Tip:
Highlight text to annotate it
X
[ Music ]
>> Hi guys, I'm Jackie.
I'm a postgrad student here at Curtin.
We hear you've been having some problems with organic chemistry
in regards to structural isomers
of the alkanes and branch chains.
So, we're going to be here talking
to Mauro Mocerino who's an expert in that kind of stuff
and hopefully that'll sort some problems out for you.
>> Hello and welcome to Curtin University.
My name is Mauro Mocerino and I'm a lecturer here
at the university in Department of Chemistry.
Today I would like to talk to you about organic chemistry
and give you a few pointers on how organic chemistry works.
Organic chemistry is a very large field of chemistry.
There are well over 10 million compounds known
and so the variety and complexity is quite large.
However, there are fundamental building blocks and rules
that you need to know and once you know that,
you can go a long way.
I'd like to start this tutorial by considering the alkanes.
They are the building blocks of organic chemistry
and for your studies, you will need to know the first 8 alkanes
and the simplest of the alkanes is methane, with 1 carbon,
a model of methane here is shown
where the black bit represents the carbon
and these things are the hydrogen atoms.
Methane is the simplest.
The next one down is ethane with 2 carbons, propane with 3,
butane with 4 and then pentane, hexane, heptane
and octane as the last 4.
Now, from experience, it seems that most students seem
to have troubles remembering the order.
A simple way of remembering it might be for the first 4,
monkeys eat peeled bananas, M standing for methane,
eat ethane, peel is the propane and the bananas for butane.
This is the bit that causes most students problems.
The next bit, most students are comfortable with their geometry
and recognise pentagons, hexagons, heptagons and octagons
and they correspond to the four other alkanes.
Okay, now I would like to consider the functional groups
in organic chemistry.
The functional groups are the reactive portions
of organic molecules.
Now, the alkanes can be thought of as the framework
that holds the functional groups together.
There are a series of important functional groups
that you will study in your Year 12 chemistry.
One of them is alcohols.
And the alcohols have the general formula a carbon bonded
to an oxygen bonded to a hydrogen.
And given that carbon has 4 bonds,
there are 3 other groups attached there.
They can be hydrogens or they can be other carbon groups,
so they are alcohols.
The alcohols are classified as primary alcohols
and a primary alcohol has 2 hydrogens and a carbon attached.
A secondary alcohol will have 2 carbons and 1 hydrogen
and a tertiary alcohol, we have 3 carbons attached
to the carbon with the OH group.
Another important functional group we would
like to consider are aldehydes
and these compounds have a carbon-carbon
or carbon-oxygen double bond, a hydrogen and another group here
which can either be a carbon or a hydrogen.
For a simple example, let's make that a methyl group
so we have ethanol or commonly known as acid aldehyde.
That is an aldehyde.
The ketones are very close cousins to aldehydes.
We have also a carbonyl group but here in ketones,
we do not have a hydrogen.
They both must be carbon groups
and a simple example here would be propranone, sometimes known
as acetone, and that's a common ingredient
in nail polish remover.
Now, if we're moving along, a,
a fourth function group we might consider are carboxylic acids
and in a carboxylic acid,
again we have a carbon-oxygen double bond.
This time we have an OH group attached to the, the carbon
and some other group here.
It can be a hydrogen.
Methanoic acid would be that compound or it can be a carbon
and here we have an example of acetic acid or ethanoic acid.
The final 2 functional groups I want
to discuss here today are esters and esters are derivatives
of carboxylic acids where you still have the carbonyl group
and the carbon-oxygen but instead of a hydrogen,
we have another carbon group.
So, a, a simple example would be an ester of ethanol
where ethanol is the alcohol portion,
reacts with the carboxylic acid portion of acetic acid
or ethanoic acid and you form ethyl acetate,
or ethyl ethanoate.
This is a common ingredient also in nail polish removers.
The final aim, functional group we want
to talk about are the amines.
Now, you may have heard in your studies of things
like amino acids and the amine part is present along
with the acid part in the molecules.
And the key feature of amines is that it's like ammonia except
that one of the hydrogens
of ammonia has been replaced with a carbon.
So, a simple example would be methyl amine or methanamine,
as an example of a, an amine.
Now. Amines are derivatives of ammonia where 1
of the hydrogens has been replaced with a carbon group.
You can have more than 1 hydrogen replaced
but that's something you'll learn about next year
when you do your university studies.
Okay. These are the key function groups.
These are the molecule, these are the portions
of the molecules that give organic compounds
their reactivity.
The structure of organic compounds is also an
important issue.
One of the compounds that you, one of the class
of the compounds that you will be working with are alkynes.
Alkynes are, can be described as a functional group and they,
they're compounds where we have a carbon-carbon double bond.
So if you have a carbon-carbon double bond, you have an alkyne.
Alkynes are a little different to alkanes because in an alkane,
and if I take a model of, say, for example butane here,
in butane, or alkanes, any alkanes, we have rotation
around the carbon-carbon bonds.
So we have complete rotation around all these bonds
and so it can take lots of shapes.
They're not fixed in one shape.
With an alkyne, if we take, consider an alkyne
with 4 carbons like a butane, they have a double bond in them
and the double bond means
that you can't rotate around that bond.
So if you consider the double bond,
and we have the compounds butane,
we can have the methyl groups opposite
and when they're opposite, we refer to that as trans
or the methyl groups can be on the same side.
And we'd refer to that as cis.
Now here I have a model of cis-butene
and a model of trans-butene.
The two compounds are different.
You cannot convert one to the other.
When we were looking at butane, we can rotate that so
that looks a little bit like cis-butane, cis-butene,
but because we have a single bond,
we have rotation around the bond.
An important reason for drawing the alkynes in this sort
of way is that it's clear,
clearly shown whether you've got, for example,
cis-butene and trans-butene.
Why do we want to show the difference?
They are different compounds.
They have different, different physical properties
and different chemical properties.
So, if you drew your alkyne in the unpreferred manner,
we can't tell which butane are we talking about.
But if you use this manner, you're clearly indicating sis
or trans and so you know which compound you are looking at.
Now. There are a, to, to round off this tutorial,
just as an exercise of the sort of questions you may be asked
and how you go about solving the question.
If you're asked to draw the structure of a compound
like 2-methyl, 2-pentane,
the first thing you would do is look at the end of the 9.
The end of the 9 tells you you have an alkyne.
So, we draw out alkyne.
You're told that it's a pentane, so we know there's 5 carbons
and we know that the alkyne starts at carbon 2,
so we can have carbon 1 here, 2, 3, 4, 5.
We now have our 5 carbons.
Now the 9th doesn't say anything about cis and trans,
but you're told that on carbon 2 we also have a methyl group.
So now we've got CH3, CH3, a hydrogen there,
that becomes a CH2, and this stays a CH3
because all carbon always has 4 bonds, we just fill
in the extra hydrogens you need to make the 4 bonds.
So now we've got our pentane, 2-pentane, and its 2-methyl.
We can see here immediately that we don't talk about cis
and trans because if we swap these things around,
we'll have the same compound.
So drawing it like this, you won't get confused.
And when asked to do a, a, problem of this form,
always start at the end.
Find out what functional group you're working with.
Then the next thing you do is find
out how many carbons you need to be dealing with, draw them out,
and then look at anything else
that might be attached to the carbon chain.
A second question that's commonly used
and helps you understand the functionalities or the bonding
within alkanes is a question
of the form is draw all isomers of C6H14.
So they're isomers of hexane.
Now, there are many ways of doing this.
One of the ways I would recommend is you start
by drawing out obviously 6 in a row, 1, 2, 3, 4, 5, 6,
then we can have 5, I'm taking a shortcut here and not putting
in the hydrogens, you can add them in later.
If we have 5 in a row, we can put a, a carbon there
or swap you with my drawing, a carbon there.
So we can have 1, 2, 3, 4, 5.
So this would be 2-methylpentane and this is 1,2,3-methylpentane.
If we move along further and put a methyl group at this position,
that's the same as this one.
So 2-methylpentane, we could count from this end.
If they had the methyl there, it would be 1,
2-methyl pentane, so they're the same.
So there are 3 isomers we can do.
The next step down you would look at 4 carbons in a row
and now we've got to put 2, 2, 2 other carbons on this chain.
We can put them both on the same carbon
or we can put them on separate carbons.
And now we have all 5 isomers.
We can't put a 2 carbon chain on if, for example,
I decided to put 2 carbon chain on,
now my longest chain is a 5-carbon chain.
So this structure here that I've drawn is actually the same
as this one here, 3-methyl pentane.
So. These are the 5 possible isomers that we, we can have.
And this is a common question
that you may encounter in your studies.
It's important to practice drawing structures
because organic chemistry seems fairly complicated
because the molecules can be large.
However, the basic rules
of putting together, it's like Legos.
If you know how to use Legos, you can do organic chemistry,
just putting the pieces together.
>> Hi guys.
Hopefully that helped you out.
If you still have any issues with chemistry in general,
just go to the link that we've given you
or ask the Chem teacher.
They are probably your best resource.
Good luck with exams
and hopefully we'll see you here at Curtin.