Which of the following alkenes would produce two products when they undergo an addition reaction with hydrogen bromide? Which of the following alkenes would produce two products when they undergo an addition reaction with hydrogen bromide?


Ethene
But-1-ene
But-2-ene
Hex-3-ene


The correct answer (according to my textbook) is But-1-ene but I do not understand why this is.
Well as mithoron said.its about symmetry.Let's see how but-1-ene comes out to be the answer

On addition of HBr markownikov rule is followed but lets draw both product.

1.)CH2=CH2 + HBr--->Br-CH2-CH3 + CH3-CH2-Br  (both products are 1-bromoethane)

2)CH2=CH-CH2-CH3 + HBr--->Br-CH2-CH2-CH2-CH3 + CH3-BrCH-CH2-CH3 (notice the                name of both the product. 1st one is 1-bromobutane. 2nd one is 2-bromobutane)

3)CH3-CH=CH-CH3 + HBr---->CH3-BrCH-CH2-CH3 + CH3-CH2-BrCH-CH3 (both products are same i.e both are 2-bromobutane)

4)CH3-CH2-CH=CH-CH2-CH3 + HBr--->CH3-CH2-BrCH-CH2-CH2-CH3 +
                    CH3-CH2-CH2-BrCH-CH2-CH3 (here also both the compounds are same i.e both are 3-bromohexane)

So what distinguishes the 2nd one from these three is that the second molecule  is unsymmetrical.You can find its unsymmetrical by looking at the no:of carbon to both side of double bond.If they are equal then molecule is symmetrical.