13.7 Functional Groups

This topic explains functional groups — the atoms or groups of atoms that give organic compounds their characteristic properties.

Definition

An atom or group of atoms that gives a family of organic compounds its characteristic chemical and physical properties is called a functional group.

Simple explanation: A functional group is the "active part" of an organic molecule — it determines how the compound behaves.

Main Idea

Most organic compounds contain elements other than carbon and hydrogen. Most of these compounds are considered hydrocarbon derivatives.

That is, they are essentially hydrocarbons, but instead of one or more hydrogen atoms, they have an additional atom or group of atoms called functional groups.

In many simple molecules, the functional group is attached to an alkyl group.

Simple explanation: Functional groups are the parts of organic molecules that are not just carbon and hydrogen — they give the molecule its special properties.

Core Concept

Why Functional Groups Are Important

Research in organic chemistry is organized around functional groups. Each functional group defines a family of organic compounds.

Although there are millions of organic compounds, there are only a few functional groups. Functional groups therefore facilitate the study of millions of organic compounds.

The concept of functional group is important in organic chemistry for three reasons:

• Basis for naming: Functional group serves as basis for naming organic compounds.
• Classification: Functional group serves to classify organic compounds into different classes. All compounds with the same functional group belong to the same class.
• Site of chemical reactivity: A functional group is a site of chemical reactivity in a molecule.

Simple explanation: Functional groups help us name, classify, and understand how organic compounds react.

Common Functional Groups

Table 13.3 from the book:

Note: R and R' represent alkyl groups.

Detailed Explanation

What Is an Alkyl Group?

An alkyl group is a hydrocarbon group derived from an alkane by removing one hydrogen atom.

• Methyl group: –CH₃ (from methane, CH₄)
• Ethyl group: –C₂H₅ (from ethane, C₂H₆)
• Propyl group: –C₃H₇ (from propane, C₃H₈)

Simple explanation: Alkyl groups are the "R" groups in general formulas — they are hydrocarbon chains.

Functional Groups Containing Carbon, Hydrogen, and Halogens: Haloalkanes

Haloalkanes are characterized by the presence of the halogen atom.

A haloalkane is a compound in which one hydrogen atom of an alkane is substituted by one halogen atom.

Examples:

Simple explanation: Haloalkanes are alkanes with one or more hydrogen atoms replaced by halogen atoms.

Functional Groups Containing Carbon, Hydrogen, and Oxygen

• Characterized by the hydroxyl group (–OH) attached to a hydrocarbon chain.
• General formula: R–OH
• Examples: CH₃CH₂CH₂OH (1-propanol), CH₃CH₂CH₂CH₂OH (1-butanol)

• When an –OH group is attached to a benzene ring, the compound is called a phenol.
• Formula: C₆H₅–OH
• Phenol was the first antiseptic used in an operation theatre.

• Organic compounds that have two alkyl groups attached to the same oxygen atom.
• Contain C–O–C linkage.
• General formula: R–O–R' (where R and R' may be the same or different)
• Examples:
• Dimethyl ether: CH₃–O–CH₃
• Ethyl methyl ether: CH₃–O–CH₂–CH₃
• Diethyl ether: CH₃–CH₂–O–CH₂–CH₃

• Both contain the carbonyl group (C=O).
• Aldehyde: has at least one hydrogen atom attached to the carbonyl carbon atom.
• Functional group: –CHO
• Example: HCHO (methanal/formaldehyde), CH₃CHO (ethanal/acetaldehyde)
• Ketone: has two hydrocarbon groups (alkyl) bonded to the carbonyl carbon atom.
• Functional group: –CO–
• Example: CH₃COCH₃ (propanone/acetone)

• The functional group of organic acids is called the carboxyl group (–COOH).
• General formula: R–COOH (where R = H or alkyl group)
• Examples:
• HCOOH (methanoic acid/formic acid)
• CH₃COOH (ethanoic acid/acetic acid)

• Compounds having the general formula R–CO–OR' are called esters.
• R and R' are alkyl groups which may be same or different.
• Functional group: –CO–OR'
• Examples:
• CH₃–CO–OCH₃ (methyl acetate/methyl ethanoate)
• CH₃–CO–OCH₂CH₃ (ethyl acetate/ethyl ethanoate)

• Compounds having the general formula R–CO–NH₂ are called amides.
• R is an alkyl group.
• Example: CH₃–CO–NH₂ (ethanamide)

Functional Groups Containing Carbon, Hydrogen, and Nitrogen: Amines

• The functional group of amines is –NH₂.
• General formula: R–NH₂
• Examples:
• CH₃–NH₂ (methyl amine)
• CH₃CH₂–NH₂ (ethyl amine)

Intermediate Level

Why Functional Groups Matter

• Chemical properties — how the compound reacts.
• Physical properties — boiling point, solubility.
• Naming — the suffix or prefix of the compound's name.
• Classification — which family the compound belongs to.

Simple explanation: The functional group is what makes each family of organic compounds unique.

Functional Groups in Detail

Alkyl Groups

Simple explanation: Alkyl groups are hydrocarbon chains attached to functional groups.

How to Identify Functional Groups

• –OH → alcohol
• –COOH → carboxylic acid
• –CHO → aldehyde
• –CO– → ketone
• –NH₂ → amine
• –X (F, Cl, Br, I) → haloalkane
• **C=C** → alkene
• C≡C → alkyne
• C–O–C → ether
• –CO–OR' → ester
• –CO–NH₂ → amide

Important Points

• - Functional group: an atom or group of atoms that gives a family of organic compounds its characteristic chemical and physical properties.
• - Importance:
• 1. Basis for naming organic compounds.
• 2. Classifies organic compounds into different classes.
• 3. Site of chemical reactivity in a molecule.
• - Common functional groups:
• - –OH (alcohol)
• - –COOH (carboxylic acid)
• - –CHO (aldehyde)
• - –CO– (ketone)
• - –NH₂ (amine)
• - –X (haloalkane)
• - C=C (alkene)
• - C≡C (alkyne)
• - C–O–C (ether)
• - –CO–OR' (ester)
• - –CO–NH₂ (amide)
• - Examples:
• - Alcohol: CH₃OH
• - Carboxylic acid: CH₃COOH
• - Aldehyde: HCHO
• - Ketone: CH₃COCH₃
• - Amine: CH₃NH₂
• - Haloalkane: CH₃Cl
• - Phenol: C₆H₅OH

Remember

• - Functional group = active part of an organic molecule.
• - Same functional group → same class → similar properties.
• - **–OH** = alcohol.
• - –COOH = carboxylic acid.
• - **–CHO** = aldehyde.
• - **–CO–** = ketone.
• - **–NH₂** = amine.
• - **–X** = haloalkane.
• - Three reasons functional groups are important: naming, classification, site of reactivity.

Common Mistakes

• 1. Thinking all organic compounds have functional groups. Alkanes have no functional group (only C–C and C–H bonds).
• 2. Confusing aldehyde and ketone. Aldehyde has –CHO (H attached to C=O); ketone has –CO– (two alkyl groups attached to C=O).
• 3. Confusing alcohol and phenol. Alcohol has –OH on an alkyl group; phenol has –OH on a benzene ring.
• 4. Forgetting the difference between ether and ester. Ether has C–O–C; ester has –CO–O–.
• 5. Thinking functional groups only affect chemical properties. They also affect physical properties (boiling point, solubility).
• 6. Confusing amine and amide. Amine has –NH₂; amide has –CO–NH₂.

Quick Revision

• Functional group: an atom or group of atoms that gives a family of organic compounds its characteristic chemical and physical properties.
• Importance of functional groups:
• Common functional groups and classes:
• Alkane: no functional group (R–H)
• **Alkene:** C=C
• Alkyne: C≡C
• Alcohol: –OH (R–OH)
• Ether: C–O–C (R–O–R')
• Aldehyde: –CHO
• Ketone: –CO–
• Carboxylic acid: –COOH (R–COOH)
• Ester: –CO–OR' (R–CO–OR')
• Amide: –CO–NH₂ (R–CO–NH₂)
• Amine: –NH₂ (R–NH₂)
• Haloalkane: –X (halogen)
• Phenol: –OH attached to benzene ring
• Examples:
• Methanol (CH₃OH) — alcohol
• Acetic acid (CH₃COOH) — carboxylic acid
• Formaldehyde (HCHO) — aldehyde
• Acetone (CH₃COCH₃) — ketone
• Methylamine (CH₃NH₂) — amine
• Chloromethane (CH₃Cl) — haloalkane
• Phenol (C₆H₅OH) — phenol

60-Second Revision

• Functional group = active part of an organic molecule.
• –OH = alcohol; –COOH = carboxylic acid.
• **–CHO** = aldehyde; **–CO–** = ketone.
• **–NH₂** = amine; **–X** = haloalkane.
• Three reasons: naming, classification, site of reactivity.
• Warning: alkanes have no functional group — they only have C–C and C–H bonds.