Write the IUPAC names of the following ketones and aldehydes. Wherever possible, give also common names. (i) $\mathrm{CH_3CO(CH_2)_4CH_3}$ (ii) $\mathrm{CH_3CH_2CHBrCH_2CH(CH_3)CHO}$ (iii) $\mathrm{CH_3(CH_2)_5CHO}$ (iv) $\mathrm{Ph\text{-}CH{=}CH\text{-}CHO}$ ![Exercise 8.4 (v): structure(s) drawn as printed in the NCERT textbook, with the labels CHO](/figures/aldehydes-ketones-and-carboxylic-acids/cyclopentane-ring-cho-iupac-naming-exercise-8-4.svg) (vi) $\mathrm{PhCOPh}$

> [!TLDR] > Each name comes from naming the longest carbon chain (or ring) bearing the carbonyl group. Aldehydes end in -al with the -CHO carbon as C-1; ketones end in -one with the lowest possible locant for the C=O; a -CHO attached straight onto a ring is a "...carbaldehyde". ### How to name each **(i) CH3CO(CH2)4CH3** — Expand: CH3-CO-CH2-CH2-CH2-CH2-CH3. The longest chain has 7 carbons with the C=O at position 2, so it is **heptan-2-one**. Common name = methyl pentyl ketone (methyl n-amyl ketone), from the two groups flanking the C=O. **(ii) CH3CH2CHBrCH2CH(CH3)CHO** — The -CHO fixes C-1. Numbering from the aldehyde: C1 = CHO, C2 = CH(CH3) (a methyl branch), C3 = CH2, C4 = CHBr (a bromo substituent), C5 = CH2, C6 = CH3. A 6-carbon aldehyde with substituents Br at C-4 and CH3 at C-2 → **4-bromo-2-methylhexanal**. **(iii) CH3(CH2)5CHO** — A straight chain of 7 carbons ending in -CHO → **heptanal** (common name heptaldehyde or enanthaldehyde). **(iv) C6H5-CH=CH-CHO** — The parent is prop-2-enal (CHO = C1, C2=C3 double bond) carrying a phenyl group on C-3 → **3-phenylprop-2-enal**; its common name is **cinnamaldehyde**. …

Write the IUPAC names of the following ketones and aldehydes. Wherever possible, give also common names. (i) m

Exercises · 8.4

Q. Write the IUPAC names of the following ketones and aldehydes. Wherever possible, give also common names.

(i)

\mathrm{CH_3CO(CH_2)_4CH_3}

(ii)

\mathrm{CH_3CH_2CHBrCH_2CH(CH_3)CHO}

(iii)

\mathrm{CH_3(CH_2)_5CHO}

(iv)

\mathrm{Ph\text{-}CH{=}CH\text{-}CHO}

Exercise 8.4 (v): structure(s) drawn as printed in the NCERT textbook, with the labels CHO — Figure

(vi)

\mathrm{PhCOPh}

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Concept understanding — IUPAC Nomenclature

IUPAC Nomenclature (Organic Compounds)

IUPAC nomenclature is a systematic way to name a compound so that its name alone tells you its exact structure, with no ambiguity. Every organic name follows the same underlying recipe, whatever the functional group.

The Recipe

Identify the principal characteristic group. If the molecule has a functional group senior enough to be named as a suffix (carboxylic acid > ester > amide > nitrile > aldehyde > ketone > alcohol > amine, and so on down the seniority order), that group decides the suffix and must be included in the parent chain. A halogen is never senior enough to be a suffix — it is always named as a prefix ("halo-"), whatever else is present.
Choose the parent chain. The parent is the longest continuous carbon chain that contains the principal characteristic group (if there is one). Among chains of the same length, the one with the most substituents wins.
Number the chain. Number from whichever end gives the LOWEST LOCANT to the principal characteristic group first. If there is no principal group (e.g. a simple haloalkane, or an alkene with a halogen substituent), lowest locant goes to the site of unsaturation (double/triple bond) first, then to substituents as a set.

Watch out

When two numbering directions give the SAME locant for the principal group/unsaturation (a genuine tie), the tie-break is the lowest locant SET for the substituents as a group — compare the two sets at their first point of difference. Only if the sets are themselves tied does the alphabetically-first substituent get the lower number.
Name and cite the substituents as prefixes, in alphabetical order (ignoring multiplying prefixes like di-/tri- but not ignoring structural prefixes like iso-/cyclo-), each with its own locant.
Assemble the name: locants + substituent prefixes (alphabetical) + parent chain name + suffix (if any).

Worked Example

$CH_3-CH(Cl)-CH(CH_3)-CH_2-CH_3$ : the longest chain is 5 carbons (pentane), no principal characteristic group (just a halogen substituent), so number for the lowest locant set. From the left: Cl at C2, methyl at C3 → set {2,3}. From the right: methyl at C3, Cl at C4 → set {3,4}. {2,3} is lower, so numbering from the left wins: 2-chloro-3-methylpentane. …

Why this formula?

