Structure of chemical compounds Bonds and isomery Richard Vytášek 2008 Presentation is only for internal purposes of 2nd Medical faculty

Lewis octet rule Atoms combine and form bonds by transferring or sharing electrons until each atom is surrounded by eight valence electrons

Types of chemical bonds covalent bonds ionic bonds

Covalent bonds electron pair (bonding electrons) is shared between two atoms molecular orbital is formed by ovelapping of atomic orbitals of two atoms electronegativity of atoms must be similar : nonpolar covalent bond – both atoms are equally electronegative polar covalent bond – one atom is more electronegative and attracts more electrons a coordinate covalent bond – both electrons of bonding pair are provided by the same atom (complexes, transition metals)

Ionic bonds strong polarization leads to formation relatively high partial charge on both atoms – the bond becames electrostatic if the character of a bond is mainly electrostatic we call this bond ionic between a polar covalent bond and an ionic bond is continous transition

Fajans´ rules predict whether a chemical bond will be covalent or ionic Covalent Large anion Small cation High positive charge Nonstabile electron structure Ionic Low positive charge Large cation Small anion Stabile electron structure (configuration of noble gas)

Ionic compounds dissociate in water to cations and anions

The change of water structure nearby the ion

Types of noncovalent intaractions

Influence of the hydrogen bond on biomolecules

Nucleobase pairing by hydrogen bonds

The unique carbon atom Carbon forms very stable single, double and triple covalent bonds forms strong covalent bonds with many other atoms (hydrogen, nitrogen, oxygen, sulfur) atoms form chains of atoms bonded to each other

Carbon elektron structure 1s 2 2s 2 2p 2 the energy of elektrons of valence shell is different (2p > 2s) hybrid atomic orbitals are formed by linear combination of original atomic orbitals and the energy of all hydrid orbitals is equal

Carbon - hybrid orbitals sp 3 the four hybrid orbitals sp 3 are formed by linear combination of orbital 2s and three orbitals 2p

Carbon - hybrid orbitals sp 2 the three hybrid orbitals sp 2 are formed by linear combination of orbital 2s and two orbitals 2p

Forming of bond  and bond 

Carbon - hybrid orbitals sp and formation of triple bond the two hybrid orbitals sp are formed by linear combination of orbital 2s and one orbital 2p remaining two orbitals 2p take part in formation of two  bonds - triple bond

Conformation orientation of a molecule in space (3D) a molecule can exist in many various conformations various conformations are formed by rotation about single bond

Rotation about single bond C-C individual conformation formed by rotation about single bond C-C (rotamers) is indistinguishing but various conformations can exibit various reactivity

Conformation of cyclohexane two stabile conformations - chair and boat - are in equilibrium chair conformation is more abundant because its energy is slightly lower

Conformation of glucose is similar to conformation of cyclohexane

Isomers Isomers are compounds with the same molecular formula but different structure constitutive – different order of atoms or different position of double (triple) bond or different site of linking of functional group (skeletal, positional, functional group isomers, tautomers) configuration

Skeletal isomers differ in their carbon skeleton

Functional group isomers isomers with different functional group(s)

Positional isomers different location of the same functional group or double (triple) bond

Tautomeric isomers (tautomers) specific case of the constitutive isomery change in the location of a hydrogen and a double bond, both tautomers are in a dynamic equilibrium (individual tautomer can´t be isolated) typical example is enol and keto form of carbonyl group

Stereochemistry studies spatial arrangement of molecules configuration isomers (stereoisomers) : geometric isomers optic isomers

Geometric isomers cis/trans isomers contain the bond which is unable of rotation (usually double or triple bond but also single bond of cyclic compounds) two different substituents on the first carbon and two different substituents on the second carbon

1,2-dichlorocyklohexane

Asymmetric (chiral) carbon is attached to four different substituents causes the optical activity - the rotation of the plane of polarized light is responsible for optic isomery

Enantiomers optic stereoisomers which are mirro-images (e.g. D,L forms of sugars or amino acids)

Diastereoisomers stereoisomers which contain multiple chiral centers and are not enantiomers in the case of sugars they differ in the name

Anomers stereoisomers of sugars in hemiacetal form differing by orientation of hydroxyl group on carbon 1

Meso compounds contains multiple chiral centers but are nevertheless symetrical