkyl or aryl groups) directly bonded to the nitrogen

atom, amines are classified as:


3


Examples of 1° amines:



4



Addition of the fourth organic group to tertiary amine yields a quaternary ammonium ion


5


Aromatic amines are formed when one or more

hydrogen atoms in the ammonia are replaced by phenyl

or substituted phenyl groups

e.g.


6

Check Point 36-1 Check Point 36-1

Classify the following amines as primary, secondary or tertiary amines.


Answer

I is a primary amine.

II is a tertiary amine.

III is a secondary amine.


7

36.2 Nomenclature of Amines (SB p.69)

1° amines are named as alkylamines

e.g.


8


• 2° and 3° amines are named in the same general way

• If the organic groups attached to the nitrogen are the same, use the prefixes ‘di-’ or ‘tri’

• If they are not the same, use the prefixes ‘N-’ or ‘N,N-’ to designate the substituents

e.g.


9


Complete the following table by filling in the structural formulae or IUPAC names of the amines.


Answer

IUPAC name Structural formula

(a)

2-Aminophenol(b)

(c)

(a) 1-Methylpropylamine

(b)

(c) Diethylamine


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36.3 Physical Properties of Amines (SB p.70)

• Lower members of amines are gases and have a characteristic ‘ammonia’ odour.

• The higher members are liquids with a distinctive ‘fishy’ odour

Amine FormulaBoiling point (°C)

Melting point (°C)

Density at 20°C (g cm–

3)

Primary amines:

Methylamine

Ethylamine

Propylamine

Butylamine

Pentylamine

Hexylamine

Phenylmethylamine

Phenylamine

CH3NH2

CH3CH2NH2

CH3(CH2)2NH2

CH3(CH2)3NH2

CH3(CH2)4NH2

CH3(CH2)5NH2

C6H5CH2NH2

C6H5NH2

–6.3

16.6

48.6

77.8

103

130

185

184

–92.5

–84

–83

–50.5

–55

–19

10

–6.2

—

—

0.719

0.741

0.761

0.768

0.981

1.022


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Amine FormulaBoiling point (°C)

Melting point (°C)

Density at 20°C (g cm–

3)

Secondary amines:

Dimethylamine

Diethylamine

N-Methylphenylamine

Diphenylamine

(CH3)2NH2

(CH3CH2)2NH2

C6H5NHCH3

(C6H5)2NH

7.4

56.3

196

302

–96

–48

–57

52.8

—

0.707

0.989

—

Tertiary amines:

Trimethylamine

Triethylamine

N,N-Dimethylphenylamine

Triphenylamine

(CH3)3NH2

(CH3CH2)3NH2

C6H5N(CH3)2

(C6H5)3N

3.5

89.4

194

365

–117

–115

2.5

126

—

0.728

0.956

—


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Boiling Point and Melting PointBoiling Point and Melting Point

1° or 2° amines can form strong hydrogen bonds with each other

the b.p. is higher than those of alkanes but lower than those of alcohols of comparable molecular masses

∵ O – H bond is more polar than N – H bond as oxygen is more electronegative stronger hydrogen bonds are formed between alcohol molecules


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• 3° amines have lower b.p. and m.p. than 1° and 2°

∵ all hydrogen in 3° amines are replaced by organic

groups

no hydrogen bond can be formed


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DensityDensity

Molecular mass of amines

densities


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SolubilitySolubility

• The solubility of amines in water:1° amines > 2° amines > 3° amines

∵ The intermolecular hydrogen bonds formed between 2° or 3° amines and water molecules decrease

• Amines are soluble in common organic solvents, e.g. hexane, ethanol and propanone


16

Example 36-1Example 36-1Match the boiling points 65°C, –6°C and –88°C with the compounds CH3CH3, CH3NH2 and CH3OH. Explain your

answer briefly.Answer

Solution:

Compounds Boiling Point (°C)

CH3CH3 –88

CH3NH2 –6

CH3OH 65

Ethane (CH3CH3) is a non-polar compound. In pure liquid form, ethane m

olecules are held together by weak van der Waals’ forces. However, both methylamine (CH3NH2) and methanol (CH3OH) are polar substances. I

n pure liquid form, their molecules are held together by intermolecular hydrogen bonds. As van der Waals’ forces are much weaker than hydrogen bonds, ethane has the lowest boiling point among the three. Besides, as the O – H bond in alcohols is more polar than the N – H bond in amines, the hydrogen bonds formed between methylamine molecules are weaker than those formed between methanol molecules. Thus methylamine has a lower boiling point than methanol.



