1.1 What are ALKANES? Their molecular
structure and nomenclature - naming of alkanes
Part 1.
ALKANES and the PETROCHEMICAL INDUSTRY
Doc Brown's Advanced A Level
Organic Chemistry Revision Notes
 This
is quite a long page because, via alkanes, its a very important introduction to
aspects of organic chemistry in general
Lots
of examples of alkanes nomenclature structure All you need to know about naming
alkanes How do you name alkanes? Nomenclature of
substituted alkanes - examples of the structure of alkanes, naming
alkanes, acceptable names, displayed formula of alkane molecules
with names, graphic
formula of alkanes, molecular formula of alkanes, skeletal formula of alkanes,
structural formulae of the
homologous series of alkanes, linear alkanes, branched alkanes, alkyl
substituted cycloalkanes etc. Everything you need on the naming and structure of
alkanes pre-university!
A basic introduction to
the structure and chemistry of
alkanes (including diagrams which haven't been repeated on this page).
A Level Organic
Chemistry Part 1 sub-index for this page only:
1.1.1
Alkane nomenclature explained and isomerism
1.1.2
Different ways of representing molecules
and classes of formula
1.1.3
Comparison of aliphatic/alicyclic/aromatic compounds -
definition & examples
1.1.4
Alkane isomerism explained
1.1.5 Homologous
series of alkane examples, general formula CnH2n+2 including isomers of molecular formulae
up to C7H16 (up to n = 7)
1.1.6 Examples of cycloalkanes
'alicyclic' compounds of general formula CnH2n
1.1.7
All 18 alkane isomers of C8H18
* 1.1.8 Examples of alkane isomers of C9H20
1.1.9
Examples of alkane isomers of C10H22
Important notes on
how to deduce empirical and molecular formula including alkane examples
Calculation of empirical formula,
deducing molecular formula of a compound starting with % mass composition)
and also
Multiple choice
Quiz on the structure and naming (nomenclature) of ALKANES
Type in name Quiz on the structure & naming of ALKANES (and
naming cycloalkanes)
Combined
multiple choice and type in name quiz on alkanes
Matching pair quiz on hydrocarbon structure
[SEARCH
BOX]
Alkanes and Petrochemical Industry INDEX
All Advanced Organic
Chemistry Notes
Index of GCSE/IGCSE Oil - Useful Products
Chemistry Revision Notes
1.1.1 A brief guide to
the structure and nomenclature of
non-cyclic alkane
hydrocarbons
The chemical bonding in alkane molecules
- Most alkanes you will come across are relatively small molecules in
which all the chemical bonds are covalent bonds.
- All the bonds in alkane molecules are single
covalent bonds i.e. C–C carbon – carbon or single C–H carbon – hydrogen bonds.
- Each carbon atom forms four single bonds and
hydrogen atoms form one single bond.
- All single covalent bonds are formed by sharing a
pair of electrons e.g. one from each of a carbon atom and a hydrogen atom,
or two carbon atoms contributing (sharing) an electron each to the covalent
bond.
- These single covalent C-C and C-H bonds are known as
sigma bonds (σ).
- Four
hydrogen
atoms (1 outer electron) and one
carbon atom
(four outer electrons) combine to form methane
so that the hydrogen atoms are electronically like helium (full outer
shell of 2 electrons) and the carbon atom becomes like neon
(with a full outer shell of 8 electrons, the two inner electrons of carbon are
not shown).
- In terms of 'dot and cross' Lewis diagrams the
structures of alkanes would be drawn as ...
-
or
the alkane methane
- Similarly six hydrogen atoms combine with
two carbon atoms to form the ethane molecule.
-
or
the alkane ethane
- BUT, these do not show the 3D shape of alkane
molecules see later.
- General formula and structural formula of alkanes and
alkane nomenclature (naming alkane compounds)
- The first 10 of the homologous series of alkanes are
tabulated below.
- Members of a homologous series have a similar
molecular structure and a common general formula.
- In their naming (nomenclature), the suffix ...ane,
tells you the molecule belongs to the homologous series of alkanes.
- Other than alkanes, all other homologous series of
organic compounds have a functional group e.g. >C=O or -COOH which gives
this set of molecules particular chemical properties (see
homologous series and functional groups).
- Reminders:
- The molecular formula sums up ALL the
atoms in the molecule e.g. an alkane hydrocarbon molecule.
- The empirical formula gives you the simplest
whole number (integer) ratio of the atoms in the molecule.
- In the table below:
- when n, the number of carbon atoms is odd, the
empirical formula and molecular formula are identical.
- when n, the number of carbon atoms is even, the
empirical formula and molecular formula are different.
n of general formula CnH2n+2 |
1 |
2 |
3 |
4 |
5 |
6 |
7 |
8 |
9 |
10 |
molecular formula of alkane CnH2n+2
|
CH4 |
C2H6 |
C3H8 |
C4H10 |
C5H12 |
C6H14 |
C7H16 |
C8H18 |
C9H20 |
C10H22 |
prefix (before ...ane) |
meth |
eth |
prop |
but |
pent |
hex |
hept |
oct |
non |
dec |
alkane name (linear chain from C4) |
methane |
ethane |
propane |
butane |
pentane |
hexane |
heptane |
octane |
nonane |
decane |
molecular empirical formula |
CH4 |
CH3 |
C3H8 |
C2H5 |
C5H12 |
C3H7 |
C7H16 |
C4H9 |
C9H20 |
C5H11 |
alkyl side chain group name (branch) |
methyl |
ethyl |
propyl |
butyl |
pentyl |
hexyl |
heptyl |
octyl |
nonyl |
decyl |
alkyl side-chain group formula CnH2n+1 |
CH3 |
C2H5 |
C3H7 |
C4H9 |
C5H11 |
C6H13 |
C7H15 |
C8H17 |
C9H19 |
C10H21 |
How to name alkanes (introduction to the nomenclature of alkanes)
-
