10.1 Fundamentals of Organic Chemistry

Syllabus

What does this mean?

Why is there so much Organic Chemistry?

Only Carbon atoms undergo catenation – join up in long chains or rings.

This gives an almost unlimited number of Carbon-containing compounds.

We classify them into similar groups called Homologous Series.

  • Learn the definitions:

o Homologous Series contains compounds with the same General Formula, the same functional group with successive members differing by –CH2- →

o Functional Groups are the reactive sites in a compound

Alkanes

Alkanes are un-reactive – they have no functional group.

C2H6 – is the Molecular Formula of Ethane (each element is represented only once)

Its Structural Formula is CH3CH3

The Displayed Formula shows all the atoms and the bonds.

The Full Structural Formula of Pentane is CH3CH2CH2CH2CH3

Its Condensed Structural Formula is CH3(CH2)3CH3

v Write Full Structural Formula and Condensed Structural Formula for Butane and Hexane

Structural Isomers

-are molecules with the same molecular formula but with a different arrangement of atoms.

eg Both these molecules are C5H12 but they are clearly not the same.

They have different structural formulae.

Question

Write structural formulae for the following molecules

Physical properties & general formulae of Homologous Series

Notice that boiling points and melting points increase with the length of the Alkane.

This is true of all homologous series since larger molecules always have stronger intermolecular forces

Notice also that every successive member of the series is CH2 larger than the previous one – this is also true of every Homologous Series.

There is a relationship of the number of Hydrogen atoms to the number of Carbon atoms.

For every C we have double the H’s plus two extra.

So the general formula is CnH2n+2

Question.

1. What is the general formula of the Homologous Series called Alkenes?

2. What is the general formula of the Homologous Series called Alcohols?

3. What is the general formula of the Homologous Series called Aldehydes?

4. What is the general formula of the Homologous Series called Ketones?

Skeletal Formulae

Displayed formulae are useful, but drawing a large one can be time consuming so Skeletal Formulae are often used.

A skeletal formula draws only C-C bonds, ignoring the atoms and assuming that (unless another atom is drawn) all Carbon atoms are saturated with Hydrogen atoms.

So, Pentane (below) becomes...

And an ethyl side-chain as....

Question

Draw the displayed formulae of the following molecules, and write its molecular and condensed structural formula

Nomenclature – Naming Compounds

The IB expect you to be able to name compounds with up to 6 Carbon atoms in a straight chain.

Beyond 4 Carbons the system is easy Pent = 5, Hex=6, Hept=7, Oct =8, Non-9 Dec=10 etc.

But for 1 -4 Carbon atoms you need to learn Meth, Eth, Prop, But.

You just have to learn these


Side chains.

Some Alkanes are branched.

Look for the longest chain of Carbons and name that.

Then count the carbons in the side chain. Side chains end with –yl.

However you look at this molecule, the longest chain contains 3 Carbon atoms.

So this is a PROPANE.

The side-chain has only 1 Carbon atom.

So this is METHYL PROPANE

The longest chain here is also PROPANE

But there are two methyl groups.

So it is DIMETHYLPROPANE

Numbers, commas and dashes

This molecule is 5 C’s long = Pentane

It has three methyl groups = Trimethylpentane

But this is also Trimethylpentane so how do we tell them apart?

We number the main chain from one end...

...and we call this molecule 2,3,4 - Trimethylpentane

Note commas between numbers, dashes between numbers and letters

But this molecule could be

2,2,4- Trimethylpentane

Or

2,4,4 – Trimethyl pentane

We use 2,2,4- Trimethylpentane because the numbers are lower.

Question.

Name the compound shown

Functional Groups

Saturated and Unsaturated compounds.

Alkanes are Hydrocarbons because they contain only Hydrogen & Carbon atoms.

They’re saturated because they contain only C-C single bonds.

Any molecule with a C=C double bond (or treble bond) is unsaturated.

The same IUPAC naming rules apply...

i) Is Ethane while (ii) is Ethene

Sometimes there may be more than one place the C=C bond can be so we number the lowest Carbon it touches...

And you might have more than 1 C=C bond

This would be But-1,3-diene.

Question.

Name the following compounds

Functional Groups.

Functional Groups are the reactive parts of molecules. Alkanes don’t have a functional group. Alkenes do – it’s the C=C bond because this is where reactions happen.

Alkynes – hydrocarbons containing C≡C bonds

Eg But-2-yne Name __________________?

Halogenoalkanes – alkanes with a F, Cl, Br or I

Eg 3-chloropentane Name_______________?

Primary Secondary and Tertiary Amines

Unfortunately, amines don’t work in quite the same way as alcohols and haloalkanes.

Aromatic Hydrocarbons.

Aromatic usually means pleasantly smelling.

In Chemistry it mean "contains a Benzene ring".

Benzene is C6H6.

This representation is called Kekulé Benzene

These representations suggest that Benzene is an Alkene (Hex-1,3,5-triene)

But modern plots of the electron density show a symmetrical molecule with all bonds the same length.

So this can’t be true.

One way to think of Benzene is that the two possible structures resonate between each other.

Another is to assume that the real molecule is somewhere between both resonance structures – the electrons are delocalised – not fixed in position

Other evidence for the delocalised structure· X-ray diffraction of Benzene crystals that show all the bonds are the same length.

· Hydrogenation of Benzene releases a lot less energy than expected for a tri-ene, suggesting the bonding is different.

This makes real Benzene 152 kJ/mol more stable than Kekulé would suggest. This is sometimes called the Resonance Energy or Delocalisation Energy.