Part 1: Introduction to organic Chemistry
Many carbon compounds are found in living organisms which is why
their study got the name organic chemistry. Today, organic
chemistry includes all carbon compounds whatever their origin,
except CO, CO2 and the carbonates, which traditionally are included
in inorganic chemistry.
Why carbon?
A carbon atom has 4 electrons in its outer shell. It could
achieve stability by losing or gaining 4 electrons; but this is too
many electrons to lose or gain and the resulting carbon ions would
be too highly charged. So when carbon forms compounds, the bonds
are covalent rather than ionic.
Carbon can also form strong covalent bonds with itself to give
chains and rings of its atoms joined by C-C covalent bonds. This
property is called catenation and leads to the limitless variety of
organic compounds possible. Each carbon atom can form 4 covalent
bonds, so the chains may be straight or branched and can have other
atoms or groups substituted on them.
Chemists cope with the vast number of organic compounds by
dividing them into groups of related compounds. Compounds that
contain a benzene ring or arene, C6H6, (to be studied at A2) are
known as aromatic compounds. All other organic compounds are known
as aliphatic. Representations of the benzene ring.
Fractional DistillationCrude oil, also called petroleum, is a
complex mixture of hydrocarbons (compounds composed only of carbon
and hydrogen atoms), which exist as a liquid in the earth's crust.
Crude oil has many compositions; some is black, thick and tar like,
while other crude oils are lighter in colour and thinner. The
carbon and hydrogen atoms in crude oil are thought to have
originated from the remains of microscopic marine organisms that
were deposited at the bottom of seas and oceans and were
transformed at moderately high temperature and high pressure into
crude oil and natural gas.
In its raw form Crude Oil is not very useful. It must first be
transported to a refinery where it is processed. The fundamental
process is fractional distillation.
Fractional distillation is the process of partially separating
the many compounds present in crude petroleum. The principle used
is that the longer the carbon chain, the higher the temperature at
which the compounds will boil. (N.B. branched chains have lower
boiling points than straight chain molecules.)The crude petroleum
is heated and changed into a gas. The gases are passed through a
distillation column which becomes cooler as the height increases.
When a compound in the gaseous state cools below its boiling point,
it condenses into a liquid. The liquid fractions may be drawn off
the distilling column at various heights. (N.B. kerosene is often
called paraffin.)Although all fractions of petroleum find uses, the
greatest demand is for petrol (gasoline). One barrel of crude
petroleum contains only 25-35% petrol. Transportation demands
require that over 50% of the crude oil be converted into petrol. To
meet this demand some petroleum fractions must be converted to
petrol. This may be done by "cracking" - breaking down large
molecules of heavy heating oil or "reforming" - changing molecular
structures of low quality petrol molecules. We will look at
cracking later.
AlkanesMany of the compounds in Crude Oil belong to a family or
homologous series of compounds known as the alkanes. The alkanes
are saturated molecules who share the same general formula:
CnH2n+2.
1. Homologous series: a series or family of organic compounds
with the same functional group, whose members differ only in the
addition of a CH2 group.
1. Functional group: the specific atom or group of atoms that
confers a particular chemical property on a molecule, e.g. the OH
group in ethanol.
1. Saturated: the molecule contains the maximum amount of
hydrogen atoms possible, with no double or triple bonds between
atoms.
Task 2The table below shows the formulae of some simple alkanes.
Can you name them?
NameFormulaName Formula
CH4C6H14
C2H6C7H16
C3H8C8H18
C4H10C9H20
C5H12C10H22
IsomerismAll the alkanes with 4 or more carbon atoms in them
show structural isomerism. This means that there are two or more
different structural formulae that you can draw for each molecular
formula.For example, C4H10 could be either of these two different
molecules:
These are called respectively butane and
2-methylpropane.CycloalkanesCycloalkanes again only contain
carbon-hydrogen bonds and carbon-carbon single bonds, but this time
the carbon atoms are joined up in a ring. The smallest cycloalkane
is cyclopropane.
