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Amines

### Syntheses Limited to Primary Amines¶

What is a benzyl group?

C6H5CH2-

Draw the structure of butyric acid with representation of alpha, beta and gamma carbon.

Define alpha carbon.

The alpha carbon (Cα) in organic molecules refers to the first carbon atom that attaches to a functional group, such as a carbonyl. The second carbon atom is called the beta carbon (Cβ),[1] and the system continues naming in alphabetical order with Greek letters.

Why N-H bond of phthalimide is a lot more acidic than it normally would be? Hint: How to know the acidity of a molecule?

because the resulting anion will be resonance stabilized.

Identify and name the nucleophile, electrophile and leaving group in the SN2 reaction of Gabriel Synthesis.

Can you say why we can only prepare primary ammines in Gabriel Synthesis?

The conversion of primary aromatic amines into diazonium salts in known as__.

Connection:
Elaborate Sandmeyer's reaction and diazotization reaction.

Write the IUPAC name of the given compound?

2,4,6-Tribromoaniline

Connection:
What the name of given compound?

Aniline

Give the IUPAC name and structure of the amine obtained when 3-chlorobutanamide undergoes Hoffmann-bromamide reaction.

Connection:
What is an amide?

Draw the structure of 3-chlorobutanamide?

Hoffmann-bromamide reaction

$$\ce{RCONH2 +Br2 + 4NaOH -> R-NH2 + Na2CO3 + 2NaBr + 2H2O}$$

Remember carbonyl group is removed, so there is decrease in C number in $\ce{R-NH2}$

3-Chloropropanamine

Complete the following reaction equation:
$$\ce{C6H5N2Cl + H3PO2 + H2O -> }$$

What is the name of $\ce{C6H5N2Cl}$ ?

Benzenediazonium chloride (Diazonium salt)

### Aryl diazonium salts as intermediates¶

Aryl diazonium salts are important intermediates. They are prepared in cold (0 º to 10 ºC) aqueous solution, and generally react with nucleophiles with loss of nitrogen. Some of the more commonly used substitution reactions are shown in the following diagram. Since the leaving group ($\ce{N2}$) is thermodynamically very stable, these reactions are energetically favored. Those substitution reactions that are catalyzed by cuprous salts are known as Sandmeyer reactions. The top reaction with hypophosphorus acid, $\ce{H3PO2}$, is noteworthy because it achieves the reductive removal of an amino (or nitro) group. Unlike the nucleophilic substitution reactions, this reduction probably proceeds by a radical mechanism.

What is the name of $\ce{H3PO2}$? State whether its oxidising or reducing agent?

Hypophosphorous acid, reducing agent

Can you say which carbon of diazonium salt under go reduction when treated with hypophosphorous acid? Explain the complete reduction process using oxidation states of carbon.

Question: Arrange the following in increasing order of basic strength: Aniline, p-Nitroaniline and p-Toluidine

What is the structure of p-Toluidine? Can you recall the structure of Aniline and p-nitoraniline?

Basity of amine depends on stability of _ ions.

Ammoinium ions

Write down the base dissociation constant along with the reaction.

Specify which of the substitute are electron withdrawing and electron donanting in the question.

Draw the ammonium ion of each structure given, and arrange according to their stability giving reasons.

Question: Arrange the following in the increasing order of their basic strength in aqueous solutions: CH3NH2, (CH3)2NH, (CH3)3N and NH3

## Primary Secondary Tertiary Amines

When we compare the basicity of the aliphatic amines, we would expect the basicity of tertiary amines to be the greatest, followed by secondary amine and then primary amine.

But this is not so. The order of basicity is

NH3< primary amine ~ tertiary amine < secondary amine

This “unexpected” order of basicity can be understood from the below stated reasons:

Steric hindrance

The size of an alkyl group is more than that of a hydrogen atom. So, an alkyl group would hinder the attack of a hydrogen atom, thus decreasing the basicity of the molecule. So, the more the number of alkyl groups attached, lesser will be its basicity.

So, the decreasing order of basicity will be:

Primary amine > secondary amine > tertiary amine

Solvation of ions

When amines are dissolved in water, they form protonated amines. Also, the number of possibilities of hydrogen bonding also increases. More the number of hydrogen bonding, more is the hydration that is released in the process of the formation of hydrogen bonds.

The more the hydration energy of the molecule, more is the stability of the amine. Experiments have indicated, in terms of hydration energy, that the tertiary amines are the least stable followed by the secondary amines followed by the primary amines.

This can be explained on the basis of the following reactions.

In primary,

RNH2 + H2O $\to$$\rightarrow$ RNH3+ + OH-

In secondary,

R2NH + H2O  $\to$$\rightarrow$ R2NH2+ + OH-

In tertiary,

R3N + H2$\to$$\rightarrow$ R2NH+ + OH-

So, the number of hydrogen bonds possible when primary amines are dissolved in water is the greatest, implying that they are most stable species of amine, the least being the tertiary amines.

The combined effect of the pushing effect of the alkyl group (+I effect), steric hindrance and the salvation of amines causes the basicity order to be: (basicity of tertiary is almost the same as that of primary).

NH3< primary amine ~ tertiary amine < secondary amine

Question: Direct nitration of aniline is not carried out. Explain why? What is done instead?

Question: Give one chemical test to distinguish between $\ce{CH3CH2NH2}$ and $\ce{C6H5NH2}$

#### Carbylamine Reaction¶

A test for detection of primary amines. In this reaction, the analyte is heated with alcoholic potassium hydroxide and chloroform. If a primary amine is present, the isocyanide (carbylamine) is formed which are foul smelling substances.

For example, the reaction with ethylamine:

What is the common name of $\ce{CHCl3}$ ?

What is the difference between cyanide and isocyanide?