•	They are generally insoluble in water.
	•	They have low boiling points and melting points - most organic compounds melt
		below 300°C.
	•	They do not conduct heat or electricity.

 

They almost all burn.

 

Organic compounds are used for fuels. Complete combustion of a fuel gives carbon dioxide, steam and lots of energy.  Incomplete combustion gives carbon (soot) and carbon monoxide (CO). These equations show an incomplete and a complete combustion of butane.

 

C4H10 + 4O2    →   CO2 + CO + 2C + 5H2O

 

2C4H10 + 13O2    →   8CO2 + 10H2O

 

You have seen these incomplete combustion products coming from the exhausts of inefficient car engines. Other products will form if other elements are present in the organic compound.

 

Organic reactions tend to proceed slowly.

 

Organic reactions require the breaking and then reforming of strong covalent bonds, and are usually slow. To speed up these slow organic reactions, heat and catalysts are used in the laboratory, while heat, catalysts and high pressure are used in industry. Extremely efficient biological catalysts, called enzymes, are used in living systems to bring about complex organic reactions at moderate temperatures.

But why are there so many carbon compounds?

 

Carbon as an element is fairly inert at room temperature; nevertheless it has the ability to combine with almost every other element in the periodic table. However, an enormous number of compounds is achieved with carbon combined with relatively few other elements. These other elements are hydrogen, oxygen, nitrogen, sulfur, phosphorus and chlorine.

 

There are a number of contributing factors that explain why carbon forms so many compounds. These are:

 

Carbon is tetravalent.

 

Tetravalent means to form four bonds. Carbon with its electron configuration of 2,4 is capable of sharing its outer shell electrons with four other atoms to form four new bonds. In doing this, it achieves the stability associated with an octet of electrons in its outermost shell. These four bonds are covalent.

 

Carbon can form single, double or triple bonds.

 

 

These are called valence bond structures. They show each covalent bond as a short straight line. This property is not unique to carbon because oxygen, for example, forms single and double bonds, and nitrogen can form single, double or triple bonds.  But carbon has one property no other element has to the same extent, and that is its ability to form carbon chains.

 

Carbon can form covalent bonds with other carbon atoms to form chains of any length.

 

To form compounds, at least one other element must be present. Hydrogen is the most common. Compounds containing carbon and hydrogen only are called hydrocarbons. The simplest hydrocarbon is methane, CH4 .

 

 

 

This is methane. Remove one hydrogen and replace with a carbon. This carbon is tetravalent too. The remaining three links bond to hydrogen.

 

This is ethane, C2H6. Remove one hydrogen and replace with a carbon. Add hydrogen as before.

 

This is propane, C3H8. Remove one hydrogen and replace with a carbon atom.

 

Can  you  draw  elect ron  dot structures for these compounds?

 

Valence bond structures are also called graphic formulae.

Chemists often refer to CH4 as the simples t    of    t he    organic compounds.

The ability of carbon to form long or short carbon chains is called catenation, from the Latin catena, meaning chain.

 

 

 

To do carbon chemistry you must be able to count up to four.

 

 

 

 

And swiftly as we see wines flow, yet sluggish olive oil will take its time undoubtedly because it is composed of atoms either longer or more hooked and mutually entangled.

 

Lucretius, De Rerum

Natura, about 57 BC

 

 

 

 

 

A.S. Couper and A. Kekulé were the first to propose that carbon is tetravalent and that it can link to form chains. Couper went further; he invented graphic formulae. In

1858, Couper asked Wurtz to present his ideas to the French Academy, but Wurtz failed to do so and  t hus  K ek ulé' s  paper  was published first.

 

 

Kekulé subsequently received most of the credit. Couper was bitterly disappointed and he never returned to the serious study of chemistry. Kekulé became one of the most famous organic chemists of all.

This is butane, C4H10.

 

This process can be repeated to form chains of any length. What you are actually adding when you extend the chain by one carbon atom is not just carbon but CH2!

 

Predicting physical properties.

 

The simplest hydrocarbon is methane. Natural gas is mainly methane. Hexane is a hydrocarbon with six carbon atoms. Your teacher will show you an example of hexane, C6H14. Your wash bottle is made from a king size hydrocarbon called polythene. The number of carbon atoms in a polythene molecule can be as high as 200 000!

What is the most obvious visual difference between these three hydrocarbons? Answer: Methane is a gas, hexane a liquid and polythene a solid. The longer the

chains are, the slower moving the molecules are.  You would expect long, slow-moving

chains of carbon to be solids at room temperature and little light molecules to be gases.

 

Predicting chemical properties.

Notice that all these molecules have C_H bonds and, apart from methane, all have C_C bonds as well. You wo

uld expect the chemistry of methane, ethane, propane and butane to be similar. Their chemistry is similar and all of these hydrocarbons belong to the same chemical family.

 

Homologous series.

 

Organic chemistry is organised to make the study of this subject as easy as possible. There are far too many compounds to allow individual studies of each, so a system must be used. The system chemists use is to put carbon compounds into families. These families of compounds are called 'homologous series'. Homologous means the same. Each family of compounds is given a name. 'Alkane' is the family name for a group of compounds of carbon and hydrogen.

 

Homologous series have four characteristics:

 

1.   Each member has similar chemical properties.

2.   The physical properties change gradually with increasing chain length:

(a)   The melting points and boiling points increase with increasing chain length. (b)         Solubility in a given solvent decreases.

 

 

 3.   Each member's formula differs from the one above and the one below by a CH2 group.

4.   The mathematically minded of you may have realised that the formula of alkanes can be represented by a general formula. This is the final characteristic of a homologous series. Each homologous series can be characterised by a general formula.

 

The general formula of the alkanes is CnH2n+2 and they are said to be 'saturated'. The terms have nothing to do with being saturated with water. They contain the maximum amount of hydrogen - unlike other hydrocarbon homologous series. They are 'saturated' with hydrogen.

 

Saturated hydrocarbons contain only C_H and C_C bonds. There are no carbon- carbon double bonds; there are no carbon-carbon triple bonds. Compounds with

 

 

Arrangement of the atoms.

 

Chemists call the arrangement of atoms the structure. The structure of a molecule refers to the arrangement in space of its atoms. Whenever carbon is singly bonded to four other atoms, the carbon is at the centre of a tetrahedron. Methane, for example, is a tetrahedral molecule; so is carbon tetrachloride.

 

Ethane can be seen as two linked tetrahedra. Can you visualise what happens in chains? A continuous carbon chain is referred to as a straight chain and, for convenience, it is usually drawn that way.

 

 

 

However, using models, which have the correct tetrahedral bond angle (109.5°), it can be seen that strictly the chain is a zigzag one - or kinked or puckered because of the tetrahedral bond angle. Your teacher will show you this.

 

But some of the carbon atoms can form branches on this chain. Here is an example of a branched chain alkane.

 

More about these later.

 

Formulae

 

Two of the representations above are examples of graphic structural formulae or valence bond formulae of straight-chain compounds. Notice that at the ends of a straight chain each carbon atom is joined to three hydrogen atoms. Those in the middle are joined to two hydrogen atoms.  We can write structural formulae in a simpler way. For example, pentane could be written:

and these are called condensed structural formulae.

 

 

The arrangement of bonded carbon atoms is referred to as the carbon 'skeleton'; the hydrogens are thus like a 'skin'.

Hydrocarbon humans

 

The twentieth century is so dominated by oil in national strategies, global politics and power, it is hard to believe that no one thought it might be useful on a large scale until

1854 when Professor Silliman of Yale University, USA was engaged to report on the properties of Pennsylvania rock oil as an illuminant and lubricant.

 

In its first decade, petrol was a useless by-product of kerosene and was often run out into the rivers at night. Kerosene was known as "the new light". Kerosene lamps pushed back the night and extended the working day.

