Spectroscopy of Amines - IR

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Spectroscopy of Amines - IR Characteristic N–H stretching absorptions 3300 to 3500 cm-1.

NH2 group shows an irregular doublet, NH - weak multiple bands. Ammonium ions show N-H at 2600 cm-1. Amine absorption bands are sharper and less intense than hydroxyl bands. 1o amines show NH2 deformation band at 1650-1590 cm-1.

C-N stretching vibrations are found at 1090-1068 cm-1 in 1o amines with a 1o carbon, 1140-1080 cm-1 with a 2o carbon, and at 1240-1170 cm-1 with a 3o carbon. 3o amines show no C-N vibrations. In aromatic amines, this band is at 1330-1260 cm-1.

N-H wagging bands are found at 850-750 cm-1 as strong, broad, multiple bands. These are weak in aromatic amines.

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Mass Spectrometry Since N is a compound with an odd number of

nitrogen atoms has an odd-numbered molecular weight and a corresponding parent ion

Alkylamines cleave at the C–C bond nearest the nitrogen to yield an alkyl radical and a nitrogen-containing cation

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Mass Spectrum of N-Ethylpropylamine The two modes of a cleavage give fragment ions at

m/z = 58 and m/z = 72.

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1H - NMR Spectroscopy

N–H hydrogens appear as broad signals, either fully coupled to neighboring C–H hydrogens, or more frequently with no coupling (just like the H-bonded OH signals)

Hydrogens on C next to N and absorb at lower field than alkane hydrogens

N-CH3 gives a sharp three-H singlet at δ 2.2 to 2.6

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C3H9NO

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C4H11NO2

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C8H11NO

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C9H13N

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C15H17N

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C6H15N - IR

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C6H15N - 1H-NMR

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Structure, Properties and Reactivity of Amines

Organic derivatives of ammonia, NH3 Nitrogen atom with a lone pair of electrons, making amines

both basic and nucleophilic Occur in plants and animals

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IUPAC nomenclature of simple amines

For simple amines, the suffix -amine is added to the name of the alkyl substituent

The suffix -amine can also be used in place of the final -e in the name of the parent compound

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

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

IUPAC nomenclature of simple amines

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Consider the NH2 as an amino substituent on the parent molecule

IUPAC nomenclature of complex amines.Amines with more than one functional group.

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IUPAC nomenclature of complex amines.

Amines with multiple alkyl groups.

Symmetrical secondary and tertiary amines are named by adding the prefix di- or tri- to the alkyl group

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IUPAC nomenclature of complex amines.

Amines with multiple different alkyl groups.

Named as N-substituted primary amines Largest alkyl group is the parent name, and other

alkyl groups are considered N-substituents

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Common Names Alkylamines do not have common names unless they are biological

molecules, such as putrycine (1,6-hexanediamine)or cadaverine (1,7-heptanediamine)

Simple arylamines have common names

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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

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Quaternary Ammonium Ions A nitrogen atom with four attached groups is

positively charged Compounds are quaternary ammonium salts

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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

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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

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

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Amines as Acids

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

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Synthesis of Amines

Ammonia and other amines are good nucleophiles

SN2 Reactions of Alkyl Halides

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Uncontrolled Multiple Alkylation are Unavoidable when Sterically Un-hindered Amines React

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Reduction of nitriles and amides

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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

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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

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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

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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

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Hofmann Rearrangement (Mechanism) Part I

RCONH2 reacts with Br2 and base to give electrondeficient nitrogen

R

O

N H

H

OH-

R

O

N H + H2O

Br2

R

O

NH + Br -R

O

NH

Br

R

O

N H

Br

OH-

R

O

NBr + H2O

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Hofmann Rearrangement (Mechanism) Part II

Alkyl group (-R) migrates to the neighboring electron-deficient

nitrogen. Hydration of the resultant isocyanate gives

carboxamic acid.

R

O

N Br R

O

N + Br -

R

O

N

R

N C O (isocyanate)

R

N C O

H2O R

N C

O

H OH

(carboxamic acid)

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Hofmann Rearrangement (Mechanism) Part III

Deprotonation-reprotonation produces a protonated

ammonium zwitterion (a good leaving group). It’s

elimination produces the amine and carbon dioxide by-product.

OH-R

N C

O

H OH

R

N C

O

H O

+ H2O

R

N C

O

H O

H2O R

N C

O

H O

H

R

N C

O

H O

H

R

N C

O

HO

H +

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Curtius Rearrangement Heating an acyl azide prepared from substitution an acid

chloride This rearrangement also involves migration of R from C=O

to the neighboring electron-deficient nitrogen with simultaneous loss of a leaving group

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Reactions of Amines

Acylation leads to amides (1o, 2o, or 3o.)

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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

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Silver Oxide Is Used for the Elimination Step

Exchanges hydroxide ion for iodide ion in the quaternaryammonium salt, thus providing the base necessary to causeelimination

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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

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Steric Effects Control the Orientation