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Part 14.1.1 ISOMERISM
in organic compounds -
Introduction
Isomerism occurs when two or
more compounds have the SAME MOLECULAR FORMULA but exist in at least
two different forms due to some structural or spatial arrangement
difference in their molecular structure (i.e. different molecules in
some way). The different molecules are known as ISOMERS.
They may be quite similar, or significantly different, in terms
of their physical and chemical properties.
The
existence of isomers is one of the reasons why such a huge array of
different organic compounds exist.
Other reasons for the diversity
of organic compounds include (i) carbon
chain catenation i.e. the ability of carbon chains to link together and
(ii) the formation of stable bonds with elements such as oxygen,
nitrogen, hydrogen etc.
In order to analyse a
molecular formula for its possible isomers, you need to know what
forms of isomerism are possible and then 'take the molecular formula apart' and reconstruct the
molecule's atoms in
as many different ways that fit according to
(i)
the valency of the atoms e.g. carbon 4 >C<, hydrogen 1 -H,
oxygen 2 -O- or O=. and nitrogen is often 3 >n- (but 5 in nitro
compounds - this is the basis of structural isomerism (this page
1).
(ii) different
spatial arrangement cannot be superimposed on each other - this
is the basis of stereoisomerism (see page 2)
So, ensure the
'reconstructs' are
genuinely different in some way and obey the isomerism criteria described on these
pages,
AND, beware!,
molecular structures that seems different on 2D paper might not be in 3D
reality, especially as there is free rotation of groups of atoms about single bonds
e.g. C-CH3, so
using models for simpler molecules is extremely useful at the start of
studying isomerism.
In the examples described
below I've tried to emphasise differences in physical properties.
In
APPENDIX 1, I've
tabulated the number of structural isomers that can be generated from
selected molecular formulae,
and in
APPENDIX 2. I've written a brief guide to
working out the 18 structural isomers of a non-cycloalkane, octane C8H18
TOP OF PAGE and
sub-index
Part 14.1.2 Introduction to
Structural Isomerism
Structural isomerism
occurs when two or more compounds have the same molecular formula but
different structural formula because of different arrangements of how the
atoms are connected.
They are
best appreciated in terms of their graphic
or full displayed structural formula - but I've chosen to display in a
variety of different styles that you need to be able to read including
skeletal formulae.
At least one atom is bonded to a different
atom in each molecule when isomers are compared.
There are four main
sub-divisions of structural isomerism, (a) to (d), are considered below.
(1a) Chain isomerism
(browse through alkanes, aromatic hydrocarbons (arenes), alkanes and
octane number)
(1b) Positional isomerism
(browse through halogenoalkanes, alkenes, aromatic sulfonic acids,
products of alkene addition reactions, alcohols, amines)
(1c)
Functional group
isomerism (browse through alcohols/ethers, carboxylic acids/esters/other
isomers, aldehydes/ketones, alkenes/cycloalkanes,
aldehydes/cycloalcohols, phenols/alcohols, aromatic amines/nitro
compounds)
(1d) Tautomerism
(hydroxy-ketones/enols)
14.1.2(a)
Chain isomerism -
changing the arrangement of the carbon atoms
CHAIN ISOMERISM is where the carbon
chain arrangement is varied for the same molecular formula.
By connecting the atoms in different
configurations you can form structural isomers, but you need a minimum
of four carbon atoms to produce a branched molecule in terms of its
carbon chain.
14.1.2(a) Structural Isomerism - Carbon chain
isomerism
Case
study 1a.1 Chain isomers of the alkane molecular formula
C5H12
- shorter/longer alkanes
 |
 |
 |
|
pentane (the
least compact) |
2-methylbutane |
2,2-dimethylpropane |
 |
 |
 |
The isomers of the molecular formula
C5H12 shown as simple 'ball and stick' models and
space filling models (above), and abbreviated (but unambiguous)
structural formula and skeletal formula (below). Note the differences in
boiling points between the isomers.
(1)
 ,
 ,
pentane,
volatile colourless liquid, bpt 34oC, linear.
(2)
 ,
 ,
methylbutane (2-methylbutane, but 2- not needed), volatile colourless liquid/gas, bpt 28oC, minimum branching.
(3), ,
 ,
2,2-dimethylpropane, colourless gas, bpt 9.5oC, maximum
branching.
One physical consequence of this isomerism,
is that as the molecule gets more branched it becomes more compact
(see the ball and stick AND space filling model diagrams above).
Therefore
the decreased surface-surface contact weakens the intermolecular
bonding (intermolecular
forces), which in this case are the instantaneous
dipole-induced dipole forces between the non-polar hydrocarbon molecules.
Hence
the weak intermolecular bonding of Van der
Waals forces are influenced by the shape of the molecule and the more compact a
molecule, the less easily it is polarised and the weaker the
intermolecular bonding.
Hence less and less thermal kinetic energy is needed to overcome them, so the boiling point
is reduced from molecule (1) to (3).
They can be separated by fractional
distillation.
Chemically they are very similar e.g. they all readily
burn to carbon dioxide and water or react with chlorine-uv light to form
isomeric halogenoalkanes.
There are no isomers for the lower alkanes CH4, C2H6 or C3H8,
but C4H10 has two chain isomers:
(4)
butane, boiling point -0.5oC, you then see
a reduction in boiling point of the isomer
(5)
2-methylpropane (methylpropane), boiling point -11.7oC.