 

By the end of the 1870s, the Standard Oil Company, owned and run by John Rockefeller, dominated world trade in kerosene. 'Petroleum,' said Standard Oil 'has forced its way into more nooks and corners of civilised and uncivilised countries than any other product in business history.'

 

The first foreign competition came from the Nobel family. Robert Nobel bought an oil refinery in Baku, an outpost of Russia, with brother Ludwig's money.

 

Ludwig Nobel expanded the business and became known as 'The oil king of Baku.' Ludwig invented the first oil tankers - little more than floating bottles - and built a crucial 42 mile long pipeline ahead of competitors by blasting through a mountain with 400 tons of dynamite supplied by his even more famous brother Alfred Nobel.

 

The emphasis on kerosene's importance diminished when the chief engineer working for Thomas Alva Edison's illuminating company in Detroit decided to branch out on his own to design and sell a petrol-powered car, named after him. This was the model T-Ford.

 

At the beginning of the twenty first century oil still dominates security, prosperity and the nature of civilisation. It provides most of the energy and much of the chemicals used to run our society, which today is truly a "hydrocarbon society."

 

 

 

 

 Crude oil or petroleum is an important source of hydrocarbons. It is a complex mixture with chain lengths of the hydrocarbons present varying from one to nearly one hundred carbon atoms.  The actual components and their proportions vary from one oil field to another.  Alkanes, with the general formula CnH2n+2  are the predominant hydrocarbons. The crude oil was formed millions of years ago from dead, sea animals and plant material, which accumulated on the sea floor, were covered by sediments and then subjected to heat from the interior of the earth, high pressure and bacterial action. Over a long period of time, crude oil formed and accumulated in the spaces between the sediment particles to become oil-bearing rock - hence the name petroleum, which means rock oil.

 

 

We use petroleum products at a rate conservatively estimated to be about one million times faster than their rate of formation.

 

"Oil - the essence of old sunshine."

 

Sir John Cornforth

 

 

 In  the  twent iet h  century,  oil, supplemented by natural gas, toppled old king coal from his throne as the power source for the industrial world.

 

Petroleum deposit

 

 

 

LPG stands for liquid petroleum gas. It is used as a camping gas and for portable gas supplies.

The oil refinery

 

Fractional distillation

 

A refinery separates crude oil by distillation into fractions, which are collections of hydrocarbons of different chain length. The process is called fractional distillation; a separation into parts or fractions. Refineries are complex operations, which are controlled by computers.  South Australia's oil refinery was at Port Stanvac.

 

 

 

 

 

All these fractions become progressively thicker, more viscous and more difficult to ignite.

Octane number

 

The quality of petrol is measured by its octane number. In an internal combustion engine, like that of the modern car, a mixture of air and vaporised hydrocarbons in the C4-C12 range (petrol) is drawn into the cylinder on the down stroke of the piston. This mixture is compressed by the piston on its upstroke and then ignited by a spark from the spark plug. A high-octane petrol will burn quickly and smoothly giving a surge of power and forcing the piston down into its next stroke. A low-octane petrol, on the other hand, will explode, setting up shock waves that batter the piston and cylinder walls, giving a knocking sound.

 

A good petrol contains branched hydrocarbons. An example is:

 

 

This is called isooctane, or 2,2,4-trimethylpentane. You will learn how to handle these names with confidence later in this chapter. Isooctane has an octane rating of 100.

 

A 'poor' petrol is heptane. This is the graphic formula for heptane:

 

 

There are no branches to this chain of carbon atoms. This petrol has been given an octane rating of zero.

 

Other fuels are assigned octane numbers by comparison with these two - for example petrol with an octane number of 75 would show the same burning characteristics as a mixture of 75 per cent isooctane, 25 per cent heptane.

 

One method of improving octane rating is to use tetraethyl lead, but this results in poisonous lead compounds being emitted from the exhaust. Cheap lead-free, high- octane fuel relies upon the addition of other hydrocarbons produced by refinery reactions. These methods were developed in the 1930s but not adopted by the petroleum industry as a whole until the 1970s. More oil is now needed for fuel than before.

