so does 2,5-dimethylhex-4-yn reacted with HBr have stereoisomers or not? I thought it would have E and Z because there is a carbo cation and HBr obeys Markovnikov and its can either go the syn and anti routes. But someone i compared with said that wasn't the case here because there is no steric hinderance so there is no stereoselectivity. I can't come to terms with this pls help

I take it that we are to assume there is only one addition in this example? Addition of HX to alkynes can give single or double addition products, but stereochemistry only applies to the former case.

You are correct that the carbocation in theory would provide no facial preference for addition of the bromide ion. However, you need to consider the stability of the products that form, as that can often determine selectivity. Which isomer do you think is more stable, and which do you think would therefore form preferentially?