Download polar covalent bonds.

The Scope of Organic Chemistry: An Overview
•Functional groups determine the reactivity of organic molecules
Alkanes – No functional groups, only carbon and hydrogen. (Chapter 2)
Alkane Reactions – Alkane bond strengths and reactions. (Chapter 3)
Cyclic Alkanes – New properties and changes in reactivity (Chapter 4)
Stereoisomerism – Same connectivity – different relative positioning of
substituents in space (Chapter 5)
Haloalkanes – Substitution Reactions and Elimination Reactions
(Chapters 6 and 7)
CHAPTER 1
Structure and Bonding in Organic
Molecules
please read this chapter and try to get
the idea of covalent bonds
1-1
The Scope of Organic Chemistry: An Overview
• Synthesis is the making of new molecules
•Wöhler’s Synthesis of Urea:
•Synthesis – Construct complex organic chemicals from simpler,
more readily available ones (Chapter 8).
• Reactions are the vocabulary, and mechanisms are the grammar of
organic chemistry
•Reactants (Substrates) – Starting compounds
•Products
•Reaction Mechanism – Underlying details of a reaction
•Reaction Intermediate – Chemical species formed and then
destroyed on the pathway between reactants and products.
1-3
Ionic and Covalent Bonds: The Octet Rule
1. Covalent Bonds are based on the sharing of electrons.
If the electrons are not shared equally, a polar covalent (partially
ionic) bond is formed, otherwise a pure covalent bond is
formed.
2. Ionic Bonds are based on the transfer of one or more electrons
from one atom to another. The resulting cation and anion are
electrostatically attracted to each other.
1-3
Ionic and Covalent Bonds: The Octet Rule
The periodic table underlies the
octet rule.
Electrons in atoms occupy levels or shells of fixed capacity.
The first has room for 2, the second 8, and the third 16.
Noble gases have 8 valence electrons (Helium 2) and are
particularly stable.
Other elements lack octets in their outer electron shells and tend
to form molecules in such a way as to create a stable octet
arrangement.
• In these and similar cases, covalent bonding occurs. Atoms
share electrons to achieve a noble gas configuration.
•In certain cases, one atoms supplies both of the electrons in the
bond:
•Often 4 electron (double) and 6 electron (triple) bonds are formed:
In most organic bonds, the electrons are not shared equally:
polar covalent bonds.
•Pure covalent bonds (perfect sharing of electrons) and ionic
bonds (complete transfer of electrons) are two extreme types of
bonding.
•Most bonds lie somewhere between these extremes and are
called polar covalent bonds.
•Each element can be assigned an electronegativity value
which represents its electron accepting ability when participating
in a chemical bond.
•The larger the difference in electronegativety between two
atoms participating in a chemical bond, the more ionic is the
bond.
•Bonds between atoms of different electronegativity are said to
be polar bonds. A partial negative charge is found on the atom
of higher electronegativity and an equal but positive charge on
the other atom.
•As a rule of thumb, electronegativity differences less than
0.3 represent pure covalent bonds, from 0.3 to 2.0 polar
covalent bonds, and greater than 2.0 ionic bonds.
•The separation of opposite charges in polar covalent molecules
results in the formation of dipoles:
1-4
Electron-Dot Model of Bonding: Lewis Structures
Lewis structures are drawn by following simple rules.
1. Draw the molecular skeleton
2. Count the number of available valence electrons
• Add one electron for each negative charge, if an anion.
• Subtract one electron for each positive charge, if a cation.
3. Depict all covalent bonds by two shared electrons, giving as
many atoms as possible a surrounding electron octet, except for
H, which requires a duet.
• Elements at the right of the periodic table (non-metals) may
contain lone pairs of electrons.
Correct Lewis Structure
Incorrect Lewis Structures
It is often necessary to use double or triple bonds to satisfy
the octet rule:
4. Assign charges to atoms in the molecule.
Charge = (# valence electrons in free, neutral atom) –
- (# unshared electrons on the atom)
– ½(# bonding electrons surrounding the atom)
In molecules such as nitric acid, charges occur on individual
atoms, even though the molecule itself is neutral.
