The IUPAC name of the given compounds,C4H3​−C3H=C2H−C1H2​−Br is 1-bromobut-2-ene. If the numbering of the carbon atoms starts from bromide attached carbon,that is C1H3​−C2H=C3H−C4H2​−Br, the numbering will be incorrect order of numbering because, functional groups are not getting least number.
