Nomenclature of Carbon Compounds (IUPAC)

Why a Naming System?

With millions of organic compounds, we need a systematic way to name them.

Otherwise:

• Same compound called different things by different chemists.
• Communication becomes impossible.
• Confusion in industry, research, medicine.

Two Naming Systems

1. Common Names (Traditional):

• Old, historical names.
• Often based on source (e.g., 'formic acid' from formica = ant).
• May not give structural information.
• Examples: ethyl alcohol, formic acid, acetone.

2. IUPAC Names (Systematic):

• Modern, internationally agreed.
• IUPAC = International Union of Pure and Applied Chemistry.
• Rules-based — name tells you the structure!
• Examples: ethanol, methanoic acid, propanone.

Throughout this chapter, we use IUPAC names mostly. Common names sometimes too.

Why IUPAC?

IUPAC names tell you:

1. How many C atoms in main chain.
2. What functional group is present.
3. Where the functional group is.
4. Whether there are branches and where.

Just from the name, you can draw the structure!

The 'Word Word' Pattern

An IUPAC name has 3 parts:

Prefix (optional) + Root (must) + Suffix (often)

Where:

• Prefix: position of branches/groups, or substituents.
• Root: number of C atoms (chain length).
• Suffix: functional group.

Step-by-Step Naming

Step 1: Count carbon atoms in the longest chain → Root. Step 2: Identify functional group → Suffix. Step 3: Identify branches/substituents → Prefix. Step 4: Number the chain (so functional group/branch gets lowest number). Step 5: Combine: Prefix + Root + Suffix.

Examples we'll see:

• 'Methane' = 1 C + alkane.
• '2-bromopropane' = 3 C, Br on C-2, alkane base.
• 'But-1-ene' = 4 C, alkene at position 1.

A Simple Hierarchy

When multiple functional groups present, IUPAC has priority:

Priority order (highest first):

1. Carboxylic acid (-COOH)
2. Aldehyde (-CHO)
3. Ketone (>C=O)
4. Alcohol (-OH)
5. Halogen (-X)

The highest priority group determines the suffix; lower priority becomes a prefix.

(Don't worry — most Class 10 questions have only one functional group.)

Roots — Numbers of Carbons

The 'root' tells how many C atoms in the main chain.

Greek/Latin Roots

Number of C	Root	Examples
1	meth-	methane, methanol
2	eth-	ethane, ethanol
3	prop-	propane, propanol
4	but-	butane, butanol
5	pent-	pentane, pentanol
6	hex-	hexane, hexanol
7	hept-	heptane
8	oct-	octane
9	non-	nonane
10	dec-	decane

First 4 (meth, eth, prop, but) — historical names. 5 onwards — Greek/Latin numbers.

Suffixes — Functional Groups

Functional Group	Suffix	Example
Alkane (only single bonds)	-ane	methane
Alkene (double bond)	-ene	ethene
Alkyne (triple bond)	-yne	ethyne
Alcohol (-OH)	-ol	ethanol
Aldehyde (-CHO)	-al	methanal
Ketone (>C=O)	-one	propanone
Carboxylic acid (-COOH)	-oic acid	ethanoic acid
Ester (-COOR)	-oate	ethyl ethanoate

Prefixes — Common Substituents

Substituent	Prefix
-F	fluoro
-Cl	chloro
-Br	bromo
-I	iodo
-CH₃	methyl
-C₂H₅	ethyl
-NO₂	nitro

Example: 'chloroethane' = ethane with Cl substituent.

Putting It Together

To name CH₃-CH₂-CH₂-OH:

• Count C: 3 → 'prop'.
• Functional group: -OH → '-ol'.
• Position of OH: at end (C-1) → no number needed for one OH.
• Name: propan-1-ol (or just 'propanol' for simple cases).

To name CH₃-CH=CH-CH₃:

• Count C: 4 → 'but'.
• Functional group: C=C double bond → '-ene'.
• Position of double bond: between C-2 and C-3 → 'but-2-ene'.
• Name: but-2-ene.

