Content

Chains and Rings

Note: From 2008, there will be greater emphasis on "green" Chemistry and environmental issues. The module will be called 'Chains, Energy and Resources'.

Foundation's now out of the way... they say this one's easier. Well, organic chemistry has a lot of "just learn it" in it, and sadly, you really have to "just learn it."

Okay, so what exactly is organic chemistry? Well, organic compounds are those composed of the elements: C, H, P, S, N and O. They make up a huge amount - around 15 million different compounds all in all. If you look to the periodic table for the remaining elements - the rest of them only make up about 50,000, so organic is pretty major...

The organic compounds fall into homologous groups with similar properties. The ones we look at are: alkanes, alkenes, alcohols and haloalkanes.

Alkanes

The first six alkanes that you MUST know:

  1. Methane - CH3
  2. Ethane - C2H6
  3. Propane - C3H8
  4. Butane - C4H10
  5. Pentane - C5H12
  6. Hexane - C6H14
  • Alkanes are hydrocarbons. This means that they contain hydrogen and oxygen ONLY.
  • They are saturated molecules, which means nothing can be added to them.
  • Their general formula is CnH2n+2 (There are twice as many hydrogens as carbons, plus two more.)
  • They all end in -ane. Like alkane.
  • All bond angles are 109.5o, so they're tetrahedral.
  • C-C and C-H bonds are non-polar and this makes them pretty unreactive.
  • Molecules are held together by Van der Waal forces. Therefore, small molecules are gases, bigger molecules are liquids. The number of branches in an isomer changes the Van der Waals - less contact area, weaker Van der Waals.

Naming Alkanes

We've already said that alkanes all end in -ane. There's more to their nomenclature though. Nomenclature is just a posh word for naming compounds. It's pretty important though - two compounds with the same atoms might have a completely different structure, and thus very different properties. Naming let's us know exactly what we have.

In alkanes, you can add more branches to the compound in a process called isomerisation. Compare the following two alkanes, both with the molecular formula: C4H10:

They look very different, just drawn out and so can't have the same name. To name them, we follow a few rules.

  1. Identify the longest carbon chain and write it down. (3 carbons=propane, 4 carbons=butane, etc).
  2. If a CH3 is "branching" from the main chain, we call it a methyl. Occasionally, you get CH2CH3 branching off and this is called an ethyl. Write methyl/ethyl come before the name from step 1.
  3. If there is more than one methyl/ethyl, we add 'di' or 'tri' before.
  4. Number each carbon on the chain and find out which ones the methyl/ethyl are connected to. Number them so that you get the lowest number. Then add these numbers before the previous name, separating numbers by commas and separating from the rest of the name with a "-".

Okay, to show how this works, look at the previous examples:

This has a chain of 4 carbons, and so it is based on butane.

There are no branches, and so it's just butane.

  1. The longest chain of carbons is 3, and so it's based on propane.
  2. There is a methyl, and so it's methylpropane.
  3. The methyl is on the second carbon, so it's 2-methylpropane.

Butane and 2-methylpropane are both structural isomers. Isomers have the same molecular formula, but a different structural display formula.

Structural isomerisation: Same molecular formula, but a different structure, caused by carbon chain differences or a different positioning of the functional group.

Stereoisomerisation: The same structural formula, but in 3D, a different arrangement of atoms. See cis and trans.

Look at the following:

  1. This has a carbon chain of 5. Therefore, it's pentane.
  2. It has methyls, so it's methylpentane.
  3. There are 2 methyls, so it's dimethylpentane.
  4. Numbering the carbon chain from left to right, we see branches coming from the 2nd carbon and the 3rd carbon. Therefore it's: 2,3-dimethylpentane.

Easy!

Burning Alkanes

Alkanes are great because they burn to produce lots of energy (exothermic reactions) and with little residue (soot). Complete combustion of an alkane produces carbon dioxide and water:

2CH3 + 3O2 ---> 2CO2 + 3H2O

Why would you want to add more branches to an alkane? Straight chain alkanes can ignite prematurely and cause knocking due to their low octane number. Adding more branching lessens knocking and allows them to burn more smoothly and steadily. Isomerisation requires a catalyst.