 

Refinery reactions

 

Physical separation is only one aspect of the function of an oil refinery. Another important role for the chemist is to change some of the less useful products into more useful ones.

 

Fuels, especially petrol, are in great demand and less useful fractions are converted into this fuel. The distillation of crude oil does not provide enough petrol, and it has a poor octane rating. Chemical technology is used to solve this problem - to create products with the properties we want them to have.

 

What sorts of alkanes are in crude oil? The answer is mostly straight-chain alkanes. The main problem for the chemist is that the branched-chained hydrocarbons are the better fuels. The more branched-chained molecules present, the better the petrol.

 

The octane number of leaded petrol is about 97, and unleaded petrol 92.

 

Knock increases engine wear and leads to wastage of petrol.

 

Octane number of some hydrocarbons

 

Pentane	62
Hexane	19
Heptane	0
2-methylpropane	122
2-methylbutane	99
2-methylpentane	83
3-methylpentane	86
2-methylhexane	41
3-methylhexane	56
2,2-dimethylpentane	87
2,3-dimethylpentane	87
Benzene	99

This is why most petrols contain up to about 30% benzene!

 

Crude petrol has an octane number of less than 60.

 

 

 

In 1921, after a three-year search, Thomas Midgley discovered that the poisonous, oil-soluble compound tetraethyl lead, Pb(CH₂CH₃)₄ was a cheap, anti-knock compound. As a result, he became a very wealthy man.

 

 

 

 

Petrol is a volatile mixture of hydrocarbons, both cyclic and non

-cyclic, in the range of pentanes to decanes. The quality of petrol is very much dependent on the hydrocarbons of which it is composed. In the C4  - C12  range, there are 661 possible alkanes. Therefore, there is no single formula for petrol.

 

 

 

 

 

 

A catalytic cracker is an important installation in an oil refinery. The USA processes 12 x 106  barrels a year (a barrel is 284 L), and of this, one third is cat. cracked.

 

Certain rocks called zeolites are now being used increasingly as catalysts. These rocks have a suitable uniform pore size and catalytic action is on the inside surface. These catalysts are more selective in producing molecules of the right chain length, and are faster, as there is less coke build up.

 

Cracking is important for two reasons:

 

•   It  produces  more  widely  used

compounds from less useful ones.

•   It    prod uces    un sa turate d

hydrocarbons.

 

Do you notice anything unusual about any of these formulae?

Methods of converting other fractions into C4 - C12 hydrocarbons

 

1.   Small alkanes are stuck together to make bigger alkanes - but the percentage of lighter alkanes is small and, as these alkanes are also in demand, this reaction is not used very much in practice.

2.   Cat. cracking - 'cat.' here stands for a catalyst. Cracking larger molecules into smaller ones of suitable size was first achieved using high temperatures alone, but with suitable catalysts (a type of clay containing Al2O3 and SiO2 is used) lower temperatures can be used.

 

Coke is also formed which slows down catalytic action, so the coke must be periodically burnt away to restore catalytic activity. This provides a source of heat for the endothermic cracking reaction.

 

3.   Isomerisation. Not only chain length is important but structure too. Isomerisation occurs when a molecule undergoes a chemical rearrangement to form a more branched isomer.

4.   Catalytic reforming. A platinum catalyst is used and the process is sometimes called 'platforming'. This process converts straight-chain alkanes boiling in the petrol range into hydrocarbon rings without altering carbon number drastically, but the number of hydrogens will be reduced. This is a widely used refinery reaction as these hydrocarbons are used to replace tetraethyl lead, making unleaded fuel.

 

We have learnt above that, under the influence of high temperature, and particularly in the presence of certain catalysts, hydrocarbons become unstable and break or 'crack' into smaller fragments, e.g. the saturated hydrocarbon dodecane, C12H26, may be broken in the following ways:

 

 

The cracking of kerosene illustrates this conversion.