Naming Examples — Step by Step

Example 1: Naming Alkanes

Methane ($CH_4$)

Step 1: 1 C → 'meth'. Step 2: only single bonds → '-ane'. Name: methane.

Ethane ($CH_3CH_3$)

1 C in chain… wait, let me re-examine. 2 C → 'eth'. Single bonds → '-ane'. Name: ethane.

Propane ($CH_3CH_2CH_3$)

3 C → 'prop'. Single bonds → '-ane'. Name: propane.

2-Methylpropane (Iso-butane)

Main chain: 3 C (prop). Branch: 1 CH₃ (methyl). Position of branch: C-2 (numbered to give lowest position). Name: 2-methylpropane.

Common name: iso-butane.

Example 2: Naming Alkenes

Ethene ($C_2H_4$)

2 C → 'eth'. Double bond → '-ene'. Name: ethene.

Propene ($CH_2=CH-CH_3$)

3 C → 'prop'. Double bond → '-ene'. Position of C=C: between C-1 and C-2. Name: prop-1-ene (or just propene — only one position possible).

But-2-ene ($CH_3-CH=CH-CH_3$)

4 C → 'but'. Double bond → '-ene'. Position: C-2 (between C-2 and C-3). Name: but-2-ene.

Example 3: Naming Alkynes

Ethyne ($C_2H_2$)

2 C → 'eth'. Triple bond → '-yne'. Name: ethyne.

Propyne ($HC≡C-CH_3$)

3 C → 'prop'. Triple bond → '-yne'. Position: between C-1 and C-2. Name: prop-1-yne (or simply propyne).

Example 4: Naming Alcohols

Methanol ($CH_3OH$)

1 C → 'meth'. -OH → '-ol'. Name: methanol.

Ethanol ($C_2H_5OH$)

2 C → 'eth'. -OH → '-ol'. Name: ethanol.

Propan-1-ol ($CH_3-CH_2-CH_2-OH$)

3 C → 'prop'. -OH on C-1 → 'propan-1-ol'. Name: propan-1-ol or 1-propanol.

Propan-2-ol ($CH_3-CHOH-CH_3$)

3 C → 'prop'. -OH on C-2 → 'propan-2-ol'. Name: propan-2-ol or iso-propanol (common name).

Example 5: Naming Aldehydes and Ketones

Methanal/Formaldehyde ($HCHO$)

1 C → 'meth'. -CHO → '-al'. Name: methanal (also called formaldehyde).

Ethanal/Acetaldehyde ($CH_3CHO$)

2 C → 'eth'. -CHO → '-al'. Name: ethanal (also called acetaldehyde).

Propanone/Acetone ($(CH_3)_2C=O$ or $CH_3-CO-CH_3$)

3 C → 'prop'. >C=O → '-one'. Name: propanone (also called acetone).

Example 6: Naming Carboxylic Acids

Methanoic Acid/Formic Acid ($HCOOH$)

1 C → 'meth'. -COOH → '-oic acid'. Name: methanoic acid (also formic acid).

Ethanoic Acid/Acetic Acid ($CH_3COOH$)

2 C → 'eth'. -COOH → '-oic acid'. Name: ethanoic acid (also acetic acid).

Note: -COOH counts as part of the chain. So 'ethanoic acid' = 2 C total (1 from CH₃ + 1 from COOH).

Naming Substituted Compounds

Halogens as Substituents

Halogens are named with prefixes:

• F: fluoro
• Cl: chloro
• Br: bromo
• I: iodo

Add position number if needed.

Examples

$CH_3Cl$: chloromethane (or methyl chloride)

$CH_3CH_2Cl$: chloroethane

$CH_3CHBrCH_3$: 2-bromopropane (Br on C-2 of propane.)

$CHCl_3$: trichloromethane (chloroform)

$CCl_4$: tetrachloromethane (carbon tetrachloride)

Numbering Rules

When numbering the chain, give the lowest possible numbers to:

1. Functional group (highest priority).
2. Then double/triple bonds.
3. Then substituents.

Read from the end that gives lower numbers.