Another way to increase octane number and make them burn better is called reforming. In this, a catalyst and a high temperature are used on a straight chain alkane. The alkane loses a hydrogen to form a cyclic alkane and then an arene.

For example:

Heptane ---> methylcyclohexane ---> methylbenzene

---> --->

Fractional Distillation

You might recall this from GCSE... it's back to haunt you, along with cracking! This time we don't need to know a great depth, though.

Basically, a fractionating column separates crude oil (the mix of oils) into fractions, depending on their boiling points. The column is heated at the bottom so all the crude oil evaporates. It's passed into the column and condenses as it rises.

The heavier fractions have higher boiling points (due to more Van der Waals) and so condense first, coming off the column lower down. The lighter alkanes come off higher up.

For more info, click here.

Cracking

Long chain carbon alkanes aren't very useful. Because of all those electrons and surface area they have lots of Van der Waals, meaning they aren't good as fuels.

Fortunately, we can crack them. Only 20% of petrol comes from fractional distillation, the rest comes from cracking.

You probably remember in from GCSE - using a hot catalyst to split a long alkane into a shorter one and an alkene (or hydrogen).

For example:


Pentane -----> Propane + Ethene

Free Radical Substitution

Both C-H and C-C bonds are similar in their electronegativities, meaning there is no polarity in their bonding. (See intermolecular bonding) This makes them pretty inert substances; in other words, they aren't readily attacked by common reagents like water, acids or alkalis, which are polar.

Alkanes will react with the halogens, though, in a process called free radical substitution. For example:

CH4(g) + Cl2(g) -----> CH3Cl(g) + HCl(g)

This requires UV light and involves a mechanism with three stages: initiation, propagation and termination. You need to learn it, too.

Initiation

Blasting the Cl2 with UV light causes photo dissociation - in English, this means the bond between the 2 Cls breaks. Two chlorine atoms are formed, each with an unpaired electron. This is called a free radical. The process is called homolytic fission ("homo" meaning the same, so both Cl are splitting equally). Note that the unpaired electron DOESN'T mean they're ions - free radicals are neutral.

Cl2 -----> 2Cl.

A free radical is indicated by a . They are very reactive and this leads to the next step...

Propogation

The Cl free radical reacts with the CH4 to produce hydrogen chloride (HCl) and a methyl free radical (CH3.)

Cl. + CH4 -----> HCl + CH3.

The methyl radical then reacts with Cl2 to produce chloromethane and a new chlorine radical.

CH3. + Cl2 -----> CH3Cl + Cl.

The propagation stage now repeats itself and continues until the reagents have been used up.

Termination

Finally, when the reagents are used up (methane and undissociated chlorine molecules) the reaction is terminated. There are various ways this can happen as shown below:

2Cl. -----> Cl2

CH3.+ Cl. -----> CH3Cl

2CH3. -----> C2H6

Free radical substitution is very difficult to control and often leads to a mix of products, such as di or tri chloromethanes.

And that's just about everything you need to know about alkanes!

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Alkenes

Another functional group, the alkenes are defined by their C=C functional group.

  • Because of the carbon double bond, they are unsaturated. This means more chemicals can be added to them easily.
  • Their general formula is CnH2n
  • The C=C bond contains a sigma (σ) bond and a pi (π) bond:

There is one pair of electrons in the σ bond. The second pair of electrons moves between the top π bond and the bottom one. The π orbitals are formed by the 'p' orbitals that those electrons are found it. Check out ionisation energies if you can't remember what 'p' orbitals look like.

  • The angle between the atoms is 120o, meaning the shape is trigonal plainer.

Naming

The general name of molecules ends in -ene. The beginning bit depends on the number of carbons, and is identical to alkanes.

For example, the first six alkenes are:

  1. Ethene, C2H4
  2. Propene, C3H6
  3. Butene, C4H8
  4. Pentene, C5H10
  5. Hexene, C6H12
  6. Heptene, C7H14

Notice there's no methene -- it's impossible, because there has to be a C=C.

