Naming organic compounds

Q: What is the molecular formula of butanoic acid?

$C_4H_8O_2$. It has 4 carbons (but-), is saturated (alkane-like backbone), so $C_nH_{2n}O_2$ applies for carboxylic acids.

Paper 1Paper 2Paper 3Paper 4

The Logic of Organic Nomenclature

Organic chemistry is the study of carbon-containing compounds. To communicate clearly, chemists use a systematic naming system (IUPAC nomenclature). The name of an organic compound tells you two things: how many carbon atoms are in the main chain and what functional group is present.

The name is built from two parts:

Prefix: Indicates the number of carbon atoms in the longest continuous chain containing the functional group.
Suffix: Indicates the type of compound (the homologous series) based on the functional group.

Number of Carbons	Prefix
1	meth-
2	eth-
3	prop-
4	but-

Building on previous concepts: In hydrocarbons (compounds with only C and H), the suffix depends on the bond type. In compounds with oxygen, the suffix depends on the specific arrangement of atoms (functional group).

Displayed Formula

A displayed formula shows the relative positions of all the atoms and all the bonds between them. Every single covalent bond must be drawn as a line.

A single bond is one line (-).
A double bond is two lines (=).
Carbon always forms 4 bonds.
Oxygen usually forms 2 bonds.
Hydrogen always forms 1 bond.

Core Compounds: Alkanes, Alkenes, Alcohols, and Carboxylic Acids

We start with the simplest organic molecules. These form the basis for naming more complex derivatives.

Homologous Series	Example (2 Carbons)
Alkane	Ethane (C_2H_6)
Alkene	Ethene (C_2H_4)
Alcohol	Ethanol (C_2H_5OH)
Carboxylic Acid	Ethanoic acid (CH_3COOH)

Why this matters: The suffix changes the chemical properties completely. Ethane is a fuel gas; Ethanol is a liquid solvent; Ethanoic acid is vinegar (acidic).

Drawing and Naming Core Compounds (LO1)

1. Methane (CH_4)

Prefix: 'meth-' (1 carbon)
Suffix: '-ane' (alkane, single bonds only)
Displayed Formula: A central Carbon atom bonded to 4 Hydrogen atoms.

H
|
H-C-H
|
H

2. Ethane (C_2H_6)

Prefix: 'eth-' (2 carbons)
Suffix: '-ane' (alkane)
Displayed Formula: Two Carbons bonded to each other, with 3 Hydrogens on each Carbon.

H H
| |
H-C - C-H
| |
H H

3. Ethene (C_2H_4)

Prefix: 'eth-' (2 carbons)
Suffix: '-ene' (alkene, double bond)
Displayed Formula: Two Carbons bonded by a double bond, with 2 Hydrogens on each Carbon.

H H
\ /
C = C
/
H H

4. Ethanol (C_2H_5OH)

Prefix: 'eth-' (2 carbons)
Suffix: '-ol' (alcohol, -OH group)
Displayed Formula: Two Carbons bonded together. One Carbon has 3 Hydrogens. The other Carbon has 2 Hydrogens and is bonded to an Oxygen, which is bonded to a Hydrogen.

H H
| |
H-C - C - O - H
| |
H H

5. Ethanoic Acid (CH_3COOH)

Prefix: 'eth-' (2 carbons)
Suffix: '-oic acid' (carboxylic acid, -COOH group)
Displayed Formula: The first Carbon has 3 Hydrogens. It is bonded to a second Carbon. This second Carbon has a double bond to one Oxygen and a single bond to an OH group.
```
O
```
//
H-C - C - O - H
|
H (Note: The first C is attached to the second C)

Correct Drawing Structure for Ethanoic Acid:
```
  O
 //
```
H - C - C - O - H
| |
H H
(Wait, standard representation:)
```
  O
 //
```
H3C - C - OH