Example: 2-Methylbutane

Structure:

CH₃ — CH — CH₂ — CH₃
      |
      CH₃

Number from left: C-1, C-2 (with methyl), C-3, C-4. Methyl at C-2.

Number from right: C-1, C-2, C-3 (with methyl), C-4. Methyl at C-3.

Choose the numbering with lower number → C-2 from left. Hence: 2-methylbutane (not 3-methylbutane).

Multiple Substituents

If multiple branches, list them alphabetically.

Example: 2-bromo-3-methylpentane

If two same substituents: Use prefixes: di- (2), tri- (3), tetra- (4).

Example: 2,2-dimethylpropane (= neo-pentane).

Naming with Functional Group Priority

If multiple functional groups present:

Use the highest priority as suffix. Others become prefixes.

Priority order:

1. -COOH (-oic acid) [highest]
2. -CHO (-al)
3. >C=O (-one)
4. -OH (-ol)
5. -X (-halo, prefix only)

Example with two groups

Compound: $HOCH_2COOH$

• Has -COOH (highest priority, suffix).
• Also -OH (lower priority, prefix as 'hydroxy').
• Main chain: 2 C.
• Name: 2-hydroxyethanoic acid (also called glycolic acid).

Practice Problems

What's CH₃-CH(OH)-CH₃?

3 C → prop. -OH → -ol. Position 2. Name: propan-2-ol (or iso-propanol).

What's CH₃-CH₂-CH₂-CHO?

4 C (count: CH₃-CH₂-CH₂-CHO = 4) → but. -CHO → -al. Position 1 (always at end for aldehydes). Name: butanal.

What's CH₃-CO-CH₂-CH₃?

4 C (CH₃-CO-CH₂-CH₃ = 4) → but. >C=O → -one. Position 2 (between C-2 and C-3 in the 4-C chain). Name: butan-2-one.

Notice: ketones must have the C=O between two C atoms. Position is always the carbon of C=O.

Common Names — Important Examples

While IUPAC is preferred, you should know common names too — they appear in real-world contexts and exams.

Alcohols

Common Name	IUPAC Name	Formula
Methyl alcohol	Methanol	CH₃OH
Wood alcohol	Methanol	CH₃OH
Ethyl alcohol	Ethanol	C₂H₅OH
Grain alcohol	Ethanol	C₂H₅OH
Iso-propyl alcohol	Propan-2-ol	(CH₃)₂CHOH

Aldehydes

Common Name	IUPAC Name	Formula
Formaldehyde	Methanal	HCHO
Acetaldehyde	Ethanal	CH₃CHO
Propionaldehyde	Propanal	C₂H₅CHO

Ketones

Common Name	IUPAC Name	Formula
Acetone	Propanone	(CH₃)₂CO
Methyl ethyl ketone	Butan-2-one	CH₃-CO-C₂H₅

Carboxylic Acids

Common Name	IUPAC Name	Formula
Formic acid	Methanoic acid	HCOOH
Acetic acid	Ethanoic acid	CH₃COOH
Propionic acid	Propanoic acid	C₂H₅COOH
Butyric acid	Butanoic acid	C₃H₇COOH

Source of historical names:

• Formic acid: Latin 'formica' = ant (in ant stings).
• Acetic acid: Latin 'acetum' = vinegar.
• Butyric acid: Latin 'butyrum' = butter (in rancid butter).

Hydrocarbons

Common Name	IUPAC Name
Methane	Methane
Ethane	Ethane
Propane	Propane
n-Butane	Butane
Iso-butane	2-methylpropane
Acetylene	Ethyne
Ethylene	Ethene
Toluene	Methylbenzene

Halogens (Common vs IUPAC)

Common Name	IUPAC Name
Methyl chloride	Chloromethane
Ethyl chloride	Chloroethane
Iso-propyl chloride	2-chloropropane
Chloroform	Trichloromethane
Carbon tetrachloride	Tetrachloromethane

Practical Tips

1. Recognise both names. Common names are everyday vocabulary. 2. Use IUPAC for clarity. Especially in exams and scientific writing. *3. Memorise key compounds:

• Methanol, Ethanol, Propanone, Methanal, Methanoic acid, Ethanoic acid.

