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Doc Brown's
Advanced Chemistry: Part 14.7
The 3
constitutional structural chain isomers of molecular formula C5H12
[Author
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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 C5H12
[updated Feb 26th
2026 *]
Associated organic chemistry page links
Index of sets of isomers for a given
molecular formula
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The 3
alkane constitutional structural isomers of molecular formula C5H12
(Mr = 72)
Introduction
to isomerism of the saturated open constitutional isomer alkanes of
molecular formula C5H12
Percent composition of
C5H12 based on atomic masses C= 12.01 H =
1.01 and Mr(C5H12) = 72.17
Element composition by mass (to two dp): carbon = 83.21%
hydrogen = 16.79%
Empirical formula = C5H12
=
molecular formula = C5H12
Structural isomerism
includes carbon chain variation (usually need a minimum of 4 C atoms, applicable
to C5H12 isomers), 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.
3 constitutional
isomers based on 3 different arrangements of the carbon chain.
Stereoisomerism is where
molecules have the same basic constitutional structural formula, but isomers
differ in the 2D/3D arrangement of the atoms. Neither is applicable to C5H12
isomers.
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.
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.
These are all examples of
carbon chain
structural isomerism - three structural isomers possible and
all of them saturated open chain aliphatic compounds.
There is no stereoisomerism i.e.
no E/Z or R/S isomers possible for formula C5H12.
Details of the isomers of C5H12
There are only 3 structural isomers of molecular formula C5H12
originating from 3 different arrangements of the carbon chain
and no other types of isomerism are possible.
(1)
 or
 are abbreviated structural formula for
pentane and the skeletal formula is
,
no E/Z or R/S isomerism possible.
Number of low resolution
NMR chemical shift
δ
signal peaks: 3 1H
and 3 13C
(email
if disagree?)
1H NMR ratio of peaks: 6
(3+3) : 4 (2+2) : 2 = 3
: 2 : 1 (for equivalent protons)
See also spectra that can distinguish one C5H12
isomer from another for (1) to (3)
The infrared spectrum of pentane
The infrared spectrum for
2-methylbutane
The infrared spectrum for 2,2-dimethylpropane
The mass spectrum of Pentane
The mass spectrum for 2-methylbutane
The mass spectrum for 2,2-dimethylpropane
The H-1 NMR spectrum of pentane
The H-1 NMR spectrum for
2-methylbutane
The H-1 NMR spectrum for 2,2-dimethylpropane
The C-13 NMR spectrum of Pentane
The C-13 NMR spectrum for
2-methylbutane
The C-13 NMR spectrum for 2,2-dimethylpropane
Index of 1H NMR spectra organic
compounds and
Index of 13C NMR spectra organic
compounds
(2)
 or 
are abbreviated structural formula for
methylbutane
(prefix 2- isn't strictly needed, but 2-methybutane
helps at the beginning of studying organic nomenclature)
The
skeletal formula is
, no E/Z or R/S isomerism possible.
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) : 1 : 2 : 3 (for equivalent protons)
(3)
 or  are the abbreviated structural formula for
dimethylpropane
(2,2-dimethylpropane, the prefix 2,2- isn't strictly needed but can help initially
in learning alkane nomenclature)
The skeletal formula is
, no E/Z or R/S isomerism possible.
Number of low resolution
NMR chemical shift
δ
signal peaks: 1 1H
and 2 13C
(email
if disagree?)
EXTRA
NOTES
Number
of constitutional isomers of C5H12
There are three constitutional (structural) isomers of C5H12:
-
n‑Pentane (straight chain)
-
2‑Methylbutane (commonly called isopentane)
-
2,2‑Dimethylpropane (commonly called
neopentane)
Types of isomerism exhibited by
C5H12 isomers
-
Chain (skeletal) isomerism: different
carbon backbones produce the three isomers above.
-
Constitutional (structural) isomerism:
different connectivity of C–C bonds; all three are constitutional isomers of
one another.
-
Stereoisomerism: none for these saturated
C5 isomers (no double bonds, no stereogenic centres).
-
Conformational isomerism: present in
n‑pentane and 2‑methylbutane (rotamers about single bonds) but usually not
examinable beyond recognition that rotation changes energy and steric
strain.
Structural features to note about isomers of
C5H12
-
n‑Pentane: five‑carbon unbranched chain;
three types of H (terminal primary, internal methylene) including accessible
secondary hydrogens.
-
2‑Methylbutane: one methyl branch at C2;
contains primary and secondary H; some H are more sterically hindered than
in n‑pentane.
-
Neopentane (2,2‑dimethylpropane): highly
symmetric tetra‑substituted central carbon (quaternary), all hydrogens are
equivalent (all ten are equivalent in NMR), marked steric crowding around
centre.
Differences in physical properties and why for
isomers of
C5H12
-
Boiling points (trend and reason)
-
Melting points
-
Density and vapour pressure
-
Entropy and heat capacity
Key structural explanations to use in answers: branching reduces
surface contact and dispersion forces; molecular symmetry improves crystal
packing and raises melting point.
Differences in chemical behaviour and relative
reactivity of isomers of
C5H12
-
Combustion
-
Radical halogenation (photochemical X2 / hv)
-
Reactivity pattern follows relative stability of
radicals formed: tertiary H > secondary H > primary H.
