Examples of constitutional isomers of molecular formula C7H8, structural isomers carbon chain structural constitutional isomers of C7H8 formula skeletal formula of isomers alkenes alkynes cycloalkenes cycloalkynes Doc Brown's advanced level chemistry revision notes for AQA Edexcel OCR Salters IB US honors chemistry courses

isomers of C₇H₈

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Advanced level organic chemistry PART 14.7: Selected structural isomers of molecular formula C₇H₈

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

Examples of the constitutional structural isomers of molecular formula C₇H₈

[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 C₇H₈ [page updated Mar 1st 2026 *]

Index of sets of isomers for a given molecular formula

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Associated organic chemistry page links

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Introduction to examples of constitutional-structural isomerism of molecular formula C₇H₈ (M_r = 92)

Examples of constitutional isomers of C7H8 molecular mass 92 constitutional structural formula of C7H8 alkynes alkenes open chain and C7H8 cyclic compounds

Percent composition of C₇H₈ based on atomic masses C= 12.01 H = 1.01 and M_r(C₇H₈) = 92.15

Element composition (to two dp): carbon = 91.23% hydrogen = 8.77%

Empirical formula = molecular formula = C₇H₈

Structural isomerism - isomers based on different connectivity's of the constituent atoms, so cannot be spatially identical (but can be defined as having the same shape).

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

All three types of structural isomerism apply to the isomers of C₇H₈.

Stereoisomerism is where molecules have the same basic constitutional structural formula, but isomers differ in the 2D/3D arrangement of the atoms. There are non for the isomers of C₇H₈ I selected to describe in detail.

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 that are not mirror images and not super imposable.

There is one example of E/Z 'geometrical' isomerism for the isomers of C₇H₈ I selected to describe in detail.

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 no R/S optical isomers for the isomers of C₇H₈ I selected to describe in detail.

NOTE

There are open chain and ring isomers, different functional group positions, and functional group isomerism e.g. cycloalkenes, bicycloalkanes, bicycloalkenes, open chain alkynes - there are potentially several hundred theoretical isomers (see the bottom C₆H₆ isomer page link, and its a simpler formula than C₇H₈ !!!).

Some of these isomers are highly reactive and very unstable and some may not exist at all (except theoretically).

Details of the selected constitutional isomers and stereoisomers of molecular formula C₇H₈

(1) methylbenzene (toluene) C₇H₈, C₆H₅CH_{3
,} , ,

The simplest molecule where an alkyl group has replaced a hydrogen atom in the benzene ring.

It is the only benzene ring compound with the formula C₇H₈

See also spectra that can distinguish one C7H8 isomer from another

The infrared spectrum of methylbenzene

The mass spectrum of methylbenzene

The H-1 NMR spectrum of methylbenzene

The C-13 NMR spectrum of methylbenzene

Selected examples of unsaturated cycloalkene molecules of formula C₇H₈

(2) cyclohepta-1,3,5-triene or 1,3,5-cycloheptatriene,

(3) 5-methylenecyclohexa-1,3-diene or 5-methylene-1,3-cyclohexadiene,

(4) 3-methylenecyclohexa-1,4-diene or 3-methylene-1,4-cyclohexadiene,

(5) 5-ethylidenecyclopenta-1,3-diene or 5-ethylidene-1,3-cyclopentadiene,

(6) 1-methyl-5-methylenecyclopenta-1,3-diene or 1-methyl-5-methylidene-1,3-cyclopentadiene,

(7) 3-methyl-5-methylenecyclopenta-1,3-diene or 1-methyl-5-methylidene-1,3-cyclopentadiene,

Examples of open chain alkyne molecules of formula C7H8

(8) hepta-1,3-diyne or 1,3-heptadiyne, H-C≡C-C≡C-CH₂-CH₂-CH₃,

(9) hepta-1,4-diyne or 1,4-heptadiyne, H-C≡C-CH₂-C≡C-CH₂-CH₃,

(10) 3-methylhexa-1,5-diyne or 3-methyl-1,5-hexadiyne, H-C≡C-CH₂-CH₂-C≡C-H,

(11) hepta-2,4-diyne or 2,4-heptadiyne, H₃C-C≡C-C≡C-CH₂-CH₃,

Examples of open chain alkene-alkyne molecules of formula C7H8, two functional groups.

(12) hepta-1,5-diene-3-yne or 1,5-heptadiene-3-yne,

skeletal formula of the two E/Z geometrical isomers

Since the C=C double bond starts on carbon atom 5 (for the E/Z isomers), the full names of the E/Z isomers are for example

e.g. (5E)-1,5-heptadien-3-yne and (5Z)-hepta-1,5-diene-3-yne

CIP assignment priority rule for E/Z isomers: ₆C₆C > ₆C₁H > ₁H

(13) 2-methylhexa-1,5-diene-3-yne or 2-methyl-1,5-hexadiene-3-yne,

molport website https://www.molport.com/shop/molecular-formula/C7H8 shows you some of the commercially available isomers of C7H8, and, as you can see, there is quite a variety of open chain aliphatic alkynes, alkene-alkynes, unsaturated ring compounds (alicyclic) and saturated cycloalkanes (alicyclic).

PLEASE NOTE you use 'pure' skeletal formulae, with no C or H shown in hydrocarbon structures and I rarely use Kekule structures for the benzene ring. BUT, many websites use molecular structures which are a hybrid of a skeletal formula on which is super-imposed an abbreviated-condensed structural formula e.g. for methylbenzene and molport website (link above).

Learning objectives - questions to be answered?

How do you work out the structure of the isomers of molecular formula C7H8?

How do you draw the structural formula and skeletal formula of the isomers of molecular formula C7H8?

How do you name the isomers of molecular formula C7H8?

How many aliphatic structural isomers are there of molecular formula C7H8?

How many aliphatic carbon chain isomers are there of molecular formula C7H8?

How many positional isomers are there of molecular formula C7H8?

Are there any alkyne isomers of C7H8

What are the alkyne isomers of C7H8?

Any cyclic alkenes? Any cyclic alkynes?

Are there any aliphatic open chain alkene isomers of molecular formula C7H8?

Are there any alkene isomers of molecular formula C7H8?

Are there any functional group isomers with a molecular formula C7H8?

Does C7H8 have any stereoisomers?

Are there any E/Z (geometrical) isomers with a molecular formula C7H8?

Are there any R/S (optical) isomers (enantiomers) with a molecular formula C7H8?

How many E/Z (geometrical) isomers are there of molecular formula C7H8?

How many R/S (optical) isomers (enantiomers) of molecular formula C7H8?

This page will answer these questions for molecular formula C7H8

Associated organic chemistry links

Index of sets of isomers for a given molecular formula

All my advanced Level pre-university organic chemistry notes

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

The molecular structure and naming of ALKANES (how to name and draw alkane structures)

The molecular structure and naming of ALKENES (how to name and draw alkene structures)

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

INDEX of ALL revision notes on the chemistry ALKENES including reactions and polymers

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.

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