[CrazyAssasin]

Hi, I have a question about substitution reactions. I have a secondary alcohol in HBr acid. Does the alcohol condensate and form ether or does it lose OH and join Br? More accurately, does Sn1 or Sn2 mechanism dominate?

[orgo814]

Usually primary condensates. I would say lose OH and join Br would be the major reaction here.

[BARAJAN]

for compounds which can form more stable carbocation SN1 is more favorable and the reverse is true for SN2

[discodermolide]

Just a point, the compounds do not know that they can form a stable carbocation
S_n1 and S_n2 refer to the reaction kinetics and ultimately allow the suggestion of a mechanistic pathway.

[souro10]

Whether condensation will occur or not is one question , and SN1 or SN2 is another question. Temperature and molar ratios are important factors which have not been mentioned. Considering the probability of intramolecular dehydration to form ethers to be less for secondary ones, because both the substrate and the base would be hindered, substitution by bromine is more plausible after the OH group gets protonated. Here it can be in two  ways, first water molec leaves, producing a carbocation which is attacked by bromine, or a concerted mechanism. The concerted mechanism happens to be more fast, because, producing a secondary carbocation need much more energy.

[souro10]

Edit: intermolecular* dehydration in second line.
Further, intermolecular dehydration is disfavored by the fact that - both the concentration of bromide ion, and the strength of the bromide ion is more than that of the second molecule of alcohol.. plus they are hindered. Hence, SN2 substitution of C-OH2 into C-Br takes place.