[lots of named
alkane structures and how to work out the possible isomers for a
given molecular formula]
e.g. some of the possible
chain isomers of the alkane series of molecular formula C8H18
Examples of their structural formula
and skeletal formula are shown below
2,2-dimethylhexane,
 ,
2,3-dimethylhexane,
 ,
2,4-dimethylhexane,
 ,
2,5-dimethylhexane,
 ,
3,3-dimethylhexane,
 ,
3,4-dimethylhexane,
 ,
3-ethylhexane,
 ,
3-ethyl-2-methylpentane,
 ,
3-ethyl-3-methylpentane,
 ,
They will be quite similar molecules,
both physically and chemically, but there will be small differences in melting
point, boiling point (but not insignificant) and density for the reasons
explained above.
TOP OF PAGE and
sub-index
14.1.2(a) Structural Isomerism - Carbon chain
isomerism
Case
study 1a.2 Aromatic hydrocarbons based on C8H10 arenes
(1)
 ethylbenzene,
mpt -94oC, bpt 136oC, all colourless liquids.
(2) 1,2-dimethylbenzene,
(3) 1,3-dimethylbenzene, (4) 1,4-dimethylbenzene
(2)
 mpt
-25oC, bpt 144oC , (3)
mpt -47 oC, bpt 139oC ,
(4) mpt
14oC, bpt 137oC,
Isomer (1) can be synthesised
by the Friedel Crafts reaction using chloroethane/aluminium chloride with benzene and only one
monosubstituted product can be formed.
Isomers (2) to
(4) are obtained from the refining and reforming of crude oil fractions.
(2) to (4) are also positional
isomers based on the two methyl groups.
(2)-(4) are formed when methylbenzene is alkylated with chloromethane/aluminium
chloride reagent (Friedel Crafts reaction). Although (2) and (4) are the
predominant products.
The similarity of boiling points, particularly (3)
and (4), makes them very
difficult to separate even by fractional distillation.
(3), bpt 139oC)
and (4), bpt
137oC, with similar bpts, can be separated as a mixture from
(2), bpt 144oC, by fractional distillation.
Then (4,
fpt 14oC) is separated from (3, fpt -47oC) by fractional
crystallisation because on cooling to low temperatures (4) will
crystallise out well before (3) because of its higher freezing point.
Dimethylbenzenes are chemically very similar e.g.
they undergo the usual electrophilic
substitution reactions of benzene (nitration, chlorination, sulfonation
etc.) and on side chain oxidation, e.g. reflux with KMnO4(aq)/NaOH(aq),
followed by 'working up' and adding dilute hydrochloric/sulfuric acid.
On oxidation
(1) gives benzoic acid, C6H5COOH, and (2)-(4) give
1,2 or 1,3 or 1,4-benzenedicarboxylic acid respectively.
C8H6O4,
 benzene-1,2-dicarboxylic acid
(or 1,3 or 1,4), three more positional or carbon
chain isomers - both descriptions apply here.
[lots of named aromatic structures]
14.1.2(a) Structural Isomerism - Carbon chain
isomerism
Case
study 1a.3 The chain isomerisation of alkanes
and the octane number of petrol fuels
Chain Isomerization is used
in the
petrochemical industry to produce more branched alkanes with a higher
octane number from linear alkanes, for fuels more suitable for petrol
engines.
The proportions of the 'isomers', as well as the hydrocarbon
chain length, in crude oil does not match specific market demands.
Straight chain alkanes are heated with a suitable catalyst to
break up the chains and more branched alkanes, as well as lower alkanes are formed on recombination
of the fragments (see examples below).
For a given carbon number of
an alkane, the more branched the alkane, the higher the octane number.
The higher the
octane number of a fuel/molecule, the less the tendency it has
to cause auto-ignition resulting in 'knocking' or 'pinking'
damaging the car engine.
(1) ,
 , is
linear
heptane,
a chain isomer of C7H16,
and assigned an octane number of 0.
(2) ,
 , is
2,2,4-trimethylpentane,
a highly branched chain isomer of
C8H18,
octane number = 100 (used to be called 'iso-octane').
The known tendency of a
mixture
of (1) and (2) to auto-ignite are compared with other fuels/molecules to give
them their individual 'octane rating'. (In the UK petrol octane
numbers of 95 and 99 are most common)
Using skeletal formula, one
possible isomerisation reaction of pentane C5H12
is shown below.
They are reversible reactions, so changing reaction
conditions, can change the position of the equilibrium.
(3)
pentane, octane number 62
(4) 2-methylbutane, octane number 93
TOP OF PAGE and
sub-index
14.1.2(b)
Positional isomerism of substituents
- carbon atom network structure is retained
These isomers have the
same molecular formula and carbon skeleton but differ in the position of
one or more
functional groups or substituted groups.
14.1.2(b) Structural Isomerism - Positional
substituent group isomerism
Case
study 1b.1
(a) Positional isomers of
C2H4X2
and
C2H3X3
where X = halogen
In all cases there are differences in physical
properties e.g. different boiling points and liquid densities.
The molecular formula C2H4X2
will give rise to two positional isomers i.e. 1,1-di ... and 1,2-di ...
1,1-dichloroethane and
1,2-dichloroethane
Boiling points: 57.3 and 83.7oC,
densities: 1.178 and 1.253 g/cm3
1,1-dibromoethane and
1,2-dibromoethane
Boiling points: 110 and 132oC,
densities: 2.055 and 2.180 g/cm3
The molecular formula C2H3X3 also gives rise to two positional structural
isomers
e.g. 1,1,1-trichloroethane and
1,1,2-trichloroethane where X = chlorine Cl
Boiling points: 74 and ~112oC,
densities: 1.320 and 1.435 g/cm3
(b)
Positional
isomers of halogenoalkane molecular formula C3H7Br
Once an alkane has at least 3 carbon
atoms, substituent groups e.g. halogen or amino groups, can take up
different positions on the carbon chain.
In the uv light catalysed reaction of bromine and propane
gases, the free radical substitution reaction can produce two initial
mono-substitution products. [full
mechanism]
CH3CH2CH3
+ Br2
{CH3CH2CH2Br or
CH3CHBrCH3}
+ HBr
(1) 1-bromopropane,
,
 ,
bpt 71oC, primary halogenoalkane,
(2) 2-bromopropane,
 ,
 ,
bpt
59oC, secondary halogenoalkane,
Only two isomers are
possible. They are both low boiling
colourless liquids, but the more compact molecule
(2) has a lower boiling point - similar physically, but differences in
boiling point.
Chemically they are very similar e.g.
both undergoing all the
nucleophilic substitution reactions with
ammonia, cyanide ion, and hydroxide ion etc.
In the case of the latter,
(1) would give the primary alcohol, propan-1-ol and (2) would give the
secondary alcohol, propan-2-ol - these are important different, if
similar outcomes, from the same reaction.
[lots
of named halogenoalkane structures]
For higher
bromoalkanes e.g. 1-bromobutane CH3CH2CH2CH2Br
and 2-bromobutane CH3CH2CHBrCH3,
another chemical difference will show up on refluxing them with ethanolic
potassium hydroxide, by which, following an elimination reaction,
1-bromobutane can
only form but-1-ene CH3CH2CH=CH2,
but 2-bromobutane can form two isomeric elimination products:
CH3CH2CH2CH2Br
+ KOH ===> CH3CH2CH=CH2
+ KBr + H2O
but-1-ene CH3CH2CH=CH2,
and but-2-ene CH3CH=CHCH3, i.e. you can
eliminate either side of the C-Br bond.
CH3CH2CH2CH2Br
+ KOH ===> {CH3CH2CH=CH2
and CH3CH=CHCH3} + KBr
+ H2O
Other halogenoalkane
positional isomers ...
 ,
 ,
 ,
1,1-dichlorobutane
and ,
 ,
 ,
1,2-dichlorobutane
OR
1-bromo-2-chlorobutane,
 ,
and
1-bromo-3-chlorobutane,
 ,
With several different substituents, even for a
lower alkane like butane, there are many positional isomers.
TOP OF PAGE and
sub-index
14.1.2(b) Structural Isomerism - Position of
functional group isomerism
(NOT functional group isomerism)
Case
study 1b.2 Two linear positional
isomers of molecular formula C5H10
In this case it is the different
position of the C=C alkene functional group give rise to two structural
isomers.
(1)
 ,
pent-1-ene, bpt 30oC,
(2)
 ,
pent-2-ene, cis bpt 37oC, trans bpt 36oC,
They are very similar
physically e.g. relatively non-polar volatile colourless liquids, with similar low boiling points.
Chemically similar e.g. all the usual electrophilic addition reactions of
any alkene, though may, or may not be, some 'isomeric consequences' as
regards both their formation or addition reaction products and some examples are
outlined below.
Unlike pent-1-ene, pent-2-ene can also exist as
E/Z (cis/trans
isomers)
Both can be formed in
cracking pentane or higher alkanes in which various isomers of C5H10 would
be formed. In the laboratory they can be made by elimination reactions
e.g.
(a) the 'dehydration' of isomeric
pentanols with conc. sulfuric acid
or
(i) CH3CH2CH2CH2CH2OH
==> CH3CH2CH2CH=CH2 + H2O
Pentan-1-ol (above) can only give 1
isomer, pent-1-ene,
(ii) CH3CH2CH2CHOHCH3 ==> {CH3CH2CH2CH=CH2
or CH3CH2CH=CHCH3} + H2O
but pentan-2-ol
(below) can give 2
isomers, pent-1-ene and pent-2-ene, because elimination of a -H (as well
as the -OH) can take place
either side of the >CH-OH group from an adjacent C-H.
This is not possible with
pentan-1-ol with the -OH group on the end carbon.
(b) or by refluxing with ethanolic potassium hydroxide. The
formation of more than one isomer of the pentenes depends on the position
of the -OH in alcohols or the -Br in bromoalkanes e.g.
(i) CH3CH2CH2CH2CH2Br
+ KOH ==> CH3CH2CH2CH=CH2 + H2O
+ KBr
1-bromopentane can only give 1
isomer, pent-1-ene (above), but 2-bromopentane (below)
(ii) CH3CH2CH2CHBrCH3
+ KOH ==> {CH3CH2CH2CH=CH2
or CH3CH2CH=CHCH3} + H2O
+ KBr
can give two isomers, pent-1-ene and pent-2-ene, because elimination of a -H (as well
as the -Br) can take place
either side of the >CH-Br group from an adjacent C-H.
This is not possible with
1-romopentane with the -Br group on the end carbon.
You can also derive many
other isomers from the molecular formula C5H10
e.g. methylbutenes (chain/positional isomers with respect to pentenes),
methylcyclobutane and dimethylcyclopropanes (both chain/functional group
isomers with respect to pentenes).
[lots
of named alkene structures], [named
cyclo-alkane structures] or [halogenoalkane
structures]
Other alkene positional
isomers e.g.
