Structure of Amine
Consider that nitrogen has five valence electrons and is trivalent with a lone pair.
According to the VSEPR theory, the nitrogen found in amines is sp3 hybridized. Due to the existence of a lone pair, it is pyramidal in form rather than tetrahedral in nature, which is the general configuration of most sp3 hybridized molecules.
Each of the three sp3 hybridized nitrogen orbitals overlaps with hydrogen or carbon orbitals, depending on amine's configuration. Owing to the existence of a lone pair, the angle of C-N-H in amines is less than 109 degrees, a characteristic angle of tetrahedral geometry. The angle of the amines is near to 108 degrees.
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Types of Amines
Amines can be classified into four types based on how the hydrogen atoms are replaced by an ammonia molecule.
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Primary Amines
If one of the hydrogen atoms in an ammonia molecule is substituted by an alkyl or aryl group, it is a primary amine.
Example: Aniline C₆H₅NH₂, Methylamine CH₃NH₂
Secondary Amines
Two organic substitutes are used to remove the hydrogen atoms in the ammonia molecule that forms an amine.
Example: Diphenylamine (C₆H₅)2NH, Dimethylamine (CH₃)2NH
Tertiary Amines
When all the three hydrogen atoms are substituted by an organic substitution, it may be an aryl or an aromatic group.
Example: Ethylenediaminetetraacetic acid (EDTA), Trimethylamine N(CH₃)3
Cyclic Amines
Cyclic amines are the ones in which the nitrogen has been incorporated into a ring structure by making it either a secondary or a tertiary amine effectively.
Example: A three membered ring aziridine, six membered ring piperidine
Basicity of Amines
Like ammonia, the primary and secondary amines have protic hydrogens and hence display a degree of acidity. While tertiary amines do not have protein hydrogen and thus do not have a degree of acidity.
The pKa value for both primary & secondary amines is around 38, which makes them a very weak acid. Whereas if we take the pKb, it's around 4. It makes amines even more essential than acidic ones. It makes amines even more essential than acidic ones. Thus an aqueous solution of amine is strongly alkaline.
What are Aliphatic Amines?
An amine in which there are no aromatic rings directly attached on the nitrogen atom is referred to as Aliphatic amine.
One of the examples of the Aliphatic amine type is listed below.
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Preparation of Amines
Now, let us look at the preparation of amines from halogenoalkanes (which can also be called either haloalkanes or alkyl halides) and from nitriles.
It deals with only amines where the functional group is not connected directly to the benzene ring. Aroma amines, such as phenylamine (aniline), are usually developed differently.
Preparation of Amines From Halogenoalkanes
Firstly, the halogenoalkane is heated with a concentrated solution of ammonia in ethanol. This entire reaction is carried out in a sealed tube. We may not heat this mixture under reflux, because the ammonia simply would escape up the condenser as a gas.
We also can think about the reaction using 1-bromoethane as a typical halogenoalkane.
Now, we get a mixture of amines formed together with their salts. These reactions occur one after another.
Preparation of a Primary Amine
The reaction is going to happen in two steps. A salt is formed in the first stage which is known as ethylammoniam bromide. It's just like an ammonium bromide, only that one of the hydrogens in the ammonium ion is substituted by an ethyl group.
CH₃CH₂Br + NH₃ ⟶ CH₃CH₂NH₃ + Br⁻
There is also a possibility of a reversible reaction in the mixture of this salt and excess ammonia.
CH₃CH₂NH₃ + Br⁻ + NH₃ ⇋ CH₃CH₂NH₂ + NH₄ + Br⁻
Ammonia removes a hydrogen ion from the ethylammonium ion, to leave a primary amine (ethylamine).
The more amount of ammonia there in the mixture, the more the forward reaction highly favours.
Preparation of a Secondary Amine
The reaction will not end at the primary amine. Also, ethylamine reacts with bromoethane in the same two stages like before.
At the first stage, you get a salt formed, a diethylammonium bromide. Consider this as ammonium bromide with two hydrogens that have been substituted by ethyl groups.
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Again, there is the possibility of a reversible reaction between this salt and surplus ammonia in the mixture.
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Ammonia removes the hydrogen ion from the dimethyl ammonium ion, to leave a secondary amine, which is diethylamine. A secondary amine is one that has two groups of alkyl attached to the nitrogen.
Preparation of a Tertiary Amine
Still, it's yet to finish! Diethylamine also reacts with bromoethane in the same two steps as before.
You get triethylammonium bromide salt in the first stage.
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Again, there is the risk of a reversible reaction between the excess ammonia and this salt in the mixture.
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Ammonia removes a hydrogen ion from the triethylammonium ion, to leave a tertiary amine, called triethylamine. Tertiary amine is the one having three alkyl groups attached to nitrogen.
Preparation of a Quaternary Ammonia Salt
This is the final stage! Triethylamine reacts with bromoethane creating tetraethylammonium bromide, a quaternary ammonium salt (one of all four hydrogens have been substituted by alkyl groups).
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There is no hydrogen remaining on the nitrogen to be added this time. Here, the reaction stops.
Important Facts or Uses of Amines
Amines perform a significant part in the survival of life – they are actively involved in the formation of amino acids, the proteins building blocks of human creatures. Many vitamins are also made from amino acids.
Serotonin is an important amine functioning as one of the primary brain's neurotransmitters. This regulates symptoms of hunger and is vital to the level at which generally the brain works. Also, it affects the state of happiness and helps to regulate the sleep and walk cycles of the brain.
