In this lesson, we will learn:
• The definition and general formula of an alcohol with some basic examples.
• The major properties of alcohols and their difference to the hydrocarbons.
• The reactions of alcohols and how the type of alcohols affects reactivity.
• How to name alcohols using IUPAC organic nomenclature.
• Alcohols are another homologous series – a ‘family’ of organic molecules with the same general formula, each member differing from the next by CH2.
• An alcohol compound is defined as any organic compound where the hydroxyl (-OH) group is covalently bonded to a saturated carbon atom. Remember that ‘saturated’ means there is no double bond – so a molecule must have a carbon atom with four single bonds, one to an –OH group, to be called an alcohol.
• Unlike alkanes, alkenes and alkynes, alcohols are not hydrocarbons. They are not considered hydrocarbons because alcohols contain an oxygen atom in the molecule.
• Alkanes are identified by their general formula: CnH2n+2O but is normally written as CnH2n+1OH, to show the hydroxyl group that defines an alcohol. Alcohols then are very similar to alkanes of the same carbon chain length – the same amount of hydrogens and carbons, only with an oxygen atom in the molecule too. This is because a hydroxyl group is just an oxygen atom bonded to a hydrogen atom, making one bond to the rest of the molecule through a carbon atom. Simple naming of alcohols is very similar to alkanes as a result – alcohols have the suffix –ol instead of the ending -e in alkanes and alkenes. See the table below for some simple alcohols in saturated compounds.
Carbon chain length
Name of alcohol
• Alcohols are important solvents in chemistry – they are used to dissolve other chemicals. The most generally used alcohol is ethanol, C2H5OH which is used as a fuel and is the alcohol used in alcoholic drinks.
• Despite being very similar in structure to basic alkanes, the properties of alcohols are substantially different from alkanes. This is because of their –OH group allowing hydrogen bonding to exist between alcohol molecules. Oxygen is a very electronegative atom that has lone pairs, which the hydrogen atom of the –OH group, in other molecules of the alcohol, can interact with. The major properties of alcohols are:
→ A much higher melting and boiling point than their analogue alkanes – methane’s boiling point is around -164°C, while methanol’s boiling point is around 60°C.
→ Polarity – shorter alcohols are polar molecules that can dissolve in water, not fats.
→ Toxic – all alcohols are toxic, including ethanol found in alcoholic drinks (drunkenness is also known as intoxication!)
→ Flammable – alcohols burn with a relatively clean flame compared to alkanes and do not produce soot (carbon particulates).
• There are clear trends in their properties. These property trends can be explained by intermolecular forces. As the carbon chain length of alcohols gets longer, they:
→ Have a higher melting/boiling point.
→ Are less volatile / flammable.
→ Are less soluble in water.
Depending on the carbon atom the –OH is bonded to, we can describe three types of alcohols and chemical tests can be performed to determine which is present. Testing for the presence of alcohol compounds involves using acidified potassium dichromate.
→ A primary alcohol (1°) is an alcohol with the –OH group bonded to a carbon atom making only one carbon-carbon bond. This would place the –OH group at the end of a carbon chain.
→ A secondary alcohol (2°) is an alcohol with the –OH bonded to a carbon atom making two carbon-carbon bonds. This would place the –OH group in the middle of a carbon chain.
→ A tertiary alcohol (3°) is an alcohol where the –OH group bonds to a carbon atom with three carbon-carbon bonds. This would place the –OH group on a carbon where a branch in the carbon chain is found. See the table (the brackets in the middle column show branching) and examples.
Type of alcohol
Test to identify using:
1. Acidified potassium dichromate.
2. Acidified silver nitrate to the product of 1.
1. Solution changes colour from orange to green.
2. Silver mirror is observed in the test tube.
1. Solution changes colour from orange to green.
2. Add acidified silver nitrate – no observed
1. No observed change. Orange solution stays
• Alcohols are known to perform a number of reactions:
→ Alcohols can be reacted with alkali metals to form metal salts and hydrogen gas. For example with sodium the reaction is:
2C2H5OH + 2Na → 2C2H5ONa + H2
→ Alcohols can be reacted with acids to form alkyl halides and water as a side product. For example with hydrochloric acid:
C2H5OH + HCl → C2H5Cl + H2O
→ Alcohols can be oxidized and converted into aldehydes or ketones, depending on the type of alcohol. Usually in organic chemistry we represent the oxidizing agent with an [O] in the chemical equation, so the focus stays on the organic chemical. Primary alcohols will be oxidized to aldehydes, and secondary alcohols will be oxidized to ketones. Tertiary alcohols cannot be oxidized. The oxidation of ethanol to ethanol is shown below:
C2H5OH + [O] → C2H4O + H2O
→ Alcohols can also be oxidized straight to carboxylic acids, skipping the aldehyde step in between. Remember that ketones cannot be reacted to make carboxylic acids, only aldehydes can. The full oxidation of ethanol is shown below:
C2H5OH + 2[O] → H3CCOOH + H2O
• To name alcohols using IUPAC nomenclature, the –OH group is given the suffix –ol. It is also given a number to show which carbon atom in the main chain it is bonded to.
→ All the systematic rules of naming alkanes, alkyl branches and alkenes apply.
→ Alcohol groups are higher order (priority) than alkenes and alkyl branches, so numbering prioritizes alcohol groups.
Alcohols - Introduction
Recall simple alcohols, their properties and the trends in properties.
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