Alcohols: Nomenclature – Synthesis – Properties

Alcohols

Alcohols are hydrocarbon compounds which have hydroxyl group bonded to sat- urated carbon atom. (-OH) is defined as the functional group in these compounds. Their general formula is R-OH or (CnH2n+ 2O).

Alcohols are classified according to the hydroxyl group that is attached to carbon atom. If the carbon atom with hydroxyl group is attached to another carbon atom, it is a primary (1°) alcohol; if the carbon with the hydroxyl group is attached to two carbon atoms, it’s a secondary (2°) alcohol; if the carbon with the hydroxyl group is attached to three carbon atoms, it is called as a tertiary (3°) alcohol.

Nomenclature of Alcohols

Alcohols are named according to IUPAC system as follows:

1.      The longest chain which has the hydroxyl group is chosen. The carbon with the hydroxyl group is given the lowest number.

2.     Naming is done according to the number of carbons. The –ol suffix is added to the end of the corresponding alkane.

3.     The number and place of the alkyl groups on the chain is mentioned.

Synthesis of Alcohols

Alcohols can be synthesized through many methods in industry and laboratories. Here we will explain one of the laboratory methods.

Addition of Water to Alkenes

Addition of water to alkenes is achieved via hot concentrated sulfuric acid used as a catalyst. Alkene reacts first to produce alkyl sulfonic acid (Markovnikov’s rule). This compound reacts with water and forms alcohol and sulfuric acid again.

Example 1

What are the probable structural formulas for alcohols that have a molecular weight of 74 g/mol?

Solution:

Physical Properties of Alcohols

Alcohols have got low molecular weights. They are poisonous and they have sharp odors. They have strong intermolecular bonds. Therefore, their boiling points are much higher than of alkynes. Besides, alcohols can form hydrogen bonds between their molecules and with water molecules. Therefore, alcohols between (C1-C3) dissolve in water very well.

Chemical Properties of Alcohols

Alcohols are compounds that tend to react. They react with polarized or ionic compounds. Some important reactions of alcohols are given below:

A) Alcohols react with sodium or potassium metals and produce hydrogen gas.

 B) They react with phosphorous trichloride (PCl3) or phosphorous pen- tachloride (PCl5) and form alkyl halides.

C) Reactions of Alcohols with Acids

This reaction occurs fast with tertiary alcohols especially when concentrated hydrochloric acid is used. But with primary and secondary alcohols, it is quite slow. And the reaction rate also depends on the type of acid.

HI > HBr > HCl

In the reaction, solution of zinc chloride in hydrochloric acid is used. This mix- ture is called as Lucas’ reagent. It is used to differentiate primary, secondary and tertiary alcohols. Tertiary alcohols readily react with this cation and form alkyl halides which does not dissolve in the solution. While secondary alcohols react with Lucas’ reagent after 5 minutes, primary alcohols do no react with it at room temperature.

Example 2

Differentiate 1-propanol, 2-propanol and 2-methyl-2-propanol chemically.

Solution:

We can differentiate those compounds using dry ZnCl2/HCl Lucas’ reagent. In this process, it is mixed with 2-propanol (secondary alcohol) and 5 minutes later, a layer of 2-propane chloride forms. While it reacts with 2-methyl-2-pro- panol (tertiary alcohol) readily, 1-propanol (primary alcohol) does not react with Lucas’ reagent at room temperature.

This way we can differentiate primary, secondary and tertiary alcohols using Lucas’ reagent.

1.      For primary alcohols to react with Lucas’ reagent, high temperature is neces- sary and they do not react at room temperature. Therefore, no turbidity is visible in solution.

2.     In secondary alcohols, turbidity appears 2-5 minutes later.

3.     Through reaction of tertiary alcohol with Lucas’ reagent, slightly water solu- ble alkyl halide forms and it causes turbidity in solution.

D) Synthesis of water from Alcohol

Ethyl alcohol reacts with concentrated sulfuric acid at 170 °C and ethylene forms.

Mechanism:

 The mechanism of the reaction above follows the following steps:

1. Addition of proton to alcohol molecule:

 2. The cleavage of water molecule from the compound above and formation of more stable carbonium ion:

3. The cleavage of proton from carbonium ion and formation of double bond and cation:

According to Saytzeff’s rule, OH- group leaves carbon atom and hydrogen atom leaves adjacent carbon atom which has fewer hydrogen.

E) Oxidation

The structure of products which form when alcohols are oxidized depends on the type of alcohol and reaction conditions. Substances widely used in oxida- tion reactions are K2Cr2O7 /H2SO4 or KMnO4/ H2SO4 mixtures. This reaction is a method used to differentiate alcohols.

Primary alcohols (1°) oxidize to aldehydes first then to carboxylic acids.

Secondary alcohols (2°) oxidize to ketones.

Tertiary alcohols (3°) do not oxidize as the carbon atom which carries the OH- group does not have any hydrogen atoms.

References

1.      K. J. Denniston c J. J.Topping c and R. L.Caretc “General Organic and Biochemistry”c Mc-Graw- Hillc New York   (2004).

2.     K.W. Whittenc R.E. Davis  and L. M. Peckc “General Chemistry” 7th ed. Holt Rinehart and Winstonc New York (2010).

3.     Clayden, J.; Greeves, N. and Warren, S. (2012) Organic Chemistry. Oxford University Press. pp. 1–15. ISBN 0-19-927029-5.

4.     Streitwieser, Andrew; Heathcock, Clayton H.; Kosower, Edward M. (2017). Introduction to Organic Chemistry. New Delhipages=3–4: Medtech (Scientific International, reprint of revised 4th edition, Macmillan, 1998). ISBN 978-93-85998-89-8.