Amines and Amino Acids

Amines are nitrogen-containing organic compounds. In simple primary amines, the characteristic functional group can be represented as the amino group, –NH₂.

Examples include:

Methylamine: CH₃NH₂
Ethylamine: CH₃CH₂NH₂
Propylamine: CH₃CH₂CH₂NH₂

Amino acids are especially important biological molecules because they contain both an amino group and a carboxyl group. Amino acids can join together to form proteins, which are essential components of living organisms.

A simplified amino acid structure is:

H₂N–CH(R)–COOH

where R represents a side chain that differs among amino acids.

The Amino Functional Group

The functional group emphasized in simple primary amines is:

–NH₂

This is called the amino group.

It contains:

• one nitrogen atom
• two hydrogen atoms

For example:

CH₃CH₂NH₂

contains an amino group attached to a two-carbon chain.

This compound is:

ethylamine

The amino group gives amines characteristic chemical properties that are very different from those of hydrocarbons, alcohols, and carboxylic acids.

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What Are Amines?

Amines are organic compounds related to ammonia, NH₃.

Ammonia:

NH₃

If one hydrogen is replaced by a carbon-containing group, a primary amine is produced.

For example:

NH₃ → CH₃NH₂

The product is methylamine.

More broadly, amines can contain one, two, or three carbon groups attached to nitrogen, but at this level the simplest primary amines containing –NH₂ are the main focus.

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Recognizing a Simple Amine

Consider:

CH₃CH₂CH₂NH₂

Look for:

–NH₂

It is present.

Therefore, the molecule is a:

primary amine

Compare this with:

CH₃CH₂CH₃

This contains only carbon and hydrogen.

Therefore, it is a:

hydrocarbon

Adding an amino group changes the chemical behaviour of the molecule significantly.

Naming Simple Amines

One common introductory naming method uses the carbon group followed by:

amine

For example:

CH₃NH₂ → methylamine

CH₃CH₂NH₂ → ethylamine

CH₃CH₂CH₂NH₂ → propylamine

Systematic IUPAC names such as methanamine, ethanamine, and propan-1-amine may also be encountered.

The important recognition skill is identifying the nitrogen-containing amino group.

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Amines Are Basic

Many amines behave as weak bases.

A base can accept:

H⁺

Amines have a lone pair of electrons on the nitrogen atom. This allows the nitrogen to accept a hydrogen ion.

A simplified reaction is:

RNH₂ + H⁺ → RNH₃⁺

For example:

CH₃NH₂ + H⁺ → CH₃NH₃⁺

This ability to accept H⁺ explains the basic behaviour of amines.

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Amines in Water

Amines can react with water to a limited extent.

A simplified representation is:

RNH₂ + H₂O ⇌ RNH₃⁺ + OH⁻

The formation of OH⁻ makes the solution:

basic

Therefore, an aqueous solution of a simple amine typically has:

pH > 7

Because the reaction is incomplete, many common amines are described as weak bases.

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Weak Base Does Not Mean Dilute

Just as with weak acids:

weak and dilute do not mean the same thing.

A weak base only partially reacts with water or accepts H⁺ to a limited extent.

A dilute solution contains a relatively small amount of dissolved substance per unit volume.

Therefore, a concentrated solution of a weak base can still be hazardous.

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Amines React with Acids

Because amines are bases, they can react with acids.

For example:

methylamine + hydrochloric acid → methylammonium chloride

A simplified equation is:

CH₃NH₂ + HCl → CH₃NH₃Cl

The amine accepts H⁺ and forms a positively charged ammonium-type ion.

This demonstrates the acid-base behaviour of amines.

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Physical Properties of Amines

The properties of amines depend on:

• molecular size
• structure
• number of amino groups
• intermolecular forces

Many small amines are:

• soluble in water
• volatile
• basic
• strong-smelling

Some low-molecular-mass amines have odours often described as:

• fishy
• ammonia-like
• unpleasant

As molecular size increases, water solubility generally decreases because the non-polar carbon portion becomes more significant.

