Organic chemistry — Question 142

Answer
Hex-1-ene, hex-2-ene and hex-3-ene are the three position isomers. Hex-2-ene has cis and trans forms.

Working
The distinct structures are $\mathrm{C}{\mathrm{H}}_2=\mathrm{C}\mathrm{H}\mathrm{C}{\mathrm{H}}_2\mathrm{C}{\mathrm{H}}_2\mathrm{C}{\mathrm{H}}_2\mathrm{C}{\mathrm{H}}_3$ (hex-1-ene), $\mathrm{C}{\mathrm{H}}_3\mathrm{C}\mathrm{H}=\mathrm{C}\mathrm{H}\mathrm{C}{\mathrm{H}}_2\mathrm{C}{\mathrm{H}}_2\mathrm{C}{\mathrm{H}}_3$ (hex-2-ene), and $\mathrm{C}{\mathrm{H}}_3\mathrm{C}{\mathrm{H}}_2\mathrm{C}\mathrm{H}=\mathrm{C}\mathrm{H}\mathrm{C}{\mathrm{H}}_2\mathrm{C}{\mathrm{H}}_3$ (hex-3-ene). Reversing the numbering gives no new position. In cis-hex-2-ene the methyl and propyl groups lie on the same side of C=C; in trans they lie on opposite sides.

Marking criteria

• Gives correct condensed structure and name for hex-1-ene. [1 mark]
• Gives correct condensed structure and name for hex-2-ene. [1 mark]
• Gives correct condensed structure and name for hex-3-ene. [1 mark]
• Draws and names cis-hex-2-ene with carbon groups on the same side. [1 mark]
• Draws and names trans-hex-2-ene with carbon groups on opposite sides. [1 mark]