Displayed Formula (all bonds shown):
```
H   O
|  //
```
H-C--C
|
H O-H

Products of Reactions (LO1)
You must be able to name the products of common reactions involving these core compounds:

Combustion: Burning in oxygen.
- Methane + Oxygen \rightarrow Carbon dioxide + Water
- Ethanol + Oxygen \rightarrow Carbon dioxide + Water
Addition Reactions (Alkenes): Breaking the double bond to add atoms.
- Ethene + Bromine water \rightarrow 1,2-Dibromoethane
- Ethene + Steam (H_2O) \rightarrow Ethanol

⚠︎ Incorrectly Naming Alcohols and Acids

Error: Students often name alcohols simply as 'butanol' or 'propanol' without specifying the position of the -OH group.

Correct Understanding: For alcohols with 3 or more carbons, the position of the hydroxyl (-OH) group matters. You must use a number to indicate which carbon the -OH is attached to. The chain is numbered to give the -OH group the lowest possible number.

Propan-1-ol: The -OH is on the first carbon.
Propan-2-ol: The -OH is on the second carbon.

Similarly, for carboxylic acids, the carbon in the -COOH group is always Carbon-1. You do not need to number it (e.g., 'Ethanoic acid', not 'Ethan-1-oic acid').

Spelling and Hyphens in Nomenclature

Context: When writing names of organic compounds, especially in Paper 3 and Paper 4 structured questions.

Tip: You must use hyphens correctly to separate numbers from letters. There is no space between the number and the letter, and no space between the letter and the next part of the name.

Correct: propan-1-ol
Incorrect: propan 1 ol (spaces)
Incorrect: propan-1ol (missing hyphen after number)
Incorrect: propen-1-ol (wrong prefix for saturated chain)

Why examiners accept this: The markscheme is strict on spelling and punctuation. 'Propenoic acid' implies a double bond (ene) which is not present in propanoic acid. Misspelling 'methanoate' as 'methanote' results in zero marks because it is not a recognized chemical name.

Supplement: Unbranched Chains with 3 and 4 Carbons (LO3)

As the carbon chain grows, isomers appear. 'Unbranched' means the carbons form a straight line.

Alkenes:

But-1-ene: Double bond starts at Carbon 1. (CH_2=CH-CH_2-CH_3)
But-2-ene: Double bond starts at Carbon 2. (CH_3-CH=CH-CH_3)

Alcohols:

Propan-1-ol: CH_3-CH_2-CH_2-OH
Propan-2-ol: CH_3-CH(OH)-CH_3
Butan-1-ol: CH_3-CH_2-CH_2-CH_2-OH
Butan-2-ol: CH_3-CH_2-CH(OH)-CH_3

Carboxylic Acids:

Propanoic acid: 3 carbons total. (CH_3-CH_2-COOH)
Butanoic acid: 4 carbons total. (CH_3-CH_2-CH_2-COOH)

Drawing Unbranched Alcohols (LO3)

1. Butan-2-ol (C_4H_9OH)

Prefix: 'but-' (4 carbons)
Position: '-2-' (OH on 2nd carbon)
Displayed Formula:
```
H   H   H   H
|   |   |   |
```
H-C - C - C - C-H
| | | |
H O H H
|
H
(Note: The OH group is attached to the second carbon from the left)

2. Propan-1-ol (C_3H_7OH)

Prefix: 'prop-' (3 carbons)
Position: '-1-' (OH on 1st carbon)
Displayed Formula:
```
H   H   H
|   |   |
```
H-C - C - C-O-H
| | |
H H H

⚠︎ Confusing Carbon Counts in Acids and Alcohols

Error: Thinking that the carbon in the -COOH group is separate from the prefix count.

Correct Understanding: The carbon atom inside the carboxyl group (-COOH) counts towards the total number of carbons in the prefix.

Methanoic acid: 1 Carbon total (the COOH carbon).
Ethanoic acid: 2 Carbons total (1 methyl + 1 carboxyl).
Propanoic acid: 3 Carbons total.