Branching can apply as with alkanes, so remember, you name the longest carbon chain. The rules are identical to with alkanes.

For alkenes bigger than propene, you need to identify where the C=C bond is. The following two alkenes of butene are different:

Naming them is similar to giving names to branches. You number the carbons to give the smallest number, and then use the carbon the double bond is attached to. This number is written before the -ene with dashes surrounding it.

Above, the left is but-1-ene and the one to the right is but-2-ene.

Cis and Trans

Because of the C=C double bond, alkenes produce geometric isomers.

In alkanes, there was free rotation around the carbons, meaning the following are identical:

They're just rotations of each other.

This isn't the case with alkenes. Because of the C=C double bond, they produce geometric isomers. The double bond stops free rotation around the carbons, and so the following are different:


If the difference occurs on the same side of the C=C bond (like in the left example) the isomerisation is called cis-isomerisation. To the left is a cis-isomer of but-2-ene.

If the difference occurs on across the C=C double bond, (right example) the isomerisation is called trans. ('trans' is the Latin for 'across') To the right is a trans-isomer of but-2-ene.

Electrophilic Addition

Because the pi bonds produce high electron densities, positively charged ions are drawn towards them. The electronegativity can also cause some molecules to become polar (such as Br2). When ions or δ+ molecules are drawn towards them, the C=C is broken and the pi bond forms a covalent bond with the particle. The 'electron loving' molecules are called electrophiles and the reaction is called electrophilic addition.

Like free radical substitution, it has a mechanism, which you've just gotta know...

For example: C2H4 + Br2 -----> C2H4Br2

  • The type of fission here is heterolytic, which means it's unequal. Look at the bromine molecule: it's splitting, but unlike a free radical, one Br takes the two electrons becomes an ion.
  • The positive charge on the carbon in the second step is called a carbocation.
  • Other electrophiles include: H2, H2O, HBr.

Important electrophilic equations you need to know:

Reagent: HBr (formed from NaBr and H2SO4)

Conditions: Room Temp

It can produce two structural isomers of a haloalkane, as shown to the left.


OR

Reagent: H2O(g)

Catalyst: concentrated phosphoric acid

Conditions: 300oC / 70 atmos

This also produces two structural isomers of an alcohol, as can be seen below.


OR

Reagent: H2

Catalyst: Ni

Conditions: 400oC

This reaction is used in the food industry to convert polyunsaturated vegetable oil into margarine.


Reagent: Br2

Conditions: Dark

This is the classic test for alkenes. An alkene decolourises the bromine water, while an alkane does nothing.

Addition Polymerisation

"Poly-" means many, and that's what polymers are -- many monomers in a long chain. You'll remember them from GCSE, no doubt.

Alkenes undergo addition polymerisation under the presence of a Ziegler catalyst, high temperature and pressure to form polymers. It's simple enough: the double bonds open up and the electrons found in the pi-bond form sigma bonds with neighboring carbons from another compound.

There are two ways to draw it out, either with n and brackets, or by drawing a 4 carbon repeating unit (the more common way).

Or:

Polymers are really useful for all sorts of things like plastics, but there are problems with them...

  • Because of the non-polar bonds (C-H) or very strong polar bonds (C-F), they aren't biodegradable. They persist in the environment as litter.
  • When they're burned, they often produce toxic gases. Scientists are trying now to develop polymers that can be burned... and used as a fuel.
  • The best way to deal with them is to recycle them.
  • New processes are also being developed for cracking polymers back to alkenes.

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Alcohols
  • The next homologous series is alcohols, defined by the hydroxyl (-OH) functional group.
  • The general formula for an alcohol is CnH2n+1.
  • They are very soluble in water because they form hydrogen bonds:

OR

  • Because alcohols burn to produce CO2 and H2O like alkanes, they make good fuels.
    For example: C5H11OH + 7.5O2 -----> 5CO2 + 6H2O
  • Methanol is used in the manufacture of thermosetting plastics such as bakelite and in the manufacture of perspex.
  • Ethanolcan be used as a fuel for cars, as a solvent, in the preparation of esters or as a beverage.
  • They can be identified by a broad infrared spectroscopy.

Naming

  1. Find the longest carbon chain and name as you would an alkane. Methan- Ethan- Propan- etc.
  2. Like alkenes, the OH group needs to be identified in the name. Numbering to give the lowest number, give the number of the carbon the OH group is attached to.
  3. Finally add -ol to the end to show an alcohol.

For the following:

  1. The longest carbon chain is 4, therefore: butan-
  2. The OH is located on the second carbon.
  3. Then add -ol to make: Butan-2-ol

Main Reactions

Displacement of hydrogen occurs when the alcohol is reacted with sodium to give hydrogen and an alkoxide. Dehydration of an alcohol (below) requires a catalyst such as Al2O3 (pumice) or cH2SO4 and formed an alkene and water.
Substitution (above) replaces OH with Br or another halogen to form a haloalkane and water. HBr doesn't exist naturally and so needs to be formed from something like NaBr and H2SO4.

Esterification of an alcohol (left) requires a carboxylic acid, such as ethanoic acid and forms an ester and water.

Primary, Secondary and Tertiary Alcohols

As the subtitle indicates, there are three different types of alcohols: primary, secondary and tertiary. How do we distinguish between them? Well, by the number of hydrogens on the carbon that the OH group is attached to. 2 hydrogens means it's primary, 1 hydrogen means it's secondary and no hydrogens means it's tertiary.

Primary (2 H)
Secondary (1 H)
Tertiary (0 H)

We can test for the three types by oxidising them. The oxidising agent for the reaction is a mixture of sulphuric acid and potassium dichromate (VI) (H2SO4 + K2CrO7). The oxidising agent is represented with an [O]. When the alcohols are oxidised, the orange solution turns green.

Because of the hydrogens on the primary and secondary types, these oxidise.

Primary alcohols oxidise to form aldehydes:

CH3CH2OH + [O] ----> + H2O

Ethanol                       Ethanal
Alcohol                       Aldehyde

                  

If the aldehyde is not distilled off and reflux is allowed, the aldehyde can be oxidised further into carboxylic acid:

+ [O] ----->

Ethanal                              Ethanoic Acid
Aldehyde Carboxylic acid

Secondary alcohols only oxidise once and form ketones:

CH3CH(OH)CH3 + [O] ----> + H2O

Propanol                            Pronanone
Alcohol Ketone

Tertiary alcohols do not oxidise at all.

Once oxidised, we can also use infrared spectroscopy to distinguish between whether an alcohol was primary or secondary. See below.

Production of ethanol

Ethanol can be produced by the fermentation of sugars or industrially. Only the first is used as a beverage (so just learn that method!). The two methods are very different:

  Fermentation of sugars
(batch process)
Hydration of ethene
(continuous process)
Raw Materials sugar, yeast, nutrients, water

ethene + steam
concentrated phosphoric acid

Conditions 30oC, anaerobic conditions (no oxygen) 300oC / 70 atmos

Equation

C6H12O6 ---> 2C2H5OH(l) + 2CO2(g) C2H4(l) + H2O(g) ---> C2H5OH(l)

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Infrared Spectroscopy

This is useful for determining a compound's structure. Different bonds in a compound absorb different frequencies of the infrared spectrum. Using this, we can zap molecules with IR light and record the readings to see what bonds are present. Best of all, there are only three that you'll need in the exam, AND they're given in the formula booklet:

Functional Group Wave number (cm-1)
Hydroxyl group (-OH) in carboxylic acids (-COOH). Very broad between 2500-3300.
O-H hydrogen bonded in alcohols and phenols. Less broad between 3230-3550.
Carbonyl group (C=O) in aldehydes, ketones and carboxylic acids. 1680-1750.

What exactly does that MEAN? Well, in an exam, you might be given an infrared spectroscopy reading:

We can tell this is an alcohol by the infrared spectroscopy.

The hydroxyl group (highlighted by the red box) is broad and between 3230-3550.