If you count the carbons and hydrogens, you will see that they
no longer fit the general formula CnH2n+2. By joining the carbon
atoms in a ring, you have had to lose two hydrogen atoms.The
general formula for a cycloalkane is CnH2n. (This is the same as
another homologous series known as the alkenes.)Don't imagine that
these are all flat molecules. All the cycloalkanes from
cyclopentane upwards exist as "puckered rings".Cyclohexane, for
example, has a ring structure which looks like this:
This is known as the "chair" form of cyclohexane - from its
shape which vaguely resembles a chair.Naming the alkanesA modern
organic name is simply a code. Each part of the name gives you some
useful information about the compound.For example, to understand
the name 2-methylpropan-1-ol you need to take the name to
pieces.The prop in the middle tells you how many carbon atoms there
are in the longest chain (in this case, 3). The an which follows
the "prop" tells you that there aren't any carbon-carbon double
bonds.The other two parts of the name tell you about interesting
things which are happening on the first and second carbon atom in
the chain. Any name you are likely to come across can be broken up
in this same way.Counting the carbon atomsYou will need to remember
the codes for the number of carbon atoms in a chain. There is no
easy way around this - you have got to learn them. If you don't do
this properly, you won't be able to name anything!
Types of carbon-carbon bondsWhether or not the compound contains
a carbon-carbon double bond is shown by the two letters immediately
after the code for the chain length.codemeans
anonly carbon-carbon single bonds
encontains a carbon-carbon double bond
For example, butane means four carbons in a chain with no double
bond.Propene means three carbons in a chain with a double bond
between two of the carbons.
Alkyl groupsCompounds like methane, CH4, and ethane, CH3CH3, are
members of a family of compounds called alkanes. If you remove a
hydrogen atom from one of these you get an alkyl group.For
example:1. A methyl group is CH3.1. An ethyl group is CH3CH2.These
groups must, of course, always be attached to something else.Worked
example: Write the structural formula for 2-methylpentane.Start
decoding the name from the bit that counts the number of carbon
atoms in the longest chain - pent counts 5 carbons.Are there any
carbon-carbon double bonds? No - an tells you there aren't any.Now
draw this carbon skeleton:
Put a methyl group on the number 2 carbon atom:
Does it matter which end you start counting from? No - if you
counted from the other end, you would draw the next structure.
That's exactly the same as the first one, except that it has been
flipped over.
Finally, all you have to do is to put in the correct number of
hydrogen atoms on each carbon so that each carbon is forming four
bonds.
If you had to name this yourself:1. Count the longest chain of
carbons that you can find. Don't assume that you have necessarily
drawn that chain horizontally. 5 carbons means pent.1. Are there
any carbon-carbon double bonds? No - therefore pentane.1. There's a
methyl group on the number 2 carbon - therefore 2-methylpentane.
Why the number 2 as opposed to the number 4 carbon? In other words,
why do we choose to number from this particular end? The convention
is that you number from the end which produces the lowest numbers
in the name - hence 2- rather than 4-.Task 11. Write the structural
formula for 2,3-dimethylbutane.