FAQs on Amines
1. What defines an amine in organic chemistry?
An amine is an organic compound and a functional group that contains a basic nitrogen atom with a lone pair of electrons. Amines are considered derivatives of ammonia (NH₃), where one or more hydrogen atoms have been replaced by a substituent such as an alkyl or aryl group.
2. What are the different types of amines and how are they classified?
Amines are classified based on the number of hydrogen atoms in ammonia that are replaced by alkyl or aryl groups. The main types are:
- Primary (1°) Amines: One hydrogen atom is replaced by an organic substituent (e.g., CH₃NH₂, Methylamine).
- Secondary (2°) Amines: Two hydrogen atoms are replaced by organic substituents (e.g., (CH₃)₂NH, Dimethylamine).
- Tertiary (3°) Amines: All three hydrogen atoms are replaced by organic substituents (e.g., (CH₃)₃N, Trimethylamine).
- Quaternary Ammonium Salts: A fourth organic substituent is attached to the nitrogen, giving it a positive charge.
3. What is the general structure and formula of an amine?
The structure of simple amines is a trigonal pyramidal geometry, similar to ammonia, with the nitrogen atom at the apex. The nitrogen atom is sp³ hybridized. The general formulas are:
- Primary amine: R-NH₂
- Secondary amine: R₂-NH
- Tertiary amine: R₃-N
Here, 'R' represents an alkyl or aryl group.
4. Can you provide some common examples of amines?
Certainly. Some common and important examples of amines include:
- Methylamine (CH₃NH₂): A simple primary aliphatic amine.
- Aniline (C₆H₅NH₂): The simplest primary aromatic amine, widely used in the manufacturing of dyes.
- Dimethylamine ((CH₃)₂NH): A common secondary amine.
- Pyridine: A heterocyclic amine where the nitrogen is part of an aromatic ring structure.
- Amphetamine: A pharmaceutical drug that contains a primary amine functional group.
5. What are some important uses of amines in daily life and industry?
Amines are crucial in many applications. They are fundamental in the production of dyes, pharmaceuticals (such as sulpha drugs and anaesthetics like novocaine), and polymers (like nylon and polyurethane). They are also used as corrosion inhibitors, in water purification, and are the building blocks of essential biological molecules like amino acids, vitamins, and hormones.
6. Why are aliphatic amines generally more basic than aromatic amines?
This difference in basicity stems from the availability of the nitrogen's lone pair of electrons. In aliphatic amines, attached alkyl groups (-R) are electron-donating (+I effect), which pushes electron density onto the nitrogen, making its lone pair more available to accept a proton. In contrast, in aromatic amines like aniline, the lone pair on the nitrogen is delocalized into the benzene ring through resonance. This makes the electron pair less available for donation, significantly reducing the amine's basicity.
7. How does the structure of an amine (primary, secondary, tertiary) affect its boiling point?
The boiling points of amines are primarily dictated by their ability to form intermolecular hydrogen bonds.
- Primary (1°) and secondary (2°) amines possess N-H bonds, enabling them to form hydrogen bonds with each other. This requires more energy to overcome, resulting in higher boiling points.
- Tertiary (3°) amines lack N-H bonds, so they cannot form hydrogen bonds among themselves. Consequently, their boiling points are significantly lower than primary and secondary amines of similar molecular mass.
Therefore, for isomeric amines, the general order of boiling points is: Primary > Secondary > Tertiary.
8. How can you distinguish between primary, secondary, and tertiary amines in a laboratory?
The most common laboratory test for distinguishing between amine types is the Hinsberg's test. This test uses benzenesulphonyl chloride (Hinsberg's reagent) in the presence of an alkali.
- A primary amine reacts to form a sulfonamide which is soluble in alkali (like NaOH) because it still has an acidic hydrogen on the nitrogen.
- A secondary amine reacts to form a sulfonamide that is insoluble in alkali as it lacks an acidic hydrogen.
- A tertiary amine does not react with Hinsberg's reagent at all.
9. What is a diazonium salt and why is it so important in organic synthesis?
A diazonium salt is an organic compound with the general formula R-N₂⁺X⁻, where R is typically an aryl group and X is an anion like Cl⁻ or HSO₄⁻. Its immense importance in synthesis comes from the fact that the diazonium group (N₂⁺) is an excellent leaving group, departing as stable nitrogen gas (N₂). This allows the N₂⁺ group to be easily substituted by a wide variety of other functional groups (e.g., -OH, -CN, -X, -H), making it a versatile intermediate for producing a vast range of substituted aromatic compounds.
10. Why are aromatic diazonium salts more stable than aliphatic ones?
The enhanced stability of aromatic diazonium salts is due to resonance. The positive charge of the diazonium ion is delocalized over the entire benzene ring, which stabilizes the cation. This resonance stabilization does not exist in aliphatic diazonium salts. Consequently, aliphatic diazonium salts are extremely unstable and decompose instantly to form a carbocation and N₂ gas, whereas aromatic salts can be isolated at low temperatures (0-5 °C) for use in further chemical reactions.
11. What makes the Gabriel phthalimide synthesis a preferred method for preparing primary amines?
The Gabriel phthalimide synthesis is highly valued because it produces pure primary amines without the formation of secondary or tertiary amine by-products. The key is that the phthalimide anion, used as the nucleophile, can only be alkylated once due to its structure and steric hindrance. This prevents the over-alkylation that often occurs in other methods like the ammonolysis of alkyl halides. The final hydrolysis step cleanly yields only the desired primary amine, ensuring a high-purity product.