These are the most common in board exams.

Summary — Quick Reference

A 2-carbon compound's IUPAC names:

• Hydrocarbon (only single bonds): ethane.
• With double bond: ethene.
• With triple bond: ethyne.
• With -OH: ethanol.
• With -CHO: ethanal.
• With -COOH: ethanoic acid.
• With -Cl: chloroethane.
• With -Br: bromoethane.

For 1 C: methane, methanol, methanal, methanoic acid, chloromethane. For 3 C: propane, propan-1-ol or propan-2-ol, propanal, propanone, propanoic acid.

Master these patterns and you can name most simple organic compounds.

🧠 Memory Capsule

A quick glance just before the board exam.

1. IUPAC Name Structure

Prefix + Root + Suffix

Where:

• Root: number of C in main chain.
• Suffix: functional group.
• Prefix (optional): substituents (Cl, Br, methyl branches, etc.).

2. Roots — C Atoms

n	Root
1	meth-
2	eth-
3	prop-
4	but-
5	pent-
6	hex-
7	hept-
8	oct-
9	non-
10	dec-

3. Suffixes — Functional Groups

Group	Suffix
Alkane	-ane
Alkene	-ene
Alkyne	-yne
Alcohol (-OH)	-ol
Aldehyde (-CHO)	-al
Ketone (>C=O)	-one
Acid (-COOH)	-oic acid

4. Common Prefixes

• chloro- (-Cl), bromo- (-Br), fluoro- (-F), iodo- (-I)
• methyl- (CH₃-)
• ethyl- (C₂H₅-)
• hydroxy- (HO- as substituent, when -OH is not main group)

5. Numbering Rule

Number the chain to give the lowest number to functional group. If tie, then lowest to substituents.

6. Important Examples

Compound	IUPAC Name
CH₄	Methane
CH₃CH₂OH	Ethanol
CH₂=CH₂	Ethene
HC≡CH	Ethyne
HCHO	Methanal
CH₃CHO	Ethanal
(CH₃)₂CO	Propanone
CH₃COOH	Ethanoic acid
CH₃Cl	Chloromethane
CH₃CHBrCH₃	2-bromopropane

7. Common ↔ IUPAC Conversions

Common	IUPAC
Methyl alcohol	Methanol
Ethyl alcohol	Ethanol
Acetone	Propanone
Formaldehyde	Methanal
Acetaldehyde	Ethanal
Formic acid	Methanoic acid
Acetic acid	Ethanoic acid
Iso-butane	2-methylpropane
Acetylene	Ethyne
Ethylene	Ethene

8. Board's 'Golden' Questions

1. Name the IUPAC name of CH₃CH₂OH and CH₃COOH.
2. Common name of methanal? formaldehyde.
3. Difference between propanone and propanal.
4. Name CH₃-CO-CH₃ in IUPAC.
5. What is iso-propanol in IUPAC?

Final Formula: Root tells C count; Suffix tells function; Prefix tells branches/groups. Read the name → get the structure.

Solved Examples

Example 1: NCERT — Name These Compounds

Give IUPAC names for: (a) CH₃CH₂CH₂OH (b) CH₃CHO (c) HCOOH (d) CH₃-CO-CH₃ (e) CH₃Cl

Solution:

(a) CH₃CH₂CH₂OH

Identify:

• C count in main chain: 3 → 'prop'.
• Functional group: -OH (alcohol) → '-ol'.
• Position of OH: on terminal C (C-1).

IUPAC Name: Propan-1-ol (or simply propanol)

Common name: n-propyl alcohol.

(b) CH₃CHO

Identify:

• C count: 2 (1 in CH₃ + 1 in CHO) → 'eth'.
• Functional group: -CHO (aldehyde) → '-al'.
• Position: always at end (C-1) for aldehydes.

IUPAC Name: Ethanal

Common name: acetaldehyde.