1. Write the structural formula for 2,2-dimethylbutane.
1. Write the structural formula for 3-ethyl-2-methylhexane.
If you had to name this yourself:How do you know what order to
write the different alkyl groups at the beginning of the name? The
convention is that you write them in alphabetical order - hence
ethyl comes before methyl which in turn comes before propyl.The
cycloalkanesIn a cycloalkane the carbon atoms are joined up in a
ring - hence cyclo.Worked example: Write the structural formula for
cyclohexane.hexan shows 6 carbons with no carbon-carbon double
bonds. cyclo shows that they are in a ring. Drawing the ring and
putting in the correct number of hydrogens to satisfy the bonding
requirements of the carbons gives:
Molecular formulaeA molecular formula simply counts the numbers
of each sort of atom present in the molecule, but tells you nothing
about the way they are joined together.For example, the molecular
formula of butane is C4H10, and the molecular formula of ethanol is
C2H6O.Molecular formulae are very rarely used in organic chemistry,
because they don't give any useful information about the bonding in
the molecule. About the only place where you might come across them
is in equations for the combustion of simple hydrocarbons, for
example:
In cases like this, the bonding in the organic molecule isn't
important.Structural formulaeA structural formula shows how the
various atoms are bonded. There are various ways of drawing this
and you will need to be familiar with all of them.Displayed
formulae (or full structural formulae)This shows all the bonds in
the molecule as individual lines. You need to remember that each
line represents a pair of shared electrons.For example, this is a
model of methane together with its displayed formula:
Notice that the way the methane is drawn bears no resemblance to
the actual shape of the molecule. Methane isn't flat with 90 bond
angles. The commonest way to draw structural formulaeFor anything
other than the most simple molecules, drawing displayed formulae is
a bit of a bother - especially all the carbon-hydrogen bonds. You
can simplify the formula by writing, for example, CH3 or CH2
instead of showing all these bonds. So for example, ethanoic acid
could be shown as:
You could even condense it further to CH3COOH, and would
probably do this if you had to write a simple chemical equation
involving ethanoic acid. You do, however, lose something by
condensing the acid group in this way, because you can't
immediately see how the bonding works.The syllabus states that: In
candidates answers, an acceptable response to a request for a
structural formula will be to give the minimal detail, using
conventional groups, for an unambiguous structure, e.g. CH3CH2CH2OH
for propan-1-ol, not C3H7OH.How to draw structural formulae in
3-dimensionsThere are occasions when it is important to be able to
show the precise 3-D arrangement in parts of some molecules. To do
this, the bonds are shown using conventional symbols:
For example, you might want to show the 3-D arrangement of the
groups around the carbon which has the -OH group in
butan-2-ol.Butan-2-ol has the structural formula:
Using conventional bond notation, you could draw it as, for
example:
Skeletal formulaeIn a skeletal formula, all the hydrogen atoms
are removed from carbon chains, leaving just a carbon skeleton with
functional groups attached to it.For example, we've just been
talking about butan-2-ol. The normal structural formula and the
skeletal formula look like this:
In a skeletal diagram of this sort1. there is a carbon atom at
each junction between bonds in a chain and at the end of each bond
(unless there is something else there already - like the -OH group
in the example);1. there are enough hydrogen atoms attached to each
carbon to make the total number of bonds on that carbon up to
4.Beware! Diagrams of this sort take practice to interpret
correctly - and may well not be acceptable to your examiners (see
below).There are, however, some very common cases where they are
frequently used. These cases involve rings of carbon atoms which
are surprisingly awkward to draw tidily in a normal structural
formula.Cyclohexane, C6H12, is a ring of carbon atoms each with two
hydrogens attached. This is what it looks like in both a structural
formula and a skeletal formula.
And this is cyclohexene, which is similar but contains a double
bond:
But the commonest of all is the benzene ring, C6H6, which has a
special symbol of its own.
Part 2: Structural Isomerism
What are isomers?Isomers are molecules that have the same
molecular formula, but have a different arrangement of the atoms in
space. That excludes any different arrangements which are simply
due to the molecule rotating as a whole, or rotating about
particular bonds.For example, both of the following are the same
molecule. They are not isomers. Both are butane.
There are also endless other possible ways that this molecule
could twist itself. There is completely free rotation around all
the carbon-carbon single bonds.If you had a model of a molecule in
front of you, you would have to take it to pieces and rebuild it if
you wanted to make an isomer of that molecule. If you can make an
apparently different molecule just by rotating single bonds, it's
not different - it's still the same molecule.What are structural
isomers?In structural isomerism, the atoms are arranged in a
completely different order. This is easier to see with specific
examples.What follows looks at some of the ways that structural
isomers can arise. The names of the various forms of structural
isomerism probably don't matter all that much, but you must be
aware of the different possibilities when you come to draw
isomers.Types of structural isomerismChain isomerismThese isomers
arise because of the possibility of branching in carbon chains. For
example, there are two isomers of butane, C4H10. In one of them,
the carbon atoms lie in a "straight chain" whereas in the other the
chain is branched.
