Vivian Marlowe
Alcohols are organic compounds with one or more hydroxyl (OH) groups attached to a carbon atom in an alkyl group or hydrocarbon chain. They are classified as primary, secondary, or tertiary alcohols, depending on the number of carbon atoms connected to the carbon atom bearing the hydroxyl group. The numeric shorthands 1°, 2°, and 3° are sometimes used to refer to these classifications. Primary alcohols have a hydroxyl group attached to a carbon atom bonded to only one other alkyl group. Secondary alcohols have a hydroxyl group attached to a carbon atom with two alkyl groups on either side, and tertiary alcohols have a hydroxyl group attached to a carbon atom with three alkyl groups. The suffix '-ol' is used in IUPAC nomenclature to denote simple compounds that contain alcohols.
| Characteristics | Values |
|---|---|
| Definition | Any of a class of organic compounds characterized by one or more hydroxyl (―OH) groups attached to a carbon atom of an alkyl group (hydrocarbon chain) |
| IUPAC nomenclature | The name for an alcohol uses the -ol suffix with the name of the parent alkane, together with a number to give the location of the hydroxyl group |
| Common names | The common name of an alcohol combines the name of the alkyl group with the word alcohol |
| Classification | Primary, secondary, and tertiary |
| Examples | Methanol, ethanol, propanol, butanol, 1-decanol, 1-octanol, cyclohexanol, 2-chloro-3-methyl-3-hexanol, 2,2-dimethylcyclopentanol |
| Uses | Sweeteners, perfumes, preservatives, antiseptic agents, anti-freezing agents, fuels, etc. |
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What You'll Learn
Alcohols are organic compounds with hydroxyl groups attached to a carbon atom
The hydroxyl group is the functional group of all alcohols, and they are often represented by the general formula ROH, where R is an alkyl group. The suffix '-ol' is used in the IUPAC chemical name of substances where the hydroxyl group is the functional group with the highest priority. When a higher priority group is present in the compound, the prefix 'hydroxy-' is used instead. The suffix '-ol' is also used in non-IUPAC names, such as paracetamol or cholesterol, to indicate that the substance is an alcohol.
Alcohols can be classified as primary, secondary, or tertiary, based on the number of carbon atoms connected to the carbon atom bearing the hydroxyl group. Primary alcohols have a hydroxyl group attached to a carbon atom that is connected to only one other carbon atom. Secondary alcohols have a hydroxyl group attached to a carbon atom that is connected to two other carbon atoms. Tertiary alcohols have a hydroxyl group attached to a carbon atom that is connected to three other carbon atoms.
The physical and chemical properties of an alcohol depend on the location of the hydroxyl group. Alcohols are polar due to the hydroxyl group, which can form hydrogen bonds with other compounds. This polarity makes alcohols more water-soluble than simple hydrocarbons. Alcohols with low molecular weights are highly soluble in water, while those with higher molecular weights become less soluble. Alcohols are also used in various applications, including as sweeteners, in perfumes, and in the synthesis of other compounds.
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IUPAC nomenclature for alcohols
Alcohols are organic compounds characterised by the presence of one, two, or more hydroxyl groups (–OH) attached to a carbon atom in an alkyl group or hydrocarbon chain. The general formula for alcohols is ROH, where R is an alkyl group. The suffix '-ol' appears in the IUPAC chemical name of all substances where the hydroxyl group is the functional group with the highest priority. When a higher-priority group is present in the compound, the prefix 'hydroxy-' is used in its IUPAC name. The ending '-ol' indicates an alcohol, and the prefix indicates the number of carbon atoms in the parent compound.
The IUPAC nomenclature for alcohols involves changing the ending of the parent alkane name to '-ol'. The longest continuous chain (LCC) of carbon atoms containing the OH group is taken as the parent compound. The chain is numbered from the end nearest the OH group, and the number that indicates the position of the OH group is prefixed to the name of the parent hydrocarbon. The -e ending of the parent alkane is replaced by the suffix -ol. For example, the alkane chain name 'ethane' becomes 'ethanol'.
When multiple OH groups appear in the same molecule (polyhydroxy alcohols), suffixes such as '-diol' and '-triol' are used. In these cases, the -e ending of the parent alkane is retained. For example, a compound with a three-carbon chain and three hydroxyl groups would be named 1,2,3-propanetriol. The common name of this compound is glycerol or glycerin.
When naming a cyclic structure with a hydroxyl group, the -OH is assumed to be on the first carbon. When an alkene and alcohol are present in a molecule, it is named as follows: (location of the alkene)-(prefix for the parent chain + en)-(location of the hydroxyl)-ol. The common system of naming is often used when the alcohol only contains a few carbons.
Alcohols can be classified as primary (1°), secondary (2°), or tertiary (3°) depending on the number of alkyl substituents attached to the carbon bonded to the OH group. In a primary (1°) alcohol, the carbon carrying the -OH group is attached to only one alkyl group, as seen in methanol (CH3OH) and ethanol (CH3CH2OH). Secondary alcohols have the -OH group attached to two alkyl groups on either side, as in 2-propanol. Tertiary alcohols have the -OH group attached to a carbon with three alkyl groups, as in tert-butanol (2-methylpropan-2-ol).