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36.4 Preparation of Amines (SB p.72)

Reduction of NitrilesReduction of Nitriles

Hydrogen reduces a nitrile to a 1° amine in the presence of nickel catalyst

Reaction with Hydrogen

Example:


18


Nitriles are reduced to 1° amines by LiAlH4 in dry ether

Reaction with Lithium Tetrahydridoaluminate

Example:


19


Reduction of AmidesReduction of Amides

Amides can be reduced by LiAlH4 to give 1°, 2° or 3° amines

Reaction with Lithium Tetrahydridoaluminate

• The product is 1° amine if R’ and R’’ = H

• The product is 2° amine if R’ or R’’ = H


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Examples:


21


• Amides with no substituent on the nitrogen react with Br2 in

NaOH (or Cl2 in NaOH) to give amines

• The amines formed have one carbon less than the parent amides

Hofmann Degradation


22


Reduction of Ammonia with HaloalkanesReduction of Ammonia with Haloalkanes

1°, 2°, 3° and quaternary ammonium salts can be prepared by heating haloalkanes with ammonia under pressure


23


Examples:

CH3CH2Br + NH3 CH3CH2NH2 + HBr

CH3CH2Br + CH3CH2NH2 (CH3CH2)2NH + HBr

CH3CH2Br + (CH3CH2)2NH (CH3CH2)3N + HBr

CH3CH2Br + (CH3CH2)3N (CH3CH2)4N+Br–

• This method always give a mixture of products

• Successive alkylations always occur because aliphatic amines are more reactive than ammonia


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Preparation of Phenylamine through Reduction of NitrobenzenePreparation of Phenylamine through Reduction of Nitrobenzene

The reduction of the nitro group can be carried out by

• Catalytic hydrogenation

• Treatment of the nitro compound with an acid (e.g. HCl) and Fe, Zn or Sn or a metal salt such as SnCl2


25


(a) Show how the following conversions can be achieved:

(i) CH3CH2Br CH3CH2CH2NH2

(ii) CH3CH2COOH CH3CH2NH2


Answer

(a) (i) 1. NaCNCH3CH2Br CH3CH2CN

2. HCl

1. LiAlH4 / dry ether CH3CH2CH2NH2 2. water

(ii)


26


(b) Draw the structural formulae for the major organic products, A to C, in the following reactions:


Answer

(b) A:

B:

C:


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36.5 Reactions of Amines (SB p.76)

Basicity of AminesBasicity of Amines

• Amines are far weaker bases than hydroxide ions and alkoxide ions but are stronger bases than water

• The lone pair electrons on nitrogen atom can accept a proton from a water molecule amines are basic


28


Amines dissolve in mineral acids such as HCl to form salts

e.g.


29

Methylamine will partially dissociate into ions

The basic strength of amines is shown by the value of base dissociation constant (Kb)

]NHCH[

]][OHNH[CHK

23

33b


pKb is used for convenience as Kb is small for weak bases

pKb = –log Kb , the smaller the pKb value, the stronger is the b

ase


30

The basicity of amines is governed by the availability of the lone pair electrons on the nitrogen atom


Name Formula pKb

Ammonia NH3 4.75

Primary aliphatic amines:

Methylamine

Ethylamine

Propylamine

Butylamine

CH3NH2

CH3CH2NH2

CH3(CH2)2NH2

CH3(CH2)3NH2

3.36

3.27

3.16

3.39

Aromatic amines:

Phenylamine C6H5NH2 9.38


31

• Primary aliphatic amines are the most basic


∵ electron-releasing alkyl groups enhance the availability of the lone pair electrons on the nitrogen atom while ammonia does not

• Aromatic amines are weaker bases than ammonia

∵ the lone pair electrons on the nitrogen atom are delocalized over the benzene ring making it less available to a proton


32


Compare the basic strength of (CH3CH2)2NH and CH3CH2NHCOCH3.


