Ntroducing Alkanes and Cycloalkanes

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NTRODUCING ALKANES AND CYCLOALKANES

This is an introductory page about alkanes such as methane, ethane, propane, butane

and the rest. It deals with their formulae and isomerism, their physical properties, andan introduction to their chemical reactivity.

What are alkanes and cycloalkanes?

Alkanes

Formulae

Alkanes are the simplest family of hydrocarbons - compounds containing carbon andhydrogen only. They only contain carbon-hydrogen bonds and carbon-carbon singlebonds. The first six are:

methane CH4 ethane C2H6 propane C3H8 butane C4H10 pentane C5H12 hexane C6H14

You can work out the formula of any of them using: CnH2n+2

Isomerism

All the alkanes with 4 or more carbon atoms in them show structural isomerism . Thismeans that there are two or more different structural formulae that you can draw foreach molecular formula.

For example, C4H10 could be either of these two different molecules:



These are called respectively butane and 2-methylpropane .

Note: If you aren't confident about naming organic compounds, the various ways of drawing organiccompounds, or structural isomerism, then you really ought to follow these links before you go on. You should read the whole of the page about drawing organic molecules, but there is no need to read theother two beyond where they talk about alkanes. Use the BACK button on your browser to return to this page.

Cycloalkanes

Cycloalkanes again only contain carbon-hydrogen bonds and carbon-carbon singlebonds, but this time the carbon atoms are joined up in a ring. The smallest cycloalkaneis cyclopropane.

If you count the carbons and hydrogens, you will see that they no longer fit the generalformula CnH2n+2. By joining the carbon atoms in a ring, you have had to lose twohydrogen atoms.

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You are unlikely to ever need it, but the general formula for a cycloalkane is C nH2n.

Don't imagine that these are all flat molecules. All the cycloalkanes from cyclopentaneupwards 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 vaguelyresembles a chair.

Note: This molecule is constantly changing, with the atom on the left which is currently pointing down

flipping up, and the one on the right flipping down. During the process, another (slightly less stable) formof cyclohexane is formed known as the "boat" form. In this arrangement, both of these atoms are eitherpointing up or down at the same time.

Physical Properties

Boiling Points

The facts

The boiling points shown are all for the "straight chain" isomers where there are morethan one.



Notice that the first four alkanes are gases at room temperature. Solids don't start toappear until about C17H36.

You can't be more precise than that because each isomer has a different melting andboiling point. By the time you get 17 carbons into an alkane, there are unbelievable

numbers of isomers!

Cycloalkanes have boiling points which are about 10 - 20 K higher than thecorresponding straight chain alkane.

Explanations

There isn't much electronegativity difference between carbon and hydrogen, so there ishardly any bond polarity. The molecules themselves also have very little polarity. Atotally symmetrical molecule like methane is completely non-polar.

Note: If you aren't sure about electronegativity and polarity, then you really ought to follow this link beforeyou go on. Use the BACK button on your browser to return to this page.

This means that the only attractions between one molecule and its neighbours will beVan der Waals dispersion forces. These will be very small for a molecule like methane,but will increase as the molecules get bigger. That's why the boiling points of the

alkanes increase with molecular size.

Note: If you aren't sure about Van der Waals forces, then you should follow this link before you go on. Use the BACK button on your browser to return to this page.

Where you have isomers, the more branched the chain, the lower the boiling point tends

to be. Van der Waals dispersion forces are smaller for shorter molecules, and onlyoperate over very short distances between one molecule and its neighbours. It is moredifficult for short fat molecules (with lots of branching) to lie as close together as longthin ones.

For example, the boiling points of the three isomers of C5H12 are:

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boiling point (K) pentane 309.2 2-methylbutane 301.0 2,2-dimethylpropane 282.6

The slightly higher boiling points for the cycloalkanes are presumably because themolecules can get closer together because the ring structure makes them tidier and less"wriggly"!

Solubility The facts What follows applies equally to alkanes and cycloalkanes. Alkanes are virtually insoluble in water, but dissolve in organic solvents. The liquidalkanes are good solvents for many other covalent compounds. Explanations Solubility in water When a molecular substance dissolves in water, you have to

break the intermolecular forces within the substance. In the case of the alkanes,these are Van der Waals dispersion forces.

break the intermolecular forces in the water so that the substance can fit betweenthe water molecules. In water the main intermolecular attractions are hydrogenbonds.

Note: If you aren't sure about hydrogen bonds, then you should follow this link before you go on. Use the BACK button on your browser to return to this page.