The primary suffix name is based on the longest carbon
chain and ending in ...ane.
-
1 carbon, methane;
2 carbons, ethane; 3
carbons in chain,
propane; 4 carbons in chain, butane.
-
After these
four 'historic' preserved 'old trivial' names, the name is 'numerically' systematic
according to IUPAC e.g.
C5
carbon chain
pentane; C6 chain hexane,
C7 chain heptane, C8 chain octane,
C9 chain nonane, C10 chain decane
etc.
-
The IUPAC acronym stands for the
International Union of Pure and Applied Chemistry, and one responsibility of
this organisation is the development and maintenance of a systematic method of
naming compounds that is recognised by chemists from all around the world - an
truly international nomenclature.
-
The table above lists the
molecular formula and names of the first ten linear alkanes (the term linear
applies to butane onwards, i.e. from whence structural chain isomerism is possible.
-
If all the carbon atoms of the
molecule are in one continuous chain, it is referred to as linear
(unbranched
or non-branched).
-
If another chain of carbon atoms
starts out of the main carbon chain, it is referred to as branching,
giving rise to 'branched' alkane, one with a side-chain - so this is
how to name branched alkanes.
-
e.g. 3-ethylpentane is
branched, because it has an 'ethyl branch' from the 3rd carbon atom
in the main chain. Abbreviated structural formula style.
-
The longest continuous
chain of 5 carbon atoms forms the basis of the name - more on this below.
-
Note that you need a minimum of 4 carbon atoms in the alkane molecule to
create a side chain branch as in the case of methylpropane. Here the
three C-C bonds are shown, but just one of the C-H bonds.
-
You must always use the longest
continuous carbon chain as the basis of the IUPAC systematic name and
the lowest possible substituent numbers.
-
For example it would be wrong to name the
above molecules trimethylmethane, and
-
would
not be named 2-propylpropane or 4-methylpentane, but 2-methylpentane,
-
would
not be named 2-ethylbutane or 3-ethylbutane, but 3-methylpentane
-
The 3- denotes the
position of the carbon chain branch i.e. the lowest number possible for
the start of the side-chain.
-
The positions of the substituent alkyl groups
(side chains or 'branches') are denoted by using the lowest possible
number(s)
-
e.g. 2, 3 etc. for the associated carbon atoms in the main chain, where the 1st
carbon atom in the chain is considered as C atom 1 and can't have an alky
substituent, since that merely elongates the longest carbon chain.
-
If there is more than one
'type' of substituent e.g. using the prefixes: methyl..., dimethyl...,
trimethyl, ethyl..., diethyl etc., they are written out in alphabetical order (BUT
di, tri etc. are ignored in using this rule, so ).
-
Note the use of the lowest number 2 as the
basis for working out all the other numbers!
-
This is quoted from near the end of this
page, but there are numerous simple examples before that!
Some 'old'
names are quoted in (italics) though their use should be avoided if possible [but
many still used - just put one into GOOGLE!].
The alkane names in bold are the
preferred IUPAC names.
1.1.2
More on ways
of representing the structure of molecules and classes of
formulae
This is illustrated by looking at the
structure of the propane, 2-methyl propane and butane molecules. Follow the
sequence of bullet points down, and then back up, so you are quite clear on the
relationship between all the structural and formula styles.
-
The empirical formula
means the simplest possible formula showing the whole number stoichiometric
ratio of the different atoms (elements) in the compound.
-
It derives from an elemental analysis of
a compound.
-
e.g. for propane C3H8 it is
C3H8,
(same as molecular formula)
-
and for butane or 2-methylpropane, both C4H10,
it is
C2H5. (different from the molecular formula,
÷2).
-
If the number of carbon atoms is an even
number, the empirical formula is 'half' the molecular formula.
-
If the number of carbon atoms is an odd
number, the empirical formula is the same as the molecular formula.
-
For empirical formula and molecular
formula calculations see ....
-
The molecular formula
summarises all the atoms in the molecule BUT does not show their arrangement
at all.
-
C3H8
is the of molecular formula for propane and
C4H10
that of butane and 2-methylpropane.
-
Note that for propane, the empirical formula is identical to the molecular formula BUT for
2-methylpropane and butane, they are not identical. In the case of the
latter, the molecular formula is 'twice' the empirical formula.
-
Note also, that the
molecular formula, does NOT distinguish the two structural isomers butane
and 2-methylpropane ('methylpropane').
-
The 2- is not strictly required,
since the branching must occur on the middle carbon as you will see below,
where we introduce examples of the more advanced IUPAC naming system that
accompanies the
unambiguous representation and naming of molecular structures.
-
An abbreviated, condensed or shortened structural formula,
is unambiguous if you know how to
interpret it!
-
It shows how groups of atoms are linked or sequenced in a
molecule but it doesn't always show all the bonds.
-
propane
or
, abbreviated and more detailed styles of alkane structural formulae.
-
The abbreviated structural formula is the minimum possible
unambiguous representation of a molecule AND essential to distinguish different
structures of the same molecular formula i.e. structural isomers like
methylpropane and butane.
-
methylpropane