-
2‑Methylbutane produces tertiary radicals (from the
tertiary carbon at C2) so it gives higher proportion of tertiary
substitution products than n‑pentane; neopentane has no tertiary H (only
primary), so gives mainly primary substitution (but steric/electronic
factors can alter product ratios).
-
Free radical chemistry and C–H bond strengths
-
Cracking and isomerisation
-
Acid‑catalysed reactions
-
NMR behaviour
-
Neopentane shows a single proton environment at room
temperature (ten equivalent H), 2‑methylbutane shows multiple distinct
proton signals, n‑pentane shows multiple overlapping signals and
temperature‑dependent conformational averaging.
Relative reactivity summary for common reactions:
-
Radical H‑abstraction and halogenation: 2‑methylbutane
(highest selectivity for tertiary substitution) > n‑pentane > neopentane
(primaries only).
-
Ease of forming stabilized carbocations (if generating
cationic intermediates): 2‑methylbutane > n‑pentane > neopentane.
Uses and applications linked to structure of the
isomers of
C5H12
Practical note: interconversion and selectivity are
important in petrochemical industry (isomerisation to increase branching for
high‑octane fuels).
Student misconceptions to correct for questions
involving the isomers of
C5H12
-
“Same formula → same properties” — false: connectivity and
branching change boiling point, melting point, density, and reactivity.
-
“More branching always increases boiling point” — false:
branching generally lowers boiling point (but can increase melting point via
symmetry).
-
“All hydrogens in isomers are equivalent” — false: only
neopentane has chemically equivalent hydrogens; n‑pentane and 2‑methylbutane
have multiple distinct H environments.
-
“Tertiary hydrogens are the most common in pentane isomers”
— false: only 2‑methylbutane provides tertiary hydrogens; neopentane has
none.
-
Confusing melting point trends with boiling point trends —
emphasize symmetry raises melting point whereas
branching lowers boiling point.
Exam
revision tips for A level, IB and AP level students
-
Memorise and be able to draw the three isomers quickly and
name them IUPAC. Use clear wedge/dash only when stereochemistry asked.
-
For physical property questions always link the observation
to surface area, branching, symmetry, and intermolecular forces.
State which structural feature causes each effect.
-
For reactivity questions link mechanisms to radical
stability, carbocation stability, steric hindrance, and bond dissociation
energies. Quote the radical stability order tertiary > secondary >
primary.
-
Practice short, high‑impact sentences: e.g., “2‑methylbutane
shows more tertiary radical substitution than n‑pentane because abstraction
from the tertiary carbon yields a more stable radical.”
-
Remember exceptional facts: neopentane’s single proton
environment in NMR and anomalously high melting point due to symmetry —
these are frequent exam hooks.
-
When asked to compare two isomers, use a two‑column format:
property → structural reason. Keep each point one or two sentences max.
-
For calculations (enthalpy or entropy) state assumptions
(per mole of CH2, ideal gas, etc.) and connect to branching where relevant.
Learning objectives - questions to be answered?
How do you work out the structure
of the isomers of molecular formula C5H12?
How do you draw the structural formula
and skeletal formula of the isomers of molecular formula C5H12?
How many aliphatic structural isomers
are there of molecular formula C5H12?
How many aliphatic carbon chain isomers
are there of molecular formula C5H12?
How many positional isomers are there
of molecular formula C5H12?
How many E/Z (geometrical) isomers are
there of molecular formula C5H12?
How many R/S (optical) isomers
(enantiomers) of molecular formula C5H12?
Are there any aliphatic open chain
alkene isomers of molecular formula C5H12?
Are there any diene isomers of
molecular formula C5H12?
Are there any alkyne isomers of
molecular formula C5H12?
Are there any alicyclic cycloalkane
isomers of molecular formula C5H12?
Are there any alicyclic cycloalkene
isomers of molecular formula C5H12?
Are there any functional group isomers
with a molecular formula C5H12?
Are there any E/Z (geometrical) isomers
with a molecular formula C5H12?
Are there any R/S (optical) isomers
(enantiomers) with a molecular formula C5H12?
Does C5H12 have any stereoisomers?
This page
will answer these questions for molecular formula C5H12
Associated organic chemistry links
Advanced Level pre-university
organic chemistry notes
IR, mass and H-1 & C-13 NMR
spectra of organic compounds
Comparison of the ir, mass, 1H and 13C
NMR spectra of the isomers of C5H12 (via
a 1H NMR
spectrum page)
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
For isomerism in organic chemistry, see also the
notes
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
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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 the isomers of C5H12
with names, structures, types of isomerism and a few spectroscopy details 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, CCEA 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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revision notes on isomerism of C5H12 molecules,
isomers of C5H12 of molecular mass 72
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molecular formula C5H12 alkyl positional isomers of C5H12 branched
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stereoisomers of molecular formula C5H12 cycloalkane functional
group isomers of C5H12 are there alkyne isomers of C5H12? are
there cycloalkanes of formula C5H12? are there cycloalkenes of
formula C5H12 chain isomers of C5H12 functional group isomers of
C5H12 substituent and functional group positional isomers of
C5H12
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