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Alcohols are organic compounds with hydroxyl group (-OH) attached in the hydrocarbon with aliphatic carbon. Because of the attached hydroxyl group, the alcohols exhibit different physical and chemical properties. Alcohols can be classified on the basis of the hybridization into primary, secondary and tertiary alcohols.
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Key takeaways: Primary alcohol, Secondary alcohol, Tertiary alcohol, Esters, Hydroxyl group, Hydrocarbon, Aliphatic carbon, Carbon, Organic compound
Different Types of Alcohol
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Alcohols can be divided according to many different criteria.
(i) Alcohol can be divided according to mono-, di-, tri- and poly- alcohol.
When the alcohol has a single -OH group attached, the alcohol is known as monohydric. For eg- C3H7OH
Monohydric
When there are two -OH groups attached, the alcohol is known as dihydric.
Dihydric
If more than 3 -OH groups are attached, the alcohol is known as trihydric.
Trihydric
Alcohols having more than 3 hydroxyl groups are known as polyhydric alcohols.
Polyhydric Alcohols
(ii) Alcohol can also be divided according to the carbon to which -OH group is attached.
- In simpler terms, whenever the -OH group is attached to a carbon with sp3 hybridisation, which is already attached to a carbon, it is known as primary alcohol.
- When the -OH group is attached to the carbon with sp3 hybridisation, which is already attached to two carbons is known as secondary alcohol.
- When the -OH group is attached to the carbon with sp3 hybridisation, which is already attached to 3 carbons, is known as tertiary alcohol.
Types of alcohol
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Physical Properties of Alcohol
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- Alcohols are generally colorless with a pungent odor.
- Alcohol is generally a liquid at room temperature .
- The alcohols having less number of carbons have lesser boiling points. The alcohol with C5-C10 can be more viscous and oily and more carbon alcohols can be waxy solids as well.
- Alcohols are miscible in water. The hydrogen of the hydroxyl group can form hydrogen bonds with water molecules. The solubility of the alcohol decreases with increase in molecular mass.
- Due to the -OH group, the alcohols are generally more soluble in water than any other liquid.
- Alcohols are weaker acids in nature because of the attached polar -OH group, where the H+ ion can be released in reaction with any active metal like sodium, potassium etc.
- Alcohols are also flammable and produce colorful flame, generally blue in color.
Chemical Properties of Alcohol
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The alcohol exhibits different chemical properties due to the cleavage either in O and H in the -OH group or due to C and O in the attached -OH group.
Chemical properties due to cleavage in O and H group.
Alcohol reacts with metal: the alcohol reacts with active metals such as sodium, potassium to form alkoxide and hydrogen gas. This reaction shows they are mildly acidic in nature.
2R-O-H+ 2Na → 2R-O-Na + H2
Alcohols react with carboxylic acids, acid chlorides and acid anhydrides to form esters in presence of acids. Esters are fruity smelling compounds.
R-OH + R’-COOH → ROCOR’ + H2O (in presence of H+)
R-OH + R’COCl → ROCOR’ + HCl (in presence of pyridine)
R-OH + (R’CO)2O → ROCOR’ + R’COOH (in presence of H+)
Chemical properties due to cleavage in C-O bond
Alcohol reacts with hydrogen halides to form alkyl halides.
R-OH+ HX → R-X + H2O
Alcohol reacts with phosphorus trihalides to form alkyl halides.
3R-OH+ PX3 → 3R-X + H3PO3
Alcohols form alkenes on reaction with concentrated sulphuric acid and undergo dehydration.
C3H7OH → C-C=C+ H2O (in presence of H2SO4 at 443k)
Alcohols on oxidation yield ketones i.e double bond with oxygen with dehydrogenation.
Dehydrogenation
Primary alcohol in presence of a strong oxidizing agent such as potassium permanganate oxidizes it directly into carboxylic acid, after an aldehyde.
-CrO3 is a mild oxidizing agent, it oxidizes primary alcohol only till aldehydes.
R-CH2OH → RCHO (in presence of CrO3)
CrO3 oxidizes secondary alcohols into ketones.
Oxidation of alcohol
Tertiary alcohols do not undergo oxidation
Things to Remember
- The alcohol is a hydrocarbon with hydroxyl group attached to it.
- The -OH group gives unique characteristics to this organic compound.
