# Introduction to organic chemistry: functional groups

> IB Chemistry HL · Unit 6: Organic Chemistry, R3: Mechanisms of chemical change
> Source: https://www.owlsprep.com/study/ib-chemistry-hl-u6-introduction-to-organic-chemistry-functional/

This sub-topic introduces functional groups, the core foundation for classifying and predicting reactivity in organic chemistry. You will learn to identify common functional groups and apply priority rules for nomenclature, critical for all subsequent organic mechanism study.

**Prerequisites:** [Basic hydrocarbon nomenclature (alkanes, alkenes)](https://www.owlsprep.com/study/ib-chemistry-hl-u6-introduction-organic-hydrocarbons/)

## Learning objectives

- Define and identify common functional groups in organic molecules
- Classify organic compounds by their functional groups
- Apply priority rules to name molecules with multiple functional groups
- Distinguish between structurally similar functional groups

## What is a Functional Group?

**Functional Group** — A specific arrangement of atoms or covalent bonds within an organic molecule that determines the molecule's characteristic chemical reactivity. All members of a homologous series share the same functional group.

*Example:* The hydroxyl group ($-OH$) is the functional group common to all alcohols.

Functional groups allow chemists to organize millions of different organic compounds into logical families. Compounds with the same functional group undergo similar reactions, regardless of the length of their carbon backbone, making it easy to predict reaction outcomes.

**Worked example:** Identify the functional group present in $CH_3CH_2COCH_3$.

1. Step 1: Expand the condensed formula to show the full arrangement of atoms:
2. $$CH_3-CH_2-\underset{\underset{O}{||}}{C}-CH_3$$
3. Step 2: Identify the reactive group: the double-bonded oxygen (carbonyl group) is bonded to two carbon atoms, one on each side.
4. Step 3: Match to the functional group classification: a carbonyl bonded to two alkyl groups is a ketone.

## Common Oxygen-Containing Functional Groups

Most organic reactions you will study for IB HL involve oxygen-containing functional groups. These are the most commonly tested functional groups in exams, so it is important to memorize their structures.

| Name | General Structure | Example |
| --- | --- | --- |
| Hydroxyl (Alcohol) | $R-OH$ | Ethanol: $CH_3CH_2OH$ |
| Aldehyde | $R-CHO$ | Ethanal: $CH_3CHO$ |
| Ketone | $R-COR'$ | Propanone: $CH_3COCH_3$ |
| Carboxylic Acid | $R-COOH$ | Ethanoic acid: $CH_3COOH$ |
| Ester | $R-COOR'$ | Methyl ethanoate: $CH_3COOCH_3$ |

**Worked example:** Classify $HOCH_2CH_2CH_2COOH$ by its primary functional group.

1. Step 1: Identify all functional groups present: a hydroxyl group ($-OH$) on one end, and a carboxyl group ($-COOH$) on the other.
2. Step 2: Recall that functional group priority determines the primary classification: carboxylic acids have higher priority than hydroxyl groups.
3. Step 3: Confirm that the entire $-COOH$ unit is a single functional group, with very different reactivity from a standalone hydroxyl group.
4. Conclusion: This molecule is 4-hydroxybutanoic acid, classified primarily as a carboxylic acid.

## Nitrogen and Halogen-Containing Functional Groups

**Amide** — A functional group consisting of a carbonyl group bonded directly to a nitrogen atom. It is structurally distinct from amines, with very different reactivity.

*Notation:* $R-CONH_2$, $R-CONHR'$

*Example:* Ethanamide: $CH_3CONH_2$

Halogenoalkanes (alkyl halides) have the general formula $R-X$, where $X = F, Cl, Br, I$. They are common starting materials for organic synthesis reactions tested frequently in mechanism questions. Amines are derivatives of ammonia, classified as 1°, 2°, or 3° based on the number of alkyl groups bonded to nitrogen.

**Worked example:** Identify all functional groups in paracetamol, which has a benzene ring bonded to $-OH$ and $-NHCOCH_3$ on opposite carbons.

1. Step 1: Analyze the first group: $-OH$ bonded to an aromatic ring is a hydroxyl (phenol) functional group.
2. Step 2: Analyze the second group: $-NHCOCH_3$ has a carbonyl group bonded directly to nitrogen, forming an amide linkage.
3. Conclusion: The molecule contains two functional groups: hydroxyl and amide.