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Hydrogen Bonding in Primary Amines

Primary amines contain:

N–H bonds

They can therefore participate in hydrogen bonding.

Nitrogen is less electronegative than oxygen, so the hydrogen bonding in amines differs in strength from that in alcohols.

Nevertheless, hydrogen bonding affects properties such as:

• boiling point
• solubility
• interactions with water

This helps explain why small amines can dissolve reasonably well in water.

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Comparing Amines and Alcohols

Consider:

ethylamine: CH₃CH₂NH₂

and:

ethanol: CH₃CH₂OH

Both contain a two-carbon chain.

However:

Ethylamine contains:

–NH₂

Ethanol contains:

–OH

Their different functional groups give them different chemical behaviour.

Most importantly:

ethylamine is basic

while:

ethanol is not normally considered basic in the same way.

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Comparing Functional Groups

You have now encountered several important organic functional groups:

Recognizing these structures allows you to predict many properties and reactions.

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What Are Amino Acids?

Amino acids are organic compounds containing both:

an amino group, –NH₂

and:

a carboxyl group, –COOH

These two functional groups give amino acids some unusual and extremely important properties.

A general amino acid structure can be represented as:

H₂N–CH(R)–COOH

The central carbon is attached to:

• an amino group
• a carboxyl group
• a hydrogen atom
• an R group

The R group differs among amino acids.

The Central Carbon

Most amino acids used to build proteins have a central carbon atom called the:

alpha carbon

This carbon is bonded to four groups:

–NH₂

–COOH

–H

and:

–R

The R group is particularly important because it determines the identity and many properties of the amino acid.

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The R Group

The:

R group

is also called the:

side chain

Different amino acids have different side chains.

These side chains can differ in:

• size
• shape
• charge
• polarity
• ability to form bonds
• chemical reactivity

As a result, different amino acids behave differently inside proteins.

This diversity allows proteins to form an enormous range of three-dimensional structures.

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Glycine

Glycine is the simplest amino acid.

Its R group is:

H

A simplified structure is:

H₂N–CH₂–COOH

Glycine contains:

• an amino group
• a carboxyl group

It is one of the amino acids that can be incorporated into proteins.

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Alanine

Alanine has the structure:

H₂N–CH(CH₃)–COOH

Its R group is:

–CH₃

Compare:

Glycine: R = H

Alanine: R = CH₃

The difference appears small, but changing the side chain changes the amino acid's properties.

Different Amino Acids

Proteins are commonly constructed from 20 standard amino acids encoded by the genetic code.

Examples include:

• glycine
• alanine
• valine
• leucine
• serine
• cysteine
• lysine
• glutamic acid
• phenylalanine
• tryptophan

They share the basic amino-acid framework but contain different R groups.

These different side chains give amino acids different chemical characteristics.

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Amino Acids Can Behave as Acids and Bases

Amino acids contain both:

–NH₂

and:

–COOH

The amino group can behave as a base.

The carboxyl group can behave as an acid.

Therefore, amino acids can show both:

acidic and basic behaviour

A substance capable of behaving as both an acid and a base is described as:

amphoteric

This is an important property of amino acids.

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Zwitterions

In many conditions, especially in solid form and near neutral aqueous conditions, amino acids can exist largely as zwitterions.

A proton can transfer from the carboxyl group to the amino group.

Instead of writing:

H₂N–CH(R)–COOH

we can represent the zwitterionic form as:

⁺H₃N–CH(R)–COO⁻

The molecule contains:

• a positive charge
• a negative charge

but its overall charge can still be:

zero

This is called a zwitterion.

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Why Zwitterions Matter

The presence of charged groups helps explain several physical properties of amino acids.

