Rank the following alkenes in order of increasing stability of the double bond towards addition of HBr: 2,3-dimethyl-2-butene, cis-3-hexene, 3-methyl-3-hexene, 1-hexene

2,3-dimethyl-2-butene > 3-methyl-3-hexene > cis-3-hexene > 1-hexene

Explanation:

According to Saytzeff rule, the more highly substituted an alkene is, the more stable it is. Since this is so, 2,3-dimethyl-2-butene will be the most stable of all the alkenes listed because it is the most substituted alkene.

Let us also note that terminal alkenes are the least stable because the pi bonds of the alkenes are least stabilized by alkyl groups. This implies that 1-hexene is the least stable alkene among the listed alkenes.