Hydrogen bond acceptance/electron pair

Most reactive impurities are acids or bases in a broad sense. Here, an acid is a substance that has proton donor capacity, hydrogen bond donor capacity, electron pair acceptability, and electron acceptability. A base is a substance that has proton acceptability, hydrogen bond acceptability, electron pair donor capacity and electron donor capacity. Some reactive impurities have both acidic and basic properties. 

Even for a simple reaction, involving just one reactant species and one product species, such as a keto-enol tautomerism or a cis-trans isomerization, the above equation for a given solvent is complicated. StUl, in specific cases it is possible to unravel the solvent effects of cavity formation, for the solute species have different volumes, polarity/polarizability if the solute species differ in their dipole moments or polarizabilities, and solvent Lewis acidity and basicity if the solutes differ in their electron-pair and hydrogen-bond acceptance abilities. 

The fact that a Lewis acid must be able to accept an electron pair means that it must have either a vacant, low-energy orbital or a polar bond to hydrogen so it can donate H" which has an empty Is orbital). Thus, the Lewis definition of acidity is much broader than the Bronsted-Lowry definition and includes many other species in addition to H. For example, various metal cations such as are Lewis acids because they accept a pair of electrons when they form a bond to a base. In the same way, compounds of group 3A elements such as BF3 and AlCln are Lewis acids because they have unfilled valence orbitals and can accept electron pairs from Lewis bases, as shown in Figure 2.5. Similarly, many transition-metal compounds, such as TiCU, FeCla, ZnCl, and SnCl4, are Lewis acids. 

Acids Substances that yield hydrogen ions when dissolved in water these substances also accept electron pairs to form covalent bonds. 

If covalent, ionic and metallic bonds are explained in electrical terms, students are better prepared to accept that hydrogen bonds, van der Waals forces, solvent-solute interactions etc. are also types of chemical bonding. Where learners see covalent bonds as electron pairs attracted to two different positive cores, they have a good basis for subsequently learning about electronegativity and bond polarity. 

The formula for a Lewis acid need not include hydrogen. Even a silver ion can be a Lewis acid, accepting electron pairs from ammonia to form covalent bonds. 

The reaction begins with an attack on the electrophile, HBr, by the electrons of the nucleophilic tt bond. Two electrons from the 7t bond form a new u bond between the entering hydrogen and an alkene carbon, as shown by the curved arrow at the top of Figure 6.7. The carbocation intermediate that results is itself an electrophile, which can accept an electron pair from nucleophilic Br ion to form a C Brbond and yield a neutral addition product. 

We can extend the Lewis symbols introduced in Section 2.2 to describe covalent bonding by using a line (—) to represent a shared pair of electrons. For example, the hydrogen molecule formed when two H- atoms share an electron pair (H=H) is represented by the symbol H—H. A fluorine atom has seven valence electrons and needs one more to complete its octet. It can achieve an octet by accepting a share in an electron supplied by another atom, such as another fluorine atom ... 

Although Lewis and Bronsted bases comprise the same species, the same is not true of their acids. Lewis acids include bare metal cations, while Bronsted-Lowry acids do not. Also, Bell (1973) and Day Selbin (1969) have pointed out that Bronsted or protonic acids fit awkwardly into the Lewis definition. Protonic acids cannot accept an electron pair as is required in the Lewis definition, and a typical Lewis protonic add appears to be an adduct between a base and the add (Luder, 1940 Kolthoff, 1944). Thus, a protonic acid can only be regarded as a Lewis add in the sense that its reaction with a base involves the transient formation of an unstable hydrogen bond adduct. For this reason, advocates of the Lewis theory have sometimes termed protonic adds secondary acids (Bell, 1973). This is an unfortunate term for the traditional adds. 

Our focus in this chapter shall be primarily upon the surface electrostatic potential, specifically its most positive and most negative values, denoted by Vs, max and Vs,min, respectively. There may be several local and absolute maxima and minima on a given surface. They indicate the most positive and negative sites. The former are often associated with hydrogens, especially acidic ones, and the latter with lone pairs, tt electrons of unsaturated molecules and strained bonds. We have demonstrated that Vs niax and Vs nim correlate well with measures of hydrogen bond donating and accepting tendencies . 

The most widely accepted mechanism (Whitmore, 13) for the polymerization of olefins involves the so-called carbonium ions. In accordance with this mechanism a carbonium ion (usually a tertiary ion) adds to the olefin to form a higher molecular weight carbonium ion which then yields the olefin polymer by elimination of, usually, a proton. With acid catalysts (e.g., sulfuric acid) the initial carbonium ion is formed by addition of the hydrogen ion from the acid to the extra electron pair in the double bond (the pi electrons) ... 

The dissolution of a solute in a solvent always affects the solvent-solvent interactions in the vicinity of the solnte particles in addition to the solnte-solvent interactions that take place (Marcus, 1998b). This may be viewed in several stages. First, a cavity in the solvent is formed, to accommodate the solute, which breaks down the cohesive forces of the solvent. Next dispersion forces take effect. They apply to nonpolar and hardly polarizable solutes and solvents, as well as to polar and polarizable ones. Other forces that become active provide contributions from interactions of polar molecnles with polar or polarizable ones and from donor acceptor interactions, such as electron-pair or hydrogen-bond donation and acceptance, whether from or to the solute, the solvent, or both. 

In the same year that Bronsted and Lowry proposed their definition of acids and bases, an American chemist named Gilbert Lewis proposed an alternative definition that not only encompassed Bronsted-Lowry theory but also accounted for acid-base reactions in which a hydrogen ion isn t exchanged. Lewis s definition relies on tracking lone pairs of electrons. Under his theory, a base is any substance that donates a pair of electrons to form a coordinate covalent bond with another substance, while an acid is a substance that accepts that electron pair in such a reaction. As we explain in Chapter 5, a coordinate covalent bond is a covalent bond in which both of the bonding electrons are donated by one of the atoms forming the bond. 

The solvents of this class are often called dipolar aprotic solvents. They are polar and of very weak acidity (proton donor capadty, hydrogen bond donor capacity and electron pair acceptability). However, with regards to basidty (proton accept-... 

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