Br0nsted-Lowry base reactions

Identify the two Br0nsted-Lowry bases in the following reaction ... 

One very important thing to remember is that an acid and base are always present in these reactions. In order for a molecule or ion to donate a proton, there has to be another ion or molecule to receive it. In addition, in reversible acid-base reactions, the roles of the substances as proton donor or acceptor will switch in the reverse reaction. In the example below, note how water (a Br0nsted-Lowry base in this reaction) accepts a proton to form a hydronium ion in the forward reaction. In the reverse reaction, the hydronium ion (Br0nsted-Lowry acid) donates a proton and, after losing the proton, becomes the water molecule once again. 

In the chemical reaction shown below, identify the Br0nsted-Lowry acid, the Br0nsted-Lowry base, the conjugate acid, and the conjugate base. 

Some of the terms associated with acids and bases have evolved specific meanings in organic chemistry. When organic chemists use the term base, they usually mean a proton acceptor (a Br0nsted-Lowry base). Similarly, the term acid usually means a proton donor (a Br0nsted-Lowry acid). When the acid-base reaction involves formation of a bond to some other element (especially carbon), organic chemists refer to the electron donor as a nucleophile (Lewis base) and the electron acceptor as an electrophile (Lewis acid). 

What is the key structural feature of all Br0nsted-Lowry bases How does this feature function in an acid-base reaction ... 

A Br0nsted-Lowry acid is defined as a proton donor, and a Br0nsted-Lowry base is a proton acceptor. A Lewis acid accepts an electron pair from a Lewis base, which is an electron pair donor. There may be confusion about how these definitions are related rather than about just how they differ. Part of the problem may lie with removing the base (water) from acid-base reactions done in water. When HCl reacts with water, the Br0nsted-Lowry definition states that the proton (H+)... 

If 44 reacts as a Br0nsted-Lowry base with a strong acid such as HCl, the conjugate acid product is an oxonium salt (46). Note the similarity of this reaction with that of water and HCl or alcohols with HCl. The conjugate base... 

An alkene has a C=C unit with a 7t-bond that is not polarized. The 7t-bond extends in space above and below the plane of the molecule, as in Figure 6.5. The 7t-bond is an electron-rich species, and those electrons can be donated to a positive center, initiating a chemical reaction. In other words, an alkene is a weak Br0nsted-Lowry base. The 7t-bond is a weak Brpnsted-Lowry base and reaction occurs only with a strong acid. This acid-base reaction is illustrated by ethene reacting with a proton H+ to give 55A. Donation of the electrons from the 7i-bond (blue arrow, see 56) to the acid (H of HCl) leads to formation of a new o-C-H bond (see 56), and a positive carbon C+ (a carbocation see Chapter 10, Section 10.2) on the other carbon atom of the C=C unit. Note that the positive carbon atom is represented as a p-orbital that has no electrons. Note also that the double-headed arrow indicates transfer of two electrons from C=C to W to form a new o-covalent bond. 

Inspection of chlorocyclopentane indicates that a new C-Cl bond is formed, and the Cl must come from HCl. The reaction leads to cleavage of the H-Cl bond. Similarly, the proton in chlorocyclopentane must come from HCl, and it is clear that the 7i-bond is broken. Indeed, the H and Cl are incorporated on the two carbons of the C=C unit. What kind of reaction is this One instantly recognizes that HCl is an acid, and if it reacts with cyclopentene, then the C=C unit probably reacts as a Br0nsted-Lowry base. Electron donation by the acid-base reaction will break the 7i-bond (the source of the two electrons) and form a new a-covalent C-H bond to electron-deficient H of HCl (the acid). As the C-H bond is formed, the H-Cl bond breaks, and both electrons in that covalent bond are transferred to Cl to give chloride ion. The analysis of this reaction is based on the knowledge that there is an intermediate, and it leads to the conclusion that carhocation 23 must he the intermediate. The structural... 

Alkenes react as Br0nsted-Lowry bases in the presence of strong mineral acids, HX. The reaction of alkenes and mineral acids (HX) generates the more stable carbocation, leading to substitution of X at the more substituted carbon. This constitutes the mechanism of the reaction, but it is given the name Markovnikov addition 1, 2, 3, 4, 5, 6, 65, 66, 67, 69, 71, 72, 75, 82, 83, 99,100,116. More highly substituted carbocations are generally more stable. An increase in the number of resonance contributors will increase the stability of a carbocation 7, 61, 68. 

When 2-methyl-2-propanol tert-hvXyl alcohol, 65) is treated with concentrated HCl, 2-chloro-2-methylpropane (2-chloro-2-methylpropane ter -butyl chloride, 93) is isolated in 90% yield. Similarly, when 1-butanol (94) is treated with 48% HBr in the presence of sulfuric acid, a 95% yield of 1-bromobutane (96) is obtained. In both reactions, the oxygen of the alcohol reacts as a Br0nsted-Lowry base in the presence of the protonic acids, HCl, or sulfuric acid. The fact that alkyl halides are produced clearly indicates that these are substitution reactions. In previous sections, tertiary halides gave substitution reactions when a nucleophilic halide ion reacted by an Sfjl mechanism that involved ionization to a carbocation prior to reaction with the halide. Primary halides react with a nucleophilic halide ion by an 8 2 mechanism. It is reasonable to assume that tertiary alcohols and primary alcohols will react similarly, i/the OH unit is converted to a leaving group. 

How are alkynes prepared Alkynes react as Br0nsted-Lowry bases in acid-base reactions with acids such as HBr or with aqueous acid. Alkynes react as Lewis bases with dihalogens such as Brg and in hydroboration reactions (see Chapter 10). Alkynes often come from reactions involving other alkynes, using the fact that a terminal alkyne is a weak acid, as introduced in Chapter 6 (Section 6.2.6). Treatment of a terminal alkyne with a base such as sodium... 