-
butane
or

-
Note that methylpropane
and butane can now be distinguished, BUT, you must be able to envisage these
correctly into a full displayed formula structure that shows how all the atoms are
'connected', and this is explained next.
-
A full structural/graphic/displayed formula
gives a '2D' projection-representation of the molecule and must clearly show how all
the atoms are connected i.e. in this case all the C-C and C-H covalent bonds, but does
it not give
the full 3D structural, or spatial arrangement, of the atoms, though for
most purposes, this level of detail is quite sufficient.
-
propane
, unfortunately it does give the impression that the bond angles are 90o,
rather than the true angles, many of which are ~109o based on the
tetrahedrally arrangement of four single bonds around the carbon atoms in
alkane molecules.
-
methylbutane

-
butane

-
A stereochemical formula
is the full displayed formula in terms of a 3D
structural or spatial arrangement of the atoms (albeit on a '2D' screen or
paper).
-
Only e.g. ball and stick models can fully show the 'true' spatial
arrangement of all the atoms, more than adequately simulated by modern
computer software for 'molecular modelling'. In the 'picture' of propane
below.
-
Imagine
the single thin lines as the C-C bonds lying in the plane of the screen/paper, the
dotted C-H bonds point away from you, and the triangular wedge C-H bonds
point towards you out of the plane of the screen/paper. All the C-C-C, C-C-H or H-C-H bond angles are 109o
in this case, and similarly for all other non-cyclic alkanes.
-
propane
This kind of representation is essential for displaying e.g. mirror image
R/S isomers (optical isomers - enantiomers). See
Isomerism Part 2a.
-
This kind of '3D' molecular
representation of alkanes, emphasises the tetrahedral arrangement of
the four bonds from any carbon atom to other carbon or hydrogen atoms - in
fact all the C-C or C-H bonds are usually ~109o.
-
An important point to appreciate from a
3D representation is that for sigma bonds (single C-C bonds here) there is
free rotation of any alkyl group about any C-C bond.
-
Ethane
is also represented by a ball-and-stick model molecular diagram AND a
space-filling model molecular diagram.
-
is
the skeletal formula
for propane. Wow, that simple!
-
Skeletal formula (e.g. of alkanes) are
very simplified organic formulae shown by removing hydrogen atoms from alkyl
chains leaving just lines for the carbon skeleton bonds and associated
functional groups (the latter does not apply to alkanes.
-
This is derived by drawing a short line
– to represent a
C-C single bond, so the V shape for propane comes from the C-C-C carbon chain skeleton
and the C-C-C bond angle of ~109o. (Note: = means a double
bond,
≡
a triple bond)
-
The symbols for carbon atoms and
hydrogen atoms are NOT usually shown in a skeletal formula.
-
No lines are shown for C-H
bonds, they are assumed.
-
However, bond lines should be drawn for C-X bonds,
where X is not a hydrogen atom, and the symbol for the X atom should be
shown too (see e.g. see
halogenoalkanes or
alcohols and ethers).
-
methylpropane is
showing the main C-C-C chain and a single carbon branch from the
middle carbon.
-
butane is
showing the 'linear', but actually zig-zag C-C-C-C chain of 4 carbon atoms with no branching.
-
represents 3-methylhexane. A zig-zag line of 5 C-C bonds (but 6 C
atoms) and a dash from the 3rd carbon atom indicating the C-C bond to the
methyl group.
-
is 3-ethylpentane, you need an extra
to indicate ethyl.
-
See also ball and stick/ball and spring models
and space-filling models for ethane.
-
A general formula CnH2n+2 for non-cyclic
alkanes, represents a member of a homologous series when n is
designated an integer value for the number of carbon atoms (in the alkane
molecule)
-
e.g. if n = 5, it gives the molecular formula of
pentane C5H12.
-
A homologous series
is a series of compounds with the same functional group in which each member differs from the next member
by a constant amount e.g. for alkanes, the addition of a -CH2-
'unit' as the series is ascended n = 1, 2, 3 etc.
-
Consequently, they have a
very similar molecular structure, very similar physical and chemical
properties.
-
However, within a homologous series, the members will show trends
in physical properties like increasing boiling point or decreasing
solubility, which are a function of intermolecular forces that increase with
chain length.
-
A functional group is a group of atoms
responsible for the characteristic reactions of a compound - but alkanes
don't have a functional group like the rest of the homologous series of
organic molecules.
-
BUT, alkanes may not have a specific
functional group, but they do form a homologous series of compounds with the
same sort of molecular structure and similar chemistry, namely chains of
carbon atoms combined with the maximum number of hydrogen atoms - so they
are saturated hydrocarbon molecules - no
double bonds
like alkenes or triple bonds like alkynes.
1.1.3 A simplified structural
comparison of
aliphatic, alicyclic and aromatic HYDROCARBON compounds
