Functional Isomerism

When two or more compounds have the same molecular formula but distinct structural formulas, this is referred to as structural isomerism.

They’re best appreciated in terms of their visual or fully shown structural formula, but I’ve opted to exhibit them in a number of formats that you’ll need to read, including skeleton formulae.

When isomers are compared, each molecule has at least one atom linked to a distinct atom.

The structural isomerism sub-divisions (a) through (d) are discussed below.

a.Chain isomerism  

b.Tautomerism  

c.Positional isomerism  

d.Functional group isomerism

Functional group isomers of C2H6O

(1) ethanol, an alcohol, boiling point 79oC, ,

(2) methoxymethane, an ether, boiling point -25oC, ,

The highly polarised Oδ–Hδ+ bond arises from the difference in oxygen/hydrogen electronegativity (O>>H).

As a result, alcohol molecules are far more polar than ethers, and ‘hydrogen bonding’ (although misnamed!) occurs between alcohol molecules. The greatest intermolecular force is hydrogen bonding, and the enhanced intermolecular forces that arise elevate the boiling point of alcohol significantly when compared to the isomeric ether.

Lower alcohols (water-alcohol H bonding) are more soluble in the highly polar solvent water than the less polar ether molecules.

The Cδ+-Oδ- is polar, but the two dipoles of the C-O-C linkage tend to cancel each out.

Apart from burning, these structural variations result in quite distinct chemical reactions and products! Alcohols have a wide chemistry due to the C-OH group, whereas ethers have a restricted chemistry due to the absence of the C-OH group. Ethers, on the other hand, are extremely helpful as solvents for other reactants due to their lack of reactivity and chemistry!

(a) Alcohols, such as (1), can produce esters by reacting with carboxylic acids via the -OH group, but ethers cannot.

CH3CH2OH + CH3COOH ==> CH3COOCH2CH3 + H2O

When ethanol is burned with ethanoic acid and a little conc. sulfuric acid, the ester ethyl ethanoate is formed.

(b) Alcohols, but not ethers, can be dehydrated to create alkenes.

CH3CH2OH ==> CH2=CH2 + H2O

When ethanol is burned with concentrated sulfuric acid, it produces ethene.

b) Alcohols react quickly with sodium metal, whereas ethers do not.

2CH3CH2OH + 2Na ==> CH3CH2O-Na+ + H2 

Ethanol forms the salt sodium ethoxide and hydrogen.

Functional group isomers of  C3H6O2 

(1) , Propanoic acid (a carboxylic acid), bpt 141oC, very polar, high bpt relative to others except (4) owing to hydrogen bonding (throughOδ–Hδ+), acidic characteristics via -COOH group, fizzing with metals/carbonates, and forms esters with alcohols

(2) , methyl ethanoate (an ester), bpt 57.5oC, pleasant odour liquid, hydrolyzes to form ethanoic acid and methanol Hydrogen bonding isn’t an option. (no Oδ–Hδ+ )

(3) ,ethyl methanoate (an ester), bpt 54°C, pleasant odour liquid, hydrolyzes to create methanoic acid and ethanol Hydrogen bonding isn’t an option. (no Oδ–Hδ+)

(4) , 1-hydroxypropanone (a bifunctional alcohol/ketone), very polar, bpt 146oC, high bpt relative to others except (1) because of hydrogen bonding (through  Oδ–Hδ+). The >Cδ+=Oδ- is also a highly polar bond.

It is a bi-functional group molecule that exhibits I alcohol chemistry, such as reacting with sodium, forming esters with carboxylic acids, and… (ii) ketone chemistry, such as nucleophilic addition of HCN, which produces yellow-orange ppt with 2,4-DNPH, but no reaction with ammoniacal silver nitrate (Tollen’s reagent) or Fehling’s solution.

(5) , 3-hydroxypropanal (a bi-functional alcohol/aldehyde), bpt ?,  Hydrogen bonding is possible (via Oδ–Hδ+) and the >Cδ+=Oδ- is also a highly polar bond.

