Consider the compounds represented as A and B below: (a) What is the celationship between A and B? ( b) Name A and B. (c) Will the chemical properties of A ...

Both A and B have the same molecular formula, C2H2Cl2, with one H and one Cl on each doubly bonded carbon. They differ only in how the two chlorine atoms are arranged about the rigid C=C double bond.

• A: the two Cl atoms are on opposite sides of the double bond (one up, one down).
• B: the two Cl atoms are on the same side of the double bond (both down).

(a) Relationship between A and B

They are geometric (cis-trans) isomers, a form of stereoisomerism. Restricted rotation about the C=C double bond fixes the positions of the substituents, so the two arrangements are distinct compounds with the same structural formula.

(b) Names

• A = trans-1,2-dichloroethene (Cl atoms on opposite sides).
• B = cis-1,2-dichloroethene (Cl atoms on the same side).

(c) Are the chemical properties the same?

Yes, A and B have essentially the same chemical properties. Reason: they possess the same functional group (a C=C double bond) and the same number and type of atoms and bonds, so they undergo the same chemical reactions. Their differences are mainly physical (for example melting point, boiling point and polarity: the cis form is polar while the trans form is non-polar), not chemical.