chapter 24. amines based on mcmurry’s organic chemistry, 6 th edition

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Chapter 24. Amines Based on McMurry’s Organic Chemistry, 6 th edition

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Page 1: Chapter 24. Amines Based on McMurry’s Organic Chemistry, 6 th edition

Chapter 24. Amines

Based on McMurry’s Organic Chemistry, 6th edition

Page 2: Chapter 24. Amines Based on McMurry’s Organic Chemistry, 6 th edition

Amines – Organic Nitrogen Compounds

• Organic derivatives of ammonia, NH3,

• Nitrogen atom with a lone pair of electrons, making amines both basic and nucleophilic

• Occur in plants and animals

Page 3: Chapter 24. Amines Based on McMurry’s Organic Chemistry, 6 th edition

24.1 Naming Amines

• Alkyl-substituted (alkylamines) or aryl-substituted (arylamines)

• Classified: 1° (RNH2), methyl (CH3NH2), 2° (R2NH), 3° (R3N)

Page 4: Chapter 24. Amines Based on McMurry’s Organic Chemistry, 6 th edition

Quaternary Ammonium Iions

• A nitrogen atom with four attached groups is positively charged

• Compounds are quaternary ammonium salts

Page 5: Chapter 24. Amines Based on McMurry’s Organic Chemistry, 6 th edition

Common Names of Heterocyclic Amines

• If the nitrogen atom occurs as part of a ring, the compound is designated as being heterocyclic

• Each ring system has its own parent name

Page 6: Chapter 24. Amines Based on McMurry’s Organic Chemistry, 6 th edition
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24.2 Structure and Bonding in Amines

• Bonding to N is similar to that in ammonia– N is sp3-hybridized

– C–N–C bond angles are close to 109° tetrahedral value

Page 9: Chapter 24. Amines Based on McMurry’s Organic Chemistry, 6 th edition

Chirality Is Possible (But Not Observed)

• An amine with three different substituents on nitrogen is chiral (in principle but not in practice): the lone pair of electrons is the fourth substituent

• Most amines that have 3 different substituents on N are not resolved because the molecules interconvert by pyramidal inversion

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24.3 Properties and Sources of Amines

• Simple methylated amines from reaction of NH3 with CH3OH and alumina catalyst

• Yields a mixture of monomethylated, dimethylated, and trimethylated products that are easily separated by distillation

Page 13: Chapter 24. Amines Based on McMurry’s Organic Chemistry, 6 th edition
Page 14: Chapter 24. Amines Based on McMurry’s Organic Chemistry, 6 th edition

Amines Form H-Bonds

• Amines with fewer than five carbons are water-soluble

• Primary and secondary amines form hydrogen bonds, increasing their boiling points

Page 15: Chapter 24. Amines Based on McMurry’s Organic Chemistry, 6 th edition

24.4 Basicity of Amines

• The lone pair of electrons on nitrogen makes amines basic and nucleophilic

• They react with acids to form acid–base salts and they react with electrophiles

Page 16: Chapter 24. Amines Based on McMurry’s Organic Chemistry, 6 th edition

Relative Basicity

• Amines are stronger bases than alcohols, ethers, or water

• Amines establish an equilibrium with water in which the amine becomes protonated and hydroxide is produced

• The most convenient way to measure the basicity of an amine (RNH2) is to look at the acidity of the corresponding ammonium ion (RNH3

+)• High pKa → weaker acid and stronger conjugate base

.

Page 17: Chapter 24. Amines Based on McMurry’s Organic Chemistry, 6 th edition

General Patterns of Basicity

• Table 24.1: pKa values of ammonium ions

• Most simple alkylammmonium ions have pKa's of 10 to 11

• Arylamines and heterocyclic aromatic amines are considerably less basic than alkylamines (conjugate acid pKa 5 or less)

Page 18: Chapter 24. Amines Based on McMurry’s Organic Chemistry, 6 th edition
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Amides

• Amides (RCONH2) in general are not proton acceptors except in very strong acid

• The C=O group is strongly electron-withdrawing, making the N a very weak base

• Addition of a proton occurs on O but this destroys the double bond character of C=O as a requirement of stabilization by N

Page 24: Chapter 24. Amines Based on McMurry’s Organic Chemistry, 6 th edition
Page 25: Chapter 24. Amines Based on McMurry’s Organic Chemistry, 6 th edition

Amines as Acids

• Loss of the N–H proton requires a very strong base

Page 26: Chapter 24. Amines Based on McMurry’s Organic Chemistry, 6 th edition

24.5 Basicity of Substituted Arylamines

• The N lone-pair electrons in arylamines are delocalized by interaction with the aromatic ring electron system and are less able to accept H+ than are alkylamines

Page 27: Chapter 24. Amines Based on McMurry’s Organic Chemistry, 6 th edition

Substituted Arylamines

• Can be more basic or less basic than aniline

• Electron-donating substituents (such as CH3, NH2, OCH3) increase the basicity of the corresponding arylamine