Breaking either of these attractions costs energy, although the amount of energy tobreak the Van der Waals dispersion forces in something like methane is prettynegligible. That isn't true of the hydrogen bonds in water, though.

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As something of a simplification, a substance will dissolve if there is enough energyreleased when new bonds are made between the substance and the water to make upfor what is used in breaking the original attractions.

The only new attractions between the alkane and water molecules are Van der Waals.

These don't release anything like enough energy to compensate for what you need tobreak the hydrogen bonds in water. The alkane doesn't dissolve.

Note: The reason that this is a simplification is that you also have to consider entropy changes whenthings dissolve. If you don't yet know about entropy, don't worry about it!

Solubility in organic solvents

In most organic solvents, the main forces of attraction between the solvent moleculesare Van der Waals - either dispersion forces or dipole-dipole attractions.

That means that when an alkane dissolves in an organic solvent, you are breaking Vander Waals forces and replacing them by new Van der Waals forces. The two processesmore or less cancel each other out energetically - so there isn't any barrier to solubility.

Chemical Reactivity

Alkanes Alkanes contain strong carbon-carbon single bonds and strong carbon-hydrogen bonds.The carbon-hydrogen bonds are only very slightly polar and so there aren't any bits ofthe molecules which carry any significant amount of positive or negative charge whichother things might be attracted to. For example, you will find (or perhaps already know) that many organic reactions startbecause an ion or a polar molecule is attracted to a part of an organic molecule whichcarries some positive or negative charge. This doesn't happen with alkanes, becausealkane molecules don't have this separation of charge. The net effect is that alkanes have a fairly restricted set of reactions. You can

burn them - destroying the whole molecule; react them with some of the halogens, breaking carbon-hydrogen bonds; crack them, breaking carbon-carbon bonds.



These reactions are all covered on separate pages if you go to the alkanes menu (seebelow). Cycloalkanes Cycloalkanes are very similar to the alkanes in reactivity, except for the very small ones- especially cyclopropane. Cyclopropane is much more reactive than you would expect. The reason has to do with the bond angles in the ring. Normally, when carbon formsfour single bonds, the bond angles are about 109.5°. In cyclopropane, they are 60°.

With the electron pairs this close together, there is a lot of repulsion between thebonding pairs joining the carbon atoms. That makes the bonds easier to break. The effect of this is explored on the page about reactions of these compounds withhalogens which you can access from the alkanes menu below.

THE COMBUSTION OF ALKANES ANDCYCLOALKANES

This page deals briefly with the combustion of alkanes and cycloalkanes. In fact, thereis very little difference between the two.

Complete combustion

Complete combustion (given sufficient oxygen) of any hydrocarbon produces carbondioxide and water.



Equations

It is quite important that you can write properly balanced equations for these reactions,because they often come up as a part of thermochemistry calculations. Don't try to learnthe equations - there are far too many possibilities. Work them out as you need them.

Some are easier than others. For example, with alkanes, the ones with an even numberof carbon atoms are marginally harder than those with an odd number!

For example, with propane (C3H8), you can balance the carbons and hydrogens as youwrite the equation down. Your first draft would be:

Counting the oxygens leads directly to the final version:

With butane (C4H10), you can again balance the carbons and hydrogens as you writethe equation down.

Counting the oxygens leads to a slight problem - with 13 on the right-hand side. Thesimple trick is to allow yourself to have "six-and-a-half" O2 molecules on the left.

If that offends you, double everything:

Note: You might well come across either version of these equations. The ones with the halves left in areoften used in calculation work. Forgive me if you find this last bit on equations unbearably trivial - not everybody does! Just be gratefulthat you have been well taught.

Trends

The hydrocarbons become harder to ignite as the molecules get bigger. This is becausethe bigger molecules don't vaporise so easily - the reaction is much better if the oxygen



and the hydrocarbon are well mixed as gases. If the liquid isn't very volatile, only thosemolecules on the surface can react with the oxygen.

Bigger molecules have greater Van der Waals attractions which makes it more difficultfor them to break away from their neighbours and turn to a gas.

Note: If you aren't sure about Van der Waals forces, then you should follow this link before you go on. Use the BACK button on your browser to return to this page.

Provided the combustion is complete, all the hydrocarbons will burn with a blue flame.However, combustion tends to be less complete as the number of carbon atoms in themolecules rises. That means that the bigger the hydrocarbon, the more likely you are toget a yellow, smoky flame.

