The major product of the reaction between 2-methyl-2-pentanol and concentrated HBr is 2-bromo-2-methylpentane.The reaction proceeds via an S_N1 mechanism. In the first step, the hydroxyl group of 2-methyl-2-pentanol is protonated by HBr, forming a good leaving group water . The protonated alcohol then undergoes a unimolecular nucleophilic substitution S_N1 reaction, in which the water molecule leaves, forming a carbocation intermediate. The carbocation is stabilized by the two methyl groups adjacent to the positively charged carbon atom. Finally, the bromide ion Br- from the HBr attacks the carbocation, resulting in the formation of 2-bromo-2-methylpentane as the major product.