why does 2-butene have cis and trans isomers but 1-butene doesn't?
To understand why 2-butene (C4H8) has cis and trans isomers while 1-butene (C4H8) does not, let's first examine their molecular structures.
Both 1-butene and 2-butene are alkenes, which means they have a double bond between two carbon atoms. Let's represent the carbon atoms as C1, C2, C3, and C4 in the butene molecules.
In 1-butene, the double bond exists between C1 and C2:
CH3CH2CH=CH2
Since the two substituents (CH3 and CH2CH3) attached to the carbon atoms forming the double bond (C1 and C2) are on the same side of the double bond, there is no possibility of rotation around the double bond to give different spatial arrangements. Therefore, 1-butene does not exhibit cis-trans isomerism.
In 2-butene, the double bond exists between C2 and C3:
CH3CH=CHCH3
Now, there are two different substituents attached to the double bond: CH3 and CH3CH3. The spatial arrangement of these substituents can differ based on the orientation of the double bond.
If the two CH3 groups are on the same side of the double bond, we have the cis isomer:
CH3CH=CHCH3 (cis-2-butene)
If the two CH3 groups are on opposite sides of the double bond, we have the trans isomer:
CH3CH=CHCH3 (trans-2-butene)
Since cis- and trans- isomers exist only in compounds where there is restricted rotation around the double bond, 2-butene can exhibit cis-trans isomerism.
In summary, the presence of different substituents on the carbon atoms forming the double bond in 2-butene allows for the possibility of cis and trans isomers, while the same-side substituents in 1-butene prevent this isomerism.