Incomplete combustion

Incomplete combustion (where there isn't enough oxygen present) can lead to theformation of carbon or carbon monoxide.

As a simple way of thinking about it, the hydrogen in the hydrocarbon gets the firstchance at the oxygen, and the carbon gets whatever is left over!

The presence of glowing carbon particles in a flame turns it yellow, and black carbon isoften visible in the smoke. Carbon monoxide is produced as a colourless poisonousgas.

Why carbon monoxide is poisonous

Oxygen is carried around the blood by haemoglobin (US: hemoglobin). Unfortunatelycarbon monoxide binds to exactly the same site on the haemoglobin that oxygen does.

The difference is that carbon monoxide binds irreversibly - making that particular

molecule of haemoglobin useless for carrying oxygen. If you breath in enough carbonmonoxide you will die from a sort of internal suffocation.

Note: There is more about haemoglobin towards the bottom of the page about complex ions. If you want some description of catalytic converters which help to remove carbon monoxide and someother pollutants, see the introductory page on catalysis.




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If you want full details about any of the environmental problems associated with burning hydrocarbons,you can't do better than explore the excellent US Environmental Protection Agency site. If you want details about the role of hydrocarbons in the formation of photochemical smog, a Googlesearch on photochemical smog will quickly lead you to some detailed chemistry. There is a Googlesearch box on the Main Menu (link below). Don't forget that you want to search the whole web - not

chemguide.

Use the BACK button (or the HISTORY or GO menus) on your browser if you want to return to this pagelater.

THE HALOGENATION OF ALKANES ANDCYCLOALKANES

This page describes the reactions between alkanes and cycloalkanes with the halogensfluorine, chlorine, bromine and iodine - mainly concentrating on chlorine and bromine.

Alkanes

The reaction between alkanes and fluorine This reaction is explosive even in the cold and dark, and you tend to get carbon and

hydrogen fluoride produced. It is of no particular interest. For example:

The reaction between alkanes and iodine Iodine doesn't react with the alkanes to any extent - at least, under normal labconditions. The reactions between alkanes and chlorine or bromine There is no reaction in the dark. In the presence of a flame, the reactions are rather like the fluorine one - producing amixture of carbon and the hydrogen halide. The violence of the reaction dropsconsiderably as you go from fluorine to chlorine to bromine. The interesting reactions happen in the presence of ultra-violet light (sunlight will do).These are photochemical reactions , and happen at room temperature.

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We'll look at the reactions with chlorine. The reactions with bromine are similar, butrather slower. Methane and chlorine Substitution reactions happen in which hydrogen atoms in the methane are replacedone at a time by chlorine atoms. You end up with a mixture of chloromethane,dichloromethane, trichloromethane and tetrachloromethane.

Note: Follow this link if you aren't happy about naming organic compounds. Use the BACK button on your browser to return to this page.

The original mixture of a colourless and a green gas would produce steamy fumes ofhydrogen chloride and a mist of organic liquids. All of the organic products are liquid atroom temperature with the exception of the chloromethane which is a gas.

If you were using bromine, you could either mix methane with bromine vapour, orbubble the methane through liquid bromine - in either case, exposed to UV light. Theoriginal mixture of gases would, of course, be red-brown rather than green.

You wouldn't choose to use these reactions as a means of preparing these organiccompounds in the lab because the mixture of products would be too tedious to

separate.

The mechanisms for the reactions are explained on separate pages.

Note: If you want the methane-chlorine mechanism, follow this link. If you want the methane-bromine mechanism, follow this one.




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Use the BACK button on your browser to return to this page.

Larger alkanes and chlorine

You would again get a mixture of substitution products, but it is worth just looking brieflyat what happens if only one of the hydrogen atoms gets substituted (monosubstitution) -

just to show that things aren't always as straightforward as they seem!

For example, with propane, you could get one of two isomers:

Note: If you aren't sure about isomerism, you might like to follow this link. Use the BACK button on your browser to return to this page.

If chance was the only factor, you would expect to get 3 times as much of the isomerwith the chlorine on the end. There are 6 hydrogens that could get replaced on the endcarbon atoms compared with only 2 in the middle. In fact, you get about the same amount of each of the two isomers. If you use bromine instead of chlorine, the great majority of the product is where thebromine is attached to the centre carbon atom. The reasons for this are beyond UK A level chemistry.

Cycloalkanes

The reactions of the cycloalkanes are generally just the same as the alkanes, with theexception of the very small ones - particularly cyclopropane.







