Unit 11: Amines and Amino Acids
Exploring nitrogen-containing organic compounds and the building blocks of proteins.
11.15 Introduction to Amines (Classification, Naming)
Amines are organic derivatives of ammonia ($NH_3$) where one or more hydrogen atoms have been replaced by an alkyl or aryl group. They contain the amino (-$NH_2$, -NHR, or -$NR_2$) functional group.
Classification of Amines:
Amines are classified based on the number of alkyl/aryl groups attached to the nitrogen atom.
- Primary ($1^\circ$) Amines: One alkyl group is attached to the nitrogen atom (e.g., $CH_3NH_2$).
- Secondary ($2^\circ$) Amines: Two alkyl groups are attached to the nitrogen atom (e.g., $(CH_3)_2NH$).
- Tertiary ($3^\circ$) Amines: Three alkyl groups are attached to the nitrogen atom (e.g., $(CH_3)_3N$).
Naming Amines:
For simple primary amines, the alkyl group name is followed by the suffix -amine (e.g., ethylamine). For more complex amines, the -$NH_2$ group is named as a substituent, using the prefix amino- with a number to indicate its position (e.g., 2-aminopropane).
Solved Examples:
- What is an amine?
Solution: An organic derivative of ammonia where one or more hydrogens are replaced by alkyl or aryl groups. - Classify ethylamine ($CH_3CH_2NH_2$).
Solution: It is a primary amine because the nitrogen is bonded to one alkyl group. - Classify trimethylamine
($(CH_3)_3N$).
Solution: It is a tertiary amine because the nitrogen is bonded to three alkyl groups. - What is the IUPAC name for
$CH_3CH(NH_2)CH_3$?
Solution: The longest chain is 3 carbons (propane) and the amino group is on carbon 2. The name is 2-aminopropane. - Draw the structure of diethylamine.
Solution: A nitrogen atom bonded to two ethyl groups and one hydrogen atom: $(CH_3CH_2)_2NH$. - What is the functional group in an
amine?
Solution: The amino group. - How many hydrogen atoms are bonded to the nitrogen in a secondary
amine?
Solution: One. - Name the amine $CH_3CH_2CH_2NH_2$.
Solution: Propylamine (or 1-aminopropane). - What is the classification of the amine
$(CH_3CH_2)_2NCH_3$?
Solution: It is a tertiary amine. - What is the difference between an amine and an
amide?
Solution: An amine has a nitrogen atom bonded to alkyl/aryl groups. An amide has a nitrogen atom bonded to a carbonyl group (C=O).
11.16 Physical & Basic Properties of Amines
Physical Properties
Similar to alcohols, primary and secondary amines can form hydrogen bonds with each other due to the polar N-H bond. This gives them higher boiling points than alkanes of similar size, but generally lower than corresponding alcohols because the N-H bond is less polar than the O-H bond. Short-chain amines are soluble in water.
Basic Properties
The lone pair of electrons on the nitrogen atom allows amines to act as weak bases, similar to ammonia. They are Brønsted-Lowry bases because they can accept a proton.
- Reaction with Water: They establish an equilibrium in water, producing
hydroxide ions.
$CH_3NH_2(aq) + H_2O(l) \rightleftharpoons CH_3NH_3^+(aq) + OH^-(aq)$ - Reaction with Acids: They react with acids in a neutralisation reaction
to form an alkylammonium salt.
$CH_3CH_2NH_2(aq) + HCl(aq) \rightarrow CH_3CH_2NH_3^+Cl^-(aq)$ (ethylammonium chloride)
Solved Examples:
- Why do primary amines have relatively high boiling
points?
Solution: Because they can form hydrogen bonds between molecules. - What feature of an amine molecule makes it a
base?
Solution: The lone pair of electrons on the nitrogen atom. - Write an equation for the reaction of ethylamine with hydrochloric
acid.
Solution: $CH_3CH_2NH_2 + HCl \rightarrow CH_3CH_2NH_3^+Cl^-$. - Is a solution of methylamine in water acidic, basic, or
neutral?
Solution: Basic (alkaline), because the amine accepts a proton from water, leaving excess $OH^-$ ions. - Which would have a higher boiling point: ethane or
methylamine?
Solution: Methylamine, due to hydrogen bonding. - What is the name of the salt formed when propylamine reacts with nitric
acid?
Solution: Propylammonium nitrate. - Can tertiary amines form hydrogen bonds with each
other?
Solution: No, because they do not have a hydrogen atom bonded directly to the nitrogen. However, they can accept hydrogen bonds from water. - Are amines Brønsted-Lowry acids or
bases?
Solution: They are Brønsted-Lowry bases because they are proton acceptors. - What is the ion formed when methylamine accepts a
proton?
Solution: The methylammonium ion, $CH_3NH_3^+$. - Why are short-chain amines soluble in
water?
Solution: Because the amino group can form hydrogen bonds with water molecules.
11.17 Amino Acids (Structure, Bifunctional Nature)
Amino acids are the building blocks of proteins. They are bifunctional molecules, meaning they contain two different functional groups:
- An amino group (-$NH_2$), which is basic.
- A carboxyl group (-COOH), which is acidic.
In the 20 common naturally occurring amino acids, both of these groups are attached to the same carbon atom, known as the alpha-carbon. The alpha-carbon is also attached to a hydrogen atom and a variable side chain, denoted by 'R'.
Because they possess both an acidic group and a basic group, amino acids can react as either an acid or a base. This property is described as amphoteric.
Solved Examples:
- What are the two functional groups present in every amino
acid?
Solution: An amino group (-$NH_2$) and a carboxyl group (-COOH). - What are amino acids the monomers of?
Solution: Proteins. - What does it mean for an amino acid to be
"bifunctional"?
Solution: It means it has two different functional groups. - What term describes a substance that can act as both an acid and a
base?
Solution: Amphoteric. - Draw the general structure of an alpha-amino
acid.
Solution: A central carbon atom bonded to an -$NH_2$ group, a -COOH group, a -H atom, and an -R group. - What part of the amino acid structure is
basic?
Solution: The amino group (-$NH_2$). - What part of the amino acid structure is
acidic?
Solution: The carboxyl group (-COOH). - What is the "R group" in an amino
acid?
Solution: It is the variable side chain that is different for each of the 20 common amino acids. - What is the simplest amino acid, where the R group is just a hydrogen
atom?
Solution: Glycine. - Why are amino acids considered
amphoteric?
Solution: Because they contain an acidic group (-COOH) that can donate a proton and a basic group (-$NH_2$) that can accept a proton.
11.18 Reactions of Amino Acids & Dipeptide Formation
Due to their bifunctional nature, amino acids can undergo reactions characteristic of both amines and carboxylic acids.
- As an acid: The carboxyl group can react with bases.
$H_2N-CH_2-COOH + NaOH \rightarrow H_2N-CH_2-COO^-Na^+ + H_2O$ - As a base: The amino group can react with acids.
$H_2N-CH_2-COOH + HCl \rightarrow Cl^-H_3N^+-CH_2-COOH$
Dipeptide Formation
The most important reaction of amino acids is the condensation reaction between two amino acid molecules. The carboxyl group of one amino acid reacts with the amino group of another, eliminating a molecule of water and forming a new covalent bond.
The resulting molecule is a dipeptide, and the newly formed bond (-CONH-) is called an amide link or, in a biological context, a peptide bond. This process can continue, adding more amino acids to form a long chain called a polypeptide, which is the basis of a protein.
Solved Examples:
- What is a peptide bond?
Solution: It is an amide link (-CONH-) formed between two amino acid molecules. - What type of reaction forms a peptide
bond?
Solution: A condensation reaction. - What small molecule is eliminated when two amino acids form a
dipeptide?
Solution: Water. - Show how the amino acid glycine ($H_2N-CH_2-COOH$) reacts with a strong acid
like $HCl$.
Solution: The basic amino group accepts a proton: $H_2N-CH_2-COOH + HCl \rightarrow [H_3N-CH_2-COOH]^+Cl^-$. - Show how glycine reacts with a strong base like
$NaOH$.
Solution: The acidic carboxyl group donates a proton: $H_2N-CH_2-COOH + NaOH \rightarrow [H_2N-CH_2-COO]^-Na^+ + H_2O$. - What is a dipeptide?
Solution: A molecule formed from the condensation of two amino acids. - What is a polypeptide?
Solution: A long chain polymer made of many amino acids linked by peptide bonds. - Which functional groups are involved in forming a peptide
bond?
Solution: The carboxyl group of one amino acid and the amino group of another. - What is the repeating backbone of a polypeptide
chain?
Solution: A repeating sequence of -N-C-C- atoms. - Can an amino acid react with an alcohol under acidic
conditions?
Solution: Yes, the carboxyl group can undergo esterification to form an ester.