Many amino acids:

• are crystalline solids
• have relatively high melting points
• are soluble in water to varying degrees
• have much lower volatility than many small organic molecules

These properties differ greatly from compounds such as small esters or hydrocarbons.

The ionic character of zwitterions creates strong attractions between particles.

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Amino Acids Are the Building Blocks of Proteins

One of the most important roles of amino acids is their ability to join together.

Amino acids can form long chains called:

polypeptides

These chains can fold into:

proteins

Therefore:

amino acids → polypeptides → proteins

Proteins are essential to almost every living organism.

Forming a Peptide Bond

Two amino acids can react together.

The carboxyl group of one amino acid reacts with the amino group of another.

A molecule of water is removed.

A simplified reaction is:

amino acid + amino acid → dipeptide + water

The new bond formed between the amino acids is called a:

peptide bond

The characteristic linkage can be represented as:

–CO–NH–

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Peptide Bond Formation

Suppose we represent two amino acids as:

H₂N–CH(R₁)–COOH

and:

H₂N–CH(R₂)–COOH

They can join to form:

H₂N–CH(R₁)–CO–NH–CH(R₂)–COOH + H₂O

The:

–CO–NH–

linkage is the peptide bond.

Because water is produced, this can be described as a type of condensation reaction.

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Dipeptides

When two amino acids join, the product is called a:

dipeptide

A dipeptide contains:

two amino-acid residues

joined by:

one peptide bond

If another amino acid joins:

tripeptide

can form.

When many amino acids join:

polypeptide

forms.

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From Amino Acids to Proteins

The basic sequence is:

amino acids → peptide bonds → polypeptide chain → folding → functional protein

A protein may contain:

• dozens
• hundreds
• or even thousands

of amino-acid residues.

The exact sequence of amino acids is extremely important.

Changing the sequence can change how the protein folds and therefore how it functions.

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Protein Structure

A polypeptide chain does not usually remain as a simple straight chain.

Interactions between different parts of the molecule cause it to fold.

The final shape of a protein depends partly on interactions between amino-acid side chains.

These interactions can include:

• hydrogen bonding
• ionic attractions
• hydrophobic interactions
• disulfide bonds

The resulting three-dimensional shape is closely connected to the protein's function.

Why Protein Shape Matters

Proteins perform highly specific jobs.

Their shape allows them to interact with particular molecules.

For example, an enzyme may have an active site whose shape allows particular substrate molecules to bind.

If protein shape changes significantly, its function can also change.

This is why:

amino-acid sequence → protein structure → protein function

is such an important relationship in biology.

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Functions of Proteins

Proteins perform an enormous range of functions.

Examples include:

Enzymes

Enzymes catalyse biochemical reactions.

Examples include digestive enzymes such as amylase and proteases.

Structural Proteins

Examples include:

• collagen
• keratin

They provide strength and support.

Transport Proteins

Hemoglobin helps transport oxygen in blood.

Antibodies

Antibodies are proteins involved in immune defence.

Hormones

Some hormones, such as insulin, are proteins or peptides.

Movement

Proteins such as actin and myosin contribute to muscle contraction.

Receptors

Many cell receptors are proteins that allow cells to detect chemical signals.

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Proteins in Food

Dietary protein provides amino acids that the body can use.

Protein-rich foods include:

• meat
• fish
• eggs
• dairy products
• beans
• lentils
• soy products
• nuts
• seeds

During digestion, proteins are broken down into smaller peptides and amino acids.

The amino acids can then be absorbed and used by cells.

Essential Amino Acids

Humans can manufacture some amino acids from other molecules.

However, some amino acids cannot be produced by the body in sufficient amounts.

These must be obtained through the diet.

They are called:

essential amino acids

For adults, nine amino acids are generally classified as essential.

A varied diet can provide the amino acids needed for protein synthesis.

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Protein Digestion

Proteins in food are too large to be absorbed intact in the same way as individual amino acids.