• Electron-withdrawing substituents (such as Cl, NO2, CN) decrease arylamine basicity

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Page 29: Chapter 24. Amines Based on McMurry’s Organic Chemistry, 6 th edition

24.6 Synthesis of Amines

• Reduction of nitriles and amides (review)

Page 30: Chapter 24. Amines Based on McMurry’s Organic Chemistry, 6 th edition
Page 31: Chapter 24. Amines Based on McMurry’s Organic Chemistry, 6 th edition

Reduction Aryl Nitro Compounds

• Arylamines are prepared from nitration of an aromatic compound and reduction of the nitro group

• Reduction by catalytic hydrogenation over platinum is suitable if no other groups can be reduced

• Iron, zinc, tin, and tin(II) chloride are effective in acidic solution

Page 32: Chapter 24. Amines Based on McMurry’s Organic Chemistry, 6 th edition

SN2 Reactions of Alkyl Halides

• Ammonia and other amines are good nucleophiles

Page 33: Chapter 24. Amines Based on McMurry’s Organic Chemistry, 6 th edition

Uncontrolled Multiple Alkylation

• Primary, secondary, and tertiary amines all have similar reactivity, the initially formed monoalkylated substance undergoes further reaction to yield a mixture of products

Page 34: Chapter 24. Amines Based on McMurry’s Organic Chemistry, 6 th edition

Selective Preparation of Primary Amines: the Azide Synthesis

• Azide ion, N3 displaces a halide ion from a primary or

secondary alkyl halide to give an alkyl azide, RN3

• Alkyl azides are not nucleophilic (but they are explosive)

• Reduction gives the primary amine

Page 35: Chapter 24. Amines Based on McMurry’s Organic Chemistry, 6 th edition

Gabriel Synthesis of Primary AMines

• A phthalimide alkylation for preparing a primary amine from an alkyl halide

• The N-H in imides (CONHCO) can be removed by KOH followed by alkylation and hydrolysis

Page 36: Chapter 24. Amines Based on McMurry’s Organic Chemistry, 6 th edition
Page 37: Chapter 24. Amines Based on McMurry’s Organic Chemistry, 6 th edition

Reductive Amination of Aldehydes and Ketones

• Treatment of an aldehyde or ketone with ammonia or an amine in the presence of a reducing agent

Page 38: Chapter 24. Amines Based on McMurry’s Organic Chemistry, 6 th edition

Reductive Amination Is Versatile

• Ammonia, primary amines, and secondary amines yield primary, secondary, and tertiary amines, respectively

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Mechanism of Reductive Amination

• Imine is intermediate

Page 43: Chapter 24. Amines Based on McMurry’s Organic Chemistry, 6 th edition

Reducing Step

• Sodium cyanoborohydride, NaBH3CN, reduces C=N but not C=O

• Stable in water

Page 44: Chapter 24. Amines Based on McMurry’s Organic Chemistry, 6 th edition
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Hofmann and Curtius Rearrangements

• Carboxylic acid derivatives can be converted into primary amines with loss of one carbon atom by both the Hofmann rearrangement and the Curtius rearrangement

Page 48: Chapter 24. Amines Based on McMurry’s Organic Chemistry, 6 th edition

Hofmann Rearrangement

• RCONH2 reacts with Br2 and base

• Gives high yields of arylamines and alkylamines

Page 49: Chapter 24. Amines Based on McMurry’s Organic Chemistry, 6 th edition

Curtius Rearrangement

• Heating an acyl azide prepared from substitution an acid chloride

• Migration of R from C=O to the neighboring nitrogen with simultaneous loss of a leaving group

Page 50: Chapter 24. Amines Based on McMurry’s Organic Chemistry, 6 th edition

Reactions of Amines

• Alkylation and acylation have already been presented

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Hofmann Elimination

• Converts amines into alkenes

• NH2 is very a poor leaving group so it converted to an

alkylammonium ion, which is a good leaving group

Page 58: Chapter 24. Amines Based on McMurry’s Organic Chemistry, 6 th edition

Silver Oxide Is Used for the Elimination Step

• Exchanges hydroxide ion for iodide ion in the quaternary ammonium salt, thus providing the base necessary to cause elimination

Page 59: Chapter 24. Amines Based on McMurry’s Organic Chemistry, 6 th edition

Orientation in Hofmann Elimination

• We would expect that the more highly substituted alkene product predominates in the E2 reaction of an alkyl halide (Zaitsev's rule)

• However, the less highly substituted alkene predominates in the Hofmann elimination due to the large size of the trialkylamine leaving group

• The base must abstract a hydrogen from the most sterically accessible, least hindered position

Page 60: Chapter 24. Amines Based on McMurry’s Organic Chemistry, 6 th edition

Steric Effects Control the Orientation

Page 61: Chapter 24. Amines Based on McMurry’s Organic Chemistry, 6 th edition
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Definition of Phase Transfer Catalysis

• The transfer of an inorganic ion such as OH from one phase to another is called phase transfer, and the tetraalkylammonium salt is a phase-transfer catalyst

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