Be careful not to draw "false" isomers which are just twisted
versions of the original molecule. For example, this structure is
just the straight chain version of butane rotated about the central
carbon-carbon bond.You could easily see this with a model. This is
the example we've already used at the top of this page.
Pentane, C5H12, has three chain isomers. If you think you can
find any others, they are simply twisted versions of the ones
below. Task 1: Can you name and draw the isomers of pentane, C5H12
below?
Task 2: How many isomers of hexane can you find?
Position isomerismIn position isomerism, the basic carbon
skeleton remains unchanged, but important groups are moved around
on that skeleton.For example, there are two structural isomers with
the molecular formula C3H7Br. In one of them the bromine atom is on
the end of the chain, whereas in the other it's attached in the
middle.Task 3Can you draw and name these two structural
isomers?
If you made a model, there is no way that you could twist one
molecule to turn it into the other one. You would have to break the
bromine off the end and re-attach it in the middle. At the same
time, you would have to move a hydrogen from the middle to the
end.Another similar example occurs in alcohols such as C4H9OH.Task
4Can you name and draw two straight-chain structural isomers with
this formula?
These are the only two possibilities provided you keep to a four
carbon chain, but there is no reason why you should do that. You
can easily have a mixture of chain isomerism and position isomerism
- you aren't restricted to one or the other.Task 5Can you draw two
more alcohols with the molecular formula C4H9OH?
You can also get position isomers on benzene rings. Consider the
molecular formula C7H8Cl. There are four different isomers you
could make depending on the position of the chlorine atom. In one
case it is attached to the side-group carbon atom, and then there
are three other possible positions it could have around the ring -
next to the CH3 group, next-but-one to the CH3 group, or opposite
the CH3 group.
Functional group isomerismIn this variety of structural
isomerism, the isomers contain different functional groups - that
is, they belong to different families of compounds (different
homologous series).For example, a molecular formula C3H6O could be
either propanal (an aldehyde) or propanone (a ketone).
There are other possibilities as well for this same molecular
formula - for example, you could have a carbon-carbon double bond
(an alkene) and an -OH group (an alcohol) in the same molecule.
Another common example is illustrated by the molecular formula
C3H6O2. Amongst the several structural isomers of this are
propanoic acid (a carboxylic acid) and methyl ethanoate (an
ester).
Task 6
Can you complete the activity sheet DF4.1 Modelling and naming
alkanes.ReferencesA-level Chemistry pages 280-281Chemistry in
Context pages 391-392
Learning ObjectivesCandidates should be able to:1. describe
structural isomerism1. deduce the possible isomers for an organic
molecule of known molecular formula.
Part 3: Combustion of alkanes
Task 1
Can you solve the anagrams to find the correct words in
bold?
Alasken contain only carbon-carbon and carbon-hydrogen bonds:
these are fairly strong and plan on nor. As a result alkanes are
unaffected by polar reagents such as acids and alkalis, clues
pinhole and police shelter. Indeed they were once known as the fans
rap if, from the Latin words parum (little) and affinitas
(affinity).
However, the few reactions, which they do undergo, are of great
importance.
The alkanes are used as fuels. They are kinetically bleats in
the presence of oxygen, but are energetically unstable with respect
to their oxidation products. Given the necessary vacation it
energy, they burn readily in air or oxygen. These reactions are
highly cheer mix to. The reaction has a free-radical mechanism, and
it occurs rapidly in the gas phase. Because it is a gas-phase acne
riot, liquid and solid alkanes must first be vaporised. That is why
less volatile alkanes burn less readily.
In the presence of a plentiful supply of oxygen, complete cubism
onto occurs. The only products are carbon dioxide and water.
Task 2
Write balanced equations for the complete combustion of methane
and hexane.
H = -890 kJmol-1H = -4195 kJmol-1
As the number of carbon atoms increase, what trends do you
observe?
1.
1. Often the flame is yellow and luminous. Insufficient oxygen
is available, leading to incomplete combustion. If you hold a
beaker over this flame it will become sooty. Water is being formed
together with carbon monoxide and carbon.Task 3
Write balanced equations for the combustion of methane gas to
carbon dioxide, carbon monoxide and carbon.
