What is an Alcohol?
Alcohol is an organic compound that possesses at least one hydroxyl group that is attached to the saturated carbon atom present in the alcohol molecule. The alcohol that is generally used to denote the primary alcohol (i.e. attached to one carbon atom) is ethanol. Ethanol is present in the drugs and is the main component of alcoholic drinks. Alcohol denotes an entire class of alcohol of which methanol and ethanol are the primary members and the higher alcohols including the primary ones are expressed generally with the formula CₙH₂ₙ₊₁OH.
The suffix (-ol) appears where the -OH group, that is the hydroxyl group, is present as one of the main functional groups and is the highest priority functional group present according to the IUPAC chemical naming. When another highest priority group is present in the compound, then the -OH group is referred to as hydroxy. The subject of this article includes the primary, secondary and tertiary alcohols along with their properties.
Properties of Alcohols
Physical Properties of Alcohols
The physical properties of alcohol can be explained by the following points –
Alcohols are colourless.
They generally give a sweet smell except for glycerol and a few lower alcohols.
They are flammable and produce a blue flame.
They don’t produce any smoke while burning.
They are generally liquid at room temperature. Although glycerol is a viscous liquid.
Boiling Point of Alcohol - They generally have higher boiling points if we compare them with other hydrocarbons. For example, ethanol shows a boiling point of 78.29℃ while hexane shows a boiling point of 69℃. This is because of the presence of intermolecular hydrogen bonding between hydroxyl groups of alcohols. The boiling point of alcohol increases with an increasing number of carbon atoms in organic alcoholic compounds.
Alcohols work as suitable solvents for many organic compounds as organic compounds are insoluble in water.
Alcohols are acidic in nature. They react with metals such as sodium, potassium etc. It is due to the polarity of the bond between a hydrogen atom and the oxygen atom of the hydroxyl group. Primary alcohols are more acidic than secondary and tertiary alcohols.
Due to the polar -OH bond in alcohols, they are more soluble in water than other simple hydrocarbons. For example, methanol and ethanol are miscible in water.
Chemical Properties of Alcohols
The chemical properties of alcohol can be explained by the following points –
Oxidation Reaction of Alcohol – Alcohols produce aldehydes and ketones on oxidation. The reaction is given below –
C₂H₅OH + 2O 🡪 CH₃COOH + H₂O
Ethanol Methanoic acid
Combustion of Alcohol – On heating ethanol gives carbon dioxide and water and burns with a blue flame. The reaction is given below –
C₂H₅OH + 3O₂ 🡪 2CO₂ + 3H₂O
Dehydration of Alcohol – Alcohol can be dehydrated in an acidic medium. The removal of water molecules from a compound is called dehydration. Upon dehydration of alcohol alkenes are produced. The reaction is given below –
C2H5OH (conc. H₂SO₄,▵)→C₂H₄ + H₂O
The reaction of Alcohol With Metal – Due to its acidic nature alcohol reacts with metal and forms alkoxide. For example, when ethanol reacts with sodium forms sodium ethoxide and hydrogen gas. The reaction is given below –
2C₂H₅OH + 2Na →2C₂H₅ONa + H₂
Formation of Halides From Alcohols – Alcohol reacts with HCl and forms alkyl halides by removal of hydroxyl groups. The reaction is given below
ROH + HCl (Zn) 🡪 R-Cl + H₂
Esterification – Alcohol reacts with a carboxylic acid in presence of a catalyst and forms ester. It is called Fischer esterification. The reaction is given below –
C₂H₅OH + C₂H₃COOH →C₄H₈O₂ + H₂O
Application of Alcohol
There is a long history of alcohol for myriad uses. For instance, mono-alcohols are the most important alcohols that are used in industries. Some of the applications of alcohol are as follows:-
For formaldehyde and fuel additive production, methanol is mainly used.
For alcoholic beverages, fuel additives, solvent production, ethanol is mainly used.
1-propanol, 1-butanol, and isobutyl alcohol are used as a solvent and precursors to solvents.
C6–C11 alcohols are used for plasticizers in polyvinylchloride and many more.
fatty alcohol (C12–C18) are utilized as precursors to detergents.
FAQs on Properties of Alcohol
1. What is an alcohol and how is it identified in IUPAC nomenclature?
An alcohol is an organic compound that contains at least one hydroxyl (–OH) functional group attached to a saturated carbon atom. In the IUPAC naming system, the presence of this group is indicated by replacing the final '-e' of the parent alkane's name with the suffix '-ol'. For example, ethane becomes ethanol. If a higher priority functional group is present, the –OH group is named using the prefix 'hydroxy-'.
2. What are the key physical properties of alcohols?
The main physical properties of alcohols are determined by the hydroxyl group. Key properties include:
- Boiling Point: Alcohols have significantly higher boiling points than hydrocarbons of similar molecular mass due to intermolecular hydrogen bonding.
