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Functional Group

A functional group is the atom or group of atoms in an organic compound that gives it its characteristic properties and reactions.

In one line: Functional group is the chemical identity tag.

Practice This Concept

Understand it

The carbon chain gives the basic framework, but the functional group decides the chemical identity of the compound. For example, -OH makes a compound an alcohol, and -COOH makes it a carboxylic acid. This is why compounds with the same chain but different functional groups behave differently in school experiments and exam questions.

Example

Ethanol has the functional group -OH, while ethanoic acid has the functional group -COOH.

Rule to remember

Common functional groups in this chapter include -OH, -COOH, -CHO, and halogen groups such as -Cl or -Br in other contexts.

The functional group decides the family and many reactions.

See it worked out

Follow the example line by line, then use the same method on new questions.

Worked example

1CH₃CH₂OH and CH₃OH both contain the -OH group, so both are alcohols.
2Even though the carbon chain length is different, the group gives them similar chemical behavior.

Method you can reuse

1Draw or read the structure carefully.
2Locate the special atom or group.
3Name the compound family from that group.
4Use the group to predict properties and reactions.

Sketch it in your answer

Mark the functional group clearly at the end or side position of the carbon chain in the structure.

Check yourself

Answer the question in your head before opening the answer.

Try it yourself

Why do ethanol and ethanoic acid behave differently even though both contain carbon, hydrogen, and oxygen?

Think first, then show answer

They behave differently because they have different functional groups. Ethanol has the hydroxyl group -OH, while ethanoic acid has the carboxyl group -COOH.

Common wrong answer

A common wrong answer is to say the carbon chain alone decides the properties. That is incomplete because the functional group controls the key reactions.

Don't mix this up

Students often treat the whole molecule as the functional group. In reality, only a specific part controls the special properties.

Exam tip

First spot the functional group, then decide the family, name, and likely reaction.

Write it in the exam

Each point is worth about one mark. Write point 1 for a 1-mark question and keep adding points as the marks go up.

How CBSE asks it

Questions may ask you to identify the functional group, classify a compound, or explain why two compounds show different properties.

1
Enough for 1 mark
A functional group is the atom or group of atoms in an organic compound that gives it its characteristic properties and reactions.
2
Add for 2 marks
Example: Ethanol has the functional group -OH, while ethanoic acid has the functional group -COOH.
3
Add for 3 marks
The carbon chain gives the basic framework, but the functional group decides the chemical identity of the compound.

MCQ Quiz

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