Alcohols (2) Structure, bonding and nomenclature of some simple alcohols

 Alcohols contain the hydroxyl functional group —O–H

They are organic molecules containing covalent and polar covalent bonds.

Here is a dot and cross diagram for methanol:

Three types of alcohol

There are three types of alcohol depending on the hydrocarbon chain attached to the –O—H group.

Primary alcohols have a single carbon atom attached to the carbon atom of the 

–O—H group

Secondary alcohols have two carbon atoms attached to the carbon of the –O—H group.

Tertiary alcohols have three carbon atoms attached to the carbon of the –O—H group.

There are also cyclic alcohols like this one—cyclobutanol:

Often drawn in skeletal form like this:

There are also alcohols with more than one –O—H group like this:

This is glycerol or propan-1,2,3-triol

Again often drawn in skeletal form like this:

Comparison of alkane and alcohol boiling points

It’s instructive to compare alkane boiling points with alcohol boiling points:

Alkane	Boiling point (oC)	Alcohol	Boiling point (oC)
Methane	-162	Methanol	62
Ethane	-89	Ethanol	78
Propane	-42	Propan-1-ol	97
Butane	0.5	Butan-1-ol	117
Pentane	36	Pentan-1-ol	138
Hexane	69	Hexan-1-ol	156

Alcohols have the higher boiling point compared to their corresponding alkane because of the H bonding that exists between the molecules now they contain an oxygen atom. 

Here is a diagram to show hydrogen bonding in methanol:

Naming alcohols

You ought to be able to name the three alcohols above and similar molecules like them. 

Nomenclature rules are simple for these small molecules.

1) Find the longest carbon chain

2) Name the chain using the alkane root

3) Give the carbon with the –O—H group the lowest number possible

4) Add this number to end of the name followed by –ol

5) Put other substituents at the front of the name in alphabetical order.

Here’s an example using structural formulae (and we all know that none of us like to use structural formulae!!)

Alcohol Structural Formula:   CH₃CH₂CHClC(CH₃)₂CH₂CHOH

Systematic Name: 4-chloro-3,3-dimethylhexan-1-ol

Can you see how the name is worked out and do it for yourself?

Drawing out the structural formula as a displayed formula should help.

The next thing you need to do is to set yourself some examples like these below to work out the name from the formula and the formula from the name. 

Here are a name and a structural formula for you to try and work out first (a) the formula and (b) the name

a) 3-bromo-2-methyl heptan-1-ol

b) CH₃C(CH₃)₂CH₂CHOHCH(Cl)CH₂CH₃