## Functional Group Priority for Nomenclature

When a molecule contains more than one functional group, you must assign the highest priority group as the main functional group for naming. All other groups are treated as substituents, with lower priority. IB requires you to know the standard priority order for common functional groups.

> **tip**
>
> The general priority order (highest to lowest) is: Carboxylic acids > Esters > Amides > Aldehydes > Ketones > Alcohols > Amines > Alkenes > Alkynes > Halogens

**Worked example:** Identify the main functional group in $OHC-CH_2-CH(OH)-CH_3$ and give its correct name.

1. Step 1: List all functional groups: aldehyde ($-CHO$) and hydroxyl ($-OH$).
2. Step 2: Compare priority: aldehydes have higher priority than alcohols.
3. Step 3: Number the 4-carbon chain starting from the aldehyde carbon, giving the aldehyde C1 and the hydroxyl C3.
4. Conclusion: The main functional group is aldehyde, and the name is 3-hydroxybutanal.

> **Exam tip**
>
> Always number the carbon chain to give the highest priority functional group the lowest possible number, even if this gives a double bond or substituent a higher number.

## Common pitfalls

- **Wrong:** Confusing aldehydes and ketones, misclassifying the functional group
  - Why it fails: Both contain a carbonyl ($C=O$) group, so students often mix them up
  - Correct: Check what the carbonyl carbon is bonded to: if bonded to a hydrogen ($R-CHO$) it is an aldehyde; if bonded to two carbons ($R-COR'$) it is a ketone
- **Wrong:** Treating the hydroxyl group in a carboxylic acid as an alcohol functional group
  - Why it fails: Carboxylic acids contain a hydroxyl group, leading students to incorrectly count it as an alcohol
  - Correct: Recognize the entire $-COOH$ unit is the carboxylic acid functional group, which has higher priority than alcohol
- **Wrong:** Confusing amines and amides
  - Why it fails: Both contain nitrogen, so students often misidentify them
  - Correct: Amides have a carbonyl group bonded directly to nitrogen ($-CONH-$), while amines have only nitrogen bonded to carbon/hydrogen
- **Wrong:** Assigning the lowest number to a double bond instead of the higher priority functional group
  - Why it fails: Students remember numbering rules for alkenes, but forget functional group priority comes first
  - Correct: Always number the chain to give the highest priority functional group the lowest possible number
- **Wrong:** Forgetting to list all functional groups when the question asks for multiple
  - Why it fails: Students often only name the highest priority group, even when asked for all present
  - Correct: Carefully scan the entire molecule to find all non-alkane functional groups and list all requested ones

## Cheatsheet

| Functional Group | Structure | Priority (High → Low) |
| --- | --- | --- |
| Carboxylic acid | $-COOH$ | 1 |
| Ester | $-COOR'$ | 2 |
| Amide | $-CONH_2$ | 3 |
| Aldehyde | $-CHO$ | 4 |
| Ketone | $-COR'$ | 5 |
| Alcohol | $-OH$ | 6 |
| Amine | $-NH_2$ | 7 |
| Alkene | $C=C$ | 8 |
| Alkyne | $C \equiv C$ | 9 |
| Halide | $-X$ | 10 |

## What's next

Mastering functional groups is the first critical step to succeeding in organic chemistry for IB HL. All subsequent topics, from nomenclature to reaction mechanisms, rely on your ability to quickly identify and classify functional groups. You will next learn to apply what you know about functional groups to name more complex organic molecules, then study the characteristic reactions of each functional group family. Functional group identification is a common first question in organic sections of Paper 2, so solidifying this knowledge will earn you easy marks on every exam.

- [Nomenclature of organic compounds](https://www.owlsprep.com/study/ib-chemistry-hl-u6-nomenclature-of-organic-compounds/)
- [Types of organic reactions](https://www.owlsprep.com/study/ib-chemistry-hl-u6-types-of-organic-reactions/)
- [Redox reactions and electrochemistry basics](https://www.owlsprep.com/study/ib-chemistry-hl-u6-redox-reactions-and-electrochemistry-basics/)

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