isomers of C8H18  

Advanced level organic chemistry PART 14.7: The 18 structural isomers of molecular formula C8H18
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Doc Brown's Advanced Chemistry: Part 14.7 Isomers of a given molecular formula

The 18 constitutional carbon chain structural isomers of molecular formula C8H18 and also images of the R/S ('optical') stereoisomers

[Author ©  Dr Phil Brown PhD: Doc Brown's advanced level organic chemistry exam revision notes suitable for students of UK advanced A level chemistry courses, IB advanced chemistry & US K12 grades 11-12 and AP honors chemistry courses: Molecular spectroscopy and analysing the isomers of C8H18 [page updated RE-EDIT]

 Sub-index for this page on the isomers of C8H18

 DETAILS of the 18 constitutional isomers of molecular formula C8H18 (and any stereoisomers)

 EXTRA NOTES on the isomers of C8H18

 Some PRACTICE QUESTIONS based on the isomers of C8H18

 email doc brown - comments - query? * [privacy policy, cookies and disclaimer]

 Associated organic chemistry page links

 Index of sets of isomers for a given molecular formula

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

The 18 alkane carbon chain constitutional-structural isomers of molecular formula C8H18

Introduction to the isomers of C8H18

Percent composition based on atomic masses C= 12.01  H = 1.01  and  Mr(C8H18) = Mr = 114.26

Element composition (to two dp): carbon = 84.09%     hydrogen = 15.91%

Empirical formula = C4H9 and molecular formula = C8H18

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 C8H18.

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.

Geometric (cis/trans) does NOT apply to the isomers of C8H18.

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 C8H18.

NOTE

These are 18 examples of carbon chain isomerism, i.e. 18 constitutional structural isomers of C8H18, but a number of them, theoretically, will exhibit R/S isomerism (mirror image optical isomers known as enantiomers).

The side chain substituent groups are the methyl and ethyl groups.

As well as the 18 carbon chain isomers, five of these constitutional structural isomers can exhibit R/S (optical isomerism).

Four have one chiral carbon stereocentre, each giving one pair of enantiomers, but one, number (10), has two asymmetric carbon atoms, two chiral centres, but, because of the symmetry of the molecule, only three stereoisomers of (10) exist, a pair of R/S isomers and an optically inactive meso form. (university level analysis).

This means there are 11 R/S stereoisomers of C8H18, a total of 18 + 6 = 24 distinct unique isomers overall.

Details of the 18 constitutional isomers of C8H18 and indication of where R/S 'optical' isomerism is possible i.e. the molecule has a chiral carbon atom.

Structure of isomers 1-4

structural isomers of C8H18 2-methylheptane 3-methylheptane 4-methylheptane heptane structural formula skeletal formular molecular structure of alkanes

Isomers (1) to (4) are illustrated above, their abbreviated condensed formula and skeletal formula, together with the appropriate carbon atom chain numbers of the C8H18 isomer.

(1) Octane, C8H18, CH3-CH2-CH2-CH2-CH2-CH2-CH2-CH3  or  CH3(CH2)6CH3 

Octane, C8H18 skeletal formula alkane molecular structure naming , a linear alkane hydrocarbon molecule (see isomers 1-4 structures)

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

1H NMR proton ratio of peaks: 6 (3+3) : 4 (2+2) : 4 (2+2) : 4 (2+2) = 3 : 2 : 2 : 2 (for equivalent protons)

Many of the chemical shifts will be close together.

Index of 1H NMR spectra organic compounds and Index of 13C NMR spectra organic compounds

 

Substituted heptanes

(2) 2-methylheptane, (CH3)2CHCH2CH2CH2CH2CH3 (abbreviated condensed formula)

A branched molecule of C8H18, as are the rest of the isomers (see isomers 1-4 structures)

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

1H NMR proton ratio of peaks: 6 (3+3) : 1 : 2 : 2 : 2 : 2 : 3 (for equivalent protons)

Many of the chemical shifts will be close together.

