Exercises: Structural isomerism and stereoisomerism

Organic compounds of molecular formula $\mathrm{C_3H_6O}$ show different types of structural isomerism depending on the connectivity of their atoms and the functional groups present.

1. a) Write the semi-structural formula and the systematic name of an aldehyde and of a ketone that share this molecular formula, stating the type of isomerism between them.
2. b) Write the semi-structural formula and the systematic name of an unsaturated acyclic alcohol with the same molecular formula $\mathrm{C_3H_6O}$.

Butan-2-ol is a secondary alcohol of molecular formula $\mathrm{C_4H_{10}O}$.

1. a) Write its semi-structural formula and mark with an asterisk (*) the carbon that acts as the chiral centre (asymmetric carbon), justifying why it meets this condition.
2. b) Deduce whether an enantiomerically pure sample of butan-2-ol shows optical activity, and state what the two non-superimposable spatial forms that are mirror images of each other are called.

The following illustration shows the spatial configurations of two unsaturated hydrocarbons labelled A and B. Observe carefully the arrangement of their atoms.

1. a) Identify the type of isomerism between compounds A and B, and assign to each structure its complete systematic name, including the cis or trans descriptors.
2. b) Justify which of the two isomers will have an appreciable net dipole moment (polar molecule) and which will have zero dipole moment (non-polar molecule).

The saturated molecular formula $\mathrm{C_4H_{10}O}$ includes various constitutional isomers belonging to different oxygen-containing functional groups.

1. a) Write the semi-structural formula and the systematic name of the four possible isomeric alcohols with this formula, and state what type of structural isomerism (chain or position) exists between butan-1-ol and butan-2-ol, and between butan-1-ol and 2-methylpropan-1-ol.
2. b) Write the semi-structural formula and the systematic name of three isomeric ethers with this molecular formula. What type of isomerism exists between butan-1-ol and diethyl ether?

The E/Z nomenclature system proposed by Cahn, Ingold and Prelog (CIP) assigns priorities to the substituents attached to each carbon of the double bond according to decreasing atomic number ($Z$). If the two highest-priority groups on each carbon lie on the same side of the double bond, it is designated (Z), and if they are on opposite sides, (E).

Consider 2-chlorobut-2-ene, whose molecular formula is $\mathrm{C_4H_7Cl}$.

Atomic number data: $Z(\mathrm{Cl}) = 17$; $Z(\mathrm{C}) = 6$; $Z(\mathrm{H}) = 1$.

1. a) Determine the order of priority of the two substituents on carbon 2 and on carbon 3 of the double bond.
2. b) Draw the semi-structural spatial arrangement and name, with justification, the two possible geometric stereoisomers using the (E) and (Z) notation.

You can do this on paper and send a photo with your answer.

Lactic acid (2-hydroxypropanoic acid) is a fundamental metabolite in anaerobic muscle biological processes and in lactic fermentation.

1. a) Write its semi-structural formula, identify its chiral centre and represent the pair of enantiomers in three dimensions using wedge formulae (bond in the plane, solid wedge towards the front and dashed wedge towards the back).
2. b) Explain which physical and chemical properties both pure enantiomers share in an achiral environment, in which measurable physical property they differ quantitatively, and what the macroscopic behaviour of an equimolar mixture of both (racemic mixture) is towards polarised light in a polarimeter.