(c) HCOOH

Identify:

• C count: 1 → 'meth'.
• Functional group: -COOH (carboxylic acid) → '-oic acid'.

IUPAC Name: Methanoic acid

Common name: formic acid.

Source: 'formic' from Latin 'formica' = ant. Ants produce this acid.

(d) CH₃-CO-CH₃

Identify:

• C count: 3 → 'prop'.
• Functional group: >C=O (ketone) → '-one'.
• Position: on the middle C (C-2 in the 3-C chain).

IUPAC Name: Propanone (or propan-2-one if specified)

Common name: acetone.

Familiar use: nail polish remover.

(e) CH₃Cl

Identify:

• C count: 1 → 'meth'.
• Functional group: alkane base, Cl as prefix.
• Suffix: -ane.
• Prefix: chloro-.

IUPAC Name: Chloromethane

Common name: methyl chloride.

Summary Table

Formula	IUPAC Name	Common Name
CH₃CH₂CH₂OH	Propan-1-ol	n-Propyl alcohol
CH₃CHO	Ethanal	Acetaldehyde
HCOOH	Methanoic acid	Formic acid
CH₃-CO-CH₃	Propanone	Acetone
CH₃Cl	Chloromethane	Methyl chloride

How to Approach

For each compound:

1. Count C atoms → choose root (meth, eth, prop, but…).
2. Identify functional group → choose suffix (-ol, -al, -one, -oic acid).
3. Find position (if needed) → number lowest position.
4. Check for substituents → add prefixes.
5. Combine: prefix + root + suffix.

Practice with many examples — soon naming becomes second nature.

[NCERT — important]

Example 2: NCERT — Draw the Structures

Draw the structural formulas for: (a) Butanone (b) Hexanal (c) Pentan-2-ol (d) Propanoic acid (e) 2-Bromoethane

Solution:

Approach

From IUPAC name to structure:

1. Root tells C count.
2. Suffix tells functional group.
3. Prefix/numbers tell substituent positions.
4. Build the structure.

(a) Butanone

Decode:

• 'But' = 4 C.
• '-one' = ketone.
• 'Butan-2-one' implies C=O at C-2.

Structure:

    CH₃ — CO — CH₂ — CH₃

Or detailed:

    H   H   O   H   H
    |   |   ||  |   |
    H — C — C — C — C — H
        |       |
        H       H

4 C in chain. C=O at C-2.

(b) Hexanal

Decode:

• 'Hex' = 6 C.
• '-al' = aldehyde.
• Aldehyde always at C-1.

Structure:

    CH₃ — CH₂ — CH₂ — CH₂ — CH₂ — CHO

Or:

         O
         ||
... — C — H

at the end.

Total: 6 C in chain. CHO at one end.

(c) Pentan-2-ol

Decode:

• 'Pentan' = 5 C.
• '-ol' = alcohol.
• Position 2 = -OH on C-2.

Structure:

    CH₃ — CH(OH) — CH₂ — CH₂ — CH₃

Or:

         OH
          |
    CH₃ — CH — CH₂ — CH₂ — CH₃

5 C in chain. OH on C-2.

(d) Propanoic Acid

Decode:

• 'Propan' = 3 C.
• '-oic acid' = carboxylic acid.

Structure:

    CH₃ — CH₂ — COOH

Or:

                  O
                  ||
    CH₃ — CH₂ — C — OH

3 C in chain. -COOH at end.

(e) 2-Bromoethane

Wait — 'ethane' has only 2 C. Where would position 2 be?

Actually, 2-bromoethane would have Br on C-2 of ethane, but ethane only has 2 C → C-2 is one of the only two carbons.

The compound is just CH₃-CH₂Br (bromoethane), since for ethane there's no need to specify position.

Probably the question means 2-bromopropane or just bromoethane.

Let's assume 'bromoethane':

Decode:

• 'Eth' = 2 C.
• 'Bromo' = -Br.
• Suffix '-ane' = single bonds only.

Structure:

    CH₃ — CH₂ — Br

Or:

    H   H
    |   |
H — C — C — Br
    |   |
    H   H

Summary

Drawing structures from IUPAC names:

Name	Structure
Butanone	CH₃COC₂H₅
Hexanal	CH₃(CH₂)₄CHO
Pentan-2-ol	CH₃CH(OH)C₃H₇
Propanoic acid	CH₃CH₂COOH
Bromoethane	CH₃CH₂Br

Practice Tip

The reverse process is also important — given a structure, name it. Try drawing structures of:

• Methanol
• Acetone
• Acetic acid
• 2-chloropropane

With practice, you'll be able to:

• Convert IUPAC name → structure.
• Convert structure → IUPAC name.
• Recognise compounds by either name.

[NCERT + Board]

Example 3: Naming Compounds with Branches

Name the following: (a) (CH₃)₂CHCH₃ (b) CH₃-CH(CH₃)-CH₂-CH₃ (c) CH₃-CHCl-CH₃ (d) (CH₃)₃C-OH

Solution:

(a) (CH₃)₂CHCH₃

Structural formula:

    CH₃
     |
  CH₃ — CH — CH₃

This is iso-butane.

IUPAC analysis:

• Main chain: 3 C (longest = 3).
• Branch: 1 CH₃ on C-2.
• Suffix: -ane (single bonds).

IUPAC Name: 2-Methylpropane

Common name: iso-butane.

(b) CH₃-CH(CH₃)-CH₂-CH₃

Structural formula:

         CH₃
          |
    CH₃ — CH — CH₂ — CH₃

IUPAC analysis:

• Longest chain: 4 C.
• Branch: 1 CH₃ on C-2.
• Suffix: -ane.

IUPAC Name: 2-Methylbutane

Common name: iso-pentane.

(c) CH₃-CHCl-CH₃

Structural formula:

         Cl
          |
    CH₃ — CH — CH₃

IUPAC analysis:

• Main chain: 3 C → 'prop'.
• Substituent: Cl on C-2 → '2-chloro' prefix.
• Suffix: -ane.

IUPAC Name: 2-Chloropropane

Common name: iso-propyl chloride.

(d) (CH₃)₃C-OH

Structural formula:

         CH₃
          |
    CH₃ — C — OH
          |
         CH₃

IUPAC analysis:

• Longest chain… here, 3 C is part of the main chain (with the carbon bearing OH).
• Wait, let me recount.
• Carbon with OH + 3 CH₃ = 4 C total.
• Longest chain: actually only 2 C straight (CH₃-C(CH₃)₂OH).

Hmm, this is a tricky one. Let me re-analyze:

(CH₃)₃C-OH

• Central C connected to: 3 CH₃ groups + 1 OH.
• Longest C chain: from a CH₃ through central C to another CH₃ = 3 C.
• The other CH₃ is a branch.

Naming:

• Main chain: 3 C → 'prop'.
• -OH on C-2 → 'propan-2-ol'.
• Branch: 1 CH₃ on C-2 (also).

IUPAC Name: 2-Methylpropan-2-ol

Common name: tert-butanol or tert-butyl alcohol.

Summary Table

Compound	IUPAC Name	Common Name
(CH₃)₂CHCH₃	2-Methylpropane	Iso-butane
CH₃CH(CH₃)CH₂CH₃	2-Methylbutane	Iso-pentane
CH₃CHClCH₃	2-Chloropropane	Iso-propyl chloride
(CH₃)₃COH	2-Methylpropan-2-ol	Tert-butanol

Tips for Naming Branched Compounds

Step 1: Find the longest continuous chain.

Step 2: Number atoms from the end nearest to the substituent (gives lowest number).

Step 3: Identify branches and their positions.

Step 4: Combine: position-prefix-root-suffix.

Example: in 2-methylbutane:

• Longest chain = 4 C (butane).
• Methyl branch at position 2.
• Combined: 2-methylbutane.

Practice: Convert Common to IUPAC

Try yourself:

• Iso-pentane: answer is 2-methylbutane.
• Neo-pentane: answer is 2,2-dimethylpropane.
• Tert-butanol: answer is 2-methylpropan-2-ol.

