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Carboranes: the strongest Bronsted acids in alcohol dehydration

机译：碳硼烷:最强的布仑斯惕酸酒精脱水

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Alcohol dehydration is an important reaction for the production of olefins (polymer feedstock) and known to evolve on solid acids through the formation of carbenium ions. In this work, we employed density functional theory calculations to study the dehydration mechanisms of biomass-derived alcohols catalyzed by fully halogenated carborane superacids, the strongest known Bronsted acids to date. We considered three carborane acids with shells consisting of fluorine, chlorine and bromine as well as alcohols of different substitution primary, secondary and tertiary). Our results demonstrate that the fluorinated carborane is the most active alcohol dehydration catalyst, with the reaction following an intramolecular H-beta elimination mechanism. We found that water, a dehydration product, competes with alcohols for the catalytic Bronsted acid) sites, increasing at the same time the dehydration barriers. Due to the increased stability of the conjugate base of the carboranes, the acid can dissociate under dehydration conditions and H3O+ can act as the acid catalyst, in addition to the non-dissociated carborane. Solvation by polar protic solvents decreases the reaction barriers due to stabilization of the carbenium ions formed at the transition states. We show strong linear correlations between calculated reaction barriers and carbenium ion stability, a known alcohol reactivity descriptor. Interestingly, by calculating the alcohol dehydration slopes on various Lewis and Bronsted catalysts, we reveal a slope dependence on the degree of carbenium ion character at the transition state, which in turn, depends on the reaction mechanism. This slope dependence can be attributed to the degree of charge separation at the transition states. As alcohol dehydration mechanisms can be very difficult to elucidate experimentally, this observation opens new avenues for identifying potential dehydration mechanisms.
机译：酒精脱水是一个重要的反应烯烃聚合物原料和生产已知在固体酸通过进化carbenium离子的形成。采用密度泛函理论计算研究了脱水的机制由完全biomass-derived醇催化卤化碳硼烷超强酸,最强的迄今为止已知的布仑斯惕酸。碳硼烷酸和壳组成的氟、氯、溴不同的替换主要醇,二、三级)。氟化碳硼烷是最活跃的酒精脱水催化剂,反应后一个分子内hβ消除机制。产品,可与醇类催化布仑斯惕酸)的网站,同时增加脱水的壁垒。稳定的共轭碱碳硼烷,酸可以分离脱水条件和H3O +可以作为酸催化剂,除了non-dissociated碳硼烷。由于降低了反应障碍稳定carbenium离子形成的过渡状态。之间的相关性计算反应的障碍和碳正离子稳定性,一个已知的酒精反应性描述符。计算酒精脱水斜坡各种各样的刘易斯和布仑斯惕催化剂,我们揭示了斜率的依赖程度的碳正离子角色的过渡状态,反过来,取决于反应机理。依赖可以归因于的程度电荷分离的过渡状态。酒精脱水机制非常很难通过实验阐明,这一点观察了识别的新途径潜在的脱水机制。

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