During digestion, enzymes called:

proteases

break peptide bonds.

A simplified sequence is:

protein → shorter peptides → amino acids

The amino acids can then be absorbed through the small intestine and transported around the body.

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Protein Synthesis

Cells use amino acids to build their own proteins.

The sequence of amino acids in a protein is ultimately determined by genetic information in:

DNA

A simplified flow is:

DNA → RNA → amino-acid sequence → protein

This creates a major connection between:

• chemistry
• genetics
• cell biology

The properties of proteins ultimately depend on the chemistry of their amino acids.

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Amines in Biology

Amines also occur in many biologically important compounds.

Nitrogen-containing organic molecules are found in:

• neurotransmitters
• hormones
• medicines
• natural products

Examples of biologically important molecules containing amine-related functional groups include compounds involved in communication between nerve cells.

This demonstrates why nitrogen chemistry is extremely important in biochemistry and medicine.

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Amines in Medicines

Many pharmaceutical molecules contain amine groups.

The basic properties of amines can influence:

• water solubility
• interaction with biological molecules
• absorption
• how medicines are formulated

Amines can also react with acids to form salts.

Some medicines are therefore manufactured or supplied as amine salts, which can have useful physical properties.

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Comparing Amines and Carboxylic Acids

Amines and carboxylic acids often show opposite acid-base behaviour.

Amine

Can accept H⁺.

Therefore:

basic

Carboxylic Acid

Can donate H⁺.

Therefore:

acidic

An amino acid contains both functional groups.

This explains why amino acids can behave as both acids and bases.

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Comparing the Organic Families

This illustrates why identifying functional groups is so useful.

Once the functional group is recognized, many properties of the molecule can be predicted.

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Worked Example: Identify the Functional Group

Compound:

CH₃CH₂NH₂

Look for:

–NH₂

Functional group:

amino group

Family:

amine

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Worked Example: Amine or Alcohol?

Compound A:

CH₃CH₂OH

contains:

–OH

Therefore:

alcohol

Compound B:

CH₃CH₂NH₂

contains:

–NH₂

Therefore:

amine

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Worked Example: Identify an Amino Acid

Consider:

H₂N–CH₂–COOH

It contains:

–NH₂

and:

–COOH

Therefore, it is an:

amino acid

Specifically, it is:

glycine

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Worked Example: Identify Both Functional Groups

Consider:

H₂N–CH(CH₃)–COOH

Functional group 1:

–NH₂ → amino group

Functional group 2:

–COOH → carboxyl group

Therefore:

amino acid

This particular amino acid is:

alanine

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Worked Example: Predict Acid-Base Behaviour

Consider:

CH₃NH₂

Would you expect the compound to behave as an acid or a base?

The molecule contains an amine group.

The nitrogen can accept H⁺.

Therefore:

CH₃NH₂ behaves as a weak base.

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Worked Example: Forming a Peptide

Two amino acids react.

What type of bond forms?

Peptide bond

What small molecule is produced?

Water

What type of reaction is this?

Condensation reaction

Therefore:

amino acid + amino acid → dipeptide + water

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Amino Acid Sequence Matters

Imagine two polypeptides containing the same amino acids but arranged in different sequences.

Protein A:

A–B–C–D–E

Protein B:

A–C–B–E–D

Although they contain the same types of amino acids, their different sequences can cause different folding patterns.

Different folding can result in:

different shapes → different functions

This is why the order of amino acids is critical.

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Denaturation

Protein structure can be disrupted by conditions such as:

• high temperatures
• extreme pH
• certain chemicals

This process is called:

denaturation

When a protein is denatured, its three-dimensional shape changes.

If the shape is important for function—as it usually is—the protein may stop working properly.

For example, an enzyme's active site may change shape.

Importantly, denaturation does not usually mean that every peptide bond in the protein has been broken.

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Common Mistakes

Confusing –NH₂ and –OH

Amine:

–NH₂

Alcohol:

–OH

Confusing an Amine with an Amino Acid

An amine contains an amine functional group.

An amino acid contains both:

–NH₂ and –COOH

Thinking Amines Are Acids

Many simple amines behave as:

weak bases

Thinking Weak Means Harmless

A weak base can still be concentrated, corrosive, toxic, or otherwise hazardous.

Forgetting the Carboxyl Group in Amino Acids

A typical amino acid contains both:

amino group + carboxyl group

Thinking the R Group Is Always the Same

The R group varies between different amino acids.

Thinking All Amino Acids Have Identical Properties

Different R groups give amino acids different properties.

Confusing Amino Acids with Proteins

Amino acids are the building blocks.

Proteins are large molecules made from amino-acid residues.

Saying Amino Acids Join Directly with No Product

Peptide bond formation releases:

water

Confusing Peptide Bonds with Ester Bonds

Peptide bond:

–CO–NH–

Ester linkage:

–COO–

Thinking Protein Shape Is Unimportant

Protein shape is closely connected to function.

Thinking Denaturation Always Breaks the Protein into Amino Acids

Denaturation mainly changes protein folding and shape. Breaking a protein completely into amino acids requires hydrolysis of peptide bonds.

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Key Terms

Amine — A nitrogen-containing organic compound related to ammonia.

Primary amine — An amine in which nitrogen is attached to one carbon-containing group and retains two hydrogens.

Amino group — The –NH₂ functional group emphasized in simple primary amines and amino acids.

Nitrogen — The element present in the amino functional group.

Ammonia — NH₃, a nitrogen compound closely related structurally to amines.

Base — A substance capable of accepting H⁺ according to the Brønsted–Lowry definition.

Weak base — A base that reacts only partially with water or accepts H⁺ to a limited extent under given conditions.

Lone pair — A pair of electrons not shared in a covalent bond.

Hydrogen bonding — An intermolecular attraction involving hydrogen bonded to an electronegative atom such as nitrogen or oxygen.

Amino acid — An organic molecule containing amino and carboxyl functional groups.

Carboxyl group — The –COOH functional group.

Alpha carbon — The central carbon in the basic structure of most protein-forming amino acids.

R group — The variable side chain that distinguishes one amino acid from another.

Side chain — Another term for the R group.

Glycine — The simplest amino acid, with H as its side chain.

Alanine — An amino acid with CH₃ as its side chain.

Amphoteric — Able to behave as either an acid or a base.

Zwitterion — A molecule containing both positive and negative charges while potentially having zero overall charge.

Peptide bond — The –CO–NH– linkage joining amino-acid residues.

Condensation reaction — A reaction in which molecules join while a small molecule such as water is produced.

Dipeptide — A molecule formed when two amino acids join by a peptide bond.

Polypeptide — A chain containing many amino-acid residues joined by peptide bonds.

Protein — A biological macromolecule consisting of one or more folded polypeptide chains.

Protein synthesis — The biological process by which cells assemble amino acids into proteins according to genetic information.

Essential amino acid — An amino acid that must be obtained in sufficient amounts through the diet because the body cannot synthesize enough of it.

Protease — An enzyme that catalyses the breakdown of proteins or peptides.

Hydrolysis — A reaction involving water that can break chemical bonds such as peptide bonds.

Denaturation — Disruption of a protein's normal three-dimensional structure.

Enzyme — A biological catalyst, usually a protein.

Functional group — An atom or group of atoms responsible for characteristic properties and reactions of an organic compound.

