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Zippers: Base Catalysis

Peter Atkins

in Reactions: The private life of atoms

A base, you should recall from Reaction 2, is the second hand clapping to the acid’s first. That is, whereas an acid is a proton donor, a base is its beneficiary as a proton acceptor. The paradigm ... More

A base, you should recall from Reaction 2, is the second hand clapping to the acid’s first. That is, whereas an acid is a proton donor, a base is its beneficiary as a proton acceptor. The paradigm base is a hydroxide ion, OH–, which can accept a proton and thereby become H2O. However, in the context of catalysis, the topic of this section, its role is rather different: instead of using its electrons to accept the proton, it uses them to behave as a nucleophile (Reaction 15), a searcher out of positive charge. Instead of forming a hydrogen–oxygen bond with an incoming proton, it sets the electronic fox among the electronic geese of a molecule by forming a new carbon–oxygen bond and thereby loosening the bonds to neighbouring atoms so that they can undergo rearrangement. The OH– ion in effect unzips the molecule and renders it open to further attack. Base catalysis has a lot of important applications. An ancient one is the production of soap from animal fat. To set that scene, I shall consider a simple model system, the ‘hydrolysis’ (severing apart by water) of the two components of an ester, 1 (the same compound I used in Reaction 17, a combination of acetic acid and ethanol), and then turn to soap-making itself. You saw in Reaction 17 how esters can be broken down into their components, a carboxylic acid and an alcohol, by an acid; here we see the analogous reaction in the presence of a base. To be specific, the reagent is a solution of sodium hydroxide, which provides the OH– ions that catalyse the reaction. We watch what happens when a solution of sodium hydroxide is added to an ester and the mixture is boiled. The O oxygen atoms of the ester have already ripened the molecule for nucleophilic attack by drawing some of the electron cloud away from the C atom to which they are both attached, leaving it with a partial positive charge, 2. The negatively charged OH– ion sniffs out that positive charge and jostles in to do its business. Less

Venice and Milan (1811–1814)

Richard Osborne

in Rossini: His Life and Works

Gioachino Rossini returned home from Venice with money in his pocket and the hope of a new commission. Unfortunately, Bologna was no longer the best place to be. By 1811 Milan had taken over as ... More

Gioachino Rossini returned home from Venice with money in his pocket and the hope of a new commission. Unfortunately, Bologna was no longer the best place to be. By 1811 Milan had taken over as continental Europe’s principal meeting place for impresarios and agents. Bologna was becoming a bit of a backwater. While he waited, Rossini rehearsed and directed an Italian-language performance of Joseph Haydn’s The Seasons sponsored by the Accademia dei Concordi. He may also have written the six-movement showpiece cantata for soprano, chorus, and orchestra, La morte di Didone (“The Death of Dido”), which he presented to Domenico Mombelli’s daughter, Ester Mombelli. If he did write it in 1811, it offers a remarkable glimpse of things to come.Less

Miscellanea

in Jewish Poet and Intellectual in Seventeenth-Century Venice: The Works of Sarra Copia Sulam in Verse and Prose Along with Writings of Her Contemporaries in Her Praise, Condemnation, or Defense

This part includes sonnets by Gabriele Zinano and Sarra Copia Sulam's response to one of them. Leon Modena, the rabbi closely involved in Sulam's biography, was privy to the content of the letters ... More

This part includes sonnets by Gabriele Zinano and Sarra Copia Sulam's response to one of them. Leon Modena, the rabbi closely involved in Sulam's biography, was privy to the content of the letters she wrote to Ansaldo Cebà and received from him, and to her problems with her adversaries. Also included in this part are a dedication to Copia from Modena's play Ester (1619) and Modena's epitaph for her tombstone (1641).Less

Fasteners: Acid Catalysis

Peter Atkins

in Reactions: The private life of atoms

I explained the general basis of catalysis in Reaction 11, where I showed that it accelerated a reaction by opening a new, faster route from reactants to products. One of the ways to achieve ... More

I explained the general basis of catalysis in Reaction 11, where I showed that it accelerated a reaction by opening a new, faster route from reactants to products. One of the ways to achieve catalysis in organic chemistry is to carry out a reaction in an acidic or basic (alkaline) environment, and that is what I explore here. In Reaction 27 you will see the enormous importance of processes like this, not just for keeping organic chemists productive but also for keeping us all alive; I give a first glimpse of that later in this section too. Various kinds of acid and base catalysis, sometimes both simultaneously, are going on throughout the cells of our body and ensuring that all the processes of life are maintained; in fact they are the very processes of life. I deal with acid catalysis in this section and base catalysis in the next. The point to remember throughout this section is that an acid is a proton donor (Reaction 2) and a proton is an aggressive, nutty little centre of positive charge. If a proton gets itself attached to a molecule, it can draw electrons towards itself and so expose the nuclei that they formerly surrounded. That is, a proton can cause the appearance of positive charge elsewhere in the molecule where the nuclei shine through the depleted fog of electrons. Because positive charge is attracted to negative charge, one outcome is that a molecule may be converted into a powerful electron-sniffing electrophile (Reaction 16). Another way of looking at the outcome of adding a proton is to note that a C atom with a positive charge is a target for nucleophilic missile attack (Reaction 15). Therefore, if a proton draws the electron cloud away from a nearby atom, then its presence is like a fifth-column agent preparing a target for later attack. Let’s shrink and watch as some acid is added to a molecule that contains a –CO– group, such as acetic acid. The protons provided by the added acid are riding on water molecules, as H3O+ ions, and arrive in the vicinity of the acetic acid molecule. Less

The Attractions of the North: Early Film Expeditions to the Exotic Snowscape

Marina Dahlquist

in Films on Ice: Cinemas of the Arctic

This chapter examines a large number of actualités - short non-fiction films from the early history of cinema - set in the Scandinavian Arctic. Exemplifying with early documentaries made by primarily ... More

This chapter examines a large number of actualités - short non-fiction films from the early history of cinema - set in the Scandinavian Arctic. Exemplifying with early documentaries made by primarily Swedish and French film companies, including Pathé Frères, Svenska Biografteatern, Svensk Kinematograf and Svensk Filmindustri, Dahlquist discusses the exoticization and racialization of the Sámi population that were constituent of the these early films. Such tropes, Dahlquist shows, were common also in other forms of visual mass media at the time, exhibits at the Stockholm open-air museum Skansen included. The chapter traces production, distribution, circulation, and reception history of these films. Dahlquist also presents key thematic and visual components of Victor Sjöström’s silent drama film Daughter of the Peaks (1914).Less

Farm Worker Futurisms in Speculative Culture: George Lucas and Ester Hernandez

Curtis Marez

in Farm Worker Futurism: Speculative Technologies of Resistance

In Chapter Three, I suggest that the technological visions of California agribusiness in the films of George Lucas and the art of Ester Hernandez connect agribusiness to influential forms of white ... More

In Chapter Three, I suggest that the technological visions of California agribusiness in the films of George Lucas and the art of Ester Hernandez connect agribusiness to influential forms of white male agrarian populism that supported the Cold War militarism of the 1980s.Less

Reactivity of superoxide ion

Donald T. Sawyer and R. J. P. Williams

in Oxygen Chemistry

Reduction of dioxygen by electron transfer yields superoxide ion (O2-.), which has its negative charge and electronic spin density delocalized between the two oxygens. As such it has limited ... More

Reduction of dioxygen by electron transfer yields superoxide ion (O2-.), which has its negative charge and electronic spin density delocalized between the two oxygens. As such it has limited radical character [H-OO bond energy ΔGBF, 72 kcal]2 and is a weak Bransted base in water . . . HOO· → H+ + O2-. Kdiss, 2.0 × 10-5 (7.1) . . . The dynamics for the hydrolysis and disproportionation of O2-. in aqueous solutions have been characterized by pulse radiolysis. For all conditions the rate-limiting step is second order in O2-. concentration, and the maximum rate occurs at a pH that is equivalent to the pKa for HOO· (it decreases monotonically with further decreases in the hydrogen ion concentration). Less

Rate Coefficients and Mechanisms of Atmospheric Oxidation of the Esters

Jack Calvert, Abdelwahid Mellouki, John Orlando, Michael Pilling, and Timothy Wallington

in Mechanisms of Atmospheric Oxidation of the Oxygenates

Esters are emitted directly into the atmosphere from both natural and anthropogenic sources and are produced during the atmospheric oxidation of ethers. Methyl acetate and ethyl acetate have found ... More

Esters are emitted directly into the atmosphere from both natural and anthropogenic sources and are produced during the atmospheric oxidation of ethers. Methyl acetate and ethyl acetate have found widespread use as solvents. Vegetable oils and animal fats are esters. Transesterification of vegetable oils and animal fats with methanol gives fatty acid methyl esters (FAMEs) which are used in biodiesel. Many esters have pleasant odors and are present in essential oils, fruits, and pheromones, and are often added to fragrances and consumer products to provide a pleasant odor. Table VII-A-1 provides a list of common esters and their odors. It is surprising to note that despite their ubiquitous nature, volatility, and fragrance, it is only very recently that quantitative measurements of esters in ambient air have been reported (Niedojadlo et al., 2007; Legreid et al., 2007). The atmospheric oxidation of saturated esters is largely initiated by OH radical attack. Reaction with O3 and NO3 radicals contributes to the atmospheric oxidation of unsaturated esters. As discussed in chapter IX, UV absorption by esters is only important for wavelengths below approximately 240 nm and, hence, photolysis is not a significant tropospheric loss mechanism. When compared to the ethers from which they can be derived, the esters are substantially less reactive towards OH radicals. The ester functionality —C(O)O— in R1C(O)OR2 deactivates the alkyl groups to which it is attached with the deactivation being most pronounced for the R1 group attached to the carbonyl group. The atmospheric oxidation mechanisms of the esters are reviewed in the present chapter. The reaction of OH with methyl formate has been studied by Wallington et al. (1988b) and Le Calvé et al. (1997a) over the temperature range 233–372 K. Data are summarized in table VII-B-1 and are plotted in figure VII-B-1. The room temperature determination of k(OH + CH3OCHO) by Wallington et al. is in agreement with that by Le Calvé et al. (1997) within the experimental uncertainties. Significant curvature is evident in the Arrhenius plot in figure VII-B-1. Less