10-1 chemistry 2060, spring 2060, lsu chapter 10: amines sections 10.1-10.8

10-1Chemistry 2060, Spring 2060, LSU

Chapter 10: AminesChapter 10: Amines

Sections 10.1-10.8Sections 10.1-10.8


SectionsSectionsChapter 10: AminesChapter 10: Amines

1.1. IntroductionIntroduction2.2. Structure and classificationStructure and classification3.3. NomenclatureNomenclature4.4. Physical PropertiesPhysical Properties5.5. Basicity of aminesBasicity of amines6.6. Reaction with acidsReaction with acids7.7. Synthesis of arylamines: Reduction of –NO2 groupSynthesis of arylamines: Reduction of –NO2 group8.8. Reactions of primary amines with nitrous acidReactions of primary amines with nitrous acid


Structure & ClassificationStructure & ClassificationClassificationClassification

• 1°, 2°, or , 3° amines1°, 2°, or , 3° amines: amines in which 1, 2, or 3 hydrogens of NH3 are replaced by alkyl or aryl groups

• 4° amines4° amines: ions in which nitrogen is bonded to four carbons and bears a positive charge

Methylamine(a 1° amine)

Trimethylamine(a 3° amine)

CH3

CH3

CH3-NH2 CH3-N

Tetramethylammonium bromide

(a 4° ammonium salt)

+CH3

CH3

CH3-N-CH3

Br-


Structure & ClassificationStructure & ClassificationAmines are further divided into aliphatic, aromatic, Amines are further divided into aliphatic, aromatic,

and heterocyclic aminesand heterocyclic amines• aliphatic aminealiphatic amine: an amine in which nitrogen is bonded

only to alkyl groups• aromatic aminearomatic amine: an amine in which nitrogen is bonded

to one or more aryl groups

NH2 N-H

CH3

CH2-N-CH3

CH3

N-Methylaniline(a 2° aromatic

amine)

Benzyldimethylamine(a 3° aliphatic amine)

Aniline(a 1° aromatic

amine)


Structure & ClassificationStructure & Classification• heterocyclic amineheterocyclic amine: an amine in which nitrogen is one

of the atoms of a ring

PyrrolePiperidinePyrrolidine Pyridine(heterocyclic

aliphatic amines)(heterocyclic

aromatic amines)

NN N N

HH H


Structure & ClassificationStructure & ClassificationExample:Example: classify each amino group by type classify each amino group by type

(b)

(S)-(+)-Coniine

(a)

(S)-(-)-Nicotine

NNH

H N

CH3

H

N

O

O

OCH3

OH3C

Cocaine

(c)


NomenclatureNomenclatureAliphatic amines: replace the suffix Aliphatic amines: replace the suffix -e-e of the parent of the parent

alkane by alkane by -amine-amine

The IUPAC system retains the common name The IUPAC system retains the common name anilineaniline

NH2

H2NNH2

C6H5

NH2

1,6-Hexanediamine(S)-1-Phenylethanamine2-Propanamine

NH2CH3

NH2

NO2

OCH3

NH2NH2

3-Methoxyaniline(m-Anisidine)

2-Methylaniline(o-Toluidine)

Aniline 4-Nitroaniline(p-Nitroaniline)


NomenclatureNomenclatureAmong the various functional groups discussed in Among the various functional groups discussed in

the text, -NHthe text, -NH22 has one of the lowest priorities has one of the lowest priorities

H2NOH

COOH

NH2

2-Aminobenzoic acid(Anthranilic acid)

2-Aminoethanol


NomenclatureNomenclatureCommon names for most aliphatic amines are Common names for most aliphatic amines are

derived by listing the alkyl groups bonded to derived by listing the alkyl groups bonded to nitrogen in one word ending with the suffix -nitrogen in one word ending with the suffix -amineamine

TriethylamineDicyclopentylamineMethylamine

N

H

CH3NH2

tert-Butylamine

NNH2


Physical PropertiesPhysical PropertiesAmines are polar compounds, and both 1° and 2° Amines are polar compounds, and both 1° and 2°

amines form intermolecular hydrogen bondsamines form intermolecular hydrogen bonds• N-H- - - -N hydrogen bonds are weaker than O-H- - - -O

hydrogen bonds because the difference in electronegativity between N and H (3.0 - 2.1 = 0.9) is less than that between O and H (3.5 - 2.1 = 1.4)

CH3CH3 CH3NH2 CH3OH

-6.3 65.0-88.6

32.031.130.1molecular weight (g/mol)

boiling point (°C)


BasicityBasicityAll amines are weak bases, and aqueous solutions All amines are weak bases, and aqueous solutions

of amines are basicof amines are basic

Kb = = = 4.37 x 10-4Keq[H2O][CH3NH3

+][OH

-]

[CH3NH2]

H

H

CH3-N: H-O-HH

H

CH3-N-H -:O-H

Methylammoniumhydroxide

Methylamine

++


BasicityBasicityit is also common to discuss the basicity of amines it is also common to discuss the basicity of amines

by reference to the acid ionization constant of the by reference to the acid ionization constant of the corresponding conjugate acidcorresponding conjugate acid

• for any acid-conjugate base pair

pKa + pKb = 14.00

CH3NH3+

H2O

[CH3NH2][H3O+]

[CH3NH3+]

CH3NH2 H3O+++

2.29 x 10-11==Ka pKa = 10.64


BasicityBasicityUsing values of pUsing values of pKKaa, we can predict the position of , we can predict the position of

equilibrium in acid-base reactionsequilibrium in acid-base reactions

• acetic acid is the stronger acid and, therefore, the position of this equilibrium lies to the right

CH3NH2 CH3COOH CH3NH3+ CH3COO-+ +

pKa 10.64pKa 4.76

(stronger acid)

(weaker acid)

(stronger base)

(weaker base)


BasicityBasicity• aliphatic amines have about the same base strength,

pKb 3.0 - 4.0, and are slightly stronger bases than ammonia

CH3NH2

NH3

CH3CH2NH2

C6H11NH2

(CH3)2NH(CH3CH2)2NH

(CH3)3N

(CH3CH2)3N

Tertiary Aminesdiethylaminedimethylamine

cyclohexylamine

ethylaminemethylamine

Secondary Amines

Primary AminesAmmonia

pKaStructureAmine

trimethylaminetriethylamine

9.26

10.6410.81

10.66

10.7310.98

9.8110.75

pKb

4.74

3.363.19

3.34

3.273.02

4.193.25


BasicityBasicity• aromatic amines are considerably weaker bases than

aliphatic amines

Cyclohexyl-amine

NH2

NH2

NH3+

NH3+

pKb 9.37

Aniline

OH-

OH-

pKb 3.34+ H2O

+ H2O

Cyclohexylammoniumhydroxide

Anilinium hydroxide


BasicityBasicity• aromatic amines are weaker bases than aliphatic

amines because the resonance stabilization of the free base is lost on protonation

N N NH H H H

H NH HH +++

Interaction of the electron pair on nitrogen with the pi system of the aromatic ring


BasicityBasicityElectron-withdrawing groups, such as halogen, Electron-withdrawing groups, such as halogen,

nitro, and carbonyl, decrease the basicity of nitro, and carbonyl, decrease the basicity of aromatic amines by decreasing the availability of aromatic amines by decreasing the availability of the electron pair on nitrogenthe electron pair on nitrogen

NH2 NH2O2N

Aniline(pKb 9.37)

4-Nitroaniline(pKb 15.0)


BasicityBasicityExample:Example: select the stronger base in each pair of select the stronger base in each pair of

aminesamines

(D)(C)

(A) (B)

NH2CH3

CH2NH2

N

O

NH

(a)

(b)

or

or


GuanidineGuanidineGuanidine, pGuanidine, pKKbb 0.4, is the strongest base among 0.4, is the strongest base among

neutral organic compoundsneutral organic compounds

• its basicity is due to the delocalization of the positive charge over the three nitrogen atoms

H2N-C-NH2

NH

H2OpKb 0.4

H2N-C-NH2

NH2+

OH-

++Guanidine Guanidinium ion

Three equivalent contributing structures

+ +

+

C

NH2

NH2H2N H2N C NH2

NH2

C

NH2

NH2H2N


Reaction with AcidsReaction with AcidsAll amines, whether soluble or insoluble in water, All amines, whether soluble or insoluble in water,

react quantitatively with strong acids to form react quantitatively with strong acids to form water-soluble saltswater-soluble salts

NH2

HO

HO

OH

HClH2O

NH3+Cl

-

HO

HO

OH

(R)-Norepinephrine hydrochloride

(a water-soluble salt)

+

(R)-Norepinephrine(only slightly

soluble in water)


Synthesis of 1° ArylaminesSynthesis of 1° ArylaminesMost are synthesized by nitration of an aromatic Most are synthesized by nitration of an aromatic

ring followed by catalytic reduction of the nitro ring followed by catalytic reduction of the nitro group to a primary aminegroup to a primary amine

COOH

NO2

3H2Ni

COOH

NH2

2H2O+(3 atm)

+

3-Nitrobenzoicacid

3-Aminobenzoicacid


Reaction with Nitrous AcidReaction with Nitrous AcidNitrous acid is a weak acid, most commonly Nitrous acid is a weak acid, most commonly

prepared by treating an aqueous solution of prepared by treating an aqueous solution of NaNONaNO22 with H with H22SOSO44 or HCl or HCl

• nitrous acid reacts with amines of different types, depending on whether the amine is 1°, 2°, 3° and whether it is aliphatic or aromatic

• we concentrate on the reaction of nitrous acid with 1° aromatic amines because this reaction is useful in organic synthesis

HNO2 H2O H3O+ NO2

-+ Ka = 4.26 x 10-4

pKa = 3.37

+


Reaction with Nitrous AcidReaction with Nitrous AcidTreating a 1° aromatic amine with HNOTreating a 1° aromatic amine with HNO22 gives a gives a

diazonium saltdiazonium salt

• warming the aqueous solution of the diazonium salt gives a phenol and N2 gas

NH2 NaNO2 HClH2O

N N Cl- H2O0°C

++

Benzenediazoniumchloride

+ +

Aniline(a 1° aromatic

amine)

Sodiumnitrite

N N Cl- H2O OH N2

Benzenediazoniumchloride

++

+

Phenol


Reaction with Nitrous AcidReaction with Nitrous AcidExample:Example: show how to bring about each step in this show how to bring about each step in this

conversionconversion

CH3 CH3

NO2

COOH

NO2

COOH

NH2

COOH

OH

(1) (2) (3) (4)

Toluene 4-Hydroxy-benzoic acid

10-1 chemistry 2060, spring 2060, lsu chapter 10: amines sections 10.1-10.8

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