Amino Functional Group | ChemTalk
The amino group is an important functional group for many biological systems. Below we discuss the structure of an amino group, some quick stats on the functional group, reactions with amino groups, and the amino functional group role in peptide bonds.
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What is an Amino Functional Group?
An amino group is a nitrogen atom bonded to two hydrogen atoms. A molecule that contains an amino functional group is called an amine.
Amino groups are sometimes also referred to as amine groups. Amino is a subcategory of amine. Amine is any group that contains nitrogen with a lone pair. Amino group is specifically nitrogen with a lone pair and at least two hydrogens bonded to it. An amino group is also a primary amine. Primary refers to only having one non-hydrogen bond.
Quick Facts on Amino Group
- Structure: Nitrogen atom with single bonds to two hydrogens.
- Formula: R-NH2
- Atomic weight: 16.02
- pH: Generally basic. Depends on R-group. Arylamines are less basic than alkylamines
- pKa: Depends on functional group, generally about 10.
- Ammonia (NH3): 9.26
- Methylamine (CH3NH2): 10.64
Naming Conventions
In IUPAC naming conventions the amino functional group is referred to using ‘-amino’. The group is referred to similarly as other functional groups. That is, the position is labeled with a number followed by ‘-amino’. If there is more than one amino group then a prefix such as ‘di’ may be added to amino.
In some other naming conventions, ‘-amine’ refers to the amino group.
Here are several examples of molecules their name.
Ammonia NH3:
1-aminobutane:
2-amino-2-methylbutane:
Amino Group Reactions
General Reaction Trends
Due to the lone pair on the nitrogen atom, amino groups easily react. They are also very basic, more so than many alcohols.
An NH2 group is an activating group
An amino group can accept H+ to form R-NH3+.
Reactions Forming an Amino Group
Reducing nitriles and amides forms amino groups. This reaction is most commonly done by utilizing LiAlH4. An example of this reaction is below:
In the above reaction, an amide conversion to an amine utilizes a strong base, LiAlH4. This is a common way to perform this reaction.
To prepare amino groups on an aryl group, nitration of an aromatic compound followed by reduction works really well. Platinum or other metal catalysts, such as zinc or iron, catalyze this type reaction.
Named Reactions
Gabriel Amine Synthesis: Used to form a primary amine from alkyl halides.
Hofmann Rearrangement: Forms a primary amino group with the loss of one carbon starting from a carboxylic acid derivative
Curtius Rearrangement: This reaction mechanism is similar to the Hofmass rearrangement but reacts with an acyl azide.
Hoffman Elimination: This reaction converts an amino group to an alkene. The amino group is methylated to make it a better leaving group and then heated with silver oxide to complete the reaction.
Amino Group vs Carboxyl Group
Amino groups and carboxyl groups are the two main functional groups in amino acids. They also have a similar structure in some ways.
Carboxyl and amino groups are often grouped in the mind because they are both critical parts of amino acids. And between two amino acids they react to form a peptide bond. Reactions and characteristics of amino acids are discussed further below and in this article.
Amino Acids
Amino acids are the basic building blocks of proteins. There are 20 common amino acids. Amino acids connect to form peptide bonds through a reaction between the amino group and the carboxyl group. The amino functional group is therefore critical to building all the proteins that makeup life on earth.
Peptide bond formation reacts an amino group with a carboxylic acid to form a peptide bond and water.
To read more on amino acids, see our article on Amino Acids and Proteins!
Example Questions
- Pick out the amino group in this molecule:
In this case, the amino group is the NH2 on the right side of the molecule.
2. Which one of these has an amino functional group?
Look carefully at the molecules above. Only one contains an amino group.
If you picked molecule one on the far left, you got it right! The nitrogen atoms in the other molecules do not have two hydrogen atoms attached to them. The second molecule contains an amine not an amino.
