organic nitrogen compounds 2

8/12/2019 Organic Nitrogen Compounds 2

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Organic Nitrogen CompoundsLearning Objectives Describe with equations and the required reagents and conditions , the formationof ethylamine (by nitrile reduction) and phenylamine (by reduction ofnitrobenzene ) Describe basicity of amine relative to the proton accepting tendency of the lonepair of nitrogen Explain relative basicity of ammonia, ethylamine and phenylamine Describe reactions of phenylamine with aqueous bromine and nitrous acid toproduce diazonium salt and phenol. Write equations , reagents and conditions Describe the coupling reaction of benzene diazonium chloride and phenol and ofsimilar reaction to form dye stuff Explain stability of diazonium salt and importance in the dye industry Describe formation of amides from reaction between alkylamine and acylchlorides Describe amide hydrolysis on treatment with aqueous alkali or acid Describe the acid base properties of amino acids and the formation of zwitterions Describe formation of peptide bond between amino acids and hence formation ofprotein Describe hydrolysis of protein


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STRUCTURE & CLASSIFICATION

Structure Amines contain the NH 2 group

Classification

primary (1 ) amines secondary (2 ) amines

tertiary (3 ) amines quarternary (4 ) ammonium salts

Aliphatic methylamine, ethylamine, dimethylamine

Aromatic NH2 group is attached directly to the benzene ring (phenylamine)

R N:

H

H

R N:

R

H

R N:

R

R

R

+ R N R

R


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NOMENCLATURE

Nomenclature

C2H5NH2 ethylamine

(CH 3)2NH dimethylamine

(CH 3)3N trimethylamine

C6H5NH2 phenylamine (aniline)


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PHYSICAL PROPERTIES

The LONE PAIR on the nitrogen atom in 1 , 2 and 3 amines makes them ...

LEWIS BASES - can be lone pair donors

BRNSTED-LOWRY BASES - can be proton acceptors

RNH 2 + H + > RNH 3+

NUCLEOPHILES - provide a lone pair to attack an electron deficient centre


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PHYSICAL PROPERTIES

Boiling point Boiling points increase with molecular mass

Amines have higher boilingpoints than correspondingalkanes because of theirintermolecular hydrogen bonding

Quarternary ammoniumsalts are ionic and exist as salts

Solubility Lower mass compounds aresoluble in water due to hydrogen

bonding with the solvent.

Solubility decreases as themolecules get heavier.

Soluble in organic solvents.


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BASIC PROPERTIES

Bases The lone pair on the nitrogen atom makes amines basic;

RNH 2 + H+ > RNH 3

+a proton acceptor

Strength depends on the availability of the lone pair and its ability to pick up protons the greater the electron density on the N, the better it can pick up protons this is affected by the groups attached to the nitrogen

electron withdrawing substituents (benzene rings)decrease basicity as the electron density on N islowered and the lone pair is less effective

electron releasing substituents (CH 3 groups)increase basicity as the electron density isincreased and the lone pair is more effective

CH 3 N:

H

H

C6H5 N:

H

H


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BASIC PROPERTIES

Measurement the strength of a weak base is given by its pK b valuethe smaller the pK b the stronger the base

the pK a value can also be used;it is worked out by applying pK a + pK b = 14the smaller the pK b , the larger the pK a .

Compound Formula pK b Comments

ammonia NH 3 4.76

methylamine CH 3NH2 3.36 methyl group is electron releasing

phenylamine C 6H5NH2 9.38 electrons delocalised into the ring

strongest base methylamine > ammonia > phenylamine weakest basesmallest pK b largest pK b


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CHEMICAL REACTIONS - WEAK BASES

Amines which dissolve in water produce weak alkaline solutions

CH3NH2(g) + H 2O(l) CH3NH3+(aq) + OH(aq)