 ,
but-1-ene
and
 ,
but-2-ene (the latter can also exhibit
E/Z
isomerism (cis/trans)
14.1.2(b) Structural Isomerism - Positional
substituent group isomerism
Case
study 1b.3 Aromatic examples based on CH3C6H4SO2OH
(C7H8SO3)
(1)
 ,
(2)  and
(3) 
2/3/4-methylbenzenesulfonic
acid
All these three are formed
when methylbenzene undergoes sulfonation when heated with fuming
sulfuric acid. The methyl group increases electrophilic substitution
activity, particularly at the 2 and 4 positions more than the 3 position, so isomers
(1)
and (3) predominate.
C6H5CH3
+ H2SO4 ==> CH3C6H4SO2OH
+ H2O
They are all physically very
similar e.g. colourless crystalline solids and chemically similar e.g.
they are all very strong acids because of the ease of release of the
proton from the sulfonic acid group, -SO2-OH (as in
sulfuric acid).
There are two other
structural
isomers, (4) C6H5CH2SO2OH,
which is an alkyl sulfonic acid, and
(5) C6H5SO2OCH3, the
methyl ester of benzenesulfonic acid, but in your aromatic chemistry
studies, you are only likely to come across (1) to (3).
[lots of named aromatic structures]
and I've often
quoted the three positional isomers for disubstituted
benzene compounds
14.1.2(b) Structural Isomerism - Positional
substituent group isomerism -
some chemical consequences
Case
study 1b.4 Addition of (i) hydrogen bromide or (ii) water to alkenes
If the alkene is symmetrical
about the >C=C< bond, only one product is possible
no matter which way round the electrophilic addition reagent adds
onto the C=C double bond e.g.
(i) CH3-CH=CH-CH3
+ HBr ==> CH3-CH2-CHBr-CH3
(ii) CH3-CH=CH-CH3
+ H2O
==> CH3-CH2-CH(OH)-CH3
so but-2-ene can only form
one product (i) 2-bromobutane and (ii) butan-2-ol.
Other symmetrical alkenes
e.g. ethene or hex-3-ene behave in a similar way.
However, unsymmetrical
alkenes can form two positional isomers depending on which way
round the reagent adds e.g.
(i) CH3-CH=CH2
+ HBr ==> {CH3-CH2-CH2-Br
or CH3-CHBr-CH3}
(ii) CH3-CH=CH2
+ H2O ==> {CH3-CH2-CH2-OH
or CH3-CH(OH)-CH3}
hence, propene can form (i)
1-bromopropane
or 2-bromopropane and (ii) propan-1-ol or propan-2-ol.
Other non-symmetrical alkenes
e.g. 2-methylpropene, but-1-ene, 2-methylbut-2-ene, pent-1-ene,
pent-2-ene, hex-1-ene and hex-2-ene behave in a similar way.
[alkene
addition reactions]
TOP OF PAGE and
sub-index
14.1.2(b) Structural Isomerism - Positional
substituent group isomerism
Case
study 1b.5 The alcohols based on C4H10O
or C4H9OH
(this also involves carbon chain isomerism too)
The molecular formula C4H10O
can lead to a multitude of isomers including different 'types' or 'classes' of
alcohols based on the formula C4H9OH,
resulting in some differences in physical and chemical properties which
are summarised below.
You have two positional isomers for
the linear configuration of the carbon chain.
(1) butan-1-ol, bpt
118oC,
 ,
is
a primary alcohol and oxidised to an aldehyde (butanal) using aqueous
sulfuric acid/potassium dichromate(VI). Its fully linear structure gives
it the maximum intermolecular attractive force, hence the highest boiling point.
(2) butan-2-ol, bpt
100oC,
 ,
(more compact molecule)
is
a secondary alcohol and oxidised to a ketone (butanone) using aqueous
sulfuric acid/potassium dichromate(VI).
You have two more positional isomers
for the branched configuration of the carbon chain.
(3) 2-methylpropan-1-ol,
bpt 108oC,
 ,
 , is
primary alcohol and oxidised to an aldehyde (2-methylpropanal) using
aqueous sulfuric acid/potassium dichromate(VI)
(4) 2-methylpropan-2-ol,
bpt 83oC,
 ,
 , is
a tertiary alcohol and not readily oxidised because the strong C-C
chain would have to be broken. It gives the lowest boiling point because
it has the most compact structure (for explanation see
case study 1a.1)
Again, note physical
difference in boiling points, but a significant chemical difference in
relative ease of oxidation.
From the molecular formula
C4H10O you can also derive three ethers, (5)
ethoxyethane, (6) 1-methoxypropane and (7) 2-methoxypropane. This is now
an example of
functional group isomerism
i.e. alcohol/ether isomerism.
(5)
 ,
(6) , 
(7) ,
These structural isomers are derived from either
changing the position of the ether linkage or configuration of the
carbon chain.
So (5) to (7) are
also
functional
group isomers of alcohols/ethers (see
case
study 1c.1),
and some examples are shown alongside the
alcohols on the
naming and
structure of alcohols/ethers page.
14.1.2(b) Structural Isomerism - Positional
substituent group isomerism
Case
study 1b.6 Halogenoalkane (haloalkane) isomers of C4H9Cl
(this also involves carbon chain isomerism too)
From this molecular
formula, both chain and positional isomers can be derived, as
well as illustrating the three classes of halogenoalkanes and a
few physical and chemical differences are summarised below. The alcohols
formed by hydrolysis of C4H9Cl are considered in
case
study 1b.5 above. Skeletal formulae are used in the examples
below.
You have two positional isomers for
the linear configuration of the carbon chain.
(1) 1-chlorobutane,
bpt 79oC,
 ,
a primary halogenoalkane, in a HCl elimination reaction only
but-1-ene is formed. The most 'linear' structure gives the highest
boiling point. On hydrolysis with aqueous sodium hydroxide, the primary
alcohol butan-1-ol is formed.