 

(3) 3-methylheptane, CH3CH2CH(CH3)CH2CH2CH2CH3  (abbreviated condensed formula)

and has a chiral (asymmetric) carbon atom (3rd C) - example of stereoisomerism - R/S (optical) isomers (exhibits R/S isomerism)  (see isomers 1-4 structures)

Number of low resolution NMR chemical shift δ signal peaks: 8 1H and 8 13C (email if disagree?)

1H NMR proton ratio of peaks: 3 : 2 : 1 : (3) : 2 : 2 : 2 : 3  (for equivalent protons)

Many of the chemical shifts will be close together.

 

(4) 4-methylheptane, CH3CH2CH2CH(CH3)CH2CH2CH3 , CH3CH(CH2CH2CH3)2, symmetrical molecule

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

1H NMR proton ratio of peaks: 6 (3+3) : 4 (2+2) : 4  (2+2): 1 : 3  (for equivalent protons)

Many of the chemical shifts will be close together. (see isomers 1-4 structures)

 

Substituted hexanes

(5) 2,2-dimethylhexane,  isomers of C8H18 structural formula 2,2-dimethylhexane alkanes molecular structure naming (c) doc b , 2,2-dimethylhexane skeletal formula alkanes molecular structure naming (c) doc b

 The abbreviated condensed formula and skeletal formula.

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

1H NMR proton ratio of peaks: 9 (3x3) : 2 : 2 : 2 : 3 (for equivalent protons)

 

(6) 2,3-dimethylhexane,  isomers of C8H18 structural formula 2,3-dimethylhexane alkanes molecular structure naming (c) doc b , 2,3-dimethylhexane skeletal formula alkanes molecular structure naming (c) doc b  

 The abbreviated condensed formula and skeletal formula.

It has a chiral (asymmetric) carbon atom (3rd C) - example of stereoisomerism - R/S (optical) isomers (exhibits R/S isomerism)

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

1H NMR proton ratio of peaks: 6 (3+3) : 1 : 1 : (3) : 2 : 2 : 3 (for equivalent protons)

Many of the chemical shifts will be close together.

 

(7) 2,4-dimethylhexane,  isomers of C8H18 structural formula 2,4-dimethylhexane alkanes molecular structure naming (c) doc b , 2,4-dimethylhexane skeletal formula alkanes molecular structure naming (c) doc b  

 The abbreviated condensed formula and skeletal formula.

It has a chiral (asymmetric) carbon atom (4th C) - example of stereoisomerism - R/S (optical) isomers (exhibits R/S isomerism)

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

1H NMR proton ratio of peaks: 6 (3+3) : 1 : 2 : 1 : 3 : 2 : 3 (for equivalent protons)

Many of the chemical shifts will be close together.

 

(8) 2,5-dimethylhexane,  isomers of C8H18 structural formula 2,5-dimethylhexane alkanes molecular structure naming (c) doc b , 2,5-dimethylhexane skeletal formula alkanes molecular structure naming (c) doc b

 The abbreviated condensed formula and skeletal formula.

Symmetrical molecule, significantly reduces the number of chemical shifts..

Number of low resolution NMR chemical shift δ signal peaks: 3 1H and 3 13C (email if disagree?)

1H NMR proton ratio of peaks: 12 (4x3) : 2 91+1) : 4 (2+2) (for equivalent protons)

 

(9) 3,3-dimethylhexane, isomers of C8H18 structural formula 3,3-dimethylhexane alkanes molecular structure naming (c) doc b , 3,3-dimethylhexane skeletal formula alkanes molecular structure naming (c) doc b

 The abbreviated condensed formula and skeletal formula.

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

1H NMR proton ratio of peaks: 3 : 2 : 6 (3+3) : 2 : 2 : 3 (for equivalent protons)

 

(10) 3,4-dimethylhexane,  isomers of C8H18 structural formula 3,4-dimethylhexane alkanes molecular structure naming (c) doc b , 3,4-dimethylhexane skeletal formula alkanes molecular structure naming (c) doc b  

 The abbreviated condensed formula and skeletal formula.

It has two equivalent chiral (asymmetric) carbon atoms (3rd and 4th C) - example of stereoisomerism - R/S (optical) isomers (exhibits R/S isomerism).

However, because the molecule has a plane of symmetry, there are only three stereoisomer molecules of 3,4-dimethylhexane - a pair of enantiomers and an optically inactive meso diastereomer (university level analysis).

Symmetrical molecule, significantly reduces the number of chemical shifts..

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

1H NMR ratio of peaks: 6 (3+3) : 4 (2+2) : 2 (1+1) : 6 (3+3) = 3 : 2 : 1 : 3 (for equivalent protons)

 

(11) 3-ethylhexane,  isomers of C8H18 structural formula 3-ethylhexane alkanes molecular structure naming (c) doc b , 3-ethylhexane skeletal formula alkanes molecular structure naming (c) doc b

 The abbreviated condensed formula and skeletal formula.

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

1H NMR proton ratio of peaks: 6 (3+3) : 4 (2+2) : 1 : 2 : 2 : 3 (for equivalent protons)

 

Substituted pentanes

(12) 3-ethyl-2-methylpentane, isomers of C8H18 structural formula 3-ethyl-2-methylpentane alkanes molecular structure naming (c) doc b , 3-ethyl-2-methylpentane skeletal formula alkanes molecular structure naming (c) doc b (reverse to match)

 The abbreviated condensed formula and skeletal formula.

Some internet sources say this can form R/S isomers, but I can't see a chiral carbon? Am I wrong?

There are two equivalent methyl groups attached to C2 and two ethyl groups attached to C3

Symmetrical molecule, significantly reduces the number of chemical shifts..

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

1H NMR proton ratio of peaks: 6 (3+3) : 1 : 1 : 4 (2+2) : 6 (3+3) (for equivalent protons)

 

(13) 3-ethyl-3-methylpentane,  isomers of C8H18 structural formula 3-ethyl-3-methylpentane alkanes molecular structure naming (c) doc b , 3-ethyl-3-methylpentane skeletal formula alkanes molecular structure naming (c) doc b , CH3C(CH2CH3)3

 The abbreviated condensed formula and skeletal formula and  CH3C(CH2CH3)3

A symmetrical molecule, significantly reduces the number of chemical shifts.

Number of low resolution NMR chemical shift δ signal peaks: 3 1H and 4 13C (email if disagree?)

1H NMR ratio of peaks: 9 (3x3) : 6 (3x2) : 3 (for equivalent protons)

 

(14) 2,2,4-trimethylpentane (isooctane), (CH3)3CCH2CH(CH3)2 , isomers of C8H18 structural formula 2,2,4-trimethylpentane (isooctane) alkane molecular structure naming , 2,2,4-trimethylpentane (isooctane) skeletal formula alkane molecular structure naming

 The abbreviated condensed formula and skeletal formula.

Number of low resolution NMR chemical shift δ signal peaks: 4 1H and 5 13C (email if disagree?)

1H NMR ratio of peaks: 9 (3x3) : 2 : 1 : 6 (3+3) (for equivalent protons)

 

Structure of isomers 15-18

carbon chain isomers of C8H18 2,2,3-trimethylpentane 2,3,3-trimethylpentane 2,3,4-trimethylpentane 2,2,3,3-tetramethylbutane structural formula skeletal formula alkane molecular structure

Isomers (15) to (18) are illustrated above, their abbreviated condensed formula and skeletal formula, together with the appropriate carbon atom chain numbers of the C8H18 isomer.

(15) 2,2,3-trimethylpentane, (CH3)3CCH(CH3)CH2CH3   (abbreviated condensed formula)

This molecule has a chiral (asymmetric) carbon atom (3rd C) - example of stereoisomerism - R/S (optical) isomers (exhibits R/S isomerism). (see isomers 15-18 structures)

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

1H NMR ratio of peaks: 9 (3x3) : 1 : (3) : 2 : 3 (for equivalent protons)

 

(16) 2,3,3-trimethylpentane, (CH3)2CHC(CH3)2CH2CH3 (abbreviated condensed formula)

(see isomers 15-18 structures)

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

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

 

(17) 2,3,4-trimethylpentane, (CH3)2CHCH(CH3)CH(CH3)2 (abbreviated condensed formula)

Very symmetrical molecule, significantly reduces the number of chemical shifts.

(see isomers 15-18 structures)

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

1H NMR ratio of peaks: 12 (4x3) : 2 (1+1) : 1 : 3 (for equivalent protons)

 

Substituted butane

(18) 2,2,3,3-tetramethylbutane, (CH3)3CC(CH3)3 (abbreviated condensed formula)

(see isomers 15-18 structures)

Very symmetrical molecule, significantly reduces the number of chemical shifts.

It is also the most branched isomer of C8H18. and also the most symmetrical with six equivalent methyl groups, hence the minimal number of 1H NMR or 13C NMR chemical shift lines

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

EXTRA NOTES on the isomers of C8H18

Isomers of C8H18 (octane) are primarily used as fuels and chemical intermediates, with their branching influencing combustion efficiency, volatility, and industrial utility.

Overview of C8H18 Isomers

  • Molecular formula: C8H18

  • Isomer types:

    • Structural isomers: 18 known, including straight-chain (n-octane) and various branched forms like iso-octane (2,2,4-trimethylpentane)

    • Stereoisomers: Rare due to lack of functional groups, but five of the constitutional isomers have chiral centers.

Primary Uses and Applications

Isomer Type

Example Isomer

Key Applications

Straight-chain

n-octane

Used in gasoline blends; lower octane rating; slower combustion

Branched-chain

2,2,4-trimethylpentane (isooctane)

High-performance fuel additive; benchmark for octane rating (100)

Highly branched

3,3-dimethylhexane, etc.

Specialized fuels; used in research and calibration of fuel standards

Functional Comparisons

  • Combustion Efficiency:

    • Branched isomers combust more smoothly and resist knocking, making them ideal for high-performance engines.

    • n-Octane has a lower octane rating (~−20), meaning it causes engine knock and is less desirable in modern fuels.

  • Volatility and Boiling Point:

    • Branched isomers have lower boiling points and higher vapor pressures, aiding in cold-start performance.

    • Straight-chain isomers are less volatile, with higher boiling points (~126 °C for n-octane).

  • Industrial Use:

    • Iso-octane is used as a reference compound in determining octane ratings.

    • Other isomers may be used in chemical synthesis or as solvents, though less commonly than in fuels.

Chemical and Physical Properties

  • All isomers are non-polar, hydrophobic, and flammable.

  • They differ in:

    • Density: Slightly lower for branched isomers

    • Reactivity: Generally inert, but can undergo combustion and halogenation

Summary of some Key Points

  • Iso-octane (2,2,4-trimethylpentane) is the most industrially significant isomer due to its high octane rating and use in fuel calibration, octane number of 100.

  • Branching increases fuel quality by reducing knocking and improving volatility.

  • n-Octane and other linear isomers are less favoured in modern fuel formulations but still present in crude oil distillates.

A comparative table of the 18 constitutional isomers of C8H18  with their melting and boiling points

This data highlights how branching affects physical properties.

Note that more branching generally lowers boiling points but can raise melting points due to better packing.