It is a bi-functional group molecule that performs I alcohol chemistry, such as reacting with sodium, forming esters with carboxylic acids, and… (ii) aldehyde chemistry, such as nucleophilic addition of HCN, yellow-orange ppt with 24DNPH, silver mirror formation with ammoniacal silver nitrate (Tollen’s reagent), and brown ppt with Fehling’s solution.

(6) ,2-methoxyethanol (a bi-functional ether/aldehyde, bpt 56oC; the ether group does not add to or inhibit its reactions as an aldehyde; for example, it undergoes nucleophilic addition of HCN, gives yellow-orange ppt with 2,4DNPH, forms silver mirror with ammoniacal silver nitrate (Tollen’s reagent), and forms brown ppt with Fehling’s solution;

(7) , 1,3-dioxolane (a di-ether, -C-O-C-O-C- in ring) bpt 75oC, two ether linkages, limited to chemistry, shows none of the functional group chemistry of (1) to (4). Hydrogen bonding is not possible (no Oδ–Hδ+).

(8) , 1,2-dioxolane (an organic cyclic peroxide, -C-O-O-C in ring), bpt ?, a very unstable and reactive compound. Hydrogen bonding not possible (no  Oδ–Hδ+ )

 Functional group isomers of C3H6O

Although there are certain physical similarities, such as low boiling colourless polar liquids or gases, chemical similarities exist between (1) and (2), as well as (3) and (4), but there are substantial chemical differences between all four indicated below.

(1) , Propanal (an aldehyde), bpt 49oC, yields yellow-orange ppt with 24DNPH, forms the principal alcohol, propan-1-ol, on reduction, rapidly oxidised to propanoic acid, gives silver mirror with ammoniacal silver nitrate and red-brown ppt with Fehlings/reagent. Benedict’s What is the I2 reaction?

(2) , Propanone (a ketone), bpt 56oC, adds HCN to generate hydroxynitrile, provides yellow-orange ppt with 24DNPH, forms secondary alcohol, propan-2-ol, on reduction, NOT easily oxidised, NO silver mirror with ammoniacal silver nitrate, and NO red-brown ppt with Fehlings/reagent Benedict’s What is the I2 reaction?

(3) ,Prop-2-ene-1-ol (a bi-functional molecule alkene/alcohol or enol), bpt 97oC, higher bpt due to hydrogen bonding via -OH (not possible with 1 and 2 above), gives electrophilic addition reactions of Br2, H2O, HI, etc. like no other alkene, reacts with sodium to give H2 and forms esters with carboxylic acids or acid chlorides just like alcohols.

(4) , cyclopropanol (an alicyclic secondary alcohol, bpt?, very unstable, difficult to investigate, and easily isomerises to (1) propanal (see case study 1c.4 below).

A secondary alcohol, when oxidised to the ketone cyclopropanone, interacts with salt to create H2, and forms esters with carboxylic acids or acid chlorides in theory.

(5) , methoxyethene (a bi-functional ether-alkene), bpt 5oC, gives electrophilic addition reaction of Br2, H2O, HI etc. like any other alkene, but no aldehyde, ketone or alcohol chemistry.

(6) , 1,2-epoxypropane (a cyclic-ether), bpt 35oC, no alkene, aldehyde, ketone or alcohol chemistry.

(7) , 1,3-epoxypropane (a cyclic-ether), bpt 49oC,  no alkene, aldehyde, ketone or alcohol chemistry.

Conclusion 

The existence of various functional groups causes functional isomerism. The molecular formula of the functional isomers is the same, but the functional groups are different.

Physical and chemical characteristics differ amongst functional isomers.

1) The functional isomer of ethyl alcohol is dimethyl ether. C2H6O is the same chemical formula for both. They do, however, belong to separate functional groupings.

2) Acetaldehyde is an acetone functional isomer. They share the same chemical formula (C3H6O), but they have distinct functional groups. Acetaldehyde has an aldehyde functional group (-CHO), whereas acetone has a ketone functional group (>C=O).

3) Functional isomers include acetic acid and methyl formate. C2H4O2 is the same chemical formula for both. Methyl formate is an ester with a -COOCH3 group, whereas acetic acid is a carboxylic acid with a -COOH group.