Amines react with acids to produce salts

C6H5NH2(l) + HCl(aq) > C6H5NH3+Cl(aq) phenylammonium chloride

This reaction allows one to dissolve an amine in water as its salt

Addition of aqueous sodium hydroxide liberates the free aminefrom its salt

C6H5NH3+Cl(aq) + NaOH(aq) > C6H5NH2(l) + NaCl(aq) + H 2O


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SN2 reaction

Synthesis of Amines via Nitriles

CH3CH2CH2CH2Br

Halogenoalkane

Na CN

CH3CH2CH2CH2CH2NH2Primary alkyl amine

CH3CH2CH2CH2CN

Nitrile

H2 (100 atm), Nifollowed byreduction, gives


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SN2 reaction, followed by reduction, gives alkylamine

Synthesis of Amines via Nitriles

CH3CH2CH2CH2Br

Na CN

CH3CH2CH2CH2CH2NH2

CH3CH2CH2CH2CN

lithium aluminum hydridein dry ether

Reduction of nitrile may alsobe brought about by LiAlH 4 indry ether.


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Amines Replacing one hydrogen atom in NH 3 with an alkyl

group gives primary amine, replacing 2 H atomsgives secondary amine

R NH 2Primary amine

R NH

R'Secondary amine

R N

R'R''

Tertiary amine

R N +

R''

R'''

R'

Quaternary ammonium salt


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Reduction of nitrobenzene to give phenylamine

NO 2

+ 6[H]i) conc. HCl/Sn

ii) NaOH(aq)

NH 2

+ H2O

Conditions are reflux, this is important in the production ofcompounds called azo-dyes

The NaOH is essential to liberate the phenylamine from the saltC6H5 NH3+Cl formed during the reaction with HCl


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Reactions of amines

As bases As nucleophiles With nitrous acid

R NH 2

as bases

RNH 3+Cl-

e.g. HCl

as

nucleophiles RNHR'

AliphaticAromatic

ROH

with HNO 2

RN 2+


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Reactivity of Amines

Is due to The availability of the lone pair of electrons

on the nitrogen Though Nitrogen is less electronegative than

oxygen The lone pair is more available on nitrogen in

amines than it is on oxygen in alcohols


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Amines are

fairly good bases (donating a lone pair to anH+ atom)

Excellent ligands (in transition metalchemistry)

Good nucleophiles able to attack thepositive end of a polarised bond


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Amines as bases

Bases are proton acceptors. Amines accept protons, by donating a lone pair

of electrons to the hydrogen atom to form a

dative bond. Ammonia and bases can do this with any

suitable acid to give a salt.

H3 N H Cl NH 4+Cl-


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Amines an bases An alkyl group is slightly electron donating This is because the electron pairs around the

carbon repel the electron pair in the bond

between the carbon and the functional group

C X


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Replacing a hydrogen in ammonia has thefollowing effect

Causes increased electron donation in the C-N bond

Becomes polar, and nitrogen becomesslightly negative

Lone pair on nitrogen becomes moreavailable

Can be donated to a proton more easily 1 amines are more basic than ammonia


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What about secondary and tertiary?

Following the same argument, 2 amines willbe more basic still, as the lone pair will beeven more available, due to greater electron

donating effect of alkyl groups. In phenylamine, the lone pair becomes

involved in the aromaticity, so it is less basic.The lone pair as part of the rings delocalisedsystem, is less readily donated to a proton .

A tertiary amine will be more basic still.


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Reaction of amines with acids

A standard Acid Base reaction

CH3CH2 NH2 + HCl CH 3CH2 NH3+

+ Cl-


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Diazonium Salts

Diazonium: there are 2 nitrogen atoms joinedtogether in the positive ion.

In French, nitrogen is still called by its old name

azote which means unable to support life.

N N Cl

diazonium chloride


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Diazonium Salts

Notice the triple bond between the nitrogenatoms

The positive charge is on the nitrogen that is

attached to the benzene ring

N N Cl

diazonium chloride


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Why are diazonium salts important? They look pretty

weird!

They are essential in the dye industry.

A Diazonium salt is produced then reacted with aphenol.

If the correct phenol is used, almost any colour canbe produced.

you need to know this reaction with the structuresof the molecules. (The way it was taught in the classwith structures drawn on the board)


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Formation of the Diazonium salt The Diazonium salt is unstable above 5C, so the reaction is

normally carried out in ice.

