Carbylamine Reaction Mechanism

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What is the Mechanism of Carbylamine reaction Class 12?

Carbylamine reaction mechanism is the synthesis of the addition of amines to the intermediate. Here, the resultant intermediate is formed by dehydrohalogenation of the chloroform molecule. This intermediate is called dichlorocarbene. Also, the carbylamine reaction is referred to as the synthesis of Hofmann isocyanide, where it is the reaction of chloroform, primary amine,  and a base to synthesize these isocyanides. 

Dichlorocarbene intermediate is quite important for this type of conversion. We cannot use this carbylamine reaction to synthesize isocyanides from the secondary or tertiary amines. 

Note: In general, the carbylamine reaction can be represented as follows.

R-NH2 + CHCl3 + 3KOH → RNC + 3KCl + 3H2O

Let us discuss a few examples of carbylamine reactions.

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Hofmann’s Isocyanide Test

Because the carbylamine reaction is only effective for primary amines, we can use it as a chemical test for the presence of primary amines. When used as a test, the carbylamine reaction is also known as Hofmann's isocyanide test. Here, the test substance is heated with alcoholic potassium hydroxide and chloroform. 

In the presence of a primary amine case, there will take place an isocyanide (carbylamine) formation, which can be identified easily by its extremely foul smell. This Hofmann isocyanide test does not emit a foul odor either with the secondary or tertiary amines because they do not undergo a carbylamine reaction.

Mechanism of the Carbylamine Reaction

The first step is given as the dehydrohalogenation (hydrogen halide removal from a given substrate) of chloroform to produce dichlorocarbene intermediate, which is very reactive. The electrophilic dichlorocarbene attacks the primary amine's nucleophilic nitrogen. The elimination of hydrochloric acid leads to an isonitrile formation. A representation of the mechanism of carbylamine reaction is given below.

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The carbylamine reaction is used for isocyanides synthesizing from primary amines by using chloroform and a base. Also, the carbylamine reaction is employed to test the primary amine's presence in a given substrate.

Why does the Carbylamine reaction not Work in Secondary and Tertiary Amines?

As we all know, the carbylamine reaction is used to detect the chloroform and primary amine. This carbylamine test is used to distinguish primary amine from both secondary and tertiary amines. 

As per the mechanism, this is one of the best examples of Alpha elimination reaction, where the carbanion is formed in the very first step, which later loses the chloride Ion to form highly reactive dichlorocarbene. Then, the remaining two chlorine atoms will be removed in the form of HCL. For that, hydrogen is given from the nitrogen in amine.

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Concerning secondary amine and tertiary amine, nitrogen does not hold more hydrogen atoms, and also, there expected a sterically highly unstable product if the secondary amines and tertiary amines would be reacted. And notably, alkyl isocyanides are the compounds that exhibit unpleasant smelling. Hence, we use this test for the detection of only the primary amines.

Why is the Secondary Amine more basic than that of the Primary and Tertiary Amines?

There are some reasons why the secondary amine is the most basic one compared to both primary and secondary ones. Let us discuss in detail about this.

Amines are the derivatives of ammonia, where hydrocarbon groups replace one, two, or all the three hydrogen atoms.

When hydrogen is substituted by one group, it is known as the primary amine [For example, methylamine (CH3-NH2)]; by two groups, it is known as the secondary amine [For example, Dimethylamine (CH3-NH-CH3)]; and by three groups, it is known to as a tertiary amine [For example, trimethylamine is CH3-N(CH3)-CH3].

Amines are basic in nature due to the reason they hold a lone pair of electrons on nitrogen. Thus, they have a strong tendency to donate the lone pair of electrons to the electron acceptors.

Basicity of Amines factors

The basicity of amines depends on the below given factors.

  • The solvation degree of the protonated amine, that includes the steric hindrance by the nitrogen groups.

  • The substituents' electronic properties (the alkyl groups enhance basicity, whereas the aryl groups diminish it).

Because the primary and secondary amine's ammonium salts undergo solvation effects (because of the hydrogen bonding) to a much greater degree compared to the ammonium salts of tertiary amines in the water, the solvation effects increase the density of electrons on the amine nitrogen to a greater degree compared to that of the inductive effect of alkyl groups. 

Although the primary amines' conjugate acid is more stable with the greatest number of hydrogen bonding, its basicity is less stable due to its low electron density on the nitrogen atom. In contrast, its lone pair of electrons are not available for protonation readily.

Thus, it is said that the Secondary amine is more basic compared to the primary and tertiary because it is more stable than both [it means, the stability of the conjugate acid that is formed with it], contains high electron density, and the easy availability of electrons for protonation, that balances in the two groups, which are attached to it.

FAQ (Frequently Asked Questions)

1. What is Carbylamine Reaction?

Ans: Primary amine (both aromatic and aliphatic), when warmed with alcoholic and chloroform KOH, produces isocyanides (otherwise called carbylamines). This is known as the carbylamine reaction. Carbylamines produces an offensive smell. This reaction is only given by the primary amine and distinguishes the primary amine from the other amine classes.

R-NH2 + CHCl3 + 3KOH → RNC (which is, carbylamine) + 3KCl + 3H2O

2. Why are most Chemical reactions reversible?

Ans: If a reaction attains equilibrium with the products, reactants, and released energy, it is called reversible. If the same conditions are impractical, it cannot be reversed unless the outside conditions are imposed. 

Reversibility does not mean the original conditions will be restored magically. At equilibrium, a reaction happens in both directions at equal rates, and the equilibrium can be perturbed by changing any of the factors involved in the reaction: changing the temperature, changing a concentration, or adding additional reactants.

3. Explain the Factors that determine the Rate of reaction?

Ans: Mainly, there exist 5 factors that affect the rate of reaction, which are listed below.

  • Pressure

  • Temperature

  • Concentration of mixture

  • Presence of catalyst

  • The surface area of the mixture molecules

4. List some important Mechanisms of Organic Chemistry?

Ans: Some of the named reactions are listed below:

  • Diazotisation,

  • Demjanov rearrangement,

  • Kucherov reaction,

  • Hydroboration oxidation,

  • Oxymercuration demarcation.