Equivalent protons, definition

Depending upon your definition of an acid and base, chemists can characterize almost all chemical reactions as some form of an acid-base reaction. Because proton transfers are ubiquitous in chemistry and biochemistry, we start this chapter focusing upon this single kind of acid-base reaction. However, a later portion of this chapter concentrates on filled and empty orbitals as our base and acid equivalents (Lewis definitions), thereby broadening our acid-base reactions considerably. Hence, the reason for discussing acid-base chemistry thoroughly, and even devoting an entire chapter to this one reaction type in Part I of this book, is that it plays a central role in all chemical disciplines. 

Any spin system that contains fluorine substituents that are chemically equivalent, but not magnetically equivalent is, by definition, second order. Such spectra can appear deceptively simple, or more commonly they can be amazingly complex. The fluorine and proton spectra of the simple, symmetrical compound, 1,1-difluoroethene exemplify the latter situation (Figures 2.5 and 2.6). 

Lewis defined a base as an electron pair donor and an acid as an electron pair acceptor. Lewis electron pair donor was the same as Bronsted-Lowry s proton acceptor, and therefore, was an equivalent way of defining a base. Lewis acids were defined as a substance with an empty valence shell that could accommodate a pair of electrons. This definition broadened the Bronsted-Lowry definition of an acid. The three definitions of acids and bases are summarized in Table 13.3. 

Note that this is equivalent to the definition that the free energy of formation of the proton in aqueous solution is equal to zero [i.e., AfG , (aq) = 0 at any temperature (for more details see, e.g., Stumm and Morgan, 1996)]. Hence, we can rewrite Eq. 8-2 as ... 

When K[PtX(acac)2] (X = Cl, Br) is treated with a strong proton add the uncoordinated 0,0 site is protonated and complex (170) is formed.1606,1607 If the alkyl groups on the 0,0 -bonded acac ligand are non-equivalent, exchange can be observed. Although the mechanism has not been definitively proven, a dissociative mechanism is favored.160 This proposal correlates with the observations that the 0,0 -bonded chelate complex Pt(acac)2 will react with tertiary phosphines and nitrogen bases with substitution of one of the oxygen-bonded chelate arms.1609,1610 A variety of products are formed as outlined in Scheme 17. 

Protons are magnetically equivalent if they have the same chemical shift and are coupled equally to otiier equivalent nuclei in die molecule. This is similar to chemical equivalence but is a more rigorous definition of equivalence. For example, the methyl protons of isobutane are chemically and magnetically equivalent since they absorb at die same frequency and are all coupled equally to the methine proton (which should be split into a 10-line multiplet ). Likewise die two mediyl groups of /i-xylene are chemically and magnetically equivalent because they are coupled equally (J = 0) to the aromatic protons both ortho and meta to diem. 

The definition of chemical shift equivalence given for protons also applies to carbon atoms interchangeability by a symmetry operation or by a rapid mechanism. The presence of equivalent carbon atoms (or coincidence of shift) in a molecule results in a discrepancy... 

An attractive alternative in which spectrometer line-listing routines are u.sed is as follows. After the spectral data have been Fourier transformed and saved, the proton FID is zero filled three to four times. If np = 32K, then Fn should be set to 256 or 512K. The resulting spectral lines should then have sufficient definition for peak picking. This expectation can easily be tested by examining a known multiplet, such as a triplet or quartet. The two- or three-line spacings are, of course, identical, and their equivalence must be reflected in the line listings displayed on the monitor. No matter what technique is used, J values between spin-coupled nuclei should always be reported as identical. 

MCAT 1 MORGAN if. Chemistry Remember that solution concentrations are always given in terms of the form of the solute before dissolution. For instance, when 1 mole of NaCl is added to 1 liter of water, it is approximately a 1 molar solution and NOT a 2 molar solution even though each NaCl dissociates into two ions. Normality measures the number of equivalents per liter of solution. The definition of an equivalent will depend upon the type of reaction taking place in the solution. The only time normality is likely to appear on the MCAT is with an acid-base reaction. In an acid-base reaction an equivalent is defined as the mass of add or base that can furnish or accept one mole of protons. For instance, a 1 molar solution of H2S04 would be called a 2 normal solution because it can donate 2 protons for each h2so4. 

The proton NMR spectra of the tetrakistropolonates of zirconium and hafnium in dimethyl sulfoxide has also been examined. The spectra are consistent with a complex in which all four ligands are equivalent, but there is no evidence for rapid exchange between the chelates and free ligands (379). Similarity in chemical shifts for the tropolone and chelate resonances, however, makes a definitive statement difficult. Attempts to form a 10-coordinate species with tropolone were unsuccessful (377). 

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