-
Reminder - a hydrocarbon is a molecule
composed of ONLY carbon and hydrogen atoms.
-
You often study the details of alkanes when
dealing with the fractional distillation of crude oil, uses of the fractions
and cracking and reforming to make other hydrocarbon products.
-
You will therefore encounter four types
of hydrocarbon, whose basic structures you should make yourself familiar
with.
-
The names in bold are the preferred
IUPAC names.
-
1. ALIPHATIC - have no
benzene ring in their structure (see 3.) and can have an
open linear carbon chain, or branched carbon chain structure or a carbon ring
structure - they are saturated hydrocarbons, no >C=C< double bonds or
benzene ring.
-
If they have a
cyclic structure, they may be termed alicyclic, but they are
still aliphatic (see 2.).
-
Examples of linear and branched aliphatic alkane compounds.
-
heptane,
or
(a linear non-branched alkane)
-
2-methylbutane,
or
(a branched alkane)
-
and lots more
examples on this page!
-
Note the general formula for non-cyclic
alkanes is CnH2n+2 irrespective of
whether they are branched or unbranched.
-
2. ALICYCLIC (sub-group of
aliphatic hydrocarbons) - these
molecules have an aliphatic structure but contain a cyclic or ring structure of at
least 3 carbon atoms (can't be less than 3 and must be carbon!) BUT not a benzene ring e.g.
-
Cyclobutane, cyclopentane,
cyclohexane
-
,
and
,
and
,
,
no branches here
-
Methylcyclohexane,
or
,
a branched example of an alicyclic alkane.
-
Alicyclic alkanes resemble the
aliphatic molecules illustrated in 1. both physically and chemically, and
there are lots more of them on this page!
-
NOTE that later in your course
you will meet lots of other homologous series of aliphatic compounds
e.g. alkenes, alcohols and halogenoalkanes etc. some of which may have
an aliphatic/alicyclic ring structure.
-
Note the general formula for cyclic alkanes
(alicyclic) is CnH2n (two hydrogens
less than non-cyclic alkanes), take care, this is the same general
formula for non-cyclic alkenes with one double bond.
-
3. AROMATIC - These molecules
contain a benzene ring based a 'special' cyclic C6 system, which is an
unsaturated ring (BUT not an alkene system) e.g.
-
benzene C6H6,
or 
-
4. ALKENES - these are also
aliphatic hydrocarbon molecules contain at least one carbon-carbon
double bond (>C=C<), they are referred to as unsaturated hydrocarbons.
-
Again, like alkanes, you can have
aliphatic cyclic (alicyclic) alkenes.
-
The structure of cyclohexene is shown on
the right.
-
NOTE:
-
(i) Some molecules can be
classified in several ways depending on which part of their structure imparts
the functional group chemistry you might be interested in e.g.
-
(ii) Substituted hydrocarbons
e.g. a halogen replacing a hydrogen atom on the carbon chain, does not affect these basic
definitions e.g.
1.1.4 A brief guide to
working out isomers of non-cycloalkanes CnH2n+2
The alkane names in bold are the preferred
IUPAC names.
-
Isomers are molecules with the same
molecular formula but different in some way in their molecular structure.
-
With alkanes you get structural
isomerism because you can vary the arrangement of atoms in the carbon
chain.
-
Structural chain isomers of alkanes
-
No structural isomers
exist for methane, ethane or propane.
-
However, from C4H10
onwards
structural isomers exist. (for other examples and explanation see
Isomerism Part 1)
-
C4H10
can be set out as a linear carbon chain to give butane itself.
-
For C5H12
you can make 3 structural chain isomers - same molecular formula, but atoms
arranged-connected differently:
-
pentane, longest possible linear or 'unbranched chain'.
-
,
2-methylbutane, longest chain with a single branch shortening the
main chain by 1 carbon.
-
2,2-dimethylpropane, shortest possible main chain by double
branching and shortening the main chain by 2 carbons.
-
The structural chain isomerism of alkanes is
fully described on another page, including the differences in physical
properties.
-
You can extend these ideas from C6H14
onwards, working out by trial
and error all the possible branchings in terms of methyl, dimethyl,
trimethyl or ethyl groupings etc. I've described lots of alkane examples on
this page.
-
Make sure you don't:
-
(i) do the same molecule bent into
different shapes on your 2D paper, if all the bond connections are the same
its not a different structural chain isomer!
-
(ii) draw mirror images of exactly the
same structural chain isomer.
-
The
18 structural chain isomers of C8H18
are worked out for you in section 1.1.7
and anything else from C6-C7 or C9 onwards,
you can work out for yourself!
-
Optical (R/S) isomerism
occurs from molecular formula C7H16 onwards.
-
R/S isomerism occurs when you have a
chiral carbon atom or asymmetric carbon atom in the molecule,
that is a carbon atom with 4 different atoms/groups attached to it.
-
It is
then possible to have two non-superimposable mirror image forms - two
molecular versions with the same molecular formula.
-
The first possible
example i.e. with 4 different groups attached to give a chiral carbon is
3-methylhexane.
-

