Download 5.6 Structure and properties of polymers 12.2 Alkenes 5.3 Bonds

5.6
12.2
Structure and properties of polymers
Alkenes
5.3
Bonds between molecules - temporary
and permanent dipoles (revisited)
5.4
Bonds between molecules - hydrogen
bonding
13.2
Alcohols and ethers
3.4
E/Z isomerism
6.4
Infrared spectroscopy
Chemical Ideas 5.6
The structure and properties of polymers (Part 1)
A polymer molecule is a long molecule made up from lots of small molecules
called monomers. If all the monomers are the same an A-A polymer forms:
A + A + A + A  A-A-A-A
Poly(ethene) and poly(chloroethene) or PVC are examples of A-A polymers.
If two different monomers are used an A-B polymer may be formed in which A
and B monomers alternate along the chain:
A + B + A + B  A-B-A-B
Polyamides (nylons) and polyesters are examples of A-B polymers.
Addition polymerisation
Many polymers are formed in a reaction known as addition polymerisation.
The monomers usually contain C=C double bonds e.g. in alkenes. In the chain,
the same basic unit is repeated over and over again.
What decides the properties of a polymer?




Strength and flexibility of polymers depend on forces of attraction between
the polymer chains;
Generally, the stronger these inter-molecular forces are , the stronger the
polymer is; and
the less flexible the molecule is;
For a polymer to be flexible, the chains must be able to slide past each
other easily.
The forces of attraction depend on the structure of the polymer – especially:

chain length – longer chains = stronger polymers;

side groups – polar side chains = stronger attraction between chains
making the polymer stronger;

branching – highly branched chains cannot pack closely together = weaker
attraction = polymer is weak;

stereoregularity – polymer chains pack together more closely if side chains
are orientated in a particular way. These are called isotactic polymers.

chain flexibility

cross linking – this makes the polymer harder and more difficult to melt;
Thermoplastics and thermosets
Crystalline polymers
Amorphous regions = the polymer chains are arranged in a random order
Crystalline regions = the polymer chains are packed in a regular arrangement
It is important to know the degree of crystallinity in a polymer as the higher it is
the stronger and less flexible the polymer.
Cold drawing
When a polymer is stretched (cold-drawn) a neck forms. In the neck, the
polymer chains line up to form a more crystalline region. Cold drawing leads to
a large increase in the polymer’s strength.
Chemical Ideas 12.2
Alkenes
Alkenes are distinguished from other hydrocarbons by the presence of the C=C
double bond. The double bond implies that they are unsaturated
hydrocarbons. As with the alkanes, the boiling points of alkenes increase as
the number of carbon atom increases. Ethene, propene and butene are gases.
After that they are liquids and eventually solids.
Ethene is a planar molecule with a bond angle of approximately 120° but
propene is not a planar molecule.
In propene:


the part of the molecule around the double bond is planar with a bond angle
of approximately 120°
the methyl group –CH3 has three hydrogen atoms in a tetrahedral
arrangement with a bond angle of approximately 109°
There is a convention for representing the three-dimensional shape of ethene,
propene and other organic chemicals on paper:



solid line represents a covalent bond in the plane of the paper
a dashed line represents a covalent bond going into the plane of the paper
a wedge represents a covalent bond coming out of the plane of the paper
H
H
C
C
H
C
H
H
H
Naming alkenes
The first part of the name of an alkene depends upon the number of carbon
atoms in the longest chain. The names of alkenes end in –ene. For example,
ethene has two carbon atoms, butene has four and hexene has six.
For alkenes with more than three carbon atoms, positional isomers are
possible. The position of the double bond is included in the name. The carbon
atoms are numbered starting at 1, in such a way that the lowest possible
number can be used. For example:

CH3CH2CH=CH2 is but-1-ene and not but-3-ene or but-4-ene

CH3CH=CHCH3 is but-2-ene and not but-3-ene
If a branch is present, the alkene is named after the longest chain containing
the double bond. The name of the branch itself is added to the start of the
name. So, methylpropene is propene with a CH3 group attached.
Dienes have two double bonds in their structure.
Cyclic alkenes are named cyclo-, followed by the prefix determined by the
number of carbon atoms in the ring and finally the –ene part. So a six-carbon
ring with a double bond is called cyclohexene.
Chemical reactions of ethene
The four electrons in the double bond of ethene give the region between the
two carbon atoms a higher than normal density of negative charge. Positive
ions or molecules with a partial positive charge will be attracted to this
negatively charged region. These are described as electrophiles – they are
attracted to this negatively charged region and accept a pair of electrons from
the C=C double bond to form a covalent bond.
Electrophilic addition reactions
When we bubble ethene gas through bromine, the red-brown bromine becomes
decolourised – this is a good general test for unsaturation in an organic
compound.
The bromine molecule becomes polarised as it approaches the alkene – the
electrons in the bromine are repelled by the alkene electrons and are pushed
back along the molecule. The bromine atom nearest the alkene becomes
slightly positively charged and the bromine atom furthest from the alkene
becomes slightly negatively charged.
The positively charged bromine atom now behaves as an electrophile and reacts
with the alkene double bond. Notice that one of the carbon atoms now has a
share in only six outer electrons and becomes positively charged – this is called
a carbocation.
H
H
H
C
= C
H
H
H
+
C
C
H
+
..
: Br :
..
-
H
Br
a carbocation and a bromide ion
+
Br δ
Br δ
Carbocations react very rapidly with anything that has electrons to share such
as the bromide ion. A pair of electrons moves from the Br- ion to the positively
charged carbon to form a new C-Br covalent bond.
H
H
C
H
+
C
H
H
Br
..
: Br :
..
H
H
C
C
Br
Br
H
-
This is an addition reaction and since the initial attack is by an electrophile the
process is called an electrophilic addition.
Reaction with hydrogen bromide
Ethene reacts readily at room temperature with a solution of HBr in a polar
solvent. It is another example of electrophilic addition.
Reaction with water
In the presence of a catalyst (phosphoric acid adsorbed onto solid silica) and
high temperature and pressure, ethene and water (steam) undergo an addition
reaction. The process is used for the industrial manufacture of ethanol. The
addition of water to an alkene is an example of a hydration reaction.
Reaction with hydrogen
This is another example of an addition reaction. A catalyst is required (usually
platinum) to break the strong H-H bond and form H atoms. Addition of
hydrogen to an alkene is known as hydrogenation and is used in the
manufacture of margarine in which plant an animal oils (unsaturated) are
hardened as they become saturated molecules.
Chemical Ideas 5.3
Bond between molecules:
temporary and permanent dipoles (revisited)
The electrons in a molecule are moving constantly. At any one time the electron
cloud may be distributed unequally which causes an instantaneous dipole on
the molecule. This induces a dipole on a neighbouring molecule.
Permanent dipoles
Permanent dipoles arise where an atom is covalently bonded to a more
electronegative atom.
Electrostatic forces
The partial charges are shown by the symbols δ- and δ+. There is an
electrostatic force of attraction between the δ- charge on one molecule and the
δ+ charge on the neighbouring molecule.
Instantaneous dipole-induced dipole attractions
These attractions are the weakest type of intermolecular force. All simple
covalent compounds have them.
The strength of instantaneous dipole-induced dipole attractions increases:


as the size of the molecule increases. This is because the size of the
charged cloud increases so it is more easily polarised.
as the amount of contact between molecules increases. In alkanes, there is
more contact between straight chain molecules than between branched
chain molecules. Molecules with branches have weaker instantaneous dipoleinduced dipole attractions and lower boiling points than molecules without
branches.
Permanent dipole-permanent dipole attractions
These are stronger than instantaneous dipole-induced dipole attractions.
Compounds that also have these intermolecular forces have higher boiling
points than those who do not.
H
δ+
Cl
δ-
H
δ+
Cl
δ-
Chemical Ideas 5.4
Bonds between molecules: hydrogen bonding
Hydrogen bonds are intermolecular forces of attraction. Water is a common
compound with hydrogen bonds. They are permanent dipole-permanent dipole
attractions that occur in a particular set of circumstances. For hydrogen
bonding to occur in a molecule, there needs to be:


