A-level Applied Science/Synthesising Organic Compounds/Organic chemicals

Organic compounds and functional groups

The chemistry of carbon is almost entirely due to covalent bonding. Almost without limit, carbon can form four covalent bonds with a wide range of non-metallic elements. Carbon is able to form long chains of atoms to form quite stable molecules. The chains may be branched or form loops. Many other atoms will form stable bonds to carbon. Carbon can also form multiple bonds e.g. a carbon atom can form a triple bond to another carbon atom, or a nitrogen atom. Here are some examples of organic compounds. For each element, carbon, hydrogen, nitrogen, count how many covalent bonds the element forms. For each element, does the number of bonds change?

octane

butanamide

aspirin

Element	Number of bonds formed
Carbon
Hydrogen
Oxygen
Nitrogen

Because the bonding of each element is so regular, it is common to write the structures without showing every atom and bond:

Draw the full structures (showing all atoms and bonds) of these molecules:

CH₃CH₂CH₂CH₂CH₃ pentane

trans-3-methylhex-3-ene

Aliphatic compounds

‘Aliphatic’ organic compounds are based on simple chains of carbon atoms. Other organic compounds are based around the ring structure of benzene and are known as ‘aromatic’.

Classify these compounds as aromatic or aliphatic:

Pentane Butanamide Aspirin

The naming and chemistry of aromatic compounds is not covered in these notes.

Naming aliphatic compounds is fairly simple. The name is based on the longest single chain of carbon atoms:

Methane	CH4
Ethane	CH3CH3
Propane	CH3CH2CH3
Butane	CH3CH2CH2CH3
Pentane	CH3CH2CH2CH2CH3
Hexane	CH3CH2CH2CH2CH2CH3
Heptane	CH3CH2CH2CH2CH2CH2CH3
Octane	CH3CH2CH2CH2CH2CH2CH2CH3

These are simple ‘alkane’ compounds. Some other patterns of atoms are seen over and over again. These are called ‘functional groups’ and each has characteristic chemistry. They are named by attaching a suffix or prefix to the name of the relevant alkane e.g. ethanol (ethane + -ol) is an alcohol with two carbon atoms, butanamide (butane + -amide) is an amide with four carbons. Complete the following table:

Functional group	Structure	Name	Example
Alkene
Alcohol
Amine
Carboxylic acid
Primary amide
Ester
Secondary amide
Aldehyde
Ketone

Structural formulae and three-dimensional shapes. The basic shape of organic molecules is based on the tetrahedron and the equilateral triangle. Use models (either on the computer or the plastic kits available in class) to examine and draw the following shapes: Carbon with just single bonds: e.g. CH₄

Carbon with one double bond: e.g. CH₂CH₂

Nitrogen and oxygen are similar, but without all four bonds: Compare:

CH₃CH₃ CH₃OH CH₃NH₂

CH₂CH₂ CH₂O

Isomers

There are two molecules with the formula C₄H₁₀. What are their structures? What are their names?

These molecules are isomers: different compounds with the same formula. Because the bonding is different in the two C₄H₁₀ isomers, they are called ‘structural isomers’. Note that to make one isomer into another, you need to break at least one bond. There are three structural isomers of C₃H₈O. Try to draw and name them:

Numbers are included in the name when the functional group could be attached more than one carbon atom. There are six isomers with the formula C₄H₈. How many can you draw? How many can you name?

Conformations

Using models, notice how a single bond can easily rotate. Double bonds will not rotate unless broken. Because you need to break the double bond to rotate it, there are two isomers of but-2-ene:

The bonding is the same in these two molecules, so they are not ‘structural isomers’. They are ‘stereoisomers’. Draw the stereoisomers of these compounds and name them. If there are no stereoisomers then say so. Propen-1-ol

Propen-2-ol

Pent-2-ene