35 C9H20 constitutional isomers of molecular formula C9H20 structural isomers carbon chain isomers structural formula skeletal formula R/S optical isomers enantiomers stereoisomers stereoisomerism in C9H20 alkane isomers Doc Brown's advanced level chemistry revision notes for AQA Edexcel OCR Salters IB US honors courses

isomers of C₉H₂₀

Advanced level organic chemistry PART 14.7: 35 Constitutional structural isomers of molecular formula C₉H₂₀

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Doc Brown's Advanced Chemistry: Part 14.7 Isomers of a given molecular formula

The 35 constitutional-structural carbon chain isomers of molecular formula C₉H₂₀

[Author © Dr Phil Brown PhD: Doc Brown's advanced level organic chemistry exam revision notes suitable for students of UK advanced level chemistry courses, IB advanced chemistry & US K12 grades 11-12 and AP honors chemistry courses: Molecular spectroscopy and analysing the isomers of C₉H₂₀ [page updated Mar 1st 2026 *]

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Associated organic chemistry page links

Index of sets of isomers for a given molecular formula

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Introduction to 35 alkane carbon chain constitutional-structural isomers of molecular formula C₉H₂₀ (M_r = 128)

35 constitutional isomers of C9H20 alkanes skeletal formula R/S optical isomers enantiomers meso isomers of molecular formula C9H20

R/S means molecule is capable of forming R/S 'optical' isomers (enantiomer pairs) i.e. has a chiral carbon atom, more details below

Percent composition based on atomic masses C= 12.01 H = 1.01 and M_r(C₉H₂₀) = 128.29

Element composition (to two dp): carbon = 84.25% hydrogen = 15.75%

Empirical formula = C₉H₂₀ = molecular formula = C₉H₂₀

Structural isomerism includes carbon chain variation (usually need a minimum of 4 C atoms), change in position of a substituent or functional group and functional group isomerism where the atoms have a different configuration, usually with significant differences in chemical and physical properties.

Only carbon chain isomerism applies to the isomers of C₉H₂₀.

Stereoisomerism is where molecules have the same basic constitutional structural formula, but isomers differ in the 2D/3D arrangement of the atoms.

E/Z stereoisomerism was called 'geometrical isomerism' e.g. cis and trans isomers of alkenes or disubstituted cyclic alkanes where there are 3D spatial variations due to restricted bond rotation that are not mirror images and not super imposable.

Does NOT apply to the isomers of C₉H₂₀.

R/S stereoisomerism was called 'optical isomerism', the pairs of isomers are called enantiomers which are 3D non-superimposable mirror image forms of the molecule. The molecule must have a chiral centre (a stereocentre), that is an asymmetric carbon atom with four different atoms/groups attached to it.

There are several examples of R/S 'optical' isomerism in the isomers of C₉H₂₀.

NOTE

All 35 constitutional isomers of C₉H₂₀ are examples of carbon chain isomerism, but a number of them, theoretically, will exhibit R/S isomerism (non-superimposable mirror images, optical isomers known as enantiomers).

The side chains are the methyl and ethyl groups.

There are quite a few stereoisomers, 15 isomers exhibit R/S isomerism and three have two chiral carbon atoms, so things get complicated - university level analysis required.

This means there are at least 18 stereoisomers for C₉H₂₀.

Details of 35 constitutional isomers of C₉H₂₀ (and accompanying R/S 'optical' isomers indicated)

(1) nonane, CH₃CH₂CH₂CH₂CH₂CH₂CH₂CH₂CH₃ or CH₃(CH₂)₇CH₃

skeletal formula, A linear alkane hydrocarbon molecule

Details of isomers based on a carbon chain of eight (substituted octanes)

(2) 2-methyloctane, CH₃CH₂CH₂CH₂CH₂CH₂CH(CH₃)₂

skeletal formula

(3) 3-methyloctane, CH₃CH₂CH₂CH₂CH₂CH(CH₃)CH₂CH₃

skeletal formula

R/S 'optical' isomers (enantiomers), 3rd C is chiral

(4) 4-methyloctane, CH₃CH₂CH₂CH₂CH(CH₃)CH₂CH₂CH₃

skeletal formula

R/S 'optical' isomers (enantiomers), 4th C is chiral

There are lots of more highly branched structural isomers of C₉H₂₀ (31 more in fact !!!)

