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In the simple formalism proposed independently by Bronsted and Lowry in 1923 an acid was defined as a proton donor and a base was defined as a proton acceptor In the simple acid-base reaction shown below H3O is termed a Bronsted Acid and HO- a Bronsted Base In writing organic reaction mechanisms the flow of electrons is often shown using curved arrows and in the example shown the arrows are designed to show that an unshared pair of electrons from hydroxide anion moves to abstract a proton from H3O with the simultaneous movement of an electron pair from the bonding orbital to form an unshared pair of electrons on oxygen Acid-base reactions are by definition equilibria and the ratio of products and reactants from the proton transfer reaction is given by the equilibrium constant according to the equation shown below In the reactions shown above the two-carbon carboxylic acid acetic acid more correctly ethanoic acid acts as a Bronsted acid and donates a proton to the Bronsted base water The products of the reaction are the carboxylate anion acetate or ethanoate anion and H3O The equilibrium constant Ka reflects the position of this equilibrium and the larger the value of Ka the stronger a given acid will be For the example shown Ka 10-476 that is the equilibrium greatly favors the undissociated form of the carboxylic acid This is because the proton which is the product of the reaction is a stronger acid than the carboxylic acid Ka for the proton is 10174 and equilibria tend to adjust themselves to favor the formation of the most stable forms of all participants Since Ka is typically a very small number for most common organic acids it is routinely represented as the negative logarithm of Ka that is as the pKa For the example shown above a Ka of 10-476 translates to a pKa of 476 Therefore in this convention the
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