Consider an unsymmetrical olefin, say propylene....#HBr# will add to give EITHER #"1-propyl bromide"#, or #"2-propyl bromide"#, viz.
#H_2C=CHCH_3 + HBr rarr underbrace(BrH_2C-CH_2CH_3)_"minor product"# #+underbrace(H_3C-CBrHCH_3)_"major product"#
The product distribution reflects the stability of the carbocation intermediate. The olefin is a nucleophile; it seeks an electrophile, and the protium ion of #HBr# is the likely target.....now it could react at #stackrel(2)C# of the olefin to give a primary carbocation....
#H_2stackrel(+)C-CH_2CH_3#
....or it could react at #stackrel(1)C# of the olefin to give a secondary carbocation....
#H_3C-stackrel(+)CHCH_3#
#Br^-# COULD react with EITHER of these intermediates.....
In terms of carbocation stability, #underbrace(3^@>2^@>1^@>H_3C^+)_(rarr"decreasing stability"rarr)#, and thus the most likely reaction pathway is thru the most stable intermediate, and in the given example this is via the #2^@# carbocation. And thus #"2-bromopropane"# should be the species that dominates in the product mix. And this is the experimental observation.
And see this old answer.
#"TO HIM THAT HATH HYDROGEN SHALL BE GIVEN"# #"HYDROGEN"#
And thus the observed #"Markownikow's rule"# reflects carbocation stability. Sometimes reagents can be added that give #"anti-Markownikow additions"#, for instance peroxides....I urge you to read the relevant chapter in your text.