Answer

CH3CH2NHCOCH3 is much less basic than (CH3CH2)2NH

because of the presence of the electron-withdrawing acyl group in the compound. The acyl group withdraws electrons away from the –NH group by both the inductive and resonance effects. Two

resonance structures can be drawn for the amide compound as shown below:

As shown in structure II, the lone pair electrons of the nitrogen atom delocalize to form a bond with the acyl carbon atom. Thus, the availability of the lone pair electrons on the nitrogen atom is much reduced. As a result, the amide is much less basic than the amine.


33


Reactions with Ethanoyl Chloride and Benzoyl ChlorideReactions with Ethanoyl Chloride and Benzoyl Chloride

• 1° and 2° amines react readily in cold with ethanoyl chloride to form N-substituted amides

Examples:


34


• 1° and 2° amines react with benzoyl chloride in the presence of alkalis to form N-substituted amides

Examples:


35


• The reaction between ethanoyl chloride and benzoyl chloride and amines would not give a mixture of amides

∵ resulting amide is less nucleophilic than original amine due to the electron-withdrawing carbonyl group make the lone pair electrons on nitrogen atom less available

• 3° amines do not react since they do not possess a hydrogen atom

• Amines are susceptible to oxidations, conversion of amine to amide can ‘protect’ against oxidation

• Free amines can be regenerated by acid hydrolysis


36


Reactions of Primary Amines with Nitric(III) AcidReactions of Primary Amines with Nitric(III) Acid

• All amines react with nitric(III) acid

• As nitric(III) acid is a weak and unstable acid, it is prepared by treating NaNO3 with an aqueous solution of a strong

acid

NaNO2(aq) + HCl(aq) HNO2(aq) + NaCl(aq)

2NaNO2(aq) + H2SO4(aq) 2HNO2(aq) + Na2SO4(aq)


37


• 1° aliphatic amines react with HNO2 to yield aliphatic

diazonium salts

• Diazonium salts, even at low temperatures, are highly unstable, they decompose spontaneously by losing nitrogen to form carbocations

Reaction of Primary Aliphatic Amines with Nitric(III) Acid


38


• 1° aromatic amines react with HNO2 to yield benzenedia

zonium salts

• Benzenediazonium salts are unstable, but they do not decompose when the temperature of the reaction mixture is kept below 5°C

Reaction of Primary Aromatic Amines with Nitric(III) Acid


39


• The greater stability of benzenediazonium salts is due to the delocalization of the positive charge of the diazonium group

+(–N N) over the benzene ring

• The benzenediazonium salts are important in synthesis because the diazonium group can be replaced by a variety of functional groups to give a a range of organic products


40


The diazonium group of benzenediazonium ions can be re

placed by –Cl, –Br and –CN by treating with CuCl, CuBr

and CuCN respectively

Replacement of the Diazonium Group by –Cl, –Br or –CN


41


The diazonium group of benzenediazonium ions can be

replaced by –I on reacting with KI

Replacement of the Diazonium Group by –I


42


The diazonium group of benzenediazonium ions can be

replaced by –F on reacting with KBF4

Replacement of the Diazonium Group by –F


43


The diazonium group of benzenediazonium ions can be replaced by –OH by adding Cu2O to a dilute solution of the

benzenediazonium salt containing a large excess of CuNO3

Replacement of the Diazonium Group by –OH


44


Coupling Reactions of Benzenediazonium IonsCoupling Reactions of Benzenediazonium Ions

• Benzenediazonium ions react with highly reactive aromatic compounds in an alkaline medium to give azo compounds

Example:


45


• This reaction is used to identify 1° aromatic amines

• Azo compounds are usually intensely coloured

∵ – N = N – link brings the two aromatic rings into

conjugation, and the electrons are delocalized

over the entire structure

• Azo compounds are used as dyes due to intense colours


46

Example 36-2Example 36-2Draw the structural formulae of the major products formed in the following reaction:

(a) (CH3CH2)2NH + HCl

(b)

(c)

Answer


Solution:

(a) (CH3CH2)2NH2+Cl–

(b)

(c)


47


Suggest a series of reactions for the following conversion:

(a)

Answer


(a)


48



(b)

Answer


(b)