a hydrogen atom covalently bonded to a small, electronegative atom
(nitrogen, oxygen or fluorine) and
at least one lone pair of electrons on the electronegative atom;
Although chlorine is as electronegative as oxygen, it is too large to form a
hydrogen bond. Just having hydrogen atoms in a molecule is not sufficient for
hydrogen bonding: methane, CH4, does not have hydrogen bonding.
Hydrogen bonds are the strongest type of intermolecular force of attraction.
Molecules with hydrogen bonding have higher boiling points than would be
expected from their relative formula masses alone. Methane and ammonia have
similar relative formula masses and so similar instantaneous dipole-induced
dipole attractions. However, ammonia has a much higher boiling point because
it also has hydrogen bonds.
Chemical Ideas 13.2
Alcohols and ethers
Alcohols
Alcohols contain the hydroxyl group –OH. The first part of their name depends
upon the number of carbon atoms in the longest chain. The names of alcohols
end in –ol. For example, ethanol has two carbon atoms, butanol has four and
hexanol has six.
For alcohols with three or more carbon atoms, position isomers are possible.
The carbon atoms are numbered starting at 1, in such a way that the lowest
total number will be obtained. The position of the hydroxyl group is included in
the name. For example:
CH3CH2CH2OH is propan-1-ol and not propan-3-ol
There are primary, secondary and tertiary alcohols:
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in primary alcohols, the carbon atom carrying the hydroxyl group is bonded
directly to one other carbon atom;
in secondary alcohols, the carbon atom carrying the hydroxyl group is
bonded directly to two other carbon atoms; and
in tertiary alcohols, the carbon atom carrying the hydroxyl group is bonded
directly to three other carbon atoms;
Alcohols with more than one hydroxyl group are possible. These are called
polyhydric alcohols:
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
diols have two hydroxyl groups
triols have three hydroxyl groups
Physical properties of alcohols
Like water molecules, alcohol molecules are polar because of the polarised O-H
bond. In both water and alcohols, there is a special sort of strong attractive
force between the molecules due to hydrogen bonds. Hydrogen bonding
between alcohol and water molecules explains why the two liquids mix
together.
As the hydrocarbon chain in alcohols becomes longer and the molecules
become larger, the influence of the –OH group on the properties of the
molecule becomes less important. So the properties of the higher alcohols get
more and more like those of the corresponding alkane.
Reactions of alcohols
Oxidation
The –OH group can be oxidised by strong oxidising agents such as acidified
potassium dichromate(VI). The orange dichromate(VI) ion, Cr2O72-(aq) is reduced
to green Cr3+(aq). In this reaction two atoms of hydrogen are removed – one from
the oxygen atom and one from the carbon atom so oxidation will not take place
unless there is a hydrogen atom on the carbon atom to which the –OH is
attached.
The product is a carbonyl compound – an aldehyde or a ketone. The type of
product depends on the type of alcohol you start with.
Primary alcohols such as ethanol are oxidised to aldehydes but the aldehyde
is then oxidised itself to a carboxylic acid. If the aldehyde is required, the
aldehyde is distilled out from the reaction mixture before it is oxidised further.
Secondary alcohols such as propan-2-ol are oxidised to ketones.
Tertiary alcohols such as 2-methylpropan-2-ol are difficult to oxidise because
they do not have a hydrogen atom on the carbon atom to which the –OH group
is attached.
Dehydration of alcohols
Many alcohols can lose a molecule of water to form an alkene e.g. propene is
formed when vapour of propan-1-ol is passed over a hot catalyst of alumina at
300°C. The reaction is described as dehydration since it involves the removal
of a water molecule from the molecule of the reactant. Alcohols can also be
dehydrated by heating with concentrated sulphuric acid. Dehydration is an
example of an elimination reaction.
Ethers
Ethers are derived from alkanes by substituting an alkoxy group (-OR) for an H
atom. General formula of ethers R-OR’. The longer hydrocarbon chain is chosen
as the parent alkane for naming.
Ethers can be thought of as being derived from water by replacing both the H