We can tell this is either a aldehyde or a ketone by the infrared spectroscopy.

The carbonyl group (C=O) (highlighted by the blue box) is narrow and between 1680-1750.

If this further oxidises, we know we have an aldehyde; if not a ketone.

We can tell this is a carboxylic acid by the infrared spectroscopy.

It has both the carbonyl group (C=O) (highlighted by the green box) and the hydroxyl group (OH) found in carboxylic acids, which is very broad and between 2500-3300.

That's really all you need to know for infrared spectroscopy!

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Haloalkanes
  • Each haloalkane contains at least one carbon-halogen bond (abbreviated C-hal).
  • If they contain just one halogen and no C=C double bond, their general formula is CnH2n+1hal.
  • As usual, naming requires you to:
    1. Locate the carbons that the halogens are attached to and write these down, numbering to give the smallest number. e.g. 2,3-
    2. Name the halogens. If there are two flourines, it's 2,3-difloro
    3. Finally write the name of longest carbon chain alkane: methane, ethane, propane... etc. So: 2,3-diflorobutane would look like this:

Nucleophilic Substitution

The C-hal bond is polarised, which leaves the carbon atom electron deficient and open to attack by a nucleophile (electron pair donor, such as OH-). The halogen is easily substituted by a nucleophile, although the C-I bond is much easier to break than the C-Fl bond because the bond is weaker (due to electron shielding, lengthening the bond, etc.).

The mechanism that occurs is called nucleophilic substitution. For example, when bromoethane and OH- react, the following occurs:

It's that simple!

Bond structure also affects the speed with which the halogens are substituted. Like alcohols, there are three types of haloalkanes:

Primary Secondary Tertiary

1-bromobutane


2-bromobutane


2-bromo-
2-methylpropane

The ordering here goes:

  1. Tertiary
  2. Secondary
  3. Primary

To see the rate of reaction, simply experiment with AgNO3 (NO3 being a nucleophile). In the reactions, the following precipitates will be formed:

  • Cl-- Ag(aq) + Cl(aq) -----> AgCl(s) White precipitate formed, which is soluble in concentrated or dilute ammonia solution.
  • Br-- Ag(aq) + Br(aq) -----> AgBrl(s) Cream precipitate formed, which is soluble in dilute ammonia solution only.
  • I-- Ag(aq) + I(aq) -----> AgI(s) Yellow precipitate formed, which is insoluble in ammonia solution.

Using the same temperature (40oC) and structural haloalkanes, we can measure the rate of reaction by the speed with which the precipitates form. AgI will form first, followed by AgBr and finally AgCl will form.

Halogenoalkenes

These behave exactly like alkenes and can be used to make polymers. For example, chloroethene makes poly(chloroethene), commonly known as PVC:

Main Reactions

There are several nucleophilic substitutions that you should know:

Hot aqueous KOH:

C2H5Br + KOH -----> C2H5OH + KBr

Gentle reflux with KCN in alcohol:

C2H5Br + KCN -----> C2H5CN + KBr

Heat with ethanoic ammonia:

C2H5Br + NH3 ------> C2H5NH2 + HBr

Finally, if we heat with an ethanoic solution of KOH, we get an elimination reaction:

C2H5Br + KOH ------> CH2=CH2 + H2O + KBr

Uses

Before it was realised how bad CFCs (Chlorofluorocarbons) are for the environment, there were a number of different uses for them... due to the strength of the C-Cl and C-F bonds, causing them to be unreactive and stable in normal atmospheric conditions (unless in the presence of a nucleophile):

Use Properties
Refrigerant Unreactive - so they won't corrode machinery.
Volatile - will evaporate quickly.
Dry Cleaning Good solvent - will dissolve grease.
Aerosols Volatile - mix with other gases easily.
Fire extinguishers Unreactive - so do not burn easily.

Since the realisation that they were burning holes in the ozone layer, however, use of CFCs has been discontinued, and the holes are now finally shirnking...

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That's just about everything for Chains and Rings. Let me know if there's anything I've missed!