Details of isomers based on a carbon chain of seven (substituted heptanes)

(5) 3-ethylheptane, skeletal formula

(6) 4-ethylheptane, skeletal formula

(7) 2,2-dimethylheptane, skeletal formula

(8) 2,3-dimethylheptane, skeletal formula

R/S 'optical' isomers (enantiomers), carbon atom 3 is chiral

(9) 2,4-dimethylheptane, skeletal formula

R/S 'optical' isomers (enantiomers), carbon atom 4 is chiral

(10) 2,5-dimethylheptane, skeletal formula

R/S 'optical' isomers (enantiomers), carbon atom 5 is chiral

(11) 2,6-dimethylheptane, skeletal formula

(12) 3,3-dimethylheptane, skeletal formula

(13) 3,4-dimethylheptane, skeletal formula

R/S 'optical' isomers (enantiomers), carbon atoms 3 and 4 are two chiral centres - complicated, university level analysis - no plane of symmetry in the molecule, so four permutations of stereoisomers possible: RR', RS', SR' and SS'

(14) 3,5-dimethylheptane, skeletal formula

R/S isomers, carbon atoms 3 and 5 are chiral (two stereocentres), but there is a plane of symmetry, pair of enantiomers R/S and a 3rd optically inactive meso isomer - university level analysis.

(15) 4,4-dimethylheptane, skeletal formula

Details of isomers based on a carbon chain of six (substituted hexanes)

The images (16) to (19) were originally done for my learning alkanes nomenclature page, now redone.

(16) 3-ethyl-2-methylhexane , ,

skeletal formula R/S 'optical' isomers (enantiomers), the molecule has a chiral (asymmetric) carbon atom (3rd C) - example of stereoisomerism - R/S (optical) isomers

(17) 3-ethyl-3-methylhexane , ,

skeletal formula

(18) 3-ethyl-4-methylhexane , ,

skeletal formula

R/S 'optical' isomers (enantiomers), it has a chiral (asymmetric) carbon atom (4th C) - example of stereoisomerism.

(19) 4-ethyl-2-methylhexane , ,

skeletal formula

(20) 2,2,3-trimethylhexane, skeletal formula

R/S 'optical' isomers (enantiomers), it has a chiral (asymmetric) carbon atom (3rd C) - example of stereoisomerism.

(21) 2,2,4-trimethylhexane, skeletal formula

R/S 'optical' isomers (enantiomers), it has a chiral (asymmetric) carbon atom (4th C) - example of stereoisomerism.

(22) 2,2,5-trimethylhexane, skeletal formula

(23) 2,3,3-trimethylhexane, skeletal formula

(24) 2,3,4-trimethylhexane, skeletal formula

R/S 'optical' isomers (enantiomers), carbon atoms 3 and 4 are two chiral centres - complicated, university level analysis - molecule has no plane of symmetry, so four possible permutations of the stereoisomers: RR', RS', SR' and SS'

(25) 2,3,5-trimethylhexane, skeletal formula

R/S 'optical' isomers (enantiomers), carbon atom 3 is chiral

(26) 2,4,4-trimethylhexane, skeletal formula

(27) 3,3,4-trimethylhexane, skeletal formula

R/S 'optical' isomers (enantiomers), carbon atom 4 is chiral

Details of isomers based on a carbon chain of five (substituted pentanes)

(28) 3,3-diethylpentane, skeletal formula

(29) 3-ethyl-2,2-dimethylpentane, ,

skeletal formula

(30) 3-ethyl-2,3-dimethylpentane, ,

skeletal formula

(31) 3-ethyl-2,4-dimethylpentane, ,

skeletal formula

(32) 2,2,3,3-tetramethylpentane, skeletal formula

(33) 2,2,3,4-tetramethylpentane, skeletal formula

R/S 'optical' isomers (enantiomers), carbon atom 3 is chiral

(34) 2,3,3,4-tetramethylpentane, skeletal formula

(35) 2,2,4,4-tetramethylpentane, skeletal formula

EXTRA NOTES on isomers of C₉H₂₀

Alkane isomers with molecular formula C₉H₂₀ (nonane and its branched variants) are primarily used as fuels, solvents, and chemical feedstocks, with differences in volatility and combustion efficiency influencing their specific applications.

Overview of C₉H₂₀ Alkane Isomers

General formula: C_nH_2n+2 → C₉H₂₀
Isomer count: There are 35 structural isomers of nonane, including:
- Straight-chain: n-nonane
- Branched-chain: e.g., 2-methyl-octane, 3,4-dimethyl-heptane, 2,2,4-trimethyl-hexane

These isomers differ in carbon branching, which affects boiling point, flash point, and combustion characteristics.

