At low temperatures, the addition of hydrogen halides to conjugated double bonds is determined by kinetic control.  When a hydrogen halide reacts with a conjugated system, the 1,2 addition product predominates.  The accepted explanation is that the activation energy needed to add across the double bonds is lower at the secondary carbocation than at the primary carbocation from 1,4 addition. This is due to the greater charge on that carbon.  However, a recent study has suggested that proximity to the electrophilic attack site, not charge, is the driving force behind the preference.  Current organic chemistry textbooks are split between these two explanations for this phenomenon. In the current study, we are exploring whether charge does play a role in the addition reaction. In an attempt to better our understanding of this reaction, we are studying the addition of HBr to 2,5-dimethyl -2,4-hexadiene. This substrate moves the greater charge to carbon 4, allowing us to see if that charge plays a significant role in the determination of product ratios.

Addition of hydrogen halides to conjugated double bonds under kinetic control

247th National Meeting (2014)

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