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Functional Groups: The Building Blocks of Organic Chemistry

What Are Functional Groups?

Definition

Functional group

A functional group is a specific arrangement of atoms within a molecule that determines the molecule's characteristic chemical reactions.

Analogy

Think of them as the "active zones" of organic molecules, dictating how the molecule behaves and reacts with other substances.

For example, the hydroxyl group (–OH) in alcohols makes these compounds polar and capable of forming hydrogen bonds.
This influences their solubility in water and boiling points, which are critical properties in both natural processes and industrial applications.

Key Functional Groups and Their Properties

Let’s examine some of the most important functional groups in organic chemistry.
Each group has distinct properties and plays a critical role in determining the behavior of the molecules they belong to.

Halogeno Functional Group (–X)

Structure: –X, where X is a halogen (Cl, Br, or I).
Class: Halogenoalkanes (also called alkyl halides).
Properties:
1. Contain a polar carbon–halogen bond.
2. Reactivity depends on the halogen type; for example, the C–I bond is weaker and more reactive than the C–Cl bond.
Example: Chloroethane ($CH_3CH_2Cl$).

Tip

When analyzing halogenoalkanes, remember that the bond strength decreases as the size of the halogen increases.
This affects the molecule’s reactivity.

Hydroxy Group (–OH)

Structure: –OH.
Class: Alcohols.
Properties:
1. Highly polar, enabling hydrogen bonding with other molecules.
2. Increases solubility in water and raises boiling points.
Example: Ethanol ($CH_3CH_2OH$).

Example

Consider ethanol, a common alcohol.
Its hydroxyl group allows it to mix with water, making it a versatile solvent in both laboratory and industrial settings.

Carbonyl Group (C=O)

Structure: A carbon atom double-bonded to an oxygen atom.
Classes:
1. Aldehydes: The carbonyl group is at the end of the carbon chain (e.g., ethanal, $CH_3CHO$).
2. Ketones: The carbonyl group is within the carbon chain (e.g., propanone, $CH_3COCH_3$).
Properties:
1. Polar, making aldehydes and ketones soluble in polar solvents.
2. Reactivity depends on the surrounding groups.

Common Mistake

Students often confuse aldehydes and ketones.
Remember: aldehydes have the carbonyl group at the end of the chain, while ketones have it in the middle.

Carboxyl Group (–COOH)

Structure: –COOH.
Class: Carboxylic acids.
Properties:
1. Strongly polar and acidic due to the hydrogen atom in the –OH group.
2. Forms hydrogen bonds, leading to high boiling points.
Example: Ethanoic acid ($CH_3COOH$).

Self review

What functional group gives vinegar (ethanoic acid) its sour taste and acidic properties?

Amino Group ($–NH_2$)

Structure: –$NH_2$.
Class: Amines.
Properties:
1. Acts as a weak base.
2. Forms hydrogen bonds, influencing solubility and boiling points.
Example: Methylamine ($CH_3NH_2$).

Amido Group ($–CONH_2$)

Structure: –$CONH_2$.

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Note

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Tip

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Introduction to Functional Groups

Functional groups are specific groups of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules. They are the reactive parts of molecules that behave in predictable ways.

Functional groups can be thought of as the "personality" of a molecule, determining how it interacts with other substances.
They are like the active sites of enzymes, where specific reactions occur.

AnalogyThink of functional groups as the "apps" on your smartphone - each one has a specific function and determines what your phone can do.

ExampleThe hydroxyl group (-OH) in alcohols is a functional group that makes these compounds polar and capable of forming hydrogen bonds.

DefinitionFunctional GroupA specific arrangement of atoms within a molecule that determines the molecule's characteristic chemical reactions.

S3.2.2 Functional groups Notes

Functional Groups: The Building Blocks of Organic Chemistry

What Are Functional Groups?