Carbon monoxide is a toxic gas. It binds irreversibly to the
haemoglobin in your red blood cells preventing the uptake of
oxygen.
Hydrocarbons contain sulphur impurities (sulphur atoms are found
in protein molecules). These burn in air to produce sulphur dioxide
(and, to a lesser extent, sulphur trioxide). This is thought to be
the major cause of acid rain.
Internal combustion engines
Carbon monoxide is one of a number of pollutants formed by the
incomplete combustion of petrol vapour in a car engine.
Task 4
Use pages 411-413 of Chemistry in Context to complete the table
below.
EmissionSourceChemical equation (where appropriate)Problems
associated with this emission
CO2
CO
CxHy
NO
NOx
SO2
Catalytic converters
These devices help to remove CO, NOx and CxHx from car exhausts.
They are made from a honeycomb of ceramic material onto which is
spread a thin layer of metals such as Pt, Rh and Pd. These metals
catalyse reactions between the pollutants and help to remove up to
90% of the harmful gases.
Task 5Can you complete the equation below? This shows one of the
reactions which take place in a catalytic converter.CO+NOCO and
CxHx are oxidised by the air.
Overall, the pollutant gases CO and NOx and unburned
hydrocarbons are replaced by CO2, N2 and H2O, which are harmless!
The catalytic reactions do not start working until the catalyst has
reached a temperature of about 200oC, so they are not effective
until the engine has warmed up.
ReferencesA-level Chemistry pages 295 and 314-315Chemistry in
Context pages 411-413
Learning ObjectivesCandidates should be:1. aware of the general
unreactivity of alkanes, including towards polar reagents;1. able
to describe the chemistry of alkanes as exemplified by the
combustion of ethane;1. able to describe and explain how the
combustion reactions of alkanes lead to their use as fuels in
industry, in the home and transport;1. able to recognize the
environmental consequences of carbon monoxide, oxides of nitrogen
and unburnt hydrocarbons arising from the internal combustion
engine and of their catalytic removal.1. able to recognize the
environmental consequences of gases that contribute to the enhanced
greenhouse effect.
Part 4: Chlorination
Alkanes such as methane do not react with chlorine (or bromine)
at room temperature or in the dark. In the presence of sunlight
(particularly ultraviolet light) or at high temperature, an
explosive reaction occurs, producing chloromethane and hydrogen
chloride (as well as other chlorinated methanes). Because the
reaction requires ultraviolet light, it is called a photochemical
reaction.
Task 1
Write a balanced equation for the reaction mentioned above.
This is an example of a free-radical substitution reaction which
occurs in several stages. All of these stages together form the
mechanism of the reaction.
1. Initiation step
Cl22Cl
Ultraviolet light consists of very high-energy radiation. This
supplies the energy needed to split the covalent bond in a chlorine
molecule, forming atoms. This is known as homolytic fission because
each free-radical retains one electron from each covalent bond.
This process occurs first because the Cl-Cl bond in chlorine is
weaker than the C-H bond in methane.
1. Propagation steps
The chlorine atoms, being free radicals, are highly reactive.
When they collide with a methane molecule they combine with one of
its hydrogen atoms, forming a new free radical:
Using curly arrows, we can show the movement of the single
electrons in this reaction:
The CH3 free radical can then react with another chlorine
molecule:
and so the process continues. These two reactions (and other
possible combinations) enable a chain reaction to occur; they are
propagation steps. Each step is exothermic, so the chain reaction
may be explosive.
1. Termination steps
The chain reaction ends when two free radicals combine to form a
stable molecule. Possible termination steps include:
Such termination steps can lead to trace amounts of impurities,
such as ethane, in the final product.
Further substitution
The reaction of a chlorine radical with methane extracts a
hydrogen radical to form HCl, as in the first propagation stop
above. This forms chloromethane which may also be attacked by a
chlorine radical in a pair of propagation steps to form
dichloromethane, and so on..
The proportion of different products formed depends on the
proportions of chlorine and methane used.