 

 

Fuels and chemicals produced

 

There are three basic fractions that yield petroleum. Light naphtha and heavy naphtha give mainly straight-chain alkanes - called 'straight run' petrol, which can be converted by isomerisation into petrol with a higher octane number. Some light naphtha and heavy naphtha is 'catalytically reformed' into other hydrocarbons, such as benzene, methylbenzene (toluene) and dimethyl benzene. Crude oil also contains some of these compounds. These also have a high octane rating, and can be used to replace tetraethyl lead.

 

Gas oil is cracked into petroleum sized molecules and these, in turn, may be subjected to isomerisation and catalytic reforming. These same three basic fractions provide six out of seven of the most important hydrocarbons in the chemical industry. Of these seven compounds, ethene, obtained by cracking, is the most important of all.

 

There is a competition between these compounds needed for fuel and the same compounds needed in the chemical industry. Eighty eight out of every 100 barrels of oil are used to produce energy. The other 12 barrels are used for the manufacture of a wide variety of chemicals. The petrochemical industry has grown since World War II when, because of shortages of products such as silk and rubber, substitutes were developed. After the war the research effort was redirected towards consumer goods.

 

The basic seven chemicals in the petrochemical industry are given in the following table:

 

 

The most important of these seven, the 'big three,' are in bold type. All of these are hydrocarbons, i.e. contain carbon and hydrogen only. Methane comes from natural gas but the remaining six are derived chemically from crude oil fractions.

 

The point to note is that these few compounds can be converted into many, many others - thousands of sophisticated chemicals and plastics. Organic chemicals can be interconverted quite simply using a basic set of reactions. You must realise that although many compounds are possible, the basic reactions to produce them are few. Organic chemistry is not difficult!

 

 

 

 

 

The simplest of the organic chemical families is a group of saturated hydrocarbons called the alkanes. All alkanes take part of their family name, the 'ane,' into their own name.

 

Alkanes are hydrocarbons (that is they contain carbon and hydrogen only) and are said to be saturated hydrocarbons - meaning they contain only C _ C and C _ H single bonds. They contain the maximum number of hydrogen atoms per molecule.

 

 

Crude oil also contains sulfur compounds, which are removed at the refinery. This is because if left in the petrol they would burn to

form oxides of sulfur so contributing to "acid rain". These oxides also poison the catalytic converters fitted to cars.

 

When an oil spill occurs it is the BTX component that does the most damage to sea birds and mammals. The sticky, tar-like blobs are difficult to remove.

 

 

Not only do we build from carbon chains, we also depend on them everyday of our lives.   Foods are carbon chains, as are clothes, packaging, paper, fuels and plastics!

Even on standing for some time, these strong oxidants show no colour change when added to any alkane.

4.   They do not react with bromine in the cold and in the absence of light. This is shown by no loss of colour. They do react if heated or subjected to bright light.

 

Substitution reactions of alkanes

 

A substitution reaction is one where an atom or group of atoms is displaced by another atom or group of atoms. The alkanes undergo substitution reactions with difficulty. For example:

 

Other substitution products may be formed:

 

 

 

 

In sunlight the reaction occurs very slowly. This reaction can be used commercially to form carbon tetrachloride, an industrial solvent.

 

In practice, a mixture of substituted hydrocarbons is formed and these must be separated by fractional distillation. For higher alkanes, a bigger variety of products is possible.

 

Combustion of alkanes

 

From the above, it would be quite wrong to say alkanes are totally unreactive - or that they are unimportant. Commercially, they are the most important organic compounds because they occur naturally in large quantities and from them chemists make almost all other synthetic organic compounds. Their most important use is as fuels, e.g.

 

Try balancing this reaction, which occurs in the engines of motorcars:

 

 

 

Where does the energy produced come from?

 

Preparation of alkanes

 

Because alkanes occur naturally (e.g. in natural gas), it is seldom necessary to make them in a laboratory. However, methane is prepared by bacterial decomposition of organic waste. This provides a renewable energy source. Other methods for making alkanes used in fuels are being investigated.

 

Microorganisms present in the dung decompose it in the absence of air to form methane gas. At sewage works, methane generated from the sewage is used to generate the electricity to power the works.

 

Methane is a colourless, odourless gas. A smelly impurity is added to gas used for domestic use so that gas leaks can be detected.

Jean Baptiste Dumas, a 19th century Professor of Chemistry in Paris, discovered substitution when he was asked to analyse s om e   candles   whic h   had distressed guests at the King's reception by giving off acrid fumes. The gas was HCl and he traced this to some chlorine, which had been used to bleach the candles. He found that chlorine had reacted chemically with the candle wax, replacing some of the hydrogens with chlorines. He found bromine and iodine could replace hydrogen as well.  Dumas could not gain acceptance of his ideas although they were correct and he gave up his brilliant chemical career to become a politician.

 

 

 

How many hydrogens can be displaced altogether in hexane? How many compounds can be formed by reacting hexane with chlorine?

 

It is oil that makes possible where we live, how we live, how we commute to work, how we travel.

 

 

Bacterial decomposition in the absence of air is called anaerobic.

 

 

Gas was once made from coal at a gas works. Today we use natural gas, and no longer need to make gas.

 

 

 

Alkenes are sometimes called olefins, meaning "oil-forming".

 

 

The type of bond in the chain, here a double bond, controls the compound's chemical properties.

 

 

Cracking is the only commercial source of ethylene (ethene) and propylene (propene).

Ethene and propene are used extensively to make plastics.

 

 

 

Al l   al kenes   are   non-polar compounds.

Alkenes

 

The next in our family of organic compounds is named the alkenes.  The general formula of this homologous series, CnH2n, shows that the alkenes have more carbon and less hydrogen in their molecules than do the alkanes.

 

Unsaturated hydrocarbons are those with less than the maximum amount of hydrogen. This arises because they have

 

Alkenes have carbon-carbon double bonds,  

 

 

Note again that the members of this series take part of the family name.

 

 

Functional groups

 

 

 Simple alkenes have only one                  group. The rest of the hydrocarbon has only C_C and C_H bonds - like alkanes!  Hence the rest of the molecule is unreactive (like alkanes). The

 

A functional group is a reactive site in a molecule or any atom or group of atoms, which cause that molecule to take part in a chemical reaction.

 

Each functional group has a name and

 

Physical properties of alkenes

 

 

Like alkanes, they have low melting points and boiling points. The first members of the homologous series are gases; again similar to the alkanes.

 

Chemical properties of alkenes

 

Alkenes are much more reactive than alkanes. This is because alkenes have a functional group. Addition and oxidation are the typical reactions.

 

Combustion of alkenes

 

Alkenes burn with luminous smoky flames. The smokiness due to unburnt carbon is typical of unsaturated hydrocarbons. With the higher carbon to hydrogen ratio, some carbon remains unburnt.

Addition of bromine

The colour of bromine rapidly disappears when bromine water is shaken with an alkene. This is used as a test for unsaturation. The two reactants form one product with a molecular formula, which is the sum of the two reactants' formulae, e.g.

 

 

The reaction site is the alkene group. The reaction is rewritten to show this:

 

 

One bromine adds to one end, the other bromine to the other. One double bond forms two new single bonds and the product is saturated.

 

Addition of hydrogen

 

e.g.                    

 

 

Addition occurs to both ends as before. The hardening of vegetable oils, which are

'polyunsaturated' into margarine, is accomplished with this reaction.

 

 

Catalytic hydrogenation continues until the fat is of the right spreading consistency.

 

Addition of water

 

An acid catalyst (usually concentrated H2SO4) is used.

 

This is the commercial method of making ethanol. Methylated spirits, which is about 95 per cent ethanol, is the most common industrial solvent.