-
The 4 different groups
are -H, -CH3, -CH2-CH3 and -CH2CH2CH3
attached to the central asymmetric/chiral carbon atom.
-
The mirror-image forms (enantiomers)
would be extremely difficult to separate.
-
Other examples and
explanation of
Optical (R/S)
Isomerism.
1.1.5
The homologous series of alkanes - non-cyclic saturated hydrocarbons with
up to 7 carbon atoms for the general formula CnH2n+2
The alkane names in bold are the preferred
IUPAC names.
-
(a) the molecular formula of methane, two
full structural graphic/displayed formula representations are
-
(b)
(displayed formula) and (c) which gives a 3-dimensional
(3D) structural formula impression of the molecule.
-
The H-C-H bond angle is 109.5o
giving a perfect tetrahedral shape so ALL the C-C-H, H-C-H or H-C-H angles are approximately 109o in all the
non-cyclic alkanes shown below.
-
is a 3D ball and stick representation of the methane
molecule.
-
The tetrahedral shape of methane arises from the
mutual repulsion of four bonding pairs of electrons around the central
carbon atoms.
-
Although only methane can have a
tetrahedral shape, the same electron pair repulsion theory predicts,
correctly, that, apart from cyclopropane, cyclobutane and
cyclopentane, all the C-C-C, C-C-H and H-C-H bond angles will be
approximately 109o in linear alkanes, branched alkanes
and cyclic alkane molecules from cyclohexane onwards.
-
For more see
dot & cross diagrams and molecule
shapes derived from electron pair repulsion theory
-
structural formulae (a) or (b)
-
or displayed formulae (c) or (d)
ethane,
-
molecular formula (e) , the skeletal formula is (f) , yes! just a dash!
-
NOTE: from (a) to (d) you go from the most abbreviated structural formula representation to the maximum 3D structural
graphic formula representation on a 2D format!
-
The shape is essentially
derived from two tetrahedral bond networks linked by the common C-C
bond.
-
ball-and-stick
model of ethane showing the typical bond angles of
approximately 109o for H-C-H and C-C-H
bond angles in non-cyclic saturated hydrocarbons i.e. alkanes.
-
space-filling
model of ethane
-
(a) or (b) or
-
(c) or (d)
-
(a) and (b) are the
abbreviated structural formula for propane,
-
(c) and (d) full displayed structural formula, but (d) is a 3D version to indicate
the spatial arrangement of the atoms,
-
for molecular formula (e) and the skeletal formula is (f)
-
All the C-C-C, C-C-H or
H-C-H bond angles are ~109o. and this applies to all the
rest of the linear or branched alkanes which are NOT cyclic.
-
(a) or (b) are abbreviated structural formula for butane
-
(a) or (b) are the abbreviated structural formula for methylpropane
(isobutane), the prefix 2- isn't strictly needed. BUT can be added for
clarity, especially for beginners,
-
the molecular
formula is (c) and
-
the skeletal formula is (d)
and is the simplest branched hydrocarbon.
-
This molecule is isomeric
with butane above.
-
This is the first instant of the
existence of structural chain isomers in an alkane - different
carbon chain arrangements, not possible with methane, ethane or
propane
-
The structural chain isomerism of alkanes is
fully described on another page, including the differences in physical
properties.
-
(a) or (b) are abbreviated structural formula
-
(a) (b) abbreviated structural formula for methylbutane
(isopentane),
-
(a) or (b) are the abbreviated structural formula for dimethylpropane
(neopentane),
-
