Download Kekulé structure of benzene

Title: Lesson 2 Isomers
Learning Objectives:
– Describe the term structural isomer
– Draw a name the non-cyclic alkanes
– Draw and name the straight-chain alkenes
Refresh

List two characteristics of a
homologous series.
Reviewing Your Notes
You should spend 60
seconds reviewing your
notes from last lesson
before attempting this.
Your notes and mind-map
must be ready for me to
inspect.
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Isomers

Compounds with the same
molecular formula but
different structural formula

The 20 different C4H8O
compounds from last lesson
are isomers of each other
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These are all structural
isomers
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Same number of each atom, but
bonded in a different order
You would have even more if
you included geometric and
optical isomers
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Structural isomers: Different arrangements
of the same atoms
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Molecular formula shows atoms present but not arrangement.
Consider the example below for C4H10:
Same molecular formula,
different arrangements =
structural isomers
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Each isomer is a distinct compound with unique physical and chemical
properties.
Number of isomers that exist for a molecular formula increases with size of
molecule.
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Draw the possible isomers for:

C5H12
C6H14

Did you get them right?

C5H12
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C6H14
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Structural isomers in alkenes
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A different type of isomer occurs when the carbon-carbon bond is found in
different positions.
C4H8 has the following straight-chain isomers:
Note: Named using
the smallest
numbered carbon
that is part of the
double bond.
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Can you draw the isomers for the following alkenes?
C5H10
C6H12
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Check you answers
Note: The structures and names of the isomers of the alkynes can be deduced from
considering different possible structures for the triple bond.
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Structural isomers activity
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Isomers that are members of different families
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There are many different types of isomers, including those in which the
molecules have different functional groups.
Different functional groups  different classes, different reactivities.
E.g. C2H6O describes both an alcohol and ether
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Primary, secondary, and tertiary compounds

The activity of a functional group is often influenced by its position in the carbon chain

A primary carbon atom is attached to the functional group and at least two hydrogen atoms. These are
known as primary molecules.

E.g. Ethanol, C2H5OH, is a primary alcohol and Chloroethane, C2H5Cl.

A secondary carbon atom is attached to the functional group and also one hydrogen group and two alkyl
groups. There are known as secondary molecules.

E.g. propan-2-ol, CH3CH(OH)CH3, is a secondary alcohol, and 2-chloroethane, CH3CHClCH3, is a
secondary halogenoalkane.
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A tertiary carbon atom is attached to the functional group and is also bonded to three
alkyl groups and has no hydrogen atoms. These are known as tertiary molecules.

E.g. 2-methylpropan-2-ol, C(CH3)3OH, is a tertiary alcohol, and 2-chloro-2methylpropane, C(CH3)3Cl, is a tertiary halogenoalkane.
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In amines, a similar classification can be applied:
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Arenes

Class of compounds derived from benzene, C6H6

Form compounds (with rings) known as aromatics

Distinct from other organic compounds, (no rings) known as aliphatics

Arenes contain the phenyl functional group:
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To understand the properties of arenes, you must study the parent molecule –
benzene.
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Physical properties of arenes
Benzene is a colourless liquid at room temperature.
The boiling point
of benzene is
comparable to
hexane but its
melting point is
much higher.
hexane benzene
Mr
Boiling point (°C)
Melting point (°C)
86.0
69.0
-95.0
78.0
80.0
5.5
This is due to the ability of the flat benzene rings to pack
closely together when solid, increasing the strength of
intermolecular forces.
As carbon and hydrogen are similar in their electronegativity
(2.6 and 2.2, respectively), benzene is a non-polar molecule
and is therefore immiscible with water.
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Combustion of arenes
Arenes burn in air to give characteristically sooty flames.
The soot is unburnt carbon, a
result of the relatively high
proportion of carbon that
arenes contain, compared with
more saturated compounds.
However, long-chained alkanes
also burn with a sooty flame due
to their increased percentage
carbon content (not due to
unsaturation).
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Naming aromatic compounds
Benzene derivatives are named in a similar fashion to other
organic compounds, with benzene forming the main part of
the name.
The presence of other groups is denoted by the use of a prefix.
methylbenzene
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chlorobenzene
nitrobenzene
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How to name aromatic compounds
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Naming aromatic compounds
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Benzene does not behave like other unsaturated
molecules
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1:1 carbon to hydrogen ratio indicates high degree of unsaturation, greater than that of alkenes or alkynes…
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Early observations on benzene indicated that it did not show properties of these possible structures:
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Benzene has no isomers, and is reluctant to undergo addition reactions (both features that you would expect of the above
structures)

