Question

How many structures can be drawn that have the given molecular formula? How could you differentiate the molecules in laboratory?

A. C₂H₄BrClO

B. C₄H₁₀O

C. C₂H₇N

D. C₄H₁₁N

Answer 1

A. The formula of C2H4BrClO can correspond to alcohols and ethers. There are 6 isomers for C2H4BrClO.

Alcohols and ethers have different properties all because of the presence of -OH (hydroxy) group in alcohols and absence of that in ethers. Since there is no hydroxy group in ethers, they are less dense, less soluble in water, have lesser boiling points than their corresponding alcohols (because (no hydroxy group = no hydrogen bonding across = no symmetric packing) and they are aprotic solvents (because of lack of proton from hydroxy group).

Chemically, alcohols will react to a number of different entities (like acid, base, PCl5, Oxidising agents, reducing agents etc.) while ethers will mostly be inert.

Spectroscopically, alcohols and ethers can be differentiated by using IR. The characteristic IR absorption frequency of ether (C-O group) lies between 1000-1300 cm(-1) while for alcohols (-OH group), the range is from3200-3700 cm(-1).

The structures can also be differentiated using proton NMR spectroscopy.

B. The formula of C4H10O can correspond to alcohols and ethers. There are 7 isomers for C4H10O.

Alcohols and ethers can be differentiated physically, chemically and spectroscopically as discussed above.

The primary, secondary and tertiary (1⁰, 2⁰ and 3⁰ respectively) alcohols can be differentiated using Victor Meyer's test. Victor Meyer's test tells to add Phosphorus + Iodine (P + I₂ mixture), AgNO2 and HNO2 sequentially to the test tube containing the alcohol. The primary alcohol gives red colour, secondary gives blue while tertiary alcohol remains unchanged. There are many more reactions to check these alcohols.

C. The formula of C2H7N can correspond to amines. There are 2 isomers for C2H7N.

Amines can be differentiated physically, chemically and spectroscopically.

The primary, secondary and tertiary (1⁰, 2⁰ and 3⁰ respectively) amines have different physical properties. The primary amines are least basic while tertiary amines are most basic.

The primary, secondary and tertiary (1⁰, 2⁰ and 3⁰ respectively) amines can be differentiated using Hinsberg reaction which is essentially using benzene sulphonyl chloride reagent in alkaline solution. The primary amines are completely soluble, secondary amines make salts which precipitates while tertiary doesn't react

Spectroscopically, amines can be differentiated by using IR. The characteristic IR absorption frequency of amine (N-H group) lies between 3300-3500 cm(-1) and is quite medium than other functional groups.

The structures can also be differentiated using proton NMR spectroscopy.

D. The formula of C4H11N can correspond to amines. There are 8 isomers.

Amines can be differentiated physically, chemically and spectroscopically as discussed above.

Following are the structures: