Chemical bonding • Definition – the chemical bond is a l
Chemical bonding
• Definition – the chemical bond is a link between atoms, that result from
mutual electrostatic attraction of their nuclei for electrons.
• Elements are more stable when they have full valence shells. (octet rule)
• Classification of the chemical bonds
1. Intramolecular bonding (force of attraction within the molecule)
a) Covalent bonds
b) Ionic bonds
c) Hydrogen bonding
2. Intermolecular bonding (force of attraction between two or more molecules)
a) Hydrogen bonding
b) Van der Waals interactions
• dipole-dipole interactions “Keesom force”
• dipole – induced dipole interactions “Debye force”
• induced dipole – induced dipole interactions “London forces” (substances with non
polar covalent bonds)
Types of covalent bonds
• A chemical bond is a connection between two atoms. This connection
occurs due to the overlapping of molecular orbitals.
• According to the number of bonds
– Single H-H
– Double O=O
– Triple N≡N
• According to the pattern of overlapping
• Sigma – formed via linear overlap of two orbitals
• Pi – formed via parallel overlap of two orbitals
• Delta – formed via very small side overlapping of two orbitals
Localized and delocalized covalent bond
• Localized covalent bond is formed, when two atoms share
electrons with each other. These bonds are concentrated on a
limited region of a molecule in which the electron density is
very high.
• Delocalized covalent bond is formed, when several atoms
share electrons with each other. Delocalization occurs in the
conjugated pi system. (A conjugated pi system has double
bonds and single bonds in an alternating pattern.)
Van der Waals forces
• Dipole-dipole interaction– electrostatic attractions, which occur between the
opposite poles of the permanent dipoles of the different molecules. (two
polarized molecules interact because of the inherent difference in charge
distribution)
• Dipole – induced dipole interactions – interactions between molecules with
permanent dipoles and molecules without permanent dipoles. (a molecule
with a permanent dipole induces charge redistribution to neighboring
molecules with no dipole moments)
• London forces - weak interactions between molecules without permanent
dipoles caused by momentary changes in electron density in a molecule. (the
fluctuations on the electron cloud lead to temporary changes in the charge
distribution inducing a charge redistribution to neighboring molecules)
Hydrogen bonding
• Definition – electrostatic attraction between hydrogen atoms
bonded to small, strongly electronegative atoms (N, O, F) and
the lone pair of electrons of another electronegative atoms.
• H-bond is weaker than covalent bond and ionic bond.
• Types of hydrogen bonding
– Intermolecular
– Intramolecular
Hydrogen bonding
• The formation of intermolecular H-bonds leads to higher
melting and boiling points, heats of fusion and vaporization.
• Biological meaning - H-bonds are important for the structures
and shapes of many naturally occurring molecules.
Type of reactions according to the structural
change in reagent molecules
Medical chemistry, Assist. Prof. S. Ruseva 15
• Classification by structural change
– Substitution (S)
– Addition (A)
– Elimination (E)
– Rearrangement (R)
– Redox
• Classification by the way of bond-breaking
– Homolytic
– Heterolytic
• Classification by the type of attacking reagent
– Radicals (R)
– Electrophilles (Е)
– Nucleophiles (N)
Classification by structural change
• S – replacement of an atom or group
(Y) by another atom or group (Z)
•A – Number of σ-bonds in substrate
molecule increases, usually at the
expense of one or more π - bonds
•E – Number of σ-bonds in substrate
decreases and new π-bonds are formed
•R – Generation of isomers
Bond-Breaking
• Homolytic
• Heterolytic
Attacking reagent
• Radicals (
.CH3
, Br.
) –
unpaired electron.
• Electrophiles (AlCl3
, NO2
+
,
SO3
+
, BF3
) - Electron
deficient species or electron
acceptor is an electrophile .
• Nucleophiles (OH-
, ROH,
NH3
, RNH2
) - Electron rich
species or electron donors
are called nucleophiles.
Electronic effects: Inductive and mesomeric effects
• Inductive effect – the effect of transmission of charge through a chain of
atoms in a molecule by electrostatic induction. The electron density is
slightly displaced towards the more electronegative atom. This cause a
permanent state of bond polarization. Symbolized with the letter [ I ].
• [+I] - Electron releasing (alkyl groups –CH3
; -CH2
-CH3
; –CH2
-CH2
-CH3
)
• [I0
] - No effect (-Н)
• [-I] - Electron withdrawing (-ОН; -NH2
; -Cl)
• Mesomeric (conjugative) effect – The effect attributed to a substituent
due to overlap of its p- or π-orbitals with the p- or π-orbitals of the rest of
the molecular entity. Delocalization is thereby introduced or extended,
and electronic charge may flow to or from the substituent. The effect is
symbolized by [ М ].
• [+M] - Electron releasing (p,π-conjugation -X:)
• [-M] - Electron withdrawing (π,π-conjugation -C=X -C≡X)