Intro: Understanding Organic Chemistry
The Aim of this instructable is to give a light overview of Organic chemistry including; Formulae, structure and identification to aid anyone who is interested in or is taking this subject.
Organic Chemistry is a subsect of chemistry which involves organic substances and their properties, structures and compositions. The substances contained in the subject can have Hydrogen, Oxygen, Nitrogen, Sulfur, the Halogens and a variety of other molecules contained in their molecular formulae.
It is important to distinguish between Organic and Inorganic chemistry as there are carbon containing substances that are regarded as "inorganic", such chemicals and ions include;
- Carbonates (CO3 [2-])
- Hydrogen Carbonates (HCO3 [-])
- Oxides of Carbon (CO, CO2)
- Cyanos and nitriles (CN [-])
Apart from these few (plus a few more not listed) most other carbon containing compounds are organic.
By the end of this instructable you should be able to name and identify the molecule shown below
Step 1: Hydrocarbons
Hydrocarbons are a set of organic compounds which contain only Hydrogen and Carbon (as the name implies). This set of chemicals are very important in understanding organic chemistry and come in a variety of different forms.
In inorganic chemisty the combination of 2 different element generally produces only 1, 2 or 3 different molecules but in organic chemistry the combination of just Carbon and Hydrogen can produce MILLIONS of different molecules!
For ALL hydrocarbons there is a general molecular formula this is;
"CxHy" where x and y are the amounts of carbon and hydrogen respectively
Another rule all hydrocarbons obey is that any carbon atom must have four bonds to fill its outer shell (it has four valence electrons) this can be done by using Hydrogen atoms or by using double or even triple bonds between cabon atoms.
The simple fact that there are so many different hydrocarbons means that a standardised form of identifying them is necessary, this is called the IUPAC (International Union of Pure and Applied Chemistry) nomenclature; (Refer to image one)
Step 2: Aliphatic HCs
So what are aliphatic hydrocarbons?
Aliphatic refers to the fact that there are no aromatic rings, put simply they all have straight chains.
There are three subsects of aliphatic hydrocarbons, these include;
- Alkenes and
Alkanes have the molecular formula of "CnH2n+2" which means for every carbon molecule there are two hydrogens plus two more. For example when there is one Carbon there are four hydrogens, this molecule is known as "Methane"
All Alkanes are "Saturated" which means that the bonds between all of the Carbon atoms are single (using only 2 electrons) this is represented by a line between them. Most people have heard of the term "saturated" namely on brands of cooking oil which include unsaturated, monounsaturated and polyunsaturated. In this case "unsaturated" refers to compounds that contain one (mono) or more (poly) double or triple bonds (shown as 2 or 3 lines respectively).
So now we know the basics lets see the first molecule measured by it having only one carbon atom; (see image one) This moleculeis called methane with a prefix of Meth- (meaning one carbon) and a suffix of -ane (meaning it is an alkane)
The next image (2) is of ethane the next alkane, and after that in image 3 is propane as you may have noticed because they are both alkanes the suffix hasn't changed but as the number of carbon atoms increases the prefix changes.
This is a list of the prefixes representing the number of carbon atoms there are in a hydrocarbon;
Step 3: Alkenes and Alkynes
Alkenes are a set of unsaturated compounds which contain one double bonded carbon atom.
The molecular formula for all alkenes is CnH2n where put simply the number of hydrogen atoms is double that of the carbon atoms. The first alkene is not methene as we would expect but it is ethene (1), this is because it must have a double bond and this requires two or more carbon atoms. As with the alkanes they follow the same set of prefixes so the next in the series would be propene (2).
But what happens if we go to the next one butene? Well we have two places to put the double bond, one in the centre and one near the end! This is called structural isomerism and because of this we have two structural isomers of butene.
To show where the double bond (or triple in an alkyne) is, you number the longest chain of carbon atoms containing the double bond while making sure the double bond is "touching the smallest number". For image number three the lowest number the double bond touches is 2 so we would write it as "but-2-ene" (be sure to put the "-" between all numbers and letters)
Alkynes are basically alkenes but the double bond is a triple bond with an accepted molecular formula of CnHn-2, which is two less than an alkene (noticing the trend?)