The extra reactivity of cyclopropane In the presence of UV light, cyclopropane will undergo substitution reactions withchlorine or bromine just like a non-cyclic alkane. However, it also has the ability to reactin the dark. In the absence of UV light, cyclopropane can undergo addition reactions in which thering is broken. For example, with bromine, cyclopropane gives 1,3-dibromopropane.

This can still happen in the presence of light - but you will get substitution reactions aswell. The ring is broken because cyclopropane suffers badly from ring strain . The bondangles in the ring are 60° rather than the normal value of about 109.5° when the carbonmakes four single bonds. The overlap between the atomic orbitals in forming the carbon-carbon bonds is lessgood than it is normally, and there is considerable repulsion between the bonding pairs.The system becomes more stable if the ring is broken.

CRACKING ALKANES

This page describes what cracking is, and the differences between catalytic crackingand thermal cracking used in the petrochemical industry.

Cracking

What is cracking? Cracking is the name given to breaking up large hydrocarbon molecules into smallerand more useful bits. This is achieved by using high pressures and temperatureswithout a catalyst, or lower temperatures and pressures in the presence of a catalyst. The source of the large hydrocarbon molecules is often the naphtha fraction or the gasoil fraction from the fractional distillation of crude oil (petroleum). These fractions areobtained from the distillation process as liquids, but are re-vaporised before cracking.



There isn't any single unique reaction happening in the cracker. The hydrocarbonmolecules are broken up in a fairly random way to produce mixtures of smallerhydrocarbons, some of which have carbon-carbon double bonds. One possible reactioninvolving the hydrocarbon C15H32 might be:

Or, showing more clearly what happens to the various atoms and bonds:

This is only one way in which this particular molecule might break up. The ethene andpropene are important materials for making plastics or producing other organicchemicals. The octane is one of the molecules found in petrol (gasoline).

Catalytic cracking Modern cracking uses zeolites as the catalyst. These are complex aluminosilicates,and are large lattices of aluminium, silicon and oxygen atoms carrying a negativecharge. They are, of course, associated with positive ions such as sodium ions. Youmay have come across a zeolite if you know about ion exchange resins used in watersofteners. The alkane is brought into contact with the catalyst at a temperature of about 500°C and

moderately low pressures. The zeolites used in catalytic cracking are chosen to give high percentages ofhydrocarbons with between 5 and 10 carbon atoms - particularly useful for petrol(gasoline). It also produces high proportions of branched alkanes and aromatichydrocarbons like benzene.



For UK A level (and equivalent) purposes, you aren't expected to know how the catalystworks, but you may be expected to know that it involves an ionic intermediate. Note: You should check your syllabus to find out exactly what you need to know. If you are studying aUK-based syllabus and haven't got one, follow this link. Use the BACK button on your browser to return quickly to this page.

The zeolite catalyst has sites which can remove a hydrogen from an alkane togetherwith the two electrons which bound it to the carbon. That leaves the carbon atom with apositive charge. Ions like this are called carbonium ions (or carbocations).Reorganisation of these leads to the various products of the reaction.

Note: If you are interested in other examples of catalysis in the petrochemical industry, you should followthis link. It will lead you to information on reforming and isomerisation (as well as a repeat of what youhave just read about catalytic cracking). Use the BACK button on your browser if you want to return quickly to this page.

Thermal cracking

In thermal cracking, high temperatures (typically in the range of 450°C to 750°C) andpressures (up to about 70 atmospheres) are used to break the large hydrocarbons intosmaller ones. Thermal cracking gives mixtures of products containing high proportionsof hydrocarbons with double bonds - alkenes.

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Warning! This is a gross oversimplification, and is written to satisfy the needs of one of the UK A levelExam Boards (AQA). In fact, there are several versions of thermal cracking designed to produce differentmixtures of products. These use completely different sets of conditions. If you need to know about thermal cracking in detail, a Google search on thermal cracking will throw uplots of useful leads. Be careful to go to industry (or similarly reliable) sources. You will find a Google

search box at the bottom of the Main Menu (link below). Remember to search the whole web rather thanChemguide otherwise you will just end up back here again!

Thermal cracking doesn't go via ionic intermediates like catalytic cracking. Instead,carbon-carbon bonds are broken so that each carbon atom ends up with a singleelectron. In other words, free radicals are formed.

Reactions of the free radicals lead to the various products.

Documents

Ntroducing Alkanes and Cycloalkanes