An alcohol is an organic compound that contains the hydroxyl group ( – OH).
Alcohols are classified according to the number of other carbon atoms that are directly bonded to the carbon atom attached to the hydroxyl group.
The presence of the electronegative oxygen atom in the – OH group affects the polarity of the molecule.
Alcohols (unlike hydrocarbons) are polar molecules.
Naming and Drawing Alcohols
IUPAC (International Union of Pure and Applied Chemistry) conventions for naming alcohols are similar to those for naming alkenes.
An alcohol is named by replacing the final -e of the parent hydrocarbon with -ol. The name includes the number of the carbon atom to which the hydroxyl group is attached.
If the chain also has hydrocarbon or halide constituents, the lowest number is assigned to the carbon atom with the hydroxyl group.
The isomers of alcohols have different properties.
Propan-1-ol is used as a solvent for lacquers and waxes, and as brake fluid.
The other isomer, propan-2-ol (isopropyl alcohol), is also known propan-1-ol as rubbing alcohol. It is used as an antiseptic to clean the skin before injections.
Both isomers of propanol are toxic to humans and many other animals if ingested.
Alcohols containing more than one – OH group are referred to as polyalcohols.
The suffix -diol (for two – OH groups) or -triol (for three – OH groups) is added to the alkane name instead of -ol.
A common example of a polyalcohol is ethane-1,2-diol, commonly known as ethylene glycol. This compound is widely used as antifreeze in automobile engines.
Another common polyalcohol is propane-1,2,3- triol, commonly called glycerol or glycerine. Since glycerine has a low toxicity and is soluble in water, it is frequently used in pharmaceutical preparations to help dissolve less polar compounds. The solubility in water is due to the three – OH groups, which increase the polarity.
Cyclic hydrocarbons may also form alcohols. When numbering the atoms in a cyclic hydrocarbon, the – OH functional group takes precedence over other substituents in naming.
An aromatic hydrocarbon can have a bonded – OH group added, forming an aromatic alcohol. If the benzene ring has two – OH groups, the name is based on benzene and includes the numbers for the – OH groups. An example is benzene-1,2-diol.
In some cases, a different functional group takes precedence over the alcohol in naming. In this case, the –OH group treated as a substituent, called hydroxyl. This convention applies to carboxylic acids, aldehydes and ketones.
When there is a multiple bond in the chain, the –OH group takes precedence (for example: pent-4-ene-1-ol).
Properties of Alcohols
Simple alcohols with short carbon chains are more soluble in water than those with longer carbon chains.
The addition of the –OH group increases the polarity of the alcohol molecule and, therefore, its solubility in water. However, as the size of the carbon chain grows, alcohols are less soluble in water.
Because the hydrocarbon region of an alcohol molecule is non-polar, alcohols tend to be able to dissolve both polar and non-polar substances. This makes alcohols very useful as solvents.
Reactions Involving Alcohols
Many alcohols are manufactured commercially by the addition reaction of an alkene and water. The modern method for production of methanol, CH3OH, combines carbon monoxide with hydrogen gas at high temperature and pressure, using a catalyst.
Like many organic compounds, alcohols undergo combustion with oxygen. This reaction, similar to the combustion of hydrocarbons, produces carbon dioxid and water.
The reverse of the addition reaction that produces an alcohol is a dehydration reaction to form an alkene and water.
Ether - an organic compound containing an oxygen atom between 2 carbon atoms in a chain.
Ethers can be synthesized from the reaction of alcohols. When the alcohols react, a molecule of water is produced in a condensation reaction.
The IUPAC method to name an ether is to add the suffix -oxy to the smaller hydrocarbon group that is bonded to the larger alkane group.
For example, an ether made up of a methyl group and an ethyl group joined by an oxygen molecule is called methoxyethane.
A commonly used alternative nomenclature uses the names of the two hydrocarbon groups followed by the word “ether:” methylethyl ether.
If the two alkane groups are the same size, the prefix di- is used. Thus, methoxymethane would be called dimethyl ether.
A thiol is an organic compound that includes the sulfhydryl functional group, –SH.
The sulfhydryl group is similar to the hydroxy group, –OH.
Thiols generally have strong odours. For example, a thiol gives garlic its strong odour.
Hydrogen peroxide (H2O2) reacts with thiols to form odourless disulfide compounds (R-S-S-R).
Hence, hydrogen peroxide is sometimes used to neutralize the odours of many thiols.
To name thiols, the suffix -thiol is added to the end of the alkane name. For example, HSCH3 is methanethiol.