Covalent bonds can be depicted by straight lines.
Bonding pairs of electrons are most often represented as straight
lines: single bonds as a single line, double bonds as two parallel
lines, and triple bonds as three parallel lines.
Lone pairs of electrons are either shown as dots or are omitted.
Structures of this type are called Kekulé structures.
1-5
Resonance Forms
The carbonate ion has several correct Lewis structures.
Three equivalent structures must be drawn to accurately
represent the carbonate ion. The only difference between these
structures is the placement of electrons.
But what is its true structure?
The “true” structure can be thought of as the average of all
three structures which is called a resonance hybrid.
The 2 negative charges are delocalized over all three oxygen
atoms.
Not all resonance forms are equal.
1. Structures with a maximum of octets are most important.
2. Charges should be preferentially located on atoms with
compatible electronegativity. If this conflicts with rule 1, then
rule 1 takes precedence.
3. Structures with less separation of opposite charges are more
important resonance contributors than those with more
charge separation.
The overlap of atomic orbitals gives rise to sigma and pi
bonds.
When n atomic orbitals overlap, n new molecular orbitals are
formed.
When n is 2, one bonding orbital and one antibonding molecular
orbital are formed.
The energy lowering of the bonding orbital and energy raising of
the antibonding molecular orbital with respect to the atomic
orbitals is called the energy splitting.
The energy splitting indicates the strength of the bond formed.
Atomic orbitals of the same size and energy overlap to form the
strongest bonds.
Geometrical factors also affect the degree of overlap. Orbitals
exhibiting directionality in space (p orbitals) can overlap to form
sigma () bonds or pi ()bonds.
All carbon-carbon single bonds contain one sigma bond. Double
and triple bonds contain extra pi interactions.
1-8
Hybrid Orbitals: Bonding in Complex Molecules
Mixing of atomic orbitals from the same atom results in new atomic
orbitals of different energy and directionality.
sp Hybrids produce linear structures.
An incorrect structure for BeH2 is predicted if 2s and 2p
orbitals of Be are overlapped with the 1s orbitals of H:
sp2 Hybrids create trigonal structures.
Hybridization of a 2s and two 2p orbitals results in three new
hybrid orbitals that point to the corners of an equilateral triangle.
The remaining p orbital points up and down, perpendicular to each
of the three hybrid orbitals.
Bond angles in molecules using sp2 hybridization are approximately
120o
The molecule, BH3 is isoelectric with the methyl cation, CH3+.
Both involve sp2 hybridization about the central atom.
sp3 Hybridizaton explains the shape of tetrahedral carbon
compounds.
When the 2s and all three 2p orbitals are hybridized, four
hybrid orbitals called sp3 orbitals are formed. These orbitals
point to the corners of a regular tetrahedron.
Bond angles in molecules using sp3 hybridization are
approximately 109.5o
Pi bonds are present in ethene (ethylene) and ethyne
(acetylene).
Molecules containing double or triple bonds contain unhybridized
p orbitals that overlap lengthwise rather than end on.
1-9
Structures and Formulas of Organic Molecules
To establish the identity of a molecule, we determine its
structure.
The empirical formula of a substance specifies the kinds and
ratios of elements present in the substance.
The empirical formula can be from an elemental analysis the
substance.
More that one substance can have the same empirical formula.
Each of these substances will have its own set of unique physical
and chemical properties, however.
Substances having the same empirical formula but different
connectivity of atoms are called constitutional or structural
isomers.
A chemist may be able to identify an unknown substance if its
properties match those of a substance already determined.
New substances require other methods of identification such as
x-ray crystallography, or various forms of spectroscopy.
Two ways of representing the structures of know molecules are
ball and stick models and space filling models.
Several types of drawings are used to represent
molecular structures.
Tetrahedral carbon structures can be accurately represented in
three dimensions using the dashed-wedged line notation.