(2) 2-chlorobutane,
bpt 67oC,
 ,
a secondary halogenoalkane, in HCl elimination, but-1-ene and
but-2-ene are formed. On hydrolysis the secondary alcohol
butan-2-ol is formed.
You have two more positional isomers
for the branched configuration of the carbon chain.
(3) 1-chloro-2-methylpropane,
bpt 68oC,
 ,
a primary halogenoalkane, in HCl elimination 2-methylpropene is
formed. On hydrolysis the primary alcohol 2-methylprop-2-ol is
formed.
(4) 2-chloro-2-methylpropane,
bpt 51oC,
 ,
a tertiary halogenoalkane, in HCl elimination 2-methylpropene is
formed. The most 'compact' structure gives the lowest boiling
point. On hydrolysis the tertiary alcohol 2-methylprop-2-ol
is formed.
They are all volatile
colourless liquids and all undergo the usual nucleophilic substitution
reactions of any halogenoalkanes.
There can chemical differences
in e.g. tertiary haloalkane (4) is likely to react via the 2 step 'unimolecular' SN1
carbocation mechanism (carbocation stability is tert > sec >
prim), and primary haloalkane (1) is more likely to go by the SN2 'bimolecular'
one step mechanism.
Also, with ethanolic/aqueous sodium hydroxide,
con-current
elimination is much more likely with tertiary halogenoalkanes than
primary ones.
See also bromobutanes for an example of
differences in elimination products,
mechanisms of
haloalkane reactions
and [lots
of halogenoalkane structures]
14.1.2(b) Structural Isomerism - Positional
substituent group isomerism
Case study 1b.7
Aliphatic amine isomers of C3H9N
These give alkaline solutions
if soluble in water.
Kb is the
dissociation constant for a base, the larger Kb,
the greater the ionisation, the more alkaline the aqueous
solution.
B(aq)
+ H2O(l)
BH+(aq) + OH-(aq)
Kb
= [BH+(aq)] [OH-(aq)]
/ [B(aq)]
(1) CH3CH2CH2NH2
propylamine (1-aminopropane), a primary amine, bpt 49oC, Kb
= 4.1 x 10-4 mol dm-3,
(2) (CH3)2CHNH2
2-aminopropane, a primary amine, bpt 4oC, Kb
= 4.0 x 10-4 mol dm-3
(1) and (2) are positional isomers
for the same carbon chain.
(3) and (4) are two more isomers
where the carbon chain is split into sections to give other classes of
amines.
(3) CH3CH2NHCH3
N-methylethylamine, a secondary amine, bpt ?oC, Kb
= ? mol dm-3
(4) (CH3)3N
trimethylamine, a tertiary amine, bpt 3oC, Kb
= 0.6 x 10-4 mol dm-3
Note the different physical
property of boiling
point, and a chemical property e.g. strength of base - extent of
ionisation.
They are physically very
similar and are all colourless gases or liquids with a strong 'fishy'
amine odour.
The more compact molecules
(2) and (4) show the lowest boiling points, see
case
study 1a.1 for the explanation.
Chemically very similar too,
e.g.
(i) they all form salts with acids
(R = H or alkyl)
(i) R3N:(aq)
+ H+(aq)
==> [R3NH]+(aq)
and (ii) R'X + R3N: ==> [R'NR3]+
+ X-
(ii) act as nucleophiles
(via lone pair e's on N) with in the
nucleophilic
substitution reactions of halogenoalkanes
(ii) R'X + R3N: ==> [R'NR3]+
+ X-
[lots
of named organic nitrogen molecule structures]
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14.1.2(c)
Structural Isomerism - Functional Group
Isomerism
These isomers have the
same molecular formula but different functional groups.
The atoms, for the same molecular
formula, can be connected in different ways to give different functional
groups.
This usually
means they have very different sets of chemical reactions based on the
functional group and there can be significant physical differences in
melting points, boiling points and solubility.
14.1.2(c) Structural Isomerism - Functional Group
Isomerism
Case
study 1c.1 Functional group
isomers of C2H6O
(1) ethanol, an
alcohol,
bpt 79oC,
 ,
(2) methoxymethane, an
ether, bpt -25oC,
 ,
The highly polarised Oδ--Hδ+
bond arises from the difference in oxygen/hydrogen electronegativity (O>>H).
This
results in alcohol molecules being much more polar than ethers and the formation of 'hydrogen bonding' between alcohol
molecules.
Hydrogen bonding is the strongest intermolecular
force (intermolecular bonding) and the resulting increased
inter-molecular forces raises the boiling point of alcohols quite
considerably compared to the isomeric ether.
The lower alcohols tend
to be more soluble in the highly polar solvent water (water-alcohol
H bonding) than the less polar ether molecules are.
The
Cδ+-Oδ-
is polar, but the two dipoles of the C-O-C linkage tend to cancel
each out.
Their structural
differences leads to quite different chemical reactions and
products, apart from combustion! Alcohols have a diverse chemistry via
the C-OH group which ethers lack giving them quite a limited chemistry.
However the lack of
chemical reactivity of ethers makes ethers very useful as
solvents for other reactants!
(a) Alcohols like (1) react
with carboxylic acids to form esters via the -OH group, ethers cannot.
CH3CH2OH
+ CH3COOH ==> CH3COOCH2CH3
+ H2O
Ethanol forms the ester
ethyl ethanoate when heated with ethanoic acid and a little conc.
sulfuric acid.
(b) Alcohols can be dehydrated to
form alkenes, ethers cannot.
