[Chemistry Class Notes] on Carbylamine Reaction Mechanism Pdf for Exam

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.

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.

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 chloroform and a 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.

Concerning secondary amine and tertiary amine, nitrogen does not hold more hydrogen atoms, and also, there is expected a sterically highly unstable product if the secondary amines and tertiary amines would be reacted. And notably, alkyl isocyanides are 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 this in detail.

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

Tips to Improve Scores in Class 12 Chemistry 

Try to study the syllabus thoroughly for better understanding, read the lesson by yourself before attending the lecture so you can concentrate on the topics you could not understand more. Take proper notes during class and make separate brief revision notes from all the sources to read before the exam. If you could not finish the entire syllabus, at least try to study topics with more weightage to save time. While writing chemical equations, take special care when you enter chemical formulas, values, and numbers as any slight change might alter the entire reaction. Remember, Chemistry is a static subject and you can score well by reading it for a short period. Try to mention as many examples as possible for any concept or reaction you explain. Use the symbols in the equation properly. Do not neglect the practicals and labs in school as they give you practical understanding and applied knowledge. Learning from your practicals will increase the retention capacity of what you have learnt as you do the chemical reactions by yourself.