In 1865 it was suggested that benzene had a cyclical arrangement:
1,3,5-cyclohexatriene
This model explained things like no
isomers…
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Story of the discovery of benzene
Benzene was first isolated in 1825
by Michael Faraday, who deduced
that its empirical formula was CH.
In 1834, the German chemist
Eilhard Mitscherlich determined
that benzene’s Mr was 78, and its
formula was C6H6. However, it
was not until 1931 that benzene’s
structure was fully resolved.
Benzene’s molecular formula suggests it is a highly
unsaturated compound. But unlike alkenes, it does not
readily undergo addition reactions. This suggests that its
structure and bonding is fundamentally different.
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The benzene ring
Benzene undergoes hydrogenation in the presence of a
nickel catalyst to form cyclohexane:
This suggests that benzene also has a cyclic structure.
In order to fit with the molecular formula of C6H6, the ring
would have to contain three double bonds and three
single bonds.
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Kekulé structure of benzene
In 1865, the French chemist
Friederich Kekulé proposed a
cyclic structure for benzene,
consisting of alternating single
and double bonds.
Kekulé’s structure for benzene was the first
time it had been proposed that a hydrocarbon
chain formed a ring (he later claimed that his
inspiration came from a dream of a snake
eating its own tail).
There were, however, a number of problems with Kekulé’s
structure in that it didn’t fully explain the physical and chemical
properties of benzene.
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Problems with the Kekulé structure: 1
Firstly, the three double bonds in the structure suggest that
benzene should readily undergo electrophilic addition
reactions, similar to other unsaturated compounds.
However, unlike alkenes,
benzene (left) does not
decolourize bromine water.
It also does not easily take
part in other electrophilic
addition reactions.
Benzene therefore has lower chemical reactivity than would
be predicted by Kekulé’s structure.
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Problems with the Kekulé structure: 2
A second problem for Kekulé’s model arises when the
isomers of dibromobenzene are studied. According to
Kekulé’s structure, there should be four different isomers:
1,2
1,3
1,4
1,6
However, it was discovered that only three isomers of
dibromobenzene are formed: the 1,6-isomer is not
distinguishable from the 1,2-isomer.
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Delocalization in benzene
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Stability of benzene is the result of delocalized
electrons
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Benzene is a cyclic structure
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Single bonds attaches each carbon to the one on either side and to a hydrogen atom
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Each carbon atom is sp2 hybridized, forming 3 sigma bonds with angles of 120o (see diagram (a))
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Planar shape (see diagram (a))
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One unhybridized p electron on each carbon with it’s dumb-bell shape perpendicular to the plane of the ring (see diagram (b))
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Instead of pairing up to form discrete alternating pi bonds, the p orbitals effectively overlap in both directions, spreading evenly
to be shared by all six carbon atoms (delocalized pi electron cloud)
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Electron density is concentrated in 2 donut shaped rings above and below the plane of the ring (see diagram (c))
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Stable arrangement and lowers internal energy of molecule
Ring inside denotes the
delocalized electrons.
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Thermochemical data
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Enthalpy changes for the hydrogenation of benzene and
related molecules
Note: 1,3,5cyclohexatriene
does not exist –
theoretical values
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The equilibrium model
Kekulé tried to resolve these problems by suggesting that
there were two forms of benzene that rapidly interconverted
(the rapid equilibrium model).
He proposed that this model could explain the low reactivity of
benzene, as the structure was in such rapid flux that the
location of the double bonds would change before any
attracted molecules had time to react with them.
This later evolved into the idea of resonance between the
two Kekulé structures of benzene.
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Bond lengths in benzene
The final problem with the Kekulé structure of benzene was
identified by X-ray crystallography.
0.133 nm
0.154 nm
As double bonds are shorter than
single bonds, the Kekulé structure
would be asymmetrical. Based on the
alkanes and alkenes, the two different
bond lengths could be predicted to be
0.154 nm and 0.133 nm.
However, X-ray studies revealed the structure of benzene to
be a perfect hexagon: all internal bond angles were 120° and
all bonds were of an equal length – 0.140 nm; somewhere in
between that which would be expected for a single and a
double bond.
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Problems with Kekulé structure: summary
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Bonding in benzene: true or false?
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Evolution of scientific knowledge
The history of the structure of benzene exemplifies how
scientific knowledge evolves dynamically between different
ideas as new data emerges.
C6H6
Each successive model can be seen as a working hypothesis
that best explains current observations, but which is only
tentative in nature, as new information may require revision
or even replacement of the current model.
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Trends in physical properties
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Organic compounds can be thought of in two parts:
1.
Carbon and hydrogen framework (hydrocarbon skeleton) – this differs in size in different members of the same
homologous series
2.
Functional group – differs depending on the homologous series