 

Reaction with acids

 

Addition of HX

X can be F, Cl, Br or I (as a concentrated solution or as a gas), e.g.

 

With cyclohexene the reaction would be:

 

Bromine, Br2, is a dark red-brown l i q u i d ;  b ro mi n e  w a te r  i s  p a l e yellow or orange, depending on concentration.

 

 

Hydrogenation can occur at room temperat ure  when  t he  nickel catalyst is added.

 

 

A ddit ion  r eac t ions  ar e  wher e atoms are added to the double bond in an alkene, or the triple bond in an alkyne.

 

 

 

Hydrogenat ion  react ions  also occur in living cells.

 

 

'Ant ioxidant s'  are   added   t o polyunsaturated vegetable oils & margarine to stop them becoming rancid. The acids formed on oxidation have nauseating odours.

 

 

 

 

 

A polymer is a very large molecule made from simple units repeated many times.

 

 

Rubber is a polyunsaturated hydrocarbon polymer, which occurs naturally and is also made artificially. It is rapidly oxidised by ozone, a pollutant in urban areas, such as Los Angeles. Car tyres, rubber hosing, gaskets and boots are attacked.

Note that reaction only occurs at the functional group. The saturated part of the molecule, regardless of size or shape, can be ignored. Note also that these are addition reactions. Adding of atoms has occurred across the double bond.

 

Reaction with acidified potassium permanganate

 

Alkenes decolourise acidified potassium permanganate. This is another test for unsaturation but it is not as good as the bromine water test.

 

KMnO4/H+ is decolourised by other functional groups too. The reaction is an addition and an oxidation since oxygen adds to the alkene. On warming, the parent chain can be broken into smaller pieces due to reaction at the alkene group.

 

Polymer formation

 

Alkenes can add to each other to form alkanes, which form very large molecules called macromolecules. Their chains can contain 200 000 carbon atoms, or more!  For example

 

 

Polythene is a giant alkane and the world's most common plastic. Other plastics are made from reactions similar to this.

 

 

Alkynes

 

A third family of hydrocarbons is called the alkynes. The general formula, CnH2n-2, shows that they too are unsaturated hydrocarbons.

 

The functional group,

 

The two carbons of the functional group and the two atoms directly attached to it are in a straight line. Ethyne (acetylene) itself is a linear molecule. It is used as a heat source for joining metals by melting them together. This is called oxy-acetylene welding.

 

Ethyne (acetylene) is prepared in the laboratory and industry by the same reaction. Calcium dicarbide (more commonly called calcium carbide) reacts with water to produce ethyne (acetylene). The reaction is:

 

 

Calcium carbide is prepared from calcium oxide and coke at temperatures exceeding

2000°C. This makes it expensive to produce and so acetylene in turn becomes expensive. For this reason, most chemicals now being produced from ethyne (acetylene) will in future be prepared from ethene instead.

 

 

    •      Addition of hydrogen chloride gas

 

When hydrogen chloride gas is added to acetylene, chloroethene is produced - otherwise known as vinyl chloride:

 

 

 

 

 

 

Acetylene and the other members of the homologous series undergo both addition and oxidation reactions like the other family of unsaturated hydrocarbons, the alkenes.

 

 

 

 

Systematic names can be very

long   indeed.   F or   example,

1,2,3,4,10,10-hexachloro-6,7- epoxy-1,4,4a,5,6,7,8,8a- octahydro-1,4-endo-endo-5,8- dimethanonaphthalene is the syste mati c    n a me    for    a n

insecticide, far better known as dieldrin!

Vinyl chloride is used to produce the plastic, polyvinyl chloride or PVC.

 

Oxidation

 

Acetylene turns the purple alkaline potassium permanganate solution green. This is evidence that oxidation has occurred.

 

 

Nomenclature

 

Naming compounds in organic chemistry is referred to as nomenclature. Rules for naming have become necessary because there are so many compounds and their numbers are increasing.