IUPAC Nomenclature: Why the Rules Work the Way They Do

IUPAC nomenclature is not a single formula, but a system of rules designed to give every organic compound a unique, unambiguous name. The "why" behind these rules lies in clarity, consistency, and communication — ensuring that a chemist in Tokyo and one in Toronto draw the same structure from the same name.

1. The Core Principle: The Longest Carbon Chain

Rule: Identify the longest continuous chain of carbon atoms. This becomes the parent chain (e.g., pentane, hexane).

Why?

The longest chain represents the backbone of the molecule.
It gives the most stable, fundamental name — shorter chains would be branches, not the main structure.
Example: In a molecule with 5 carbons in a row and a 2-carbon branch, calling it "pentane" (not "ethane") tells you the core skeleton is 5 carbons long.

Key idea: The parent chain is the maximum continuous path — not necessarily the one that looks "straight" on paper.

2. Numbering: Lowest Locants (The "First Point of Difference" Rule)

Rule: Number the parent chain so that substituents get the smallest possible numbers. When there's a tie, compare the first point of difference.

Why?

This ensures reproducibility — two chemists will always number the same way.
It avoids ambiguity: "2-methylpentane" is unambiguous; "3-methylpentane" would be a different compound.
The first point of difference rule: If you have substituents at positions 2,4 and 3,5, choose 2,4 because 2 < 3 (the first number is smaller).

Example:

For a methyl group on carbon 2 vs. carbon 4 of a 5-carbon chain:
- 2-methylpentane (correct)
- 4-methylpentane (wrong — higher number)

3. Alphabetical Order of Substituents

Rule: List substituents in alphabetical order (ignoring prefixes like di-, tri-, sec-, tert- but not iso-).

Why?

Alphabetical order is a universal sorting convention — no need to remember priority based on size or complexity.
It makes names searchable and predictable.
Example: "3-ethyl-2-methylpentane" (e before m) — not "2-methyl-3-ethylpentane".

Exception: Prefixes like iso- and neo- are considered part of the name (e.g., isopropyl comes before methyl because "i" < "m").

4. Multiple Bonds: The "Lowest Locant" Rule for Alkenes/Alkynes

Rule: Number the chain so that the double or triple bond gets the lowest possible number, even if it means giving a substituent a higher number.

Why?

The functional group (alkene/alkyne) is more important than alkyl substituents.
The bond position defines the compound's reactivity and geometry.
Example: In pent-2-ene (not pent-3-ene), the double bond is between carbons 2 and 3 — the lower number (2) is used.

Priority order:

Principal functional group (e.g., -OH, -COOH, C=C)
Multiple bonds
Substituents (alkyl, halo, etc.)

5. The "Suffix" and "Prefix" System …

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Each name comes from naming the longest carbon chain (or ring) bearing the carbonyl group. Aldehydes end in -al with the -CHO carbon as C-1; ketones end in -one with the lowest possible locant for the C=O; a -CHO attached straight onto a ring is a "...carbaldehyde".

How to name each

CH3CO(CH2)4CH3 — Expand: CH3-CO-CH2-CH2-CH2-CH2-CH3. The longest chain has 7 carbons with the C=O at position 2, so it is heptan-2-one. Common name = methyl pentyl ketone (methyl n-amyl ketone), from the two groups flanking the C=O.
CH3CH2CHBrCH2CH(CH3)CHO — The -CHO fixes C-1. Numbering from the aldehyde: C1 = CHO, C2 = CH(CH3) (a methyl branch), C3 = CH2, C4 = CHBr (a bromo substituent), C5 = CH2, C6 = CH3. A 6-carbon aldehyde with substituents Br at C-4 and CH3 at C-2 → 4-bromo-2-methylhexanal.
CH3(CH2)5CHO — A straight chain of 7 carbons ending in -CHO → heptanal (common name heptaldehyde or enanthaldehyde).
C6H5-CH=CH-CHO — The parent is prop-2-enal (CHO = C1, C2=C3 double bond) carrying a phenyl group on C-3 → 3-phenylprop-2-enal; its common name is cinnamaldehyde. …

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Q.Write the IUPAC names of the following ketones and aldehydes. Wherever possible, give also common names.

Concept understanding — IUPAC Nomenclature

IUPAC Nomenclature (Organic Compounds)

The Recipe

Worked Example

Why this formula?

IUPAC Nomenclature: Why the Rules Work the Way They Do

1. The Core Principle: The Longest Carbon Chain

2. Numbering: Lowest Locants (The "First Point of Difference" Rule)

3. Alphabetical Order of Substituents

4. Multiple Bonds: The "Lowest Locant" Rule for Alkenes/Alkynes

5. The "Suffix" and "Prefix" System …

How to name each

Method: IUPAC Nomenclature of Ketones and Aldehydes (Chain Selection + Common Names)

Core Concept

Steps

Q.Write the IUPAC names of the following ketones and aldehydes. Wherever possible, give also common names.

How to name each

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