Cis-trans Isomerism

1. Requirements for Cis-trans Isomerism

o Carbon-to-carbon double bonds (C=C)
o There must be two nonidentical groups on each doubly bonded carbon atom.

01 Multiple Choice

2. Identifying Cis or Trans Isomers

Consider a compound with 4 different groups attached to the C = C bond

• Consider each of the double bonds separately.
• Identify the two atoms directly attached to each carbon in the double bond
• If the two atoms with larger atomic numbers are on the same side:
  • It is a cis isomer.
• If the two atoms with larger atomic numbers are on opposite sides:
  • It is a trans isomer.

Note: Show clear trigonal planar shape around the 2C atoms of C=C bond

01 Multiple Choice

3. C4H8 (Consider alkene and non-cyclical compound only)

Chain isomerism
1-Butene
	\(\xrightarrow[isomerism]{Geometric}\)
2-Butene (cis)		2-Butene (trans)
2-Methyl-1-propene

4. No. of Cis-trans Isomers

5. Polarity in Cis-trans Isomers

Cis-isomer	Trans-isomer
Dipole moments do not cancel out o Net dipole present o Polar	Dipole moments cancel out o No net dipole present o Non-polar

X is more electronegative than Y

01 Multiple Choice

6. Effect of Polarity on Boiling Point

Cis-isomer	Trans-isomer
Effect on boiling point: Boiling involves overcoming the stronger permanent dipole - permanet dipole attractions between molecules. o More energy is needed to separate the molecules o Higher boiling point	Effect on boiling point: Boiling involves overcoming the weaker induced dipole - induced dipole attractions between molecules. o Less energy is needed to separate the molecules o Lower boiling point