49



(c) CH3CH2NH2 CH3CH2NHCH3

Answer


(c)


50



(d)

Answer


(d)


51

36.6 Uses of Amines and their Derivatives (SB p.83)

As DyesAs Dyes

• 1° aromatic amines are used as a starting material for th

e manufacture of azo dyes

Note:

G is an electron-releasing substituent like –OH or –NH2


52

• Azo compounds are highly coloured and can be synthesiz

ed from relatively inexpensive compounds

widely used in dyeing industry


Azo compounds are used as dyes in garments and food


53


Examples:

1. Methyl orange (an orange dye for fabrics)

2. Sunset Yellow FCF (an orange-yellow dye for food products)


54


3. Direct Brown 138 (a brown dye for fabrics)

4. Ponceau (a red dye for food products)


55


As DrugsAs Drugs

Amine derivatives are commonly used as drugs such as painkillers (analgesics), transquillizers and antihistamines

1. Acetaminophen (also known as paracetamol)

• used to relieve pains

• less harmful to the stomach compared with aspirin


56


2. Chlorpheniramine

• helps to relieve allergic disorders caused by cold, insect bites and stings

• present in some over the counter drugs such as Coricidin, Coltaline, Piriton and Dristan


57


3. Chlorpromazine

• sedative effect without inducing sleep

• used to relieve anxiety, excitement, restlessness and even metal disorders


58

36.7 Amino Acids (SB p.85)

• Amino acids are compound containing both the basic amino group (–NH2) and the acidic carboxyl group

(–COOH)

• Amino acids found in nature as the constituents of natural proteins are mostly -amino acids

• 22 -amino acids can be obtained from proteins


59


Name Formula

Neutral amino acids:

Glycine

Alanine

Valine

Leucine

NH2CH2COOH

• According to the structures, they are classified into 3 groups: neutral amino acids, acidic amino acids and basic amino acids


60


Name Formula

Acidic amino acids:

Aspartic acid

Glutamic acid

Basic amino acids:

Lysine

Arginine


61


• All naturally occurring amino acids are optically active except glycine

contain an asymmetric carbon atom(*)


62

Amino Acids as Dipolar IonsAmino Acids as Dipolar Ions

• Amino acids are bifunctional compounds∵ contain a basic amino group and an acidic

carboxyl group

• Amino acids are neither strong bases nor strong acids

• As dipolar ion (also called zwitterions) in aqueous solution by transferring the acidic proton to the amino group



63

• The predominant form of an amino acid present in a solution depends on the pH of the solution


• The concentration of the zwitterion is at maximum when the solution is at an intermediate pH called the isoelectric point concentration of the anions and cations are equal


64


Due to the formation of dipolar ions, amino acids exhibits many abnormal properties:

• low volatility

• high melting point

• low solubility in organic solvents

• high dipole moment

• abnormal acid and base dissociation constants


65


All amino acids are solids at room temperature. Explain why that should be the case.

Answer


Ionic compounds are solids at room temperature. As amino acids in the form of zwitterions are ionic compounds, they exist as solids at room temperature.


66

Dipeptides and PolypeptidesDipeptides and Polypeptides

Dipeptide: dimers of amino acids

Two amino acid molecules join together with the elimination of a water molecule to form a dipeptide condensation reaction



67

Peptide linkage: the linkage formed between

amino acids


Polypeptide: polymer of amino acids formed by condensation polymerization reaction of a large number of amino acids


68


• The polypeptide can be hydrolyzed and broken down into their constituent amino acids

• The hydrolysis can be carried out in an acidic or alkaline medium


69


In a polypeptide, amino acids are joined together by peptide linkages which are amide groups. The peptide linkages have the same chemistry as the amides. Polypeptides are readily hydrolyzed by acids or bases. A polypeptide has the following structure:

(a) Draw the structures of the amino acids formed after complete hydrolysis of the polypeptide shown above.

(b) Indicate the asymmetric carbon of the amino acids, if there are any. Answer


(a), (b) After complete hydrolysis of the polypeptide, three amino acids are formed. Their structures are shown as follows:

* indicates the asymmetric carbon atom


70

The END

new way chemistry for hong kong a-level book 3b1 nitrogen compounds 36.1introduction...

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