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Introductory Chemistry

Alcohols: An Introduction to Organic Chemistry

Naming Carbon Compounds

The simlpest member of the hydrocarbon family are the alkanes. Alkanes are an example of a homologous series - a series of compounds of the same type with the same general formula. The general formula for the alkanes is C_nH_2n+2. There are three ways of representing the formulae of organic compounds (see Fig 1.):

• Molecular formulae show the number of atoms in a single molecule
• Structural formulae show how the atoms are grouped in the molecule
• Displayed formulae show all the atoms and all the bonds

Isomers are compounds that have the same molecular formula but different structures. Because of this, some compounds will have different names, even though they have the same molecular formula, and you must be able to name compounds containing carbon atoms.

Using the table on the left, we can see how the number of carbon atoms in a chain affect the name of the carbon compound. All of the alkanes end with -ane. The beginning of the name is Greek, and does need to be learnt (although after the first four, the names are the same as those used in geometric figures e.g. pentagon, hexagon etc.).

To name more complicated compounds, we need to follow a few rules. Fig 2. shows butane and an isomer of butane - 2 methyl propane. This shows that isomers can be named differently to each other because of the rules we use to name the compounds.

Here are the rules for naming a carbon compound:

1. The number(s) at the front of the name tell us which C atom the branched carbon hangs off. This number is the lowest number possible, i.e. you can count in from either side!
2. The second word descibes the branched carbon. In the example above, it is called methyl because the C atom has one less H atom than methane. If there is more than one number at the beginning of the name, then methyl becomes dimethyl, trimethyl etc.
3. The last part of the name is the longest unbroken chain. In the example above, the longest unbroken chain is 3 carbons (remember, you can count around corners). From the table above, we can see that 3 carbons means is called propane, so that is what it is called.

To make sure you understand, Fig 3. takes the example of Pentane and 2 isomers of pentane:

There are also names for alkanes containing a ring of carbon atoms. They are names from the corresponding straight-chain hydrocarbon by adding the prefix cyclo-. Fig 4. shows two examples of cycloalkanes:

Because of their structure, cycloalkanes have the same general formula as alkenes, that is, C_nH_2n. But because they do not have a double bond, they will not react with bromine, and this is how we can distinguish between the two.

Alcohols

Alcohols are another homologous series. They have a hydroxyl group - OH - attached to a carbon atom by a covalent bond. The -OH group is an example of what is called a functional group, and if a hydrocarbon group is connected to an OH group, it is an alcohol. We use the term functional group to descibe the atom, or groups of atoms, which gives a compound its characteristic properties.

Because methanol and ethanol only have 1 or 2 C atoms, the OH group can only be on the first C atom, because you always number it as the smallest you can. But when we get to propanol and beyond, the OH group can either be connected to the first or second C atom. Propanol can therefore be known as propan-1-ol or propan-2-ol.

You need to be able to name alcohols aswell, and to do this, you need to understand the three different types (examples in Fig 6.):

• Primary Alcohol (1^o) - the C with the OH group is only bonded to one other C
• Secondary Alcohol (2^o) - the C with the OH group is bonded to two other C's
• Tertiary Alcohol (3^o) - the C with the OH group is bonded to 3 other C's

Notice in Fig 6. that they are isomers of pentanol. You need to be able to name these, as I have here, using the rules previously mentioned. When alcohols contain more than one hydroxyl group they are known as diols or triols etc. after the number of hydroxyl groups they contain.

Solubility with Water

• A hydrocarbon chain is hydrophobic (does not mix with water)
• A hydroxyl group is hydrophilic (mixes with water)
• The solubility decreases with increasing length of the hydrocarbon chain