Isomerism: structural and stereoisomerism

State the three different types of structural isomerism

Structural isomerims are compounds that have the same molecular formula but different structural formulae.

There are three different types of structural isomerism including chain, positional and functional group isomerisms.

State the type of isomerism that occurs between the following pairs of compounds.

A. 2-methylpentane and 2,3-dimethylbutane

B. Butan-1-ol and butan-2-ol

A. The type of isomerism that occurs between these compounds is chain isomerism which has the same molecular formula but not same length of hydrocarbon chain

B. The type of isomerism that occurs between these compounds is positional isomerism which has differences in the position of a functional group in each isomer

A. Name the two types of stereoisomers

B. Describe what a chiral centre is

A. The two types of stereoisomers which are described as the same structural formula but having differences in arrangement of atoms in 30 space are geometric and optical

B. A chiral centre is a carbon atom with the four different groups attached.

The structure of one optical isomer of a chlorofluorocarbon is shown in the figure below

Draw the structure of the other enantiomer.

C₄H₈ exists as several isomers.

A. Name the homologous series that C₄H₈ belongs to.

B. Draw the skeletal formulae of two cyclic C₄H₈ isomers and two C₄H₈ isomers that show cis / trans isomerism.

A. The homologous series that C₄H₈ belongs to alkenes

B. 

Explain why compound Z-2-chloro-but-2-en-1-ol exists as two stereoisomers, but compound 3-methyl-but-2-enoic acid does not.

Skeletal formula of Z-2-chloro-but-2-en-1-ol

Skeletal formula of 3-methyl-but-2-enoic acid

Because there is restricted rotation by the presence of π bonds in the C=C double bond. It’s easily noticed that there are 4 different substituted groups on C=C bond of Z-2-chloro-but-2-en-1-ol whereas, there are same functional group of methyl on one side on C=C bond of 3-methyl-but-2-enoic acid. Therefore, compound Z-2-chloro-but-2-en-1-ol exists as two stereoisomers, but compound 3-methyl-but-2-enoic acid does not.

Draw the two isomers of compound 2-chloropropanal  to explain how this type of isomerism occurs.

This type of isomerism occurs because of the chiral centre which is a carbon atom having 4 different atoms or groups attached. Therefore, there are two molecules that are non-superimposable mirror images of each other.

A molecule can have optical isomerism and geometrical isomerism. Which of the compounds below show both?

A. CH₃CH₂CHBrCH=CHBr

B. CH₃CBr=CBrCH₃

C. CH₃CH=CHCH₂CH₃

D. CH₃CHBrCH=CH₂

The answer is A

If a molecule contains a carbon atom that is bonded to four different atoms or groups of atoms which refers to as the chiral centre of the molecule, it can form two optical isomers. 

The term geometric isomerism refers to complexes with the same molecular formula but different geometrical arrangements of their atoms, more specific is due to the restriction surrounding the C=C double bond

B is incorrect because this has no chiral carbons

C is incorrect because this has no chiral carbons

D is incorrect because this has no cis-trans isomerism due to the presence of 3 H groups surrounding C=C double bond.

Which carbon atom is the chiral centre in the following molecule?

A. A

B. B

C. C

D. D

The answer is C

A chiral centre is a carbon atom with the four different groups attached.

A is incorrect because this carbon has two hydrogen atoms attached

B is incorrect because this carbon has C=O carbonyl group attached

D is incorrect because this carbon has two hydrogen atoms attached

2,2-dimethylpropane is an isomer of pentane.

What type of isomerism is shown by 2,2-dimethylpropane?

A. Stereoisomerism

B. Functional group

C. Cyclic

D. Chain

The answer is D

A is incorrect because 2,2-dimethylpropane is not a stereoisomer

B is incorrect because there is not a different functional group in 2,2-dimethylpropane compared to pentane

C is incorrect because cyclic is not a type of isomerism.