Practically all the reactions of alkanes proceed by free-radical
mechanisms, characterised by high activation energies and a
tendency to proceed rapidly in the gas phase.
Task 2
Write the structural formulas of all the products you would
expect to be formed when ethane reacts with excess chlorine in
sunlight.
ReferencesA-level Chemistry pages 280-281Chemistry in Context
pages 391-392
Learning ObjectivesCandidates should be able to:1. describe the
mechanism of free-radical substitution at methyl groups with
particular reference to the initiation, propagation and termination
reactions.1. describe the substitution of alkanes by chlorine and
bromine.
Part 5: Cracking
The composition of crude oil varies from place to place. In
general, however, the amount of each fraction produced by
distillation does not match the demand:
FractionApproximate %
Crude oilDemand
Gases24
Petrol and naphtha1627
Kerosene138
Gas oil1923
Residue5038
It can be seen that in general there is a higher-demand for the
lighter fractions and a surplus of heavy fractions. This imbalance
is solved using a process known as cracking. Cracking is the name
given to the breaking up of large hydrocarbon molecules into
smaller and more useful bits.
Large alkanes are cracked to produce smaller alkanes, alkenes
and sometimes even hydrogen. As it involves the breaking of
carbon-carbon and carbon-hydrogen bonds it requires a large input
of thermal energy or the use of a catalyst.
Cracking always produces many different products, and these can
be separated in a fractionating column. Task 1
Write one possible equation for the cracking of C15H32.
What conclusions can you draw about cracking reactions?
............................................................................................................................................................
............................................................................................................................................................
............................................................................................................................................................Thermal
cracking
1. Produces a high proportion of alkenes1. Temperatures range
from 400-900oC 1. Pressures up to 7000kPaAs the temperature
increases, the cracking shifts from the middle of the chain towards
the ends. To avoid complete decomposition to elements, the exposure
(or residence) time is kept short (around 1 second).Thermal
cracking is initiated by the homolytic fission of a C-C bond to
form two alkyl radicals. (Note C-C bonds are weaker than C-H
bonds.)
Radicals are highly reactive atoms, or groups of atoms, with
unpaired electrons. They are represented by the presence of a dot
(e.g. Cl). They are usually formed during the cleavage of non-polar
covalent bonds.
The alkyl radical can then react in a number of ways. It could
abstract a hydrogen atom from an alkane molecule to produce a
different alkyl radical and a shorter alkane:
Or it could undergo further bond cleavage to form an alkene and
a shorter alkyl radical:
In addition dehydrogenation, isomerisation and cyclisation
reactions are also occurring. The alkenes produced, with their
reactive double bonds, are used by the petrochemical industry as
building blocks for larger organic molecules.
Catalytic cracking1. Produces a large proportion of branched
alkanes, cycloalkanes and aromatic hydrocarbons1. Uses zeolite
(crystalline aluminosilicate) catalysts1. Temperature around
450oC1. Pressure just above atmosphericProportion of alkenes is
small. This method is used mainly for the production of high-octane
motor fuels. These branched alkanes prevent auto-ignition or
knocking.
Mechanism proceeds via formation of carbocations (heterolytic
fission). In the catalytic cracker the hot, vaporised oil fraction
and the catalyst behave as a fluid. Some of the hydrocarbon mixture
is broken down to carbon, which blocks the pores of the catalyst.
This is burnt off in air at a high temperature, allowing the
catalyst to be recycled.
Task 2
When an alkane is cracked, each molecule forms at least two new
molecules.
1. What reaction conditions are needed to cause cracking
reactions in alkanes?1. Which of the following rules are true in
writing an equation for cracking?1. There are more total molecules
on the reactant side.1. There are more total molecules on the
product side.1. All the crackate molecules are unsaturated.1. Some
of the crackate molecules are unsaturated.1. Crackate molecules are
always smaller.
Task 3 Complete the question sheet Alkanes.
ReferencesA-level Chemistry pages 310-311Chemistry in Context
pages 410-411
Learning ObjectivesCandidates should be able to suggest how
cracking can be used to obtain more useful alkanes and alkenes of
lower Mr from larger hydrocarbon molecules.