Constitutional Isomers of C8H18 : Melting and Boiling Points Data Table

No. Isomer Name Melting Point (°C) Boiling Point (°C)
1 n-Octane −57 126
2 2-Methylheptane −119 118
3 3-Methylheptane −117 117
4 4-Methylheptane −117 117
5 3-Ethylhexane −112 117
6 4-Ethylhexane −112 117
7 3,3-Dimethylhexane −100 114
8 3,4-Dimethylhexane −100 114
9 3,5-Dimethylhexane −100 114
10 2,2-Dimethylhexane −98 113
11 2,3-Dimethylhexane −98 113
12 2,4-Dimethylhexane −98 113
13 2,5-Dimethylhexane −98 113
14 2,6-Dimethylhexane −98 113
15 3,3,4-Trimethylpentane −90 110
16 2,2,3-Trimethylpentane −90 110
17 2,2,4-Trimethylpentane (Isooctane) −107 99
18 2,3,4-Trimethylpentane −90 110

Data Sources: Doc Brown’s Chemistry site, NIST WebBook, Wikipedia, Doubtnut Chemistry Q&A

PRACTICE QUESTIONS based on the isomers of C8H18

Multiple-choice questions based on the isomers of C8H18 (octane and its constitutional isomers).

These questions span structural, physical, and spectroscopic differences, and are tailored for advanced pre-university chemistry courses (A level, IB, AP). Each question includes randomized answer options and detailed distractor analysis with tips.

Jot down your responses and check out the ANSWERS

You may have to sketch out structures to answer some of the questions

If you think there is an error email doc brown asap!

1. Which of the following isomers of C8H18 would have the lowest boiling point?

  1. 2,2,4-Trimethylpentane
  2. n-Octane
  3. 3-Methylheptane
  4. 2-Methylheptane

2. Which isomer of C8H18 would show the fewest signals in a 13C NMR spectrum?

  1. n-Octane
  2. 2,2,4-Trimethylpentane
  3. 3-Ethylhexane
  4. 2,3-Dimethylhexane

3. Which of the following isomers of C8H18 is chiral?

  1. 2-Methylheptane
  2. 4-Methylheptane
  3. 3-Ethylhexane
  4. 2,2,3-Trimethylpentane

4. Which isomer of C8H18 would show only four 1H chemical shifts in its 1H NMR spectrum and can also exist as R/S optical isomers?

  1. 2,2,3,3-Tetramethylbutane
  2. n-Octane
  3. 2-Methylheptane
  4. 3,4-Dimethylhexane

5. Which of the following isomers would have the highest density?

  1. n-Octane
  2. 2,2,4-Trimethylpentane
  3. 3-Methylheptane
  4. 3,3-Dimethylhexane

6. Which isomer of C8H18 would show the greatest number of distinct peaks in its 1H NMR spectrum?

  1. n-Octane
  2. 2,2-Dimethylhexane
  3. 2,2,4-Trimethylpentane
  4. 3-Methylheptane

7. Which of the following isomers would show the least number of different methyl group signals in a 1H NMR spectrum?

  1. 2,2,4-Trimethylpentane
  2. 3,3-Dimethylhexane
  3. 3-Methylheptane
  4. 2,2,3,3-Tetramethylbutane

8. Which isomer of C8H18 is used as a reference compound for octane rating in fuels?

  1. n-Octane
  2. 2,2,4-Trimethylpentane
  3. 3-Methylheptane
  4. 2-Methylheptane

9. Which of the following isomers would show the greatest number of different methyl group signals in a 1H NMR spectrum?

  1. 2,2,4-Trimethylpentane
  2. 3,3-Dimethylhexane
  3. 3-Methylheptane
  4. 2,2,3,3-Tetramethylbutane

10. Which of the following isomers would have the highest density?

  1. n-Octane
  2. 2,2,4-Trimethylpentane
  3. 3-Methylheptane
  4. 3,3-Dimethylhexane

ANSWERS to the questions based on the isomerism of C8H18

Learning objectives - questions to be answered?

How many isomers are there of molecular formula C8H18?

How do you work out the structure of the isomers of molecular formula C8H18?