Image number (4) shows what would be called "Ethyne" but is commonly known as "acetylene"
Step 4: Branches on an Aliphatic HC
This segment tries to explain what happens if there is one or more "branches" coming off of a straight chain hydrocarbon, how to draw them and how to identify them.Branches can come off of any aliphatic hydrocarbons such as Alkane, Alke... (you know the rest!). The most simple branch that comes off of a straight chain hydrocarbon belongs to the Alkyne group and if you can remember back to the prefixes it is "Methyl". For an alkane to have a distinguishable branch it must be coming off of the longest carbon chain for example; (1) 2-Methylpropane which has a methyl branch on its second carbon on the longest chain. It is important to remember that the longest chain isn't just the obvious, the longest chain is 3 but this is not necessarily left to right!
You could create the chain from the left carbon to the middle carbon to the bottom carbon!
Just be wary of this when you ever do any complex questions.
A branch is not just limited to methyl it can also include ethyl, propyl and so on and so forth ad infinitum.
Step 5: Aromatic HC
Aromatic hydrocarbons are cyclic in nature meaning that both ends meat up and bond to form a round shape. Aromatics have a molecular formula of CnHn, it is important to understand why this isnt CnH2n where each carbon would bond to the two neighbouring carbons and to two hydrogens. The reason it is CnHn is because each carbon atom releases an electron into the centre of the ring, this can also be shown as alternating single and double bonds. But either way the effect is that each carbon only has one hydrogen or free single bond. The attached image is that of "Toleune" Which is the chemical; CycloHexane with a methyl branch. Confusingly CycloHexane is commonly known as Benzene (AKA Benzene ring/s)
BRANCHING OF AROMATIC HYDROCARBONS
As seen in the image cyclic hydrocarbons branch in a similar way to aliphatic HCs and have the same numbering system although the double bond in an cycloalkene is always one.
Also you could have a massive chain coming off of the cyclic part without it influencing the "end" name for example; decthylcyclopropane
Step 6: Alicyclic HCs
Alicyclic HCs are very similar to aromatic HCs the difference being that alicyclic compounds don't have the electron sharing '"ring" in the centre of the molecule so rather than a molecular formula of CnHn they have a formula of;
- CnH2n for Alkanes
- CnH2n-2 for Alkenes
- CnH2n-4 for Alynes
The reason why they have 2 less hydrogen atoms in their formula is because the ends are tied to each other using up 2 single bonds!
The attached images are of the different Alicyclic groups in a cyclic but(suffix), the first being; cyclic propane the second; cyclic propene and the third; cyclic propyl.
Step 7: Identifying an Aliphatic HC
STEP # ONE
Identify the longest continuous carbon cain containing the double bond (if it is an Alkene). This is the "parent" chaing. It is given the stem name associated with the number of carbon atoms in this chain with the suffix added by the type of Aliphatic HC.
In this example the double bond is on the second carbon and there are 7 carbon atoms in the stem chain therefore the ending will be called; "Hept-2-ene"
STEP # TWO
Name the branches and remember their carbon number, if there are multiple branches which are the same there is a different writing style. In the example there are two methyl branches so instead of writing; 5-Methyl-6-Methyl we write "5,6-Dimethyl"
STEP # THREE
Add the two names together and check over to see if your answer is plausable;
Also you would add the structural isomerism; TRANS (see next page) to give;
Step 8: Geometric Isomerism
Geometric isomerism is a way of differenciating Hydrocarbons by the positioning of their branches and hydrogen about a double bond. If you notice image number one you will see that each hydrogen is on the opposite side of the molecule this geometric isomer is TRANS.
The opposite geometric isomer is (you guessed it) CIS which means the two hydrogens are on the same side.
TRANS = opposite
CIS = same
Step 9: Conclusion and Questions
This may be the end of the instructable but the subject goes on and on, if I had the time I would go into Halogenation, substitution and addition reactions and whatnot but this is sufficient for you to get a general idea of what Organic chemistry encompasses.
I hope you found this helpful and for more information just browse Wikipedia.org which has a large and succinct database on organic chem.