ssAmines and Urea

Introduction

Preparation

Properties of Amine

Properties of Amine

Aliphatic amines are always more basic than aromatic amines due to absence of resonance.

Aqueous solution of amines are basic in nature, express by following reaction. When this reacts with ferric chloride it forms red

precipitate of ferric hydroxide.

This reaction with benzoyl chloride is also known as

Benzoylation of amines or Schowtann Bowmann’s Reaction.

(viii) Reaction with HNO2. (Nitrosation of aliphatic amines)

When sodium nitrite reacts with H2SO4 or HCl it produces nitrous acid (HNO2) which works

as precursor of nitrosyl cation (N+ O) this species is known as nitrosating agents.

When any secondary amines reacts with nitrous acid, amine work as nucleophile which is

attacking at nitrosyl cation to form yellow liquid nitroso solution.

Some other examples of nitroso products

When primary amines undergo process of nitrosation, nitroso compounds formed first and

can react further and finally produce various products where alcohols are formed as major

product.

Alkyl diazonium ion is the final product of this sequence but not any other species can be

isolated. When any primary amine is converted into diazonium species, it is known as

diazotisation of amines. These are not very much stable due to positively charged nitrogen

atom which is strained species. It’s decompose rapidly where molecular nitrogen is a leaving

group and carbocations are formed.

As the nitrogen free products result from the formation and decomposition of diazonium salts

so these reactions are known as deamination reactions.

The nitrosation of tertiary alkylamines is rather complicated, and these reactions are also not

very useful in general chemistry primarilly they form addition product which breakdown on

heating and form nitrasoamine.

When primary amines reacts with excess of alkyl halide, it quaternize the starting amines to

form quaternary ammonium salt.

Above reaction is known as Hoffmann’s reaction prefer less substituted alkene,

an example of kinetically control product. If we study stereochemistry a preference of trans-

periplanar (Anti)-elimination of the H and the amine. It shows, E2 mechanism with anti

stereochemistry.

By simply quaternizing the 1° amines with an excess (> 3 equivalents) of alkyl halide is also

known as Hoffmann’s exhaustive alkylation. For example

In this example anti-elimination requires loss of one of the

which is trans to the nitrogen not cis

. This can be given by chair conformation where nitrogen is axial, so above product can

formed.

Some following conversions also explain the concept of Hoffmann’s quaternary ammonium

salt

(xiv) Cope elimination When tertiary alkylamine oxides in which one of alkyl group have

atom undergo thermal cleavage to form alkenes and hydroxyl amine.

This elimination does not require the presence of baseion, actually internal base already

present inside the system as N-oxide oxygen so intramolecular base promotes a Ei

elimination by path of a five membered cyclic transition state.

Above reaction follows syn stereochemistry for elimination exactly opposite to the anti

elimination in Hoffmann’s regiochemistry. For example

Second isomeric cyclohexene is not formed because it is impossible for a ring hydrogen to

becomes syn to the amine oxide group unless the cyclohexane ring is severely distorted. Cope

elimination also form less substituted alkene same as Hoffmann’s orientation.

UREA

Basic Character

First organic compound to be synthasized in lab from inorganic compounds thus discarding

“Vital force theory”

of Berzelius which said that no organic compound can be synthesized in lab. It was

synthesized by Wholar’s.

On the other hand acetic acid was the first organic compound to be synthesize from elements

in their free states.

Urea is a monoamide of carbamic acid (NH2COOH) and diamide of carbonic acid

H2CO3

Preparation

Manufacture of urea

Urine contains some amount of urea so when it reacts with HNO

3

, a ppt of urea nitrate is formed which on reaction with BaCO

3

gives urea. In this process coloured impurities are removed by acidic KMnO

4

.

Properties

Breaking of urea takes place by NaOBr but Urea on Hoffmann’s Hypobromite reaction forms

N

2

, CO

2

, H

2

O and NaBr while other amides furnish amines with lower homologue. Quantitative

estimation of urea done by cleavage in the presence of urease or fission by HNO

2s

Nitrosation of aromatic amines

(b) Nitrosation of aromatic amines

When aromatic amines undergo process of nitrosation by nitrosyl cation, electrophilic aromatic substitution occurs and then nitros o

derivatives are formed. For example:

Only very strong activating group, for example aniline and its derivatives can react with very weak electrophile nitrosyl with

nitrosyl cation it resemble with secondary amines where synthesis of N-alkyl nitrosoproduct takes place.

Alkyl diazomium ions are not stable due to positively charged nitrogen atom but aryl diazonium ions are much more stable due to

resonance and can be stored aqueous solution at 0°C to 5°C for long periods of time and loss of nitrogen form an unstable ary l

cation and is much slower than loss of nitrogen from an alkyl diazonium ion so C6H5N+

2

immediately reacts with chloride ion to form benzene diazonium chloride.

Here NaNO2and HCl are taken in 1 : 3 so excess chloride ion quickly reacts with intermediate otherwise electrophile C 6H5N+

2

immediately reacts with nucleophilic aniline to form coupling azo product.

Aryl diazonium ions undergo a variety of reactions which makes them versatile intermediates for the preparation of many ring

substituted aromatic derivatives for example

These reactions occur only when amino group attached directly with benzene ring but when benzyl derivatives type compounds

undergo these reactions they form normal aliphatic amines type products.

When methyl amines reacts with nitrous acid they form alkoxyalkanes as major product.

(x) Separation of 1o, 2

o and 3

o Amines