Organic I (Basics)
Formulae
Molecular formula
This is a list of each type of atom in a molecule, and the number of atoms present. For
example, the formula C4H10 indicates that there are four carbon atoms and ten hydrogen
atoms.
Structural formula
This indicates which atoms are bonded to which other atoms. For example, both
CH3CH2CH2CH3 and CH3CH(CH3)CH3 have the molecular formula C4H10, but they are
clearly different compounds, butane and methyl propane respectively.
Structural formula can be abbreviated as above, or shown with all bonds, which is called
the ‘full’ structural formula or ‘displayed’ structural formula.
Draw full structural formulae for CH3CH2CH2CH3 and CH3CH(CH3)CH3.
These molecules have the same molecular formula (atoms) but different structural
formulae (bonds). They are structural isomers.
Molecules can have the same structural formula and still be different. The structural
formula only tells us about the connections between atoms, not about the spatial
arrangement of the atoms. A good analogy for this idea is that the connections between
bones in right and left hands are identical, but right and left hands are not interchangeable
and have a different shape (as is obvious if you try to put a right hand into a left glove).
Similarly, the structural formula CH3CH=CHCH3 denotes but-2-ene, but does not specify
how the H and CH3 groups are distributed across the rigid C=C bond, which is rigid and
flat.
Draw the different arrangement of CH3 and H groups in but-2-ene molecules.
So, these isomers have the same molecular formula (atoms) and the same structural
formula (bonds) but differ by the spatial arrangement of their atoms. They are
stereoisomers.
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Organic I (Basics)
Homologous series and general formulae
A homologous series is a series of compounds that have a general formula and whose
molecular formulae differ by a –CH2– unit.
Alkanes
Compound
CH4
CH3CH3
CH3CH2CH3
CH3CH2CH2CH3
Name
methane
ethane
propane
butane
Molecular formula
The general formula of alkanes is CnH2n+2.
Alkenes
Compound
CH2CH2
CH3CH=CH2
CH3CH2CH=CH2
CH3CH2CH2CH=CH2
Name
ethene
propene
but-1-ene
pent-1-ene
Molecular formula
The general formula of alkanes is CnH2n.
Halogenoalkanes
Compound
CH3Br
CH3CH2Br
CH3CH2CH2Br
CH3CH2CH2CH2Br
Name
bromomethane
bromoethane
1-bromo propane
1-bromo butane
Molecular formula
The general formula of halogenoalkanes is CnH2n+1X.
Alcohols
Compound
CH3OH
CH3CH2OH
CH3CH2CH2OH
CH3CH2CH2CH2OH
Name
methanol
ethanol
propan-1-ol
butan-1-ol
Molecular formula
The general formula of alcohols is CnH2n+1OH or CnH2nO
[NB The range of compounds used here to illustrate isomerism is wider than that covered
by the unit, and also discussion of the impact of isomerism on physical properties is made
to link the topic to structure and bonding]
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Organic I (Basics)
Isomerism
Isomerism is about how different molecules can be made from the same set of atoms. The
set of atoms is called the molecular formula. For example, C4H8 means that there are 4
carbon atoms and 8 Hydrogen atoms.
•
C atoms have four bonds
•
H atoms have one bond
•
N atoms have three bonds
•
O atoms have two bonds
•
Halogen atoms have one bond (in organic chemistry)
Broadly speaking, there are two ways in which molecules can be different yet have the
same molecular formula:
1. The atoms can be joined up in a different order, for example but-1-ene and but-2-ene
have the same molecular formula C4H8 but their structural formulae are different.
This type of isomerism is structural isomerism.
2. Alternatively, the structural formula can be kept the same but the shape of the
molecule can be varied. For example, but-2-ene can exist as two different molecules,
cis-but-2-ene and trans-but-2-ene.
This type of isomerism is an example of stereoisomerism, called geometrical isomerism.
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Organic I (Basics)
Structural Isomerism
There are three types of structural isomerism, Chain (or Branch), Positional and Functional
Group.
•
Chain Isomerism
Draw all the molecules that have the molecular formula C5H12.
Simple chain isomers have very similar chemical properties has they are governed by the
chemistry of C-H and C-C bonds.
However, you need to understand how the boiling point of the above molecules is
affected by branching.
When the above substances boil, the intermolecular forces are broken by the heat energy
being supplied. The C-C and C-H bonds do not break at all. The intermolecular force in
operation between these molecules is the induced dipole force, which is studied in more
detail in Structure and Bonding. Usually we think about the size of the electron cloud with
this type of force, but since the molecules are isomers they will have the same size electron
cloud because they have exactly the same atoms. Instead, we look to for branching in the
molecule because branching lowers the surface area, which in turn lowers the points of
contact between the molecules.
Show how pentane molecules have more points of contact than dimethylpropane molecules.
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Organic I (Basics)
•
Positional Isomerism
In positional isomerism, the position of an atom or group of atoms moves along a carbon
chain that otherwise stays the same.
Draw butan-1-ol and butan-2-ol.
Draw 1-bromo butane and 2-bromo butane.
Compounds that are met later in different Units can also undergo positional isomerism.
Draw positional isomers of dichlorobenzene
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Organic I (Basics)
The chemical properties of positional isomers are broadly similar because they will
contain the same functional group, but there will be some variation because the reactions
of a functional group do depend on its environment. The physical properties such as
boiling point depend on the intermolecular forces.
Discuss the order of boiling points between 1- and 2-bromo butanes (Hint, they depend upon the
exposure of the C-Br dipole and surface area).
1-bromobutane has the higher boiling point because…
•
it has the larger surface area as it is not branched, so there are more points of contact
between the molecules as they pass by each other, which enables the induced dipole forces
of attraction to interact more.
•
Also, the polar C-Br bond at the end of the molecule can interact more freely with other
polar C-Br bonds than when it is in the body of the molecule, with bulky CH3 groups on
either side sterically hindering (getting in the way of) its interactions.
Discuss the order of boiling points between butan-1 and 2-ols (Hint, they depend upon the exposure
of the OH group and surface area).
Butan-1-ol has the higher boiling point because…
•
it has the larger surface area etc
•
its –OH groups are exposed and can H-bond more effectively to –OH groups in another
molecule.
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Organic I (Basics)
•
Functional Group Isomerism
In functional group isomerism, a structural change completely alters the functional group
of the molecule. Consequently, both chemical and physical properties change significantly.
Draw all the isomers of C2H6O, using what you know about how many bonds each atom may have.
Draw all the isomers of C3H6O that are aldehydes or ketones.
Explain why CH3CH2OH is a liquid whereas CH3OCH3 is a gas (Hint, it is something to do with
intermolecular forces!)
Molecules of ethanol have –OH groups, so they have H-bonding between them. Molecules of
ethoxyethane do not, they have a mixture of induced and permanent dipoles. The
intermolecular forces in ethanol are stronger due to the presence of H-bonding, which
explains why it is a liquid at room temperature, yet ethoxy ethane is a gas.
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Organic I (Basics)
Stereoisomerism
There are two types of stereoisomersim, geometrical and optical (Optical isomerism is
covered in a later unit). In both types, the structural formula stays the same. The molecules
differ because of the spatial arrangement of the atoms.
•
Geometrical
A restriction of rotation (such as C=C) allows the groups around it to be arranged in two
different ways. Often the words cis and trans can be used to identify the isomers.
Draw out the geometrical isomers of CH3CH=CHCH3
Draw out the geometrical isomers of CH3CH=CHC2H5
Draw out the geometrical isomers of CH3CH=C(OH)Br
The further condition is that there cannot be two identical groups on either C in the double
bond.
To illustrate this, try drawing CH2=CHCH3 as geometrical isomers.
It doesn’t work!
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Organic I (Basics)
In addition, you need to understand why there is no free rotation about a double bond.
Draw out the p orbitals that make the pi bond in a C=C, and show that there is no free rotation due
to sideways overlap of p orbitals.
NB – there are plenty of examples of geometrical isomers that do not have a C=C, and
equally they might not even be organic molecules.
Cyclic isomers of C3H4Cl2
Isomers of CH3CH=NC6H5
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