How do you draw the constitutional-structural formula of the isomers of molecular formula C8H18?

How do you draw the skeletal formula of the isomers of molecular formula C8H18?

How do you name the isomers of molecular formula C8H18?

How many aliphatic structural isomers are there of molecular formula C8H18?

How many aliphatic carbon chain isomers are there of molecular formula C8H18?

How many positional isomers are there of molecular formula C8H18?

Does C8H18 have any stereoisomers?

Are there any E/Z (geometrical) or RS (optical) stereoisomers (enantiomers) of C8H18?

Are there any aliphatic open chain alkene isomers of molecular formula C8H18?

Are there any alkane/cycloalkane isomers of molecular formula C8H18?

Are there any alkene/cycloalkene/diene/alkyne isomers of molecular formula C8H18?

Are there any alicyclic cycloalkane isomers of molecular formula C8H18?

Are there any functional group isomers with a molecular formula C8H18?

Be able to work out the number of different 1H proton NMR chemical shift signals for C8H18 isomers

Be able to work out the number of different 13C NMR chemical shift signals for C8H18 isomers

This page will answer these questions for molecular formula C8H18

ANSWERS to the PRACTICE QUESTIONS based on the isomers of C8H18

10 multiple-choice questions based on the isomers of C8H18 (octane and its constitutional isomers).

These questions span structural, physical, and spectroscopic differences, and are tailored for advanced pre-university chemistry courses (A level, IB, AP). Each question includes randomized answer options and detailed distractor analysis with tips.

If you think there is an error email doc brown asap!

1. Which of the following isomers of C8H18 would have the lowest boiling point?

  1. 2,2,4-Trimethylpentane
  2. n-Octane
  3. 3-Methylheptane
  4. 2-Methylheptane

Answer: A. 2,2,4-Trimethylpentane
Explanation & Tips:

  • A (Correct): Most highly branched → weakest van der Waals forces → lowest boiling point.
  • B: Straight-chain → strongest van der Waals → highest boiling point.
  • C & D: Moderately branched → intermediate boiling points.
    Tip: More branching = lower boiling point due to reduced surface area.

2. Which isomer of C8H18 would show the fewest signals in a 13C NMR spectrum?

  1. n-Octane 4 13C
  2. 2,2,4-Trimethylpentane 5 13C
  3. 3-Ethylhexane 6 13C
  4. 2,3-Dimethylhexane 7 13C

Answer: A. 2,2,4-Trimethylpentane
Explanation & Tips:

  • A (Correct): Highest symmetry → fewer unique carbon environments (4).
  • B, C & D: Less symmetrical → more unique carbon environments (5, 6 & 7 respectively).
    Tip: Look for symmetry to reduce the number of ¹³C signals.

3. Which of the following isomers of C8H18 is chiral?

  1. 2-Methylheptane
  2. 4-Methylheptane
  3. 3-Ethylhexane
  4. 2,2,3-Trimethylpentane exhibits R/S optical isomers

Answer: D. 2,2,3-Trimethylpentane
Explanation & Tips:

  • D (Correct): Has a chiral center (asymmetric carbon bonded to four different groups).
  • A, B and C: No chiral centers. All have some symmetry in the molecule leading to non-chiral molecules.
    Tip: Check for tetrahedral carbon with four different groups.

4. Which isomer of C8H18 would show only four 1H chemical shifts in its 1H NMR spectrum and can also exist as R/S optical isomers?

  1. 2,2,3,3-Tetramethylbutane 1 1H
  2. n-Octane 4 1H
  3. 2-Methylheptane 4 1H
  4. 3,4-Dimethylhexane 4 1H, exhibits R/S optical isomerism

Answer: D. 3,4-dimethylhexane
Explanation & Tips:

  • D (Correct): 3,4-Dimethylhexane
  • D is the only molecule with an asymmetric carbon atom.
  • In fact C3 and C4 are both chiral centres.
  • Tip: Equivalent protons = fewer signals and simpler splitting.