With practice, you'll see the pattern.

[Board: 3-5 marks]

Example 4: A Concluding Question

(a) Define IUPAC nomenclature. (b) Give 5 examples of root + suffix combinations. (c) What is methanol vs methanal vs methanoic acid? (write structures) (d) Name CH₃CH(OH)CH₃ in IUPAC.

Solution:

(a) IUPAC Nomenclature

'IUPAC nomenclature' = the systematic, internationally agreed naming system for chemical compounds.

IUPAC = International Union of Pure and Applied Chemistry.

The system:

• Uses rules that are universally accepted.
• Each name has a clear meaning.
• Reading the name → you can draw the structure.
• Used in scientific publications, education, industry.

Format: Prefix + Root + Suffix

• Root: number of C in main chain (meth, eth, prop, but…).
• Suffix: functional group (-ane, -ene, -ol, -al, -oic acid, -one).
• Prefix: substituents (chloro, bromo, methyl, etc.).

Replaces older 'common names' that varied by region/source.

(b) 5 Examples of Root + Suffix

Example 1: Methane

meth (1 C) + ane (alkane) = methane (CH₄)

Example 2: Ethanol

eth (2 C) + ol (-OH) = ethanol (C₂H₅OH)

Example 3: Propanal

prop (3 C) + al (-CHO) = propanal (C₂H₅CHO)

Example 4: Butanone

but (4 C) + one (>C=O) = butanone (C₂H₅COCH₃)

Example 5: Pentanoic acid

pent (5 C) + oic acid (-COOH) = pentanoic acid (C₄H₉COOH)

(c) Methanol vs Methanal vs Methanoic acid

All have 1 C atom (root: meth-). Different functional groups → different suffixes.

Methanol — CH₃OH

Suffix: -ol (alcohol). Functional group: -OH.

Structure:

      H
      |
  H — C — OH
      |
      H

Common name: methyl alcohol or wood alcohol. Use: industrial solvent, fuel.

Methanal — HCHO (formaldehyde)

Suffix: -al (aldehyde). Functional group: -CHO.

Structure:

         O
         ||
      H — C — H

Common name: formaldehyde. Use: preserving biological specimens (formalin).

Methanoic acid — HCOOH (formic acid)

Suffix: -oic acid (carboxylic acid). Functional group: -COOH.

Structure:

         O
         ||
      H — C — O — H

Common name: formic acid. Source: ant stings.

Comparison Summary

Compound	Functional Group	Structure
Methanol	-OH	CH₃-OH
Methanal	-CHO	H-CHO
Methanoic acid	-COOH	H-COOH

Same C count (1), different functional groups, different chemistry.

All have similar root names — only suffix differs based on functional group.

(d) Name CH₃CH(OH)CH₃

Step 1: Count C atoms = 3 → root: 'prop'.

Step 2: Identify functional group = -OH (alcohol) → suffix: '-ol'.

Step 3: Find position of -OH: Structure:

CH₃ — CH(OH) — CH₃
   |       |       |
   1       2       3

The OH is on the middle carbon (C-2).

Step 4: Combine:

IUPAC Name: Propan-2-ol (also written as 2-propanol)

Common name: iso-propyl alcohol or iso-propanol (rubbing alcohol used as disinfectant).

Note: Numbering — both ends give the same number for OH (it's symmetric: C-1 and C-3 are CH₃, C-2 has OH). So no ambiguity.

Why Iso-propyl Alcohol is Useful

Iso-propanol (CH₃CH(OH)CH₃):

• Effective disinfectant.
• Used in hand sanitisers.
• Cleans electronic equipment.
• Boiling point 82°C — convenient for use.

Final Insight

The IUPAC nomenclature system:

1. Brings order to organic chemistry. 2. Allows clear communication globally. 3. Each name encodes the structure. 4. Master root + suffix combinations → name most simple compounds.

This systematic approach is one of the great achievements of modern chemistry — turning a chaotic mass of compounds into an organised system anyone can understand.

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