Similarly for alcohols, the carbon attached to the OH is part of the chain.

Supplement: Naming Esters (LO4)

Esters are formed from an alcohol and a carboxylic acid. The name has two parts:

First part (Alkyl group): Comes from the alcohol. Drop the '-ol' and add '-yl'.
- Methanol \rightarrow Methyl
- Ethanol \rightarrow Ethyl
- Propanol \rightarrow Propyl
- Butanol \rightarrow Butyl
Second part (Carboxylate group): Comes from the carboxylic acid. Drop the '-oic acid' and add '-oate'.
- Methanoic acid \rightarrow Methanoate
- Ethanoic acid \rightarrow Ethanoate
- Propanoic acid \rightarrow Propanoate
- Butanoic acid \rightarrow Butanoate

Rule: Each parent chain (alcohol part and acid part) must contain up to four carbon atoms.

Naming and Drawing Esters (LO4)

1. Methyl ethanoate

Alcohol: Methanol (1 carbon) \rightarrow Methyl
Acid: Ethanoic acid (2 carbons) \rightarrow Ethanoate
Displayed Formula:
```
H   O
|  //
```
H-C--C
|
H O - C - H
|
H
(Structure: Methyl group attached to Oxygen, which is attached to Carbonyl, which is attached to Methyl)

2. Ethyl propanoate

Alcohol: Ethanol (2 carbons) \rightarrow Ethyl
Acid: Propanoic acid (3 carbons) \rightarrow Propanoate
Displayed Formula:
```
H   H   O
|   |  //
```
H-C - C--C
| |
H H O - C - H
|
H
(Wait, Ethyl is CH_3CH_2-. Propanoate is CH_3CH_2COO-.)
Correct Displayed Formula for Ethyl Propanoate:
```
H   H       O
|   |      //
```
H-C - C -- C
| |
H H O - C - H
|
H
(Note: The left part is the propanoate chain (3 carbons). The right part attached to the single-bonded Oxygen is the ethyl group (2 carbons).)

3. Butyl methanoate

Alcohol: Butanol (4 carbons) \rightarrow Butyl
Acid: Methanoic acid (1 carbon) \rightarrow Methanoate
Displayed Formula:
```
H   H   H   H
|   |   |   |
```
H-C - C - C - C - O - C = O
| | | | |
H H H H H

Identifying the Ester Linkage in Structures

Context: When asked to identify an ester from a displayed formula (common in Paper 1 and 2 MCQs).

Tip: Look for the ester linkage: -COO- or -C(=O)-O-. This is a Carbon double-bonded to one Oxygen and single-bonded to another Oxygen.

The Carbon side of the C=O belongs to the acid part (the second name).
The Oxygen side of the single bond belongs to the alcohol part (the first name).

Why examiners accept this: Recognizing the functional group is the primary skill tested. If you see -COO-, it is an ester. If you see -OH, it is an alcohol or acid. If you see C=C, it is an alkene.

Identifying Homologous Series and Structures

Which homologous series does the compound CH_3CH_2COOH belong to?

Carboxylic acids. The suffix '-oic acid' and the presence of the -COOH group identify it.

What is the molecular formula of butanoic acid?

C_4H_8O_2. It has 4 carbons (but-), is saturated (alkane-like backbone), so C_nH_{2n}O_2 applies for carboxylic acids.

Name the compound with the displayed formula showing a chain of 3 carbons, where the first carbon has a double bond to O and single bond to OH.

Propanoic acid. 3 carbons = propan-. Carboxylic acid group = -oic acid.

Draw the displayed formula of propan-1-ol.

A chain of 3 carbons. The first carbon is bonded to an OH group and 2 H atoms. The other two carbons are bonded to 3 H atoms each. All bonds shown as lines.

Name the ester formed from methanol and butanoic acid.

Methyl butanoate. Methanol gives 'Methyl'. Butanoic acid gives 'butanoate'.