(the prefix 2,2- isn't strictly needed but can help initially),
-
molecular formula (c) and the skeletal formula is (d)
-
Molecules 6-8 are isomeric
with each other.
-
The structural chain isomerism of alkanes is
fully described on another page, including the differences in physical
properties.
-
(a) or (b)
-
are abbreviated structural formula for
hexane
(normal or n-hexane),
-
molecular formula (c) and the skeletal formula is (d)
-
(a) or (b) are abbreviated structural formula for
-
2-methylpentane (NOTE: prefix numbers
definitely needed from now on),
-
molecular formula
and the skeletal formula is (d)
-
(a) or (b) are abbreviated structural formula
-
(a) or (b) are abbreviated structural formula for
-
2,2-dimethylbutane (NOTE: numbers needed here,
cross-check with 7.),
-
molecular formula (c) and the skeletal formula is (d)
-
(a) or (b) are abbreviated structural formula for
-
2,3-dimethylbutane, molecular formula (c) and the skeletal formula is (d)
-
Molecules 9-13 are isomeric
with each other - this is a good example of working out all the
structural chain isomers of the alkane with a particular molecular
formula, in this case C6H14.
Start with the linear molecule (hexane), then one methyl branch
(pentanes) and the two methyl branches (butanes) and you derive the
five possible structural chain isomers of C6H14.
Note that you cannot make an ethylpropane - its actually a
methylpentane! check it out for yourself!
-
The structural chain isomerism of alkanes is
fully described on another page, including the differences in physical
properties.
-
(a)
-
or (b)
-
are the abbreviated structural formula for
heptane, molecular formula (c)
-
and the skeletal formula is (d)
-
(a)
-
or (b)
-
are the abbreviated structural formula for
2-methylhexane,
-
molecular formula (c) and the skeletal formula is (d)
-
(a)
-
or (b)
-
are abbreviated structural formula for
3-methylhexane, molecular formula (c)
-
and the skeletal formula is (d)
, has an asymmetric carbon atom so will exhibit optical (R/S)
isomerism
-
(a) or (b) are abbreviated structural formula
-
(a) or (b) are abbreviated structural formula
-
(a) or (b)
are abbreviated structural formula
-
for 2,3-dimethylpentane, molecular formula (c) and the skeletal formula is (d)
, has an asymmetric carbon atom so will exhibit optical (R/S)
isomerism
-
(a) or (b)
are abbreviated structural formula
-
(a) or (b) are abbreviated structural formula
-
(a) or (b) are the abbreviated structural formula for
1.1.6 Cycloalkanes
(general formula CnH2n
& isomeric with non-cyclic
alkenes)
The alkane names in bold are the
preferred IUPAC names.
-
Cycloalkanes are named according to the rules previously described, but the name is based on the number of carbon atoms in the ring itself.
-
The 'smallest' cycloalkanes must have at least 3 carbon atoms in the ring.
-
The structures are shown as abbreviated structural formulae and skeletal formulae.
-
They are sometimes referred to as examples of
alicyclic hydrocarbons, that is, aliphatic in nature, but with a ring
i.e. cyclic-aliphatic compounds, as opposed to an aromatic ring compound based
on a benzene
ring.
-
cyclopropane, C3H6 ,
,
note C-C-C bond angle is 60o
because all three carbon atoms are in the same plane, the only
cycloalkane in which all the carbon atoms are in the same plane. It
means the C-C-C bond is highly strained and easily broken unlike the
majority of C-C-C bonds in alkane molecules.
-
methylcyclopropane, C4H8 ,
,