An aliphatic Diazonium salt is very unstable, anddecomposes readily. So only aromatics are used.

The lone pairs present on the N in the salt get delocalisedon the benzene ring, making it more stable. This is due tooverlap of p-orbitals in the diazo group with the pi-system inthe ring.

So phenylamine would give benzene diazonium chloride.


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Formation of the Diazonium salt

N N Cl NH 2

+ HONO(aq) + HCl(aq) + 2H 2O(l)


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Formation of the Diazonium salt

The conditions are 5C and remember theHNO2 (nitrous acid) is prepared in situ byreacting sodium nitrite with hydrochloricacid.

The diazonium salt decomposes to givephenol, N 2 and HCl at a temperature higher

than 5C.


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Coupling reactions

The mechanism is forinterest only, you doNot need to know it.

N N

O H

N N

OH

H

HO

H

N N

OH


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General method for synthesis of azo dyes

Add a cold aqueous solution of sodium nitriteslowly (with cooling and stirring) to a cold solutionof the amine compound in excess hydrochloric acid

The temperature must not rise above 5

C. This solution (still cold) should then be added

slowly with stirring to a solution of the couplingcompound.

The temperature should be kept below 5

C thewhole time.


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Amino Acids

These are bi-functional compounds. Theycontain 2 functions groups:

A primary amine (in most cases) NH2 The carboxylic acid group COOH An amino acid must contain both of these

functional groups.


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Amino Acids

The simplest amino acid is glycine.

C C N

H

H

H

H

O

O H


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Amino Acids

All the amino acids (the twenty vitally importantones biologically) are 2-amino acids.

The amine and acid groups are both attached to the

same carbon. All can be named systematically, but in most cases

the old names are used. Alanine is also known as 2-aminopropanoic acid,

but alanine is the acceptable name to use.


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Physical Properties

White solids With relatively high melting points glycine

(the simplest) has a melting point of 235

C. Normally readily soluble in water Almost totally insoluble in non-polar solvents


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Acid Base Properties

They are very largely ionic compounds. The carboxyl group can lose a proton The amine group can gain a proton The result is a ZWITTERION. From the

German for hermaphrodite, hybrid ormongrel!


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Zwitterions

Glycine mainly exists as

H3 N+ CH 2 COO

-


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Zwitterions

The strong attractions in the crystal cause thehigh melting point

In aqueous solution depending on the pH,they form either the neutral form,

or the carboxylate will lose a proton, or the amino group will gain a proton.


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Zwitterions

C C NH

H

O

O H

H

H

+ H+ in strong acidC CO

O H

H

H

H3 N+ -H + in strong alkali C C N

H

H

OH

H O -


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Isoelectric Point

For each amino acid there is a definite pH the isoelectric point at which the acid andbasic ionisations are equal .

The molecule is effectively neutral it carriesequal and opposite charges

This is rarely near pH 7 because the moleculeionisation tendencies are affected by theother groups in the molecule .


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Isoelectric Point

Aspartic acid which has 2 COOH groups is acidic in aqueous solution.

Lysine with more amino than carboxyl groupsis alkaline.

Due to this dual functionality, they are ableto act as buffer solutions (able to maintain a

reasonably constant pH with small additionsof acid or alkali). They also have optical activity .


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How amino acids join together

This kind of bond between 2 amino acids iscalled a peptide bond or a peptide link.

CO

NH


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N CH

H

H

R'

CO

OH N C

H

H

H

R'

CO

OH+

N C

H

R'

CO

OHC

R'

H

H

HC N

O

H


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Two joined amino acids form a dipeptide Three join to form a tripeptide Many join to form a polypeptide At some point a polypeptide becomes a

protein. This can be put at 40 amino acids.


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Acid Hydrolysis of proteins

Proteins and peptides can be hydrolysed withhot concentrated (6 mol dm-3) HCl.

The protein is refluxed for about 24 hours. This hydrolysis is the exact reverse of the

formation of the peptide bond. A molecule of water is in effect added across

the linkage to regenerate the original aminoacid and carboxyl groups.

organic nitrogen compounds 2

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