- The alcohol is generally liquid at room temperature and higher alcohols are waxy solids.
- They are mildly acidic.
- They can react with metals and produce hydrogen gas.
- The position of the -OH group also affects the solubility, acidity and boiling point of the compound.
- They can create a large number of other compounds on reaction with different reagents.
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Sample Questions
Ques: What are alcohols? (1 mark)
Ans. Alcohols are hydrocarbons in which one of the H from carbon is replaced by hydroxyl group -OH and thus exhibit different physical and chemical properties.
Ques: Define the physical properties of alcohol. (2 marks)
Ans. Alcohols have a pungent alcoholic smell and some of them have a fruity smell like glycerol. They are generally liquid at room temperature, only the higher carbon alcohols are waxy or solid like. They are miscible in water due to hydrogen bonding with H2O.
Ques: Arrange the following sets according to their increasing boiling points. (2 marks)
pentan-1-ol, butan-1-ol, butan-2-ol, ethanol
Ans. ethanol<butan-2-ol<butan-1-ol<pentan-1-ol.
The boiling point decreases in the order of 1°>2°>3° due to the vanderwall’s force of attraction.
Boiling point is inversely proportional to molecular mass and branching.
Ques: Arrange the following according to their increasing boiling points. (2 marks)
n-butane, pentan-2-ol, pentan-1-ol
Ans. n-butan<pentan-2-ol<pentan-1-ol
Alcohols have a higher boiling point as compared to other hydrocarbons.
Ques: Arrange the following according to their solubility in water. (2 marks)
butanol, ethanol, pentanol, hexanol
Ans. ethanol>butanol>pentanol>hexanol
Solubility of alcohol decreases with increase in molecular mass of the alkyl group attached.
Ques: Arrange the following according to their acidity. (2 marks)
Butanol, butan-1-ol, butan-2-ol
Ans. butanol>butan-1-ol>butan-2ol
The acidity of the alcohol increases when an electron donor group is attached to the hydroxyl group which increases the electron density on the oxygen and release of H+ is easy.
Primary alcohols are more acidic than secondary and tertiary.
Ques: Give 2 chemical properties of alcohol. (2 marks)
Ans. The alcohols are slightly acidic, as they can react with active metals to yield hydrogen gas. They can react with strong oxidizing agents to yield carboxylic acids or ketones.
Ques: Give a series of reactions to convert butan-1-ol into butene. (1 mark)
Ans. CH3CH2CH2COH in presence of concentrated sulphuric acid at 443 k is converted into CH3CH2CH=CH2 along with dehydration.
Ques: Give a series of reactions to form ethyl bromide from ethene. (2 marks)
Ans. CH2=CH2 → CH3CH2OH (in presence of H+ and H2O)
CH3CH2OH+ HBr → CH3CH2Br + H2O
Ques: A compound X is added into an alcohol in the presence of acid. There is a fruity smell on the reaction. How will you judge compound X? (1 mark)
Ans. X can be carboxylic acid, acid chloride or acid anhydride as they give out esters on reaction with alcohol in presence of acid.
Ques: Why CrO3 be used to create aldehydes even though potassium permanganate is also an oxidizing agent? (2 marks)
Ans. Potassium permanganate is a very strong oxidizing agent, and it creates an unknown amount of aldehydes and oxidizes directly into carboxylic acid. To arrest the reaction till aldehydes, CrO3 are used.
Ques. What are aldehydes' physical and chemical properties? (3 marks)
Ans: Aldehydes are electrophilic organic compounds that are highly reactive, poisonous, and carcinogenic. They contain the carbonyl functional group and can be rapidly oxidised to carboxylic acids. They are formed when primary alcohols are oxidised. They are found in nature, albeit at a smaller quantity than other functional groups such as Vanillin, which gives vanilla its distinct aroma. They form amides when they react with amines.
Ques. What makes rubbing alcohol different from other (non-beverage) alcohols? (3 marks)
Ans: The number of carbon atoms per molecule varies among alcohols. When there are more carbon atoms in a molecule, the molecular structure can alter from when there are the same amount of carbon atoms (isomers). Physical and chemical properties differ as a result of these differences. Melting and boiling temperatures, vapour pressure, solubility, and other characteristics will vary. There is only one type of alcoholic beverage: ethanol. Each molecule of ethanol has two carbon atoms.
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