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Differentiating primary, secondary, and tertiary alcohols
Alcohols are organic compounds that contain a hydroxyl group (OH) attached to an alkyl or aryl group (ROH). They are classified as primary, secondary, or tertiary alcohols based on the number of carbon atoms connected to the carbon atom bearing the hydroxyl group.
Primary alcohols have a hydroxyl carbon with only one R group attached. The general formula for primary alcohols is RCH2OH, with the simplest example being methanol (CH3OH). Other examples include ethanol and propanol.
Secondary alcohols have a hydroxyl carbon with two R groups attached. The general formula for secondary alcohols is RR'CHOH, with the simplest example being 2-propanol (R = R' = CH3).
Tertiary alcohols have a hydroxyl carbon with three R groups attached. The general formula for tertiary alcohols is RR'R"COH, with the simplest example being tert-butanol (2-methylpropan-2-ol), where each of R, R', and R" is CH3.
The number of carbon atoms attached to the carbon bearing the hydroxyl group also determines the reactivity of the alcohol. For example, in the oxidation test, primary alcohols are easily converted to aldehydes, which can then be converted to carboxylic acids. Secondary alcohols can be converted to ketones, but they cannot be further oxidised. Tertiary alcohols do not oxidise in the presence of sodium dichromate.
The physical and chemical properties of alcohols can also vary depending on the location of the hydroxyl group. For example, the boiling point of ethanol (a primary alcohol) is 78.29 °C, while the boiling point of hexane (a comparable hydrocarbon) is 69 °C.
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Uses of alcohols in different industries
Alcohols are organic compounds characterised by the presence of one, two, or more hydroxyl groups (–OH) attached to the carbon atom in an alkyl group or hydrocarbon chain. They are used across various industries, including medicine, food, and cosmetics.
In the medical field, alcohols like ethanol are used as antiseptic agents and preservatives for specimens in laboratories. They also serve as active ingredients in alcoholic beverages, acting as antiseptics in hand sanitisers and hand wipes. Additionally, ethanol is used to dissolve organic compounds that are not soluble in water, making it valuable in the cosmetic industry for perfumes, paints, detergents, and inks.
Methanol (propanol) is used as an anti-freezing agent when mixed with a solution of ethylene glycol dissolved in water. It is also employed as a fuel in internal combustion engines.
Alcohols are used in the food industry, with butanol being used in the manufacture of butyl ethanoate, a synthetic fruit flavouring agent in food and confectionery products.
Furthermore, cyclohexanol, an industrial alcohol, is used in the production of nylon. Alcohols are also used in the synthesis of other compounds, showcasing their versatility across multiple sectors.
The classification of alcohols into primary, secondary, and tertiary categories is based on the number of carbon atoms connected to the carbon atom bearing the hydroxyl group. This classification system influences the physical properties of the alcohols.
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How to identify an alcohol
Alcohols are organic compounds with a hydroxyl (OH) functional group on an aliphatic carbon atom. The OH group is the functional group of all alcohols, and they can be represented by the general formula ROH, where R is an alkyl group. Alcohols are differentiated based on the presence and location of the hydroxyl group, which changes the physical and chemical properties of the alcohol.
The International Union of Pure and Applied Chemistry (IUPAC) names all substances with the hydroxyl group as the functional group with the highest priority with the suffix -ol. For example, "ethanol" comes from the alkane chain name "ethane". The ending -ol indicates an alcohol, and the prefix hydroxy- is used in IUPAC names when a higher priority group is present in the compound. In archaic nomenclature, alcohols can be named as derivatives of methanol using "-carbinol" as the ending.
Alcohols can be classified as primary, secondary, or tertiary, based on the number of carbon atoms connected to the carbon atom bearing the hydroxyl group. Primary alcohols have a hydroxyl group attached to a carbon atom connected to only one alkyl group. The simplest primary alcohol is methanol (CH3OH). Secondary alcohols have a hydroxyl group attached to a carbon atom connected to two alkyl groups on either side. Tertiary alcohols have a hydroxyl group attached to a carbon atom connected to three alkyl groups.
Tertiary alcohols do not get oxidized by mild oxidizing agents in aqueous, alkaline, or neutral conditions. They undergo oxidation under acidic conditions, first undergoing dehydration to form alkenes, which then form a ketone with one less carbon atom. This ketone can further oxidize to form a carboxylic acid.
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Frequently asked questions
Tertiary alcohols, also known as 3o alcohols, are those which feature a hydroxyl group attached to the carbon atom which is connected to 3- alkyl groups.
Alcohols are named by both formal and common systems. The most generally applicable system is the one adopted by the International Union of Pure and Applied Chemistry (IUPAC) in 1957. Using the IUPAC system, the name for an alcohol uses the -ol suffix with the name of the parent alkane, together with a number to give the location of the hydroxyl group.
Examples of tertiary alcohols include tert-butanol (2-methylpropan-2-ol) and 2-chloro-3-methyl-3-hexanol.
