Organic chemistry and instrumental analysis Flashcards
(72 cards)
When do Molecular orbitals form?
When atomic orbitals combine
The number of molecular orbitals is equal to..?
the number of atomic orbitals that combine
The combination of two
atomic orbitals results in the formation of…?
a bonding molecular orbital and an antibonding orbital.
The bonding molecular orbital encompasses…?
both nuclei
Sigma bonds
Molecular orbitals that form by end-on overlap of atomic orbitals along the axis of the covalent bond are called sigma (σ ) molecular orbitals or sigma bonds.
pi bonds
Molecular orbitals that form by side-on overlap of parallel atomic orbitals that lie
perpendicular to the axis of the covalent bond are called pi (π ) molecular orbitals or
pi bonds.
electron config of an isolated carbon
The electronic configuration of an isolated carbon atom cannