I know, I know. The title is very vague. But to be honest this section is very vague, it has all the gibberish required for F322 but, seriously it's, well, basic. To start of with:
Because why not??
First of all we have some definitions, which you may not necessarily need to remember but you do need to understand them.
EMPIRICAL FORMULA: The simplest whole number ratio of atoms of each element present in each element.
As it says in the name, remember it's a ratio. So you need to find the moles of each stuff and divide by the smallest to get the simplest WHOLE number ratio. Simple!
MOLECULAR FORMULA: Actual number of atoms of each element in a molecule.
GENERAL FORMULA: Simplest algebraic formula of a member of a homologous series.
That sounds depressing, doesn't it?? It only means a general formula for any compound of a specific type of molecule. E.g. An alkane, alkene, alcohol. Each of these is a homologous series, meaning each member of that group has similar properties (alkenes have double bonds, alcohols have OH group).
STRUCTURAL FORMULA: The arrangement of atoms in a molecule.
Write it out as it looks. E.g. Propane: CH3CH2CH3
DISPLAYED FORMULA: Relative positioning of atoms and bonds between them.
This one is how it actually looks. So its a drawing - you display drawings don't you?? Structural you write what it looks like, displayed you draw what it looks like.
SKELETAL FORMULA: Simplified organic formula, without showing hydrogen. Leaving just carbon skeleton and associated functional groups.
HOMOLOGOUS SERIES: A series of organic compounds having the same functional group, but with each successive member differing by CH2.
Alright, all this means is that they have the same general formula, same functional group but as you go along the group of that series each member (of that group) has an extra CH2.
FUNCTIONAL GROUP: A group of atoms responsible for the characteristic reactions of a compound.
Easy enough??
Here are the first ten member of the alkanes homologous series:
- Methane
- Ethane
- Propane
- Butane
- Pentane
- Hexane
- Heptane
- Octane
- Nonane
- Decane
Akenes end with the suffix "ene".
The next part is a pain. A pain in the backside, at least for me it is. You need to be able to identify, draw and name different types of isomerisms.
Okay. So there are two main types of isomerisms:
- Structural isomerism: Same molecular formula but different structural formulae.
- Stereoisomerism: Same structural formulae but with different arrangement in space.
Structural isomerism (as in the name) has different structural formula. Remember structural formula is the one where you write what you see. So this means that the number of C, O, H, etc. is the same but the OH group (for example) could be attached to a different carbon. This is the easy one, just think about where all the functional groups are and voila!
Stereoisomerism is the tricky one. Here we have the structural formula the same. So none of the groups are moved about. This means that there MUST be a double bond present, because this prevents the molecule from twisting in the air (and being a nuisance) and therefore a functional group could be on the same carbon (structure the same) but on the top OR bottom of the double bond! Got it??
Then we have an example of stereoisomerism, which is E/Z isomerism (that bad guy - yes).
As I said a double bond has restricted rotation. Now for E/Z isomerism two different groups have to be attached to each carbon atom on the double bond.
- E isomerism is when the two (same) groups are on opposite sides.
- Z isomerism is when the two (same) groups are on the same side.
- Cis isomerism is when the two other (non hydrogen) groups are on the same side of the double bond.
- Trans isomerism is when the two other (non hydrogen) groups are on the different sides of the double bond. Think of trans as being changing gender... so different.
Next up we have different types of covalent bond fission ( breaking of covalent bonds in different way).
During HOMOLYTIC fission the two products have the same charge, they are two radicals as the shared pair (the covalent bond) is split equally so each atom gets one electron each.
During HETEROLYTIC fission the two products have different charges. A cation (positive ion) and anion (negative ion) are formed. This is because one of the two atoms is greedy (more electronegative) and steals both electrons from the covalent bond. This type of fission happens during electrophilic addition (about that later).
We can use CURLY arrows to show where the pair of electrons (from covalent bond) have moved from and to.
When drawing these diagrams, think....
- Which is more electronegative?
- Then you'll know if it's hetero/homolytic fission.
- Next you know that cations are electrophiles (attracted to electrons/ or electron acceptors) so they will attack any double bonds of a compound and steal on electron to become neutral and another one to form a covalent bond.
- This leaves that carbon a cation (its extra electron was stolen - the one from double bond).
- This means the left over anion will then attack the cation carbon and donate it's electrons, one to make carbon neutral so the anion becomes neutral too, and one to form a covalent bond.
Ta dah!
Worst for last. You really should keep in mind percentage yield and atom economy, and for crying out loud! Do understand them too, memorising isn't everything.
In real life situations, it is not always possible to get the calculated amount of product in a reaction:
- Reversible reactions may not go to completion
- Some product may be lost when it is removed from the reaction mixture
- Some of the reactants may react in an unexpected way
Percentage Yield = Actual mass of product obtained
------------------------------------- * 100
Maximum theoretical mass of product
So in short percentage yield shows you how efficient that reaction is, how close you got to what you wanted to get.
Atom economy is very similar, and that what gives me a headache. It looks at how much of all the products is what you really want. Ugh! I'll explain in a bit.
Atom economy = Molecular mass of the desired product
------------------------------------- *100
Total molecular mass of products
Now... You could have a high percentage yield (as you get nearly the same mass of - I don't know! butanol, for example - as you expected to get) but the atom economy is low as there are many waste products produced along with butanol (e.g. carbon dioxide and water).
To clear up. % yield looks at the what you wanted to get and actually do. And atom economy creates a ratio of what you actually got over all products - so how efficient the reaction is. You do not want a low atom economy as it wastes raw resources.
Phew! That's it. Hope you got that, it took me a LOOOOONG white to get my head round it.
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