[Author ©  Dr Phil Brown GRIC, 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 C₉H₁₂ [page updated RE-EDIT]

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Percent composition of C₉H₁₂ based on atomic masses C= 12.01  H = 1.01  and  M_r(C₉H₁₂) = 120.21

Element composition of C₉H₁₂ aromatic molecules: carbon = 89.92%     hydrogen = 10.08%

Empirical formula = molecular formula = C₉H₁₂ of these aromatic molecules

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.

The aromatic isomerism of C₉H₁₂ described here is based on different carbon chain arrangements, which, overlaps with the different positions of the alkyl groups when there is more than one alkyl substituent in the C₉H₁₂ molecules

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 that are not mirror images and not super imposable.

Not applicable to the aromatic isomers of C₉H₁₂ described here.

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.

Not applicable to the aromatic isomers of C₉H₁₂ described here.

(2) Details of the 8 structural-constitutional isomers of the benzene ring C₉H₁₂ aromatic molecules

Although these 8 C₉H₁₂ isomers are primarily considered as aromatic molecules - aromatic hydrocarbons called arenes, they are sometimes named as aliphatic alkane compounds with e.g. a phenyl substituent.

The abbreviated-condensed structural formula are shown alongside the skeletal formula for the aromatic (benzene ring) isomers of C₉H₁₂

The expected number of 1H or 13C NMR peaks and the integrated proton ratio quoted are based on the structural formula of the given isomer.

(1) propylbenzene or 1-phenylpropane,    

Number of low resolution NMR chemical shift δ signal peaks: 6 ¹H and 7 ¹³C (email if disagree?)

¹H NMR integrated ratio of peaks: 1 : 2 : 2 : 2 : 2 : 3 (aromatic: alkyl, for equivalent protons)

For the 1H or 12C chemical environments of the benzene ring C3 = C5 and C2 = C6.

Spectra can be used to distinguish between the aromatic isomers of C₈H₁₀ e.g.

The infrared spectrum of propylbenzene

The infrared spectrum of (1-methylethyl)benzene  or  2-phenylpropane (Cumene)

The infrared spectrum of 1,3,5-trimethylbenzene

The mass spectrum of propylbenzene

The mass spectrum of (1-methylethyl)benzene or  2-phenylpropane (Cumene)

The mass spectrum of 1,3,5-trimethylbenzene

The H-1 NMR spectrum of propylbenzene

The H-1 NMR spectrum of (1-methylethyl)benzene  or  2-phenylpropane (Cumene)

The H-1 NMR spectrum of 1,3,5-trimethylbenzene

TheC-13 NMR spectrum of propylbenzene

The C-13 NMR spectrum of (1-methylethyl)benzene  or  2-phenylpropane (Cumene)

The C-13 NMR spectrum of 1,3,5-trimethylbenzene

 

(2)  (1-methylethyl)benzene or 2-phenylpropane, ,  

Number of low resolution NMR chemical shift δ signal peaks: 5 ¹H and 6 ¹³C (email if disagree?)

¹H NMR integrated ratio of peaks: 1 : 2 : 2 : 1 : 6 (aromatic: alkyl, for equivalent protons)

For the 1H or 12C chemical environments of the benzene ring C3 = C5 and C2 = C6 and the two methyl groups are equivalent to each other.

See also spectra links

 

(3) 1-ethyl-2-methylbenzene, ,

Number of low resolution NMR chemical shift δ signal peaks: 7 ¹H and 9 ¹³C (email if disagree?)

¹H NMR integrated ratio of peaks: 1 : 1 : 1 : 1 : 3 : 2 : 3 (aromatic: alkyl, for equivalent protons)

All of the 1H or 12C chemical environments are different.

No symmetry in the molecule, so the maximum number of possible chemical shifts should be observed.

 

(4) 1-ethyl-3-methylbenzene, ,

Number of low resolution NMR chemical shift δ signal peaks: 7 ¹H and 9 ¹³C (email if disagree?)

¹H NMR integrated ratio of peaks: 1 : 1 : 1 : 1 : 3 : 2 : 3 (aromatic: alkyl, for equivalent protons)

All of the 1H or 12C chemical environments are different.

No symmetry in the molecule, so the maximum number of possible chemical shifts should be observed.

 

(5) 1-ethyl-4-methylbenzene, ,

Number of low resolution NMR chemical shift δ signal peaks: 5 ¹H and 7 ¹³C (email if disagree?)

¹H NMR integrated ratio of peaks: 2 : 2 : 3 : 2 : 3 (aromatic: alkyl, for equivalent protons)

For the 1H or 12C chemical environments of the benzene ring C2 = C4 and C3 = C5.

 

(6) 1,2,3-trimethylbenzene, ,

Number of low resolution NMR chemical shift δ signal peaks: 4 ¹H and 6 ¹³C (email if disagree?)

¹H NMR integrated ratio of peaks: 1 : 2(1+1) : 6(3+3) : 3 (aromatic: alkyl, for equivalent protons)

For the 1H or 12C chemical environments of the benzene ring C1 = C3 and C4 = C6 and the two of the methyl groups are equivalent to each other (on C1 and C3).

 

(7) 1,2,4-trimethylbenzene, ,  

Number of low resolution NMR chemical shift δ signal peaks: 6 ¹H and 9 ¹³C (email if disagree?)

¹H NMR integrated ratio of peaks: 1 : 1 : 1 : 3 : 3 : 3 (aromatic: alkyl, for equivalent protons)

All of the 1H or 12C chemical environments are different.

No symmetry in the molecule, so the maximum number of possible chemical shifts should be observed.

 

(8) 1,3,5-trimethylbenzene, ,  

Number of low resolution NMR chemical shift δ signal peaks: 2 ¹H and 2 ¹³C (email if disagree?)

¹H NMR integrated ratio of peaks: 3(3x1) : 9(3x3) (aromatic: alkyl, for equivalent protons)

A highly symmetrical molecule, greatly reduced maximum number of NMR peaks.

In fact this is the minimum chemical shifts possible in any of the benzene ring isomers.

For the 1H or 12C chemical environments of the benzene ring C1 = C3 = C5, C2 = C4 = C6 and all the methyl groups are equivalent to each other (on C1, C3 and C5).

See also spectra links

(3) EXTRA NOTES on the benzene ring isomers of molecular formula C₉H₁₂

What benzene ring molecules have the formula C9H12?

Can you write out the names of the aromatic isomers of formula C9H12?

What are the possible isomers of C9H12 aromatic molecules?

How do you work out the structure of the isomers of molecular formula C9H12 aromatic molecules?

How do you draw the structural formula and skeletal formula of the isomers of molecular formula C9H12 aromatic molecules?

How do you name the isomers of molecular formula C9H12 aromatic molecules?

How many structural isomers are there of molecular formula C9H12 aromatic molecules?

How many carbon chain isomers are there of molecular formula C9H12 aromatic molecules?

How many positional isomers are there of molecular formula C9H12 aromatic molecules?

Are there any functional group isomers with a molecular formula C9H12 aromatic molecules?

Are there any E/Z (geometrical) aromatic isomers with a molecular formula C9H12?

Are there any R/S (optical) aromatic isomers (enantiomers) with a molecular formula C9H12?

This page will answer these questions for molecular formula C9H12 aromatic molecules

 Index of sets of isomers for a given molecular formula

 Molecular structure and nomenclature of aromatic compounds

 The chemistry of aromatic compounds

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

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