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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.
The first six alkanes that you MUST know:
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.
Okay, to show how this works, look at the previous examples:
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:
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:
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.
Heptane ---> methylcyclohexane ---> methylbenzene
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.
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).
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:
This requires UV light and involves a mechanism with three stages: initiation, propagation and termination. You need to learn it, too.
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.
A free radical is indicated by a . They are very reactive and this leads to the next step...
The Cl free radical reacts with the CH4 to produce hydrogen chloride (HCl) and a methyl free radical (CH3.)
The methyl radical then reacts with Cl2 to produce chloromethane and a new chlorine radical.
The propagation stage now repeats itself and continues until the reagents have been used up.
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:
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!
Another functional group, the alkenes are defined by their C=C functional group.
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 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:
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.
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.
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
Important electrophilic equations you need to know:
"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).
Polymers are really useful for all sorts of things like plastics, but there are problems with them...
For the following:
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.
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:
If the aldehyde is not distilled off and reflux is allowed, the aldehyde can be oxidised further into carboxylic acid:
Secondary alcohols only oxidise once and form ketones:
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:
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:
What exactly does that MEAN? Well, in an exam, you might be given an infrared spectroscopy reading:
That's really all you need to know for infrared spectroscopy!
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:
The ordering here goes:
To see the rate of reaction, simply experiment with AgNO3 (NO3 being a nucleophile). In the reactions, the following precipitates will be formed:
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.
These behave exactly like alkenes and can be used to make polymers. For example, chloroethene makes poly(chloroethene), commonly known as PVC:
There are several nucleophilic substitutions that you should know:
Hot aqueous KOH:
Gentle reflux with KCN in alcohol:
Heat with ethanoic ammonia:
Finally, if we heat with an ethanoic solution of KOH, we get an elimination reaction:
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):
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...
That's just about everything for Chains and Rings. Let me know if there's anything I've missed!