- Solubility: Lower molecular weight alcohols (like methanol and ethanol) are highly soluble in water because their –OH group can form hydrogen bonds with water molecules. Solubility decreases as the size of the alkyl group increases.
- State: Most common alcohols are colourless liquids at room temperature.
- Odour: Lower alcohols typically have a distinct, often sweet smell.
3. How are alcohols classified into primary, secondary, and tertiary types?
Alcohols are classified based on the number of carbon atoms attached to the carbon atom that bears the hydroxyl (–OH) group:
- Primary (1°) Alcohol: The –OH group is attached to a carbon atom that is bonded to only one other carbon atom (e.g., Ethanol).
- Secondary (2°) Alcohol: The –OH group is attached to a carbon atom that is bonded to two other carbon atoms (e.g., Propan-2-ol).
- Tertiary (3°) Alcohol: The –OH group is attached to a carbon atom that is bonded to three other carbon atoms (e.g., 2-methylpropan-2-ol).
4. What are some important chemical reactions that alcohols undergo?
Alcohols are chemically reactive due to the polar –OH group. Some important reactions include:
- Oxidation: Primary alcohols oxidise to form aldehydes and then carboxylic acids, secondary alcohols oxidise to form ketones, and tertiary alcohols resist oxidation.
- Dehydration: Alcohols lose a water molecule in the presence of an acid catalyst to form an alkene.
- Esterification: Alcohols react with carboxylic acids in a reaction catalysed by acid to form esters, which often have fruity smells.
- Reaction with Active Metals: Alcohols behave as weak acids and react with active metals like sodium (Na) to release hydrogen gas and form a sodium alkoxide.
5. What are some common industrial uses of alcohols like methanol and ethanol?
Methanol and ethanol are two of the most commercially important alcohols. Their uses include:
- Ethanol (C₂H₅OH): Used as a solvent in paints and varnishes, in the manufacturing of alcoholic beverages, as an antiseptic, and increasingly as a biofuel additive to petrol.
- Methanol (CH₃OH): Primarily used as an industrial solvent, a feedstock for producing other chemicals like formaldehyde, and as a fuel source, especially in racing cars.
6. Why do alcohols have higher boiling points than alkanes of comparable molecular mass?
Alcohols exhibit higher boiling points primarily due to the presence of intermolecular hydrogen bonding. The hydroxyl (–OH) group is highly polar, allowing alcohol molecules to form strong hydrogen bonds with each other. These bonds require significantly more energy to break compared to the weak van der Waals forces that are the only intermolecular forces present in nonpolar alkanes. This additional energy requirement results in a higher boiling point for alcohols.
7. How does the acidic strength of primary, secondary, and tertiary alcohols compare, and why?
The acidic strength of alcohols decreases in the order: Primary > Secondary > Tertiary. This trend is explained by the positive inductive effect (+I effect) of alkyl groups. Alkyl groups are electron-donating and push electron density towards the oxygen atom of the –OH group. As the number of alkyl groups increases from primary to tertiary, the electron density on the oxygen atom increases, making the O-H bond less polar. This strengthening of the O-H bond makes it more difficult to release a proton (H⁺), thus decreasing the alcohol's acidity.
8. What is the mechanism for the acid-catalysed dehydration of an alcohol to form an alkene?
The acid-catalysed dehydration of an alcohol, like ethanol, to an alkene generally follows a three-step mechanism:
- Protonation of the alcohol: The lone pair of electrons on the oxygen atom of the alcohol attacks a proton (H⁺) from the acid, forming a protonated alcohol (oxonium ion). This makes the hydroxyl group a better leaving group (H₂O).
- Formation of a carbocation: The C-O bond breaks, and the leaving group (water) departs, resulting in the formation of a carbocation. This is typically the slowest, rate-determining step.
- Deprotonation: A base (like a water molecule or the conjugate base of the acid) removes a proton from the carbon atom adjacent to the carbocation, and the electrons from the C-H bond shift to form a C=C double bond, resulting in an alkene.
9. How can you chemically distinguish between primary, secondary, and tertiary alcohols in a lab?
A common method to distinguish between the three classes of alcohols is the Lucas test. This test uses the Lucas reagent, which is a solution of anhydrous zinc chloride (ZnCl₂) in concentrated hydrochloric acid (HCl). The test is based on the different rates of reaction of the alcohols to form cloudy alkyl halides.
- Tertiary alcohols: React almost immediately to produce a cloudy solution or immiscible layer.
- Secondary alcohols: React more slowly, typically forming a cloudy solution within 5-10 minutes.
- Primary alcohols: Do not react noticeably at room temperature. Turbidity may appear only upon heating.