Uses and Applications of isomers of C₉H₂₀

Isomer Type	Key Properties	Common Uses
n-Nonane	High boiling point (~151 °C), low volatility	- Jet fuel and diesel blending - Organic solvent - Calibration standard
Branched isomers	Lower boiling points, higher octane ratings	- Petrol additives (e.g., 2,2,4-trimethylpentane = isooctane) - Cleaner combustion fuels

Sources:

Found in crude oil and separated via fractional distillation
Can be cracked to produce smaller alkenes and alkanes for plastics and fuels

Comparative Analysis of isomers of C₉H₂₀

Feature	n-Nonane	Branched Isomers
Boiling point	Higher	Lower due to compact structure
Octane rating	Lower (poor anti-knock quality)	Higher (better for internal combustion engines)
Combustion efficiency	Slower, more complete	Faster, cleaner burn
Industrial preference	Jet fuels, solvents	Petrol additives, performance fuels

Summary of Applications of isomers of C₉H₂₀

Fuel blending: Branched isomers are preferred in gasoline due to high octane ratings.
Solvent use: n-Nonane is used in analytical chemistry and industrial cleaning.
Chemical synthesis: All isomers can serve as feedstocks for cracking and reforming processes.
Environmental considerations: Branched isomers tend to produce fewer pollutants during combustion.

Learning objectives - questions to be answered?

How many isomers are there of molecular formula C9H20?

How do you work out the structure of the isomers of molecular formula C9H20?

How do you draw the constitutional-structural formula of the isomers of molecular formula C9H20?

How do you draw the skeletal formula of the isomers of molecular formula C9H20?

How do you name the isomers of molecular formula C9H20?

How many aliphatic structural isomers are there of molecular formula C9H20?

How many aliphatic carbon chain isomers are there of molecular formula C9H20?

How many positional isomers are there of molecular formula C9H20?

Does C9H20 have any stereoisomers?

Are there any E/Z (geometrical) or RS (optical) stereoisomers (enantiomers) of C9H20?

Are there any aliphatic open chain alkene isomers of molecular formula C9H20?

Are there any alkane/cycloalkane isomers of molecular formula C9H20?

Are there any alkene/cycloalkene/diene/alkyne isomers of molecular formula C9H20?

Are there any alicyclic cycloalkane isomers of molecular formula C9H20?

Are there any functional group isomers with a molecular formula C9H20?

This page will answer these questions for molecular formula C₉H₂₀

Associated organic chemistry links

Advanced Level pre-university organic chemistry notes

IR, mass and H-1 & C-13 NMR spectra of organic compounds

Examples of effects of isomerism: The similarity or difference in the physical & chemical properties of structural isomers

Index of sets of isomers for a given molecular formula

The molecular structure and naming of ALKANES

Index of revision notes on the chemistry of ALKANES and the petrochemical industry

Isomerism: introduction, structural isomerism - chain, positional, functional group, tautomerism

Stereoisomerism: introduction, definition, priority rules, E/Z isomerism (cis/trans isomerism)

Stereoisomerism - R/S isomerism (optical isomerism) - definition - examples explained

This is a big chemistry website, please allow time to explore it

Website content © Dr Phil Brown 2000+. All copyrights reserved on revision notes, images, quizzes, worksheets etc. Copying of Doc Brown's pre-university advanced level chemistry website material is NOT permitted. Exam revision summaries & references to science course specifications are unofficial. These organic chemistry revision notes on isomerism are suitable for use of pre-university students studying AQA advanced level chemistry, Edexcel advanced level chemistry, OCR advanced level chemistry, IB advanced level chemistry, WJEC (Eduqas) advanced level chemistry, CIE advanced level chemistry, US grade 11-12 AP honors chemistry courses and they will also prove useful to 1st year undergraduate students of chemistry. The isomerism of molecules of formulae C₉H₂₀, structural constitutional isomers and R/S optical stereoisomers

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Overview of C9H20 Alkane Isomers

Uses and Applications of isomers of C9H20

Comparative Analysis of isomers of C9H20

Summary of Applications of isomers of C9H20

Overview of C₉H₂₀ Alkane Isomers

Uses and Applications of isomers of C₉H₂₀

Comparative Analysis of isomers of C₉H₂₀

Summary of Applications of isomers of C₉H₂₀