|
m/c
quiz on naming alkanes *
type in
naming quiz on alkanes
Examples of displayed formula of alkane molecules
with names, graphic
formula, molecular formula, skeletal formula, structural formula of this
homologous series
Four introductory
sections (a) to (d), precede the numerous named examples of how to draw and name
alkanes.
*
(a)
Nomenclature explained * (b) Different ways of representing molecules
and classes of formulae *
*
(c) Comparison of aliphatic/alicyclic/aromatic compounds -
definition & examples * (d) Isomers explained *
EXAMPLES of structures
and names
(i) non-cyclic
alkanes, general formula CnH2n+2
including isomers of molecular formulae
up to C7H16 (up to n = 7)
(ii)
examples of cycloalkanes
'alicyclic' compounds of general formula CnH2n and
(iii)
examples of higher non-cyclo alkanes general formula CnH2n+2 where n = 8 to 10
Some other useful
organic chemistry pages.
Organic
chemical identifying functional group tests *
Summary
of functional groups
Intermolecular forces & boiling points of
alkanes compared to other
organic molecules
The shapes
and bond angles of simple molecules - section on bond angles in organic
molecules
1.1.1 A brief guide to
the structure and nomenclature of
non-cyclic alkane
hydrocarbons
| CnH2n+2
n = |
1 |
2 |
3 |
4 |
5 |
6 |
7 |
8 |
9 |
10 |
| formula of
alkane |
CH4 |
C2H6 |
C3H8 |
C4H10 |
C5H12 |
C6H14 |
C7H16 |
C8H18 |
C9H20 |
C10H22 |
| name of alkane |
methane |
ethane |
propane |
butane |
pentane |
hexane |
heptane |
octane |
nonane |
decane |
-
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 preserved 'old trivial' names, the name is 'numerically' systematic e.g.
C5
carbon chain
pentane; C6 chain hexane,
C7 chain heptane, C8 chain octane,
C9 chain nonane, C10 chain decane
etc.
-
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 chain isomerism is possible.
-
If all the carbon atoms of the
molecule are in one continuous chain, it is referred to as unbranched
or linear.
-
If another chain of carbon atoms
starts out of the main carbon chain, it is referred to as branching,
giving rise to 'branched' alkane.
-
e.g. 3-ethylpentane is
branched, because it has an 'ethyl branch' from the 3rd carbon atom
in the main chain.
-
The longest continuous
chain of 5 carbon atoms forms the basis of the name.
-
The 3- denotes the
position of the carbon chain branch i.e. the lowest number possible.
-
The positions of the substituent alkyl groups
(side chains or 'branches') are denoted by using the lowest possible
number(s)
-
If there is more than one
'type' of substituent e.g. using the prefixes: methyl... and ethyl.... etc., they are written out in alphabetical order (di, tri are ignored in using this rule).
-
Some 'old'
names are quoted in () though their use should be avoided if possible [but
many still used - just put one into GOGGLE!].
-
The shapes
and bond angles of simple molecules - section on bond angles in organic
molecules

1.1.2 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.
-
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 a more advanced structure notation that allows the
unambiguous representation of molecular structures.
-
An
'abbreviated' 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. It is the minimum possible representation to distinguish different
structures of the same molecular formula i.e. structural isomers like
methylpropane and butane.
-
propane
or

-
methylpropane

-
butane
or 
-
Note that methylpropane
and butane can now be distinguished, BUT, you must be able to envisage these
correctly into a full 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

-
methylbutane

-
butane

-
A
stereochemical formula
is the full structural/graphic/displayed formula in terms of the 3D
structural or spatial arrangement of the atoms (albeit on '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. All the C-C-C, C-C-H or H-C-H bond angles are 109o
in this case, similarly for all other non-cyclic alkanes.
-
is
the skeletal formula
for propane. 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. 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 (see e.g.
alcohols and ethers).
-
A general formula
e.g. CnH2n+2 for non-cyclic
alkanes, represents a member of a homologous series when n is
designated an integer value e.g. if n = 5, it gives the molecular formula of
pentane.
-
A
homologous series
is a series of compounds 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.

1.1.3 A simplified structural
comparison of
aliphatic, alicyclic and aromatic hydrocarbon compounds
-
1. ALIPHATIC - have no
benzene ring in their structure (see 3.) and can have an
open linear or branched structure. If they have a
non-benzene ring cyclic structure, they may be termed alicyclic, but they are
still aliphatic (see 2.)
e.g.
-
2. ALICYCLIC - 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!) BUT not a benzene ring e.g.
-
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 
-
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
-
No structural isomers
exist for methane, ethane or propane.
-
Structural chain isomers
-
However, from C4H10,
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 isomers:
-
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.
-
You can extend these
ideas to C6H14
onwards, working out by trial
and error all the possible branchings in terms of methyl, dimethyl,
trimethyl or ethyl groupings etc.
-
The
18 structural chain isomers of C8H18
are worked out for you in section (iii)
and anything else from C6-C7 or C9 onwards,
you can work out for yourself!
-
Optical isomerism
occurs from molecular formula C7H16 onwards.
-
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
-
The mirror-image forms
(enantiomers) must be extremely difficult to separate.
-
Other examples and
explanation of Optical
Isomerism.

1.1.5
Non-cyclic Alkanes with
up to 7 carbon atoms for CnH2n+2
-
(a) the molecular formula of methane, two
full structural graphic formula representations are
-
(b) and (c) which gives a 3-dimensional
(3D) structural formula impression of the molecule.
-
The H-C-H bond angle is 109o. NOTE: all the C-C-H, H-C-H or H-C-H angles are approximately 109o in all the
non-cyclic alkanes shown below.
-
(a) or (b) or (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!
-
(a) or (b) or (c) or (d)
-
(a) and (b) are the
abbreviated structural formula for propane,
-
(c)/(d) full
displayed-graphic 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)
-
(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,
-
(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)
-
(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 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
-
(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)
-
(a) or (b) are abbreviated structural formula
-
(a) or (b) are abbreviated structural formula
-
(a) or (b)
are abbreviated structural formula
-
(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)
-
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 ,
,
note C-C-C bond angle is 60o
-
methylcyclopropane ,
,

-
1,1-dimethylcyclopropane ,
,

-
1,2-dimethylcyclopropane ,
,

-
ethylcyclopropane ,
,

-
cyclobutane ,
,
note C-C-C bond angle is 90o
-
methylcyclobutane ,
,

-
1,1-dimethylcyclobutane ,
,

-
1,2-dimethylcyclobutane ,
,

-
1,3-dimethylcyclobutane ,
,

-
ethylcyclobutane ,
,

-
cyclopentane ,
,

-
methylcyclopentane ,
,

-
1,1-dimethylcyclopentane ,
,

-
1,2-dimethylcyclopentane ,
,

-
1,3-dimethylcyclopentane ,
,

-
ethylcyclopentane ,
,

-
propylcyclopentane ,
,

-
butylcyclopentane ,
,

-
pentylcyclopentane ,
,

-
cyclohexane ,
,

-
methylcyclohexane ,
,

-
1,1-dimethylcyclohexane ,
,

-
1,2-dimethylcyclohexane ,
,

-
1,3-dimethylcyclohexane ,
,

-
1,4-dimethylcyclohexane ,
,

-
ethylcyclohexane ,
,

-
propylcyclohexane ,
,

-
butylcyclohexane ,
,

-
pentylcyclohexane ,
,


1.1.7
Larger Alkanes with 8-10 carbon atoms
CnH2n+2 continued
Illustrated as brief guide to
working out the 18 isomers of non-cycloalkanes C8H18
!!!!!
-
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-methylheptane, (CH3)2CHCH2CH2CH2CH2CH3
3-methylheptane, CH3CH2CH(CH3)CH2CH2CH2CH3
4-methylheptane, CH3CH2CH2CH(CH3)CH2CH2CH3
2,2-dimethylhexane,
,

2,3-dimethylhexane,
,

2,4-dimethylhexane,
,

2,5-dimethylhexane,
,

3,3-dimethylhexane,
,

3,4-dimethylhexane,
,

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 ...
3-ethyl-2-methylpentane,
,

3-ethyl-3-methylpentane,
,

2,2,3-trimethylpentane,
(CH3)3CCH(CH3)CH2CH3
2,2,4-trimethylpentane
(isooctane), (CH3)3CCH2CH(CH3)2
2,3,3-trimethylpentane,
(CH3)2CHC(CH3)2CH2CH3
2,3,4-trimethylpentane,
(CH3)2CHCH(CH3)CH(CH3)2
2,2,4,4-tetramethylbutane,
(CH3)3CC(CH3)3

1.1.8 Examples of isomers of C9H20
-
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
-
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 ,

CH4 C2H6 C3H8 C4H10 C5H12
C6H14 C7H16 C8H18 C9H20 C10H22
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