-
1,1-dimethylcyclopropane, C5H10 ,
,

-
1,2-dimethylcyclopropane, C5H10 ,
,

-
ethylcyclopropane, C5H10 ,
,

-
cyclobutane, C4H8 ,
,
note the C-C-C bond angle may look as if
it is 90o
but there are two slightly angled 'butterfly' conformations which
oscillate from one form to another with maximum C-C-C bond angles of ~88o.
In other words the carbon atoms are NOT in the same plane, which is the
case for every cycloalkane except cyclopropane (above). The H-C-H bond
angles are ~107o in cyclobutane.
-
methylcyclobutane, C5H10 ,
,

-
1,1-dimethylcyclobutane, C6H12 ,
,

-
1,2-dimethylcyclobutane, C6H12 ,
,

-
1,3-dimethylcyclobutane, C6H12 ,
,

-
ethylcyclobutane, C6H12 ,
,

-
cyclopentane, C5H10 ,
,
,
all bond angles ~109o?
it is NOT planar, the pentagon is bent.
-
methylcyclopentane, C6H12 ,
,

-
1,1-dimethylcyclopentane, C7H14 ,
,

-
1,2-dimethylcyclopentane, C7H14 ,
,

-
1,3-dimethylcyclopentane, C7H14 ,
,

-
ethylcyclopentane, C7H14 ,
,

-
propylcyclopentane, C8H16 ,
,

-
butylcyclopentane, C9H18 ,
,

-
pentylcyclopentane, C10H20 ,
,

-
cyclohexane, C6H12 ,
,
,
all bond angles ~109o
NOT 120o, the hexagon is bent
into its most stable double pointed chair form - its most stable
conformation.
ball-and-stick
diagram of
cyclohexane
-
methylcyclohexane, C7H14 ,
=
=

-
1,1-dimethylcyclohexane, C8H16 ,
,

-
1,2-dimethylcyclohexane, C8H16 ,
,

-
1,3-dimethylcyclohexane, C8H16 ,
,

-
1,4-dimethylcyclohexane, C8H16 ,
,

-
ethylcyclohexane, C8H16 ,
,

-
propylcyclohexane, C9H18 ,
,

-
butylcyclohexane, C10H20 ,
,

-
pentylcyclohexane, C11H22 ,
,

1.1.7 to 1.1.9
Larger Alkanes with 8-10 carbon atoms
CnH2n+2 continued
1.1.7 Illustrated as brief guide to
working out the 18 isomers of non-cycloalkanes C8H18
The alkane names in bold are the preferred
IUPAC names.
(1)
octane, CH3-CH2-CH2-CH2-CH2-CH2-CH2-CH3

Start with the linear
(unbranched) carbon chain, then make the next longest chain with a single, but
shortest, carbon branch (-CH3), to give three
methylheptanes ...
(2) 2-methylheptane, (CH3)2CHCH2CH2CH2CH2CH3
(3) 3-methylheptane, CH3CH2CH(CH3)CH2CH2CH2CH3
(4) 4-methylheptane,
CH3CH2CH2CH(CH3)CH2CH2CH3
then do double methyl
branching permutations to make 6 dimethylhexanes ...
(5) 2,2-dimethylhexane,
,

(6) 2,3-dimethylhexane,
,

(7) 2,4-dimethylhexane,
,

(8) 2,5-dimethylhexane,
,

(9) 3,3-dimethylhexane,
,

(10) 3,4-dimethylhexane,
,

then you can
make one ethylhexane ...
(11) 3-ethylhexane,
,

and don't try
2-ethylhexane, because its actually 3-methylheptane using the
nomenclature rules correctly.
Now you can do a
double branching again to make two ethylmethylpentanes ...
(12) 3-ethyl-2-methylpentane,
,
(13) 3-ethyl-3-methylpentane,
,

and you can do a
triple branching to give four trimethylpentanes ...
(14) 2,2,3-trimethylpentane,
(CH3)3CCH(CH3)CH2CH3
(15) 2,2,4-trimethylpentane
(isooctane), (CH3)3CCH2CH(CH3)2,
,

(16) 2,3,3-trimethylpentane, (CH3)2CHC(CH3)2CH2CH3
(17) 2,3,4-trimethylpentane,
(CH3)2CHCH(CH3)CH(CH3)2
Then finally,
the most branched isomer is the single tetramethylbutane (shortest
possible main chain) ...
(18) 2,2,3,3-tetramethylbutane, (CH3)3CC(CH3)3
1.1.8 Examples of isomers of C9H20
The alkane names in bold are the
preferred IUPAC names.
-
3-ethyl-2,2-dimethylpentane,
,

-
3-ethyl-2,3-dimethylpentane,
,

-
3-ethyl-2,4-dimethylpentane,
,
-
3-ethyl-2-methylhexane ,
,

-
3-ethyl-3-methylhexane ,
,

-
3-ethyl-4-methylhexane ,
,

-
4-ethyl-2-methylhexane ,
,

1.1.9 Examples of isomers of C10H22
(many have an asymmetric carbon atom so will exhibit optical (R/S) isomerism)
The alkane names in bold are the preferred
IUPAC names.
-
3-ethyl-2,2-dimethylhexane ,
,

-
3-ethyl-2,3-dimethylhexane ,
,
-
3-ethyl-2,5-dimethylhexane ,
,
-
3-ethyl-2,4-dimethylhexane ,
,
-
4-ethyl-2,2-dimethylhexane ,
,
-
4-ethyl-2,3-dimethylhexane ,
,
-
4-ethyl-3,3-dimethylhexane ,
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A Level Organic
Chemistry Part 1 sub-index:
1.1.1 Alkane nomenclature explained
1.1.2 Different ways of representing molecules
and classes of formula
1.1.3 Comparison of aliphatic/alicyclic/aromatic compounds -
definition & examples
1.1.4
Alkane isomerism explained
1.1.5 Homologous
series of alkane examples, general formula CnH2n+2 including isomers of molecular formulae
up to C7H16 (up to n = 7)
1.1.6 Examples of cycloalkanes
'alicyclic' compounds of general formula CnH2n
1.1.7
All 18 alkane isomers of C8H18
* 1.1.8 Examples of alkane isomers of C9H20
1.1.9 Examples of alkane isomers of C10H22
and also
Multiple choice
Quiz on the structure and naming (nomenclature) of ALKANES
Type in name Quiz on the structure & naming of ALKANES (and
naming cycloalkanes)
Combined
multiple choice and type in name quiz on alkanes
Matching pair quiz on hydrocarbon structure
Alkanes and Petrochemical Industry INDEX
All Advanced Organic
Chemistry Notes
Index of GCSE/IGCSE Oil - Useful Products
Chemistry Revision Notes
formula keywords: how to
name naming nomenclature empirical molecular formula graphic formula
displayed formula skeletal formula structural isomers isomerism CH4 C2H6
CH3CH3 CH3-CH3 C3H8 CH3CH2CH3 CH3-CH2-CH2-CH3 C4H10 CH3CH2CH2CH3
CH3-CH2-CH2-CH3 C4H8 C5H12
C5H10 C6H14 C6H12 C7H16 C8H18 C9H20 C10H22 chemistry revision notes structure of alkanes AS AQA
GCE A level chemistry how do you name alkanes? AS Edexcel GCE A level
chemistry alkane nomenclature rules AS OCR GCE A level chemistry what is
the molecular structure of alkanes? AS Salters GCE A level chemistry how
to work out isomers of alkanes US grades 11 & 12 chemistry IUPAC naming
of alkanes notes for revising the structure and naming of linear and
cyclic alkanes how to name alkanes
for AQA AS chemistry, how to name alkanes
for Edexcel A level AS chemistry, how to name alkanes for A level OCR AS chemistry A,
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how to name alkanes for AQA A level chemistry, how to name
alkanes for A level Edexcel A level chemistry,
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A, how to name alkanes for A level OCR Salters A
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CCEA/CEA A level chemistry notes on how to name alkanes
These detailed notes on the
structure and naming of alkanes include the general formula of alkane
molecules, empirical formula of alkane molecules, structural formula of
alkane molecules, skeletal formula of alkane molecules, displayed
formula of alkane molecules, shapes of alkane molecules, isomers of
alkane molecules IUPAC rules for alkane nomenclature. Students should be
able to draw structural formula of alkane, displayed and skeletal
formulas for alkane organic compounds apply IUPAC rules for nomenclature
to name alkane acid organic compounds including chains and rings and be
able to apply IUPAC rules for nomenclature to draw the structural,
displayed or skeletal structure of alkane organic compounds from the
alkane IUPAC name from the homologous series of alkanes
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