CH3CH2OH
==> CH2=CH2 + H2O
Ethanol forms ethene when
heated with conc. sulfuric acid.
(c) Alcohols rapidly react
with sodium metal, ethers do not.
2CH3CH2OH
+ 2Na ==> CH3CH2O-Na+ + H2
Ethanol forms the salt
sodium ethoxide and hydrogen.
Isomeric alcohols
and ethers based
on C4H10O are
considered in
case study 1b.5.
[lots
of named alcohol/ether structures]
TOP OF PAGE and
sub-index
14.1.2(c) Structural Isomerism - Functional Group
Isomerism
Case
study 1c.2 Functional group
isomers of C3H6O2
(all colourless liquids)
Quite a variety of isomers
are possible!
(1)
, propanoic
acid (a carboxylic acid), bpt 141oC, highly
polar, high
bpt compared to others except (4) due to hydrogen bonding (via Oδ--Hδ+), shows acidic
properties via -COOH group e.g. fizzing with metals/ carbonates and forms esters with
alcohols.
(2)
, methyl ethanoate
(an ester), bpt 57.5oC, pleasant
smelling liquid, hydrolyses to form ethanoic acid and methanol. Hydrogen
bonding not possible (no Oδ--Hδ+
)
(3)
, ethyl methanoate
(an ester), bpt 54oC, pleasant
smelling liquid, hydrolyses to form methanoic acid and ethanol. Hydrogen
bonding not possible (no Oδ--Hδ+)
(4)
, 1-hydroxypropanone (a bi-functional alcohol/ketone), highly
polar, bpt 146oC, high
bpt compared to others except (1) due to hydrogen bonding (via Oδ--Hδ+).
The >Cδ+=Oδ-
is also a highly polar bond.
It is a
bi-functional group molecule giving the (i) the chemistry of alcohols
e.g. reacts with sodium, forms esters with carboxylic acids and ...
(ii) the
chemistry of a ketone e.g. nucleophilic addition of HCN, gives
yellow-orange ppt with 24DNPH, but no reaction with ammoniacal silver
nitrate (Tollen's reagent) or Fehlings solution.
(5)
, 3-hydroxypropanal (a bi-functional alcohol/aldehyde), bpt
?, Hydrogen bonding is possible (via Oδ--Hδ+)
and the >Cδ+=Oδ-
is also a highly polar bond.
It is a bi-functional group molecule giving the (i) the
chemistry of alcohols e.g. reacts with sodium, forms esters with
carboxylic acids and ...
(ii) the chemistry of an aldehyde e.g. nucleophilic
addition of HCN, yellow-orange ppt with 24DNPH, forms silver mirror with
ammoniacal silver nitrate (Tollen's reagent) and brown ppt with Fehlings
solution.
(6)
, 2-methoxyethanal (a bi-functional ether/aldehyde, bpt 56oC, the
ether group will not add to, or inhibit, its reactions as an aldehyde
e.g. undergoes nucleophilic addition of HCN, gives yellow-orange ppt
with 24DNPH, forms silver mirror with ammoniacal silver nitrate (Tollen's
reagent) and a brown ppt with Fehlings solution.
(7)
, 1,3-dioxolane (a di-ether, -C-O-C-O-C- in
ring) bpt 75oC, two
ether linkages, limited to chemistry, shows non of the functional group
chemistry of (1) to (4). Hydrogen bonding not possible (no Oδ--Hδ+).
(8)
, 1,2-dioxolane (an organic cyclic peroxide, -C-O-O-C
in ring), bpt ?, very
unstable and reactive compound. Hydrogen bonding not possible (no Oδ--Hδ+
)
For a 'small' molecular
formula, C3H6O2 packs quite an isomeric punch! but don't worry too much, (1) to (3) are ones whose
detailed structure, naming, physical properties and chemical reactions you should be very familiar
with. (5) to (6) you should cope with in a functional group concept Q and
(7) to (8) I wouldn't worry too much about!
[lots
of named carboxylic acid/derivative structures] and [aldehyde
and ketone structures]
TOP OF PAGE and
sub-index
14.1.2(c) Structural Isomerism - Functional Group
Isomerism
Case
study 1c.3 Functional group isomers
of C3H6O
An
amazing variety of functional group isomers is possible for such a
simple formula!
Some physical similarities
e.g. low boiling colourless polar liquids or gases, (1) and (2) also show
chemical similarities, as do (3) and (4), but there are significant chemical differences
between all four shown below.
(1) ,
propanal
(an aldehyde), bpt 49oC, adds HCN to give
hydroxynitrile, gives yellow-orange ppt with 24DNPH, produces the primary alcohol,
propan-1-ol, on reduction, readily oxidised to propanoic acid, gives
silver mirror with ammoniacal silver nitrate and red-brown ppt with
Fehlings/Benedict's reagent. I2 reaction?
(2)
 ,
propanone
(a ketone), bpt 56oC, adds HCN to give
hydroxynitrile, gives yellow-orange ppt with 24DNPH, produces secondary alcohol,
propan-2-ol, on reduction, NOT readily oxidised, NO silver mirror with
ammoniacal silver nitrate and NO red-brown ppt with Fehlings/Benedict's
reagent. I2 reaction?
(3)
 ,
prop-2-ene-1-ol
(a bi-functional molecule alkene/alcohol or enol), bpt 97oC, higher
bpt due to hydrogen bonding via -OH (not possible with 1 and 2 above),
gives electrophilic addition reaction of Br2, H2O,
HI etc. like any other alkene, reacts with sodium to give H2 and forms esters with
carboxylic acids or acid chlorides just like alcohols do, NO reaction with ammoniacal silver
nitrate, Fehlings/Benedict's reagent or 24DNPH.
(4) 
, cyclopropanol (an alicyclic secondary alcohol, bpt
?, very unstable, difficult to study, and readily isomerises to
(1) propanal (see
case study 1c.4 below.
Theoretically has the chemistry of a secondary
alcohol e.g. oxidised to the ketone cyclopropanone,
reacts with sodium to give H2, forms esters with carboxylic
acids or acid chlorides.
(5)
, methoxyethene (a bi-functional ether-alkene), bpt 5oC, gives electrophilic addition reaction of Br2, H2O,
HI etc. like any other alkene, but no aldehyde, ketone or alcohol
chemistry.
(6)
, 1,2-epoxypropane (a cyclic-ether), bpt 35oC, no
alkene, aldehyde, ketone or alcohol chemistry.
(7)
, 1,3-epoxypropane (a cyclic-ether), bpt 49oC, no
alkene, aldehyde, ketone or alcohol chemistry.
Again, for a 'small' molecular
formula, C3H6O2 packs quite an isomeric punch! but don't worry too much, (1) to
(2) are the ones whose
detailed structure, naming, physical properties and chemical reactions you should be very familiar
with.
(3) to (5) you should cope with in a functional group concept Q and
(6) to (7) I wouldn't worry too much about at all!
14.1.2(c)
Structural Isomerism - Functional Group
Isomerism
or Case
study 1c.4 The isomerisation
reactions of cyclopropane or cyclopropanol
In these cases one isomers is changed
into another, with a different functional group, without any other
reactants or products.
Cyclopropane
is quite
unstable because the ring is very strained due to the enforced geometry,
i.e. the C-C-C bond angle of 60o, rather than the usual 'tetrahedral' bond
system producing C-C-C angles of 109o.
On heating or catalysis,
isomerization occurs and cyclopropane (a cyclo-alkane, alicyclic) is readily converted to
the much more stable propene
(linear alkene).
(1)
(2) 
similarly, unstable (3)
cyclopropanol
(a alicyclic secondary alcohol) readily isomerizes to form
the more
stable (4) propanal (an aldehyde).
(3)
(4)
14.1.2(c) Structural Isomerism - Functional Group
Isomerism
Case
study 1c.5 An alcohol,
phenols and ether based on C7H8O
These are all colourless
liquids but show great differences in chemical
properties.
(1)
, phenylmethanol
(a aliphatic primary alcohol, OH NOT attached directly to
benzene ring), OH NOT attached directly to benzene ring, mpt -25oC,
bpt 205oC
, it can
be oxidised to an aldehyde, forms esters with
carboxylic acids or acid chlorides, but can't act as ligand to form a purple complexes
with the iron(III) ion.
(2)
, methyl-3-phenol
(a aromatic phenol, OH attached directly to benzene ring),
mpt 12oC, bpt 202oC, forms esters with
carboxylic acids or acid chlorides, but phenols can act as ligands and form purple complexes
with the iron(III) ion.
As phenols, they form diazo dyes when coupled with diazonium
salts.
There two other positional isomers,
namely (3) methyl-2-phenol, mpt 31oC, bpt 191oC
and (4) methyl-4-phenol, mpt
35oC, bpt 202oC, not
shown, but very similar physically and chemically to (2).
(5)
, methoxybenzene
(a mixed aliphatic/aromatic ether), mpt -37oC,
bpt 154oC, it cannot be oxidised
to an aldehyde, cannot form esters with carboxylic acids or acid
chlorides, or purple complexes with the iron(III) ion.
The boiling
point is relatively lower than the others because hydrogen bonding via
O-H is not possible as it is in (1) and (2).
TOP OF PAGE and
sub-index
14.1.2(c) Structural Isomerism - Functional Group
Isomerism
Case
study 1c.6 Aromatic compounds based on C7H7NO2
This molecular formula can
give rise to many isomers of a wide variety of chemistry and a few
examples are quoted below.
(1)
, methyl-2-nitrobenzene (a tri-functional nitro, alkane
(via -CH3) and benzene ring), colourless liquid,
mpt -3oC, bpt 223oC.
It has two other positional isomers, ...-3-... and ...-4-...
The melting point of
(1) is significantly lower than
(2) and
(3) described below due to
lack of H-bonding via the -OH in (2) or -CONH2 group in (3).
Chemistry of (1) e.g.
(i) the nitro group
can be reduced to an (-NH2) amine by refluxing with Sn(s)/HCl(aq)
(ii) the
-CH3 can be 'free radical' chlorinated with Cl2/uv light
(iii) the four 'vacant' C-H positions around the benzene ring can
undergo electrophilic substitution (nitration, chlorination,
sulfonation, alkylation, acylation etc.) - this applies to (2) and
(3) too.
(2)
, 3-aminobenzoic acid (a tri-functional primary amine-carboxylic
acid and benzene ring), colourless solid, mpt 180oC,
bpt ?. It has
two other positional isomers, 2-... and 4-...
Chemistry of (2) e.g.
(i) the
-NH2 can forms salts and diazotised to couple with phenols to
make dyes
(ii) the -COOH group reacts with metals/carbonates to give salts +
H2/CO2 gas respectively, and with alcohols to form
esters
(iii) the benzene ring can
undergo electrophilic substitution - this applies to (1) and (3)
too.
(3)
, 4-hydroxybenzamide (a tri-functional phenol-primary amide
and benzene ring), colourless solid, mpt 162oC,
bpt ?. It has two
other positional isomers, 2-... and 3-...
Chemistry of (3) e.g.
(i) complexes
with Fe3+(aq) via -OH phenol group
(ii) couples with diazotised aromatic
amines to form dyes
(iii) the benzene ring can undergo electrophilic
substitution - this applies to (1) and (2) too.
Apart from electrophilic substitution
in the benzene ring, all three molecules have their own unique
functional group chemistry in terms of at least two reactions.
TOP OF PAGE and
sub-index
14.1.2(d)
Structural Isomerism: Tautomerism
(I don't think you find this term in contemporary
UK A level chemistry
textbooks?)
This is a special case,
quite often
of functional group isomerism, in which there is a dynamic equilibrium between
the two isomers, i.e. they co-exist at the same time in an
equilibrium situation.
14.1.2(d) Structural Isomerism - tautomerism: Case
study 1d.1
The keto-enol interchange
(functional group isomers of C3H6O)
This is a classic example
based on functional group isomers involved a dynamic equilibrium.
(1)
(2) 
propanone
(a ketone)
prop-1-en-2-ol (a bi-functional alkene-alcohol,
sometimes referred to as an enol)
The equilibrium is very much
on the left, but the small % of the 'enol' form is an important
intermediate in certain reactions of ketones.
The enol form is an
important intermediate in the iodination of propanone (see
mechanism).
It's an example of an
isomerisation reaction, and because of its 'dynamic' equilibrium
nature, its a special case of functional group isomers.
14.1.2(d) Structural Isomerism - Tautomerism:
Case
study 1d.2 hydroxy ketone
and alkene diol equilibrium systems
(3) R-CH(OH)-CO-R'
(4) HO-CR=CR'-OH
R and R' are alkyl or aryl groups
e.g. C6H5 , CH3C6H4 etc.
The hydroxy-ketones with aryl groups tend to form the alkene-diol more
readily because of resonance-delocalisation stabilisation.
(3) is a bi-functional
alcohol-ketone
(hydroxy-ketone) and
(4) is a bi-functional alkene-diol (another sort of 'enol').
TOP OF PAGE and
sub-index
APPENDIX 1 - Numbers of possible isomers
from a given molecular formula -
just out of
academic interest!
Once you a few atoms of carbon and
hydrogen combined together, plus substituents based on oxygen, nitrogen
and halogens etc. the number of possible isomers rises dramatically with
increase in carbon number.
Most of these numbers have been generated by
computer program algorithms designed to predict the number of
'theoretically possible' isomeric molecular structures!
Some
familiar aliphatic homologous series and numbers of isomers
Two series may overlap in terms of
their general molecular formula
e.g. non-cyclic mono-alkenes and cycloalkanes
|
Number of carbon atoms n in the
aliphatic molecular formula |
2 |
3 |
4 |
5 |
6 |
7 |
8 |
9 |
10 |
11 |
12 |
examples of homologous series
with this general formula |
| |
|
|
The number of possible isomers >1 structure |
|
| CnH2n+2
e.g. including C5H12 pentane, n = 5, (see
*
below) |
1 |
1 |
2 |
3 |
5 |
9 |
18 |
35 |
75 |
159 |
355 |
non-cyclic alkanes |
| CnH2n e.g.
including C3H6 propene, n = 3 or cyclobutane, n = 4 |
1 |
2 |
5 |
9 |
17 |
27+ |
66+ |
153+ |
377+ |
~915+ |
~2300+ |
non-cyclic alkenes
and cycloalkanes |
| CnH2n+2O
e.g. including CH3CH2CH2OH and CH3OCH2CH3
when n = 3 |
2 |
3 |
7 |
14 |
32 |
72 |
171 |
405 |
989 |
~2430 |
~6070 |
alcohols
(prim, sec, tert) and ethers |
| CnH2n+3N
e.g including CH3CH2NHCH3 and CH3CH2CH2NH2
when n = 3 |
2 |
3 |
8 |
17 |
39 |
89 |
211 |
507 |
1238 |
3057 |
? lots ! |
amines
(prim, sec, tert) |
As far as I can tell, the
numbers quoted I found on the internet for CnH2n
from n = 7 onwards, do not take into account all the possible non-cyclic
alkenes and cycloalkanes of that given molecular formula.
Even just considering the possible combination of
just carbon and hydrogen atoms gives a staggeringly large number of
possible isomers and once you bring in other elements like oxygen,
nitrogen, phosphorus, there seems 'infinite' possibilities for
synthesising new organic compounds!
*
On another page I've described how to work out
ALL the isomers and names for alkanes from C4H10
to C8H18
TOP OF PAGE and
sub-index
APPENDIX 2. A brief guide to
working out the 18 structural isomers of non-cycloalkanes C8H18
There are 18 basic structural isomers and all deduced from
changing the carbon chain arrangement, so they all chain isomers, but some have
four different groups (H or alkyl) around a specific carbon atom. This is a
chiral or symmetric carbon atom and therefore some
R/S
optical isomers will exist (I think I've spotted them
all?).
(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
(also optical
R/S isomers)
(4) 4-methylheptane,
CH3CH2CH2CH(CH3)CH2CH2CH3
then do double methyl
branching permutations to make 6 dimethylhexanes ...
(5) 2,2-dimethylhexane,
,
(6) 2,3-dimethylhexane,
,
(also
optical R/S isomers)
(7) 2,4-dimethylhexane,
,
(also optical
R/S isomers)
(8) 2,5-dimethylhexane,
,
(9) 3,3-dimethylhexane,
,
(10) 3,4-dimethylhexane,
,
(also
optical
R/S isomers)
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
(also
optical
R/S isomers)
(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
I only did this for fun!
structural
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