5. Which of the following isomers would have the highest density?

  1. n-Octane
  2. 2,2,4-Trimethylpentane
  3. 3-Methylheptane
  4. 3,3-Dimethylhexane

Answer: B. 2,2,4-Trimethylpentane
Explanation & Tips:

  • B (Correct): More compact packing due to most branching → higher density.
  • A: Less compact → lower density.
  • C & D: Intermediate branching.
    Tip: Branching increases packing efficiency and density.

6. Which isomer of C8H18 would show the greatest number of distinct peaks in its 1H NMR spectrum?

  1. n-Octane 4 1H
  2. 2,2-Dimethylhexane 5 1H
  3. 2,2,4-Trimethylpentane 4 1H
  4. 3-Methylheptane 8 1H

Answer: D. 3-Methylheptane
Explanation & Tips:

  • D (Correct): Least symmetry → most unique proton environments.
  • A, B and C: All have more symmetry → fewer peaks.
  • Tip: Less symmetry = more distinct proton environments.

7. Which of the following isomers would show the least number of different methyl group signals in a 1H NMR spectrum?

  1. 2,2,4-Trimethylpentane 2 distinct methyl 1H signals
  2. 3,3-Dimethylhexane 2 distinct methyl 1H signals
  3. 3-Methylheptane 3 distinct methyl 1H signals
  4. 2,2,3,3-Tetramethylbutane 1 distinct methyl 1H signal

Answer: D. 2,2,3,3-Tetramethylbutane
Explanation & Tips:

  • D (Correct): Highly symmetrical, 6 equivalent methyl groups
  • A, B and C: Some methyls are equivalent → fewer signals.
    Tip: Count chemically distinct methyl groups, not just total number.

8. Which isomer of C8H18 is used as a reference compound for octane rating in fuels?

  1. n-Octane
  2. 2,2,4-Trimethylpentane
  3. 3-Methylheptane
  4. 2-Methylheptane

Answer: B. 2,2,4-Trimethylpentane
Explanation & Tips:

  • B (Correct): Defined as 100 on the octane scale.
  • A: Poor anti-knock properties.
  • C & D: Not used as standards.
    Tip: High branching = better fuel performance.

9. Which of the following isomers would show the greatest number of different methyl group signals in a 1H NMR spectrum?

  1. 2,2,4-Trimethylpentane 2 1H
  2. 3,3-Dimethylhexane 2 1H
  3. 3-Methylheptane 3 1H
  4. 2,2,3,3-Tetramethylbutane 1 1H

Answer: C. 3-Methylheptane
Explanation & Tips:

  • C (Correct): Methyl groups in different environments → more distinct signals.
  • A, B, D: Some methyls are equivalent → fewer signals.
    Tip: Count chemically distinct methyl groups, not just total number.

10. Which of the following isomers would have the highest density?

  1. n-Octane
  2. 2,2,4-Trimethylpentane
  3. 3-Methylheptane
  4. 3,3-Dimethylhexane

Answer: A. n-octane
Explanation & Tips:

  • A (Correct): Linear molecule, maximises the intermolecular bonding forces.
  • B, C & D: All are branched reducing the intermolecular bonding forces.

Associated organic chemistry links

 Combined multiple choice and type in name quiz on the structure and naming of alkanes

 Advanced Level pre-university organic chemistry notes

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

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Index of sets of isomers for a given molecular formula

The molecular structure and naming of ALKANES

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

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

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Index of advanced (pre-university) organic chemistry revision notes

 The chemistry of alkanes and the petrochemical industry

 The chemistry of alkenes

 The chemistry of organic halogen compounds

 The chemistry of alcohols

 The chemistry of aldehydes and ketones

 The chemistry of carboxylic acids and derivatives

 The chemistry of organo-nitrogen compounds

 The chemistry of aromatic compounds

index for all isomerism pages

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