Page - (000021) - in Biomedical Chemistry: Current Trends and Developments

– the polarity of a C-H bond increases; – breaking a bond between carbon and a more electronegative atom is more difficult (e.g. the C-O bond in isopropanol is easier to cleave than the C-O bond in isopropenol). The electronegativity of an element can also be an important factor to explain some functional groups reactivity. For instance, alcohols, ethers, amines, thiols, sulfides, disulfides and phosphates (Table 1.1.1) all have a carbon forming a single bond with a more electronegative atom, causing the carbon to bear a partial positive charge (δ+). These modifications affect both the σ- and π-bonds, although in the case of some π-bonds the resonance effect should also be considered. The carbonyl group can be classically treated as a resonance hybrid represented by two resonance structures (Scheme 1.1.1), which contributes to the reactivity of the compounds that have this functional group (aldehydes, ketones, carboxylic acids, esters, thioesters, amides, acyl phosphates). Resonance is possible whenever movement of electrons are allowed within the same molecule without movement of atoms. Scheme 1.1.1: Carbonyl group resonance structures and equivalent notation, explicitly showing the bond polarity with partial charges. 1.1.2 Acids and Bases Versus Electrophiles and Nucleophiles Acids and bases are very important in biological transformations, as most require some form of acid or basic catalysis to occur. The simplest acid-base theory is the Brønsted-Lowry theory, which states that acids are molecules that donate protons (hydrogen ions, H+) and bases/alkalis are molecules that accept protons. For example, a carboxylic acid can donate a proton to a base, such as an amine, in a reversible proton- transfer reaction (Scheme 1.1.2).