Lecture № 3

Н Н

+I effect characterizes alkyl radicals, -I effect characterizes halogen atoms, functional groups, containing oxygen, nitrogen, sulphur, multiple bonds, and also groups bearing positive charge.

If a molecule has copulative systems with simple and multiple bonds or atoms with non-shared electron pairs, included into the copulative system, they could participate in formation of common π-electron cloud. Participation of groups in copulation and their influence on ht e state of π-electron cloud of a molecule, is called mesomeric effect (displacement of electron density of copulative π-bonds), or copulative effect (it is marked by the letter M, more seldom by the letter C). A curved arrow is used to mark it. Atoms of nitrogen, oxygen, fluorine has complete outer electron cover and they are able to displace p-electrons of non-shared pair closer to bensole nucleus or non-saturated carbon atom.

СН₃ - О - СН = С^d-Н₂

If copulative system has a multiple bond between different atoms, then common π-electron cloud displaces closer to more electronegative atom:

O^d-

СН₂ = СН-C^d+

Н

Positive mesomeric effect characterizes the following groups: -OH, -OR, -NH₂, -NR₂, -SH; negative effect is common for the following groups: -NO₂,-CºN, -COOH, -SO₃H (the displacement of electron density goes to hetero-atom of the functional group). The presence of inductive and mesomeric effects leads to re-distribution of electron density in a molecule. As a result, one atoms concentrate partial negative charge and others concentrate partial positive charge. In its turn, such a re-distribution of charge defines reaction ability of combinations, their acid-base features, dipolar moment of molecules, the strength of inter-molecule bonds (it means also such features as boiling and melting temperatures, solubility).

1.7. Acid and base features of organic combinations.

Two theories have the most important meaning to evaluate acid and base features of organic combinations. They are Bronsted’s and Lewis’s theories.

According to Lewis’s theory acid and base features of combinations are determined by their ability to take or give the pair of electrons with bond formation. According to Lewis, acids and bases could be divided into strong and weak.

Lewis’s acids could be an atom, molecule or cation, having vacant orbital and which are able to take an electron pair with formation of covalent bond.

Lewis’s acids are acceptors of electron pair, Lewis’s bases are donors of electron pair. Lewis’s bases (atom, molecule or anion) should have at least one pair of valent electrons, which they can donate to the partner to form covalent bond. All Lewis’s bases are nucleophil re-agents.

According to Bronsted’s theory (protolytic theory) acidity and baseness of combinations is connected to proton transmission H⁺. Acid and base form copulative acid-base pair, in which the stronger the acid, the weaker the copulative base, and visa versa: the stronger the base, the weaker copulative acid.

Bronsted’s acids (proton acids) are neutral molecules or ions, which are able to donate proton (proton donors).

Bronsted’s bases are neutral molecules or ions, which are able to accept proton (proton acceptors).

Acid and base features are not absolute but relative features of combinations: acid features appear only in the presence of base; and base features appear only in the presence of acid. As a solvent at studying acid-base balance water is usually used.

According to the nature of an element, to which proton is bonded, we differentiate between OH- acids (carboxylic acids, phenols, alcohols), SH-acids (tioles), NH-acids (amines, amides, imides), CH-acids (hydrocarbon and their derivatives). Element and bonded to it hydrogen atom are called acid centre. In all cases there is a shift of electron density from hydrogen atom to more electronegative atom, proton in this case is more or less free to split off. The higher the electronegativity of an element to which proton is bonded, the higher the acidity of the combination (thus, carboxylic acids are stronger acids than tioles or amines).

The presence in a molecule electron acceptor groups, having negative electron effects, increases the positive charge on proton, which leads to the increase of acid features.

To form a covalent bond with proton, Bronsted’s bases should donate either non-shared electron pair or electrons in π-bond. According to this fact all Bronsted’s bases are divided into n-bases and π-bases.

n-bases could be neutral or negatively charged. As a rule, anions have stronger base character than neutral substances. That is, amide-ion NH₂^- or hydroxide-ion HO^- on their base features dominate over ammonia NH₃ and water H₂O.

In π-bases, which include alkynes, alkadiens, arens, the centre of baseness, that is, the place of placing proton, are electrons of π-bonds. They are really weak bases as protonated electron pairs are not free.

The presence of electron donor substitutes increases the base features of organic combinations.

pH factor of liquid systems of the organism.

Normal functioning of living beings is possible only in conditions of a definite stability of pH and other factors of their inner environment. This stability is supported by corresponding buffer systems.

Дата добавления: 2014-01-11