 

Systematic names give the structure of the compound being referred to. Previously only unsystematic names were used which could refer to the source of the compound, e.g. jasmone from jasmine, or to a property of the compound, for example putrescine has a foul smell.

 

Unsystematic, 'trivial' or 'common names' abound. In fact, they are still probably used more than systematic names because of their brevity, and because when a new compound is first isolated, the structure, upon which its name is based, is frequently not known.

 

Lengthy cumbersome names are rarely used in day-to-day conversation, even among chemists. They are reserved for scientific publications where it is essential to give an unambiguous statement. However, the basic rules of nomenclature for simple compounds are easy to apply, and having mastered these, you will know how to convert a name to a structure and a structure to a name.

 

We start with the names of the alkanes because if you know the names of the first ten alkanes, you have a basis for naming hundreds of thousands of other compounds.

 

Parent alkanes

 

Learn the names of the first ten alkanes and the number of carbon atoms associated with each name.

 

Branches or side-chains can be short or long and these have names too. Two

important side chains are _CH3 (methyl) and _CH2CH3 (ethyl). These do not exist separately but only attached to longer carbon chains. They have one hydrogen less than the alkanes from which they are derived.

Naming branched alkanes

 

Names of organic compounds are frequently made up of three parts:

1.   The name of the 'parent' alkane chain.

2.   The name(s) of the substituent(s). (Substituent is a word that refers either to an alkyl side chain or to a functional group attached to the parent chain.)

3.   The position numbers of the substituents on the parent chain.

 

Rules

 

1.   Look for the longest continuous carbon chain. This is called the parent chain. e.g.

 

 

 

 

 

2.   Number each carbon atom in sequence from the end closer to the branches.

In the example above, there is an ethyl branch on carbon atom number 3 and a methyl branch on carbon atom 6. (Each branch must have a name and a position number.)

 

More precisely, the above compound is a methylpentane and, since the methyl group is attached to carbon atom number two, it is 2-methylpentane. (The octane above it is 3-ethyl-6-methyloctane.)

 

3.   Side chains are listed in alphabetical order. For example ethyl is written before methyl.

 

Building models is the best way of

learning about the structure of organic molecules. Borrow a model kit from your teacher.

 

 

The rule is that the substituent must be given the lowest possible number. Hence this compound is not called 4-methylpentane.

 

 

 

 

Commas separate numbers, and hyphens separate numbers and words. The final word, however long, is written as one word.

 

 

 

Every hydrocarbon with an alkene group has an 'ene' ending to its systematic name.

 

In longer alkenes and alkynes, the position number of the functional group is part of the name.

 

 

 

 

A halogen can be attached to a parent chain by a single bond like a branch. Their names become _F (fluoro), _Cl (chloro), _Br (bromo) and  _I (iodo).

 

 

 

Remember:

1     Name the parent chain.

2     Name the substituents and put in alphabetical order.

3     Put in the position number of each substituent.

4.   Two identical side chains are preceded by 'di', three by 'tri', four by 'tetra' and so on. For example:

 

If two side chains are on the same carbon atom, the number must appear twice. (Note: di, tri, tetra, penta and similar prefixes have no alphabetical significance.)

 

Exercise

According to text books there are three other isomers having the same molecular formula as the dimethylbutanes shown above. Draw graphic formulae for these compounds and name them.

 

Naming alkenes

 

Alkane   →   alkene

Pick the longest continuous chain containing the alkene group. This is the parent chain. The 'ane' in the parent alkane is changed to 'ene'. For example

 

 

 

Naming alkynes

 

Alkane   →  alkyne

Look for the longest continuous chain containing the alkyne group.  This is the parent chain.  The 'ane' part of the parent alkane is changed to 'yne'.  What is the systematic name for acetylene?

 

Naming haloalkanes

 

 

 The term halo is used to cover fluorine, chlorine, bromine and iodine. This family of elements is called the halogens. This is similar to naming branched alkanes. Some examples will make this clear:

 

 

Organic chemistry