Question

What is the IUPAC name of the following compound?
$H_{3}C-CH=CH-\stackrel{C{H}_3}{\stackrel{|}{C}{H}_2}$

• A. $Pent-2-ene$
• B. $Pent-1-ene$
• C. $Pent-3-ene$
• D. $1-methyl-but-2-ene$

Answer 1

The correct option is A $Pent-2-ene$

The correct IUPAC name is Pent−2−ene.
$H_3\underset{\text{1}}{C}-\underset{\text{2}}{C}H=\underset{\text{3}}{C}H-\stackrel{\underset{\text{5}}{C}{H}_3}{\stackrel{|}{\underset{\text{4}}{C}}{H}_2}$
The parent hydrocarbon contains 5 C atoms and a double bond. Hence, it is pentene.
The carbon carbon double bond is present between second and third C atom. Hence, the locant 2 is added as prefix. The numbering is done from the end which gives lowest locants to C atoms joined by double bond.