atoms with alkyl groups. Ether molecules are only slightly polar and the
attractive force between the molecules are relatively weak. There are no H
atoms attached to the oxygen to form hydrogen bonds between ether
molecules.
The boiling point of an ether is similar to that of the alkane with corresponding
relative molecular mass. Like alkanes, the lower ethers are very volatile and
dangerously flammable.
Ethers are only slightly soluble in water but mix well with other non-polar
molecules such as alkanes.
Chemical Ideas 3.5
E/Z isomerism
Isomers are molecules with the same molecular formula but different
arrangements of their atoms. Alkenes can show a type of stereoisomerism
called geometric isomerism. This is because the double bond resists rotation.
For an alkene to show geometric isomerism, the carbon atoms attached by the
double bond must both have different substitute groups attached to them. If
one or more has two identical groups attached twice, the alkene does not show
geometric isomerism.
If the substitute groups are on the same side of the double bond, the isomer is
called a cis isomer. If the groups are on the opposite sides of the double bond,
the isomer is called a trans isomer.
Geometric isomers have different physical properties such as melting point and
density.
H
H
H
C
H
C
H
H
C
C
H
H
cis-but-2-ene
H
H
C
H
H
C
C
H
C
H
H
trans-but-2-ene
H
Chemical Ideas 6.4
Infrared spectroscopy
The energy possessed by molecules is quantised – molecules can only have a
small number of definite energy values rather than any energy value. Analysis
of the energy (or frequency of radiation) needed to produce a change from one
energy level to another is the basis of most forms of molecular spectroscopy.
In infrared spectroscopy substances are exposed to radiation in the frequency
range 1014-1013Hz i.e. wavelengths 2.5-15μm. This makes vibrational energy
changes occur in the molecules which absorb infrared radiation of specific
frequencies.
The important point to remember about infrared spectroscopy is
that you do not try to explain the whole spectrum; you look for one
or two signals which are characteristic of particular bonds.
Infrared radiation from a heated filament is split into two parallel beams, one of
which passes through the sample, the other through a reference chamber. This
ensures that unwanted absorptions from water and carbon dioxide in the air or
from a solvent are cancelled out. The beams are then directed by mirrors so
that they follow parallel paths.
The beams are analysed by passing them through a prism of sodium chloride
which is transparent to infrared radiation or through a diffraction grating. Light
of only one particular frequency will now be focused onto the detector. The
spectrum is produced by rotating the prism so that the detector scans the
frequencies and records their intensities.
When the sample is not absorbing there will be no difference between the two
beams reaching the detector so no signal is recorded. When a vibration is being
excited, the sample absorbs radiation; the sample beam intensity will be
reduced and a signal generated.
A newer method called Fourier transform infrared (FTIR) spectroscopy uses a
single beam of infrared radiation and the phenomenon of wave interference to
produce an infrared spectrum. FTIR is cheaper and faster for collecting infrared
data.
Interpreting the spectra
In general we can match a particular bond to a particular absorption region. The
precise position of an absorption depends on the environment of the bond in
the molecule so we can only quote wavenumber regions in which we can expect
absorptions to arise.
Below 1500cm-1 the spectrum can be quite complex and it is more difficult to
assign absorptions to particular bonds. This region is characteristic of a
particular molecule and is often called the fingerprint region. It is useful for
identification purposes e.g. to compare two spectra to find out if they are
spectra of the same compound. It is only rarely used to identify functional
groups.
This spectrum shows a strong absorption at 2970cm-1 characteristic of C-H
stretching in aliphatic compounds. There is no indication of any functional
groups.
The spectrum of benzoic acid shows a sharp absorption at 3580cm-1
characteristic of an O-H bond (not hydrogen bonded). The strong absorption at
1760cm-1 shows the presence of a C=O group. The position of the C-H
absorption suggests it is an aromatic compound.