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Key Takeaways

• Simple primary amines contain the –NH₂ amino functional group.
• Amines are nitrogen-containing organic compounds related to ammonia.
• Methylamine, ethylamine, and propylamine are simple examples.
• Many simple amines behave as weak bases.
• The nitrogen atom can accept H⁺.
• Amines can react with acids to form salts.
• Small primary amines can participate in hydrogen bonding.
• Many small amines are soluble in water.
• Some small amines have strong characteristic odours.
• Amino acids contain both an amino group, –NH₂, and a carboxyl group, –COOH.
• A general amino-acid structure can be represented as H₂N–CH(R)–COOH.
• The central carbon is bonded to an amino group, carboxyl group, hydrogen, and R group.
• The R group distinguishes different amino acids.
• Glycine has H as its R group.
• Alanine has CH₃ as its R group.
• Proteins commonly use 20 standard amino acids encoded by the genetic code.
• Amino acids can behave as both acids and bases.
• This behaviour is described as amphoteric.
• Amino acids can exist as zwitterions containing both positive and negative charges.
• Amino acids are the building blocks of proteins.
• Two amino acids can join by a peptide bond.
• Peptide bond formation produces water.
• It is therefore a condensation reaction.
• Two amino acids form a dipeptide.
• Many amino acids form a polypeptide.
• Polypeptide chains can fold into functional proteins.
• Amino-acid sequence influences protein shape.
• Protein shape strongly influences protein function.
• Proteins function as enzymes, antibodies, structural materials, transport molecules, receptors, and more.
• Dietary proteins can be digested into amino acids.
• Essential amino acids must be obtained in sufficient amounts from food.
• Functional groups allow us to connect organic chemistry with biological chemistry.

The most important structural distinction is:

Amine: –NH₂

Carboxylic acid: –COOH

Amino acid: contains both –NH₂ and –COOH

And the key biological sequence is:

amino acids → peptide bonds → polypeptides → folded proteins → biological functions

Check Your Understanding

1. What functional group is characteristic of simple primary amines?

2. What element distinguishes amines from alcohols and hydrocarbons?

3. Identify the functional group in CH₃CH₂NH₂.

4. Name CH₃NH₂.

5. Name CH₃CH₂NH₂.

6. Are simple amines generally acidic or basic?

7. Explain why an amine can accept H⁺.

8. What does it mean to describe an amine as a weak base?

9. Why does "weak base" not mean "dilute base"?

10. What two important functional groups are present in an amino acid?

11. Write the general structure of a simple amino acid.

12. What is the R group?

13. Why are R groups important?

14. What is the R group in glycine?

15. What is the R group in alanine?

16. Explain why amino acids can behave as both acids and bases.

17. What does amphoteric mean?

18. What is a zwitterion?

19. Explain how a zwitterion can contain charges but have zero overall charge.

20. What type of bond joins amino acids together?

21. What small molecule is produced when a peptide bond forms?

22. Why is peptide formation a condensation reaction?

23. What is a dipeptide?

24. What is a polypeptide?

25. Explain the relationship between amino acids, polypeptides, and proteins.

26. Why is the sequence of amino acids in a protein important?

27. Give four biological functions performed by proteins.

28. What are essential amino acids?

29. What happens to dietary proteins during digestion?

30. Challenge: Consider the compounds:

A: CH₃CH₂OH

B: CH₃CH₂NH₂

C: CH₃COOH

D: H₂N–CH₂–COOH

a. Identify the functional group in A.
b. State the organic family of A.
c. Identify the functional group in B.
d. State the organic family of B.
e. Explain why B behaves as a base.
f. Identify the functional group in C.
g. State the organic family of C.
h. Identify both functional groups in D.
i. Explain why D is classified as an amino acid.
j. Name amino acid D.
k. Explain why D can behave as both an acid and a base.
l. Describe the zwitterionic form of D.
m. Explain how two amino acids can join together.
n. Name the bond formed between them.
o. Identify the other product of the reaction.
p. Explain why this is a condensation reaction.
q. Explain how many amino acids can form a polypeptide.
r. Explain why changing the order of amino acids can alter a protein's properties.
s. Explain why protein shape is important for biological function.
t. Describe how this topic connects organic chemistry to biology.