Both properties influence the physical properties of a compound.
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For example:
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Increasing carbon number  increased boiling point

Increased branching of the hydrocarbon chain  decreased boiling point/increased volatility
Increased branching have less
contact  weaker
intermolecular forces…
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Polarities of functional groups and differences in
intermolecular forces
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Solutions
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Structural Isomers of the Alkanes
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The (non-cyclic) alkanes have the general formula CnH2n+2
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Draw full and condensed structural formulas for every isomer of every one of
the alkanes up to n = 6 (10 minutes)
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If you finish early, draw each as a skeletal formula
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Ask Ms Easton for the answers when you’re done (10
minutes)…
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Did you get them all?
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And skeletally
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Naming Straight-chain alkanes
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Suffix:
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Example 1: ethane
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Example 2: butane:
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Task: write in the names of
the 4 straight chain alkanes
next to your diagrams from
last slide
Tells us the functional group of
the molecule
For alkanes it is ‘-ane’
Prefix:
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Tells us the length of the
longest carbon chain:
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1 carbon: meth2 carbons: eth3 carbons: prop4 carbons: but5 carbons: pent6 carbons: hex-
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Naming branched-chain alkanes
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Start by naming the longest chain
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Add extras to say the size of a branch, its
position and how many of that branch
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Branch Size:
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Example 1: 2-methylpropane
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Example 2: 2,3-dimethylbutane
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Task: name the remaining
alkanes
Position:
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1 carbon: methyl2 carbons: ethyl3 carbons: propyl-
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Number the carbons in the longest chain
Choose numbers to minimise the total
numbers used
Number of same branches
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One branch – nothing
Two branches – diThree branches – triFour branches – tetra-
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The straight-chain alkenes
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Alkenes are the same as alkanes but have one C=C double bond.
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The suffix for the alkene homologous series is ‘-ene’
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Task: draw full structural and skeletal formulas for each of the straight-chain
alkenes up to C6 and name them.
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Do the branched ones as well if you have time
Hint: you need to state the position of the double bond, but only if there is
the possibility of multiple isomers:
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i.e. ‘but-2-ene’ or ‘hex-1-ene’ but only ‘ethene’ not ‘eth-1-ene’
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Did you get them?
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Key Points
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Structural isomers have the same number of each atom but they are
connected differently
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When naming compounds
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The longest carbon chain forms the prefix
The functional group tells you the suffix
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Sometimes numbers need to be used to tell you where this functional group is
Side chains and other groups are named according to what they are, how many there
are and their position
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