The chemical reactivity of a C=0 unit is related to the C=C unit discussed in Chapter 10. In that chapter, the C=C unit reacts as a base, and the jr-electrons are donated to an appropriate acid. The 7t-bond of an alkene is a Br0nsted-Lowry base in reactions with a protonic acid such as HCl, whereas the alkene reacts as a Lewis base in reactions with a Lewis acid such as boron (in a borane). A 7i-bond is present in the C=0 unit, so similar reactivity is expected. The oxygen atom of a carbonyl unit also has lone electron pairs, so the oxygen atom of the... 

There is, at best, a small concentration of 36 in the reaction. The hydronium ion (H3O+) is a strong acid and, similarly, the protons on the positively charged oxygen in 36 are quite acidic. The alkoxide unit RO is a good Br0nsted-Lowry base, and if it reacts with the acidic proton of a second molecule of oxonium ion 36, a new product (37) would be formed. Because 37 is also an oxonium ion, the proton of that ion is acidic and is removed by even a weak base such as the water present in the medium. Water is amphoteric (see Chapter 6, Section 6.2.5) and reacts as a base with 37 to give 38 and the hydronium ion. 

Returning to the problem at hand, both 2 and 3 react quickly and easily with HBr, so the ti-bonds in each molecule are considered to be good Br0nsted-Lowry bases in their reactions with mineral acids. Benzene does not react with HBr, even with heating. Benzene has six 7i-electrons 2 has only two, so benzene is more electron rich relative to 2. The fact that benzene does not react suggests that it is too weak a base, which is a good indication that the six p-electrons are held tightly by the molecule and are not available for donation. The poor basicity of benzene in a reaction with HBr is presented as evidence of the special stability of benzene, and the explanation for that stability is the resonance delocalization shown for 1C. 

Benzene is particularly stable because of resonance, which means that it is not as reactive as other molecrdes containing 7i-bonds (i.e., alkenes and alkynes). This is clearly demonstrated by mixing benzene with HBr, where there is no reaction at normal temperatraes. In other words, benzene is a very weak base due to the extra stability imparted by resonance (it is aromatic). Recall from Chapter 10 (Section 10.4.1) that cyclohexene reacts rapidly with Br2 to give a frons-dibromide. It is known that benzene does not react directly with bromine, so it is easy to conclude that it is a weak Lewis base as well as a weak Br0nsted-Lowry base. How can benzene be made to react Can the bromine reagent be modified so that it can react with a weak Lewis base such as benzene It is known that diatomic... 

Aluminum hydroxide is an amphoteric substance. It can act as either a Br0nsted-Lowry base or a Lewis acid. Write a reaction showing A1(0H)3 acting as a base toward and as an acid toward OH . 

The Lewis definition implies the presence of high electron density centres in Lewis bases, and low electron density centres in Lewis acids. In a reaction between a Lewis acid and a Lewis base the electron pair donated by the base is used to form a new sigma bond to the electron-deficient centre in the acid. The identification of Lewis bases follows basically the same guidelines as the identification of Br0nsted-Lowry bases. They frequently contain atoms that have non-bonding electrons, or lone pairs. In contrast Lewis acids frequently contain atoms with an incomplete octet, a full positive charge, or a partial positive charge. 

Given the formula of a Brpnsted-Lowry acid and the formula of a Br0nsted-Lowry base, write the net ionic equation for the reaction between them. 

The classical properties of adds and bases are listed in the introduction to the chapter. As an example of how a property relates to the ion associated with it, an acid-base neutralization is H+ + OH —> HjO. 3. An Arrhenius base is a source of OH ions, whereas a Br0nsted-Lowry base is a proton remover. The two are in agreement, as the OH ion is an excellent proton remover. Other substances, howeva-, can also remove protons, so there are other bases according to the Br0nsted-Lowry concept. 5. In the reaction shown below, AICI3, a Lewis add, accepts an electron pair from Q , a Lewis base, in a Lewis add—Lewis base neutralization reaction. 

When HNO3 reacts with water, water acts as a Br0nsted-Lowry base. Write the equation for the reaction. 

When an aqueous solution of KCN is added to a solution containing Ni ions, a precipitate forms, which redissolves on addition of more KCN solution. Write reactions describing what happens in this solution. (Hint CN is a Br0nsted-Lowry base 10 ] and a Lewis base.)... 

The Lewis definitions of acids and bases provide for a more general view of acid-base reactions than either the Arrhenius or Br0nsted-Lowry pic ture A Lewis acid is an electron pair acceptor A Lewis base is an electron pair donor The Lewis approach incorporates the Br0nsted-Lowry approach as a subcategory m which the atom that accepts the electron pair m the Lewis acid is a proton... 

Although the concepts of specific acid and specific base catalysis were useful in the analysis of some early kinetic data, it soon became apparent that any species that could effect a proton transfer with the substrate could exert a catalytic influence on the reaction rate. Consequently, it became desirable to employ the more general Br0nsted-Lowry definition of acids and bases and to write the reaction rate constant as... 

The limitations of the Arrhenius theory of acids and bases are overcome by a more general theory, called the Bronsted-Lowry theory. This theory was proposed independently, in 1923, by Johannes Br0nsted, a Danish chemist, and Thomas Lowry, an English chemist. It recognizes an acid-base reaction as a chemical equilibrium, having both a forward reaction and a reverse reaction that involve the transfer of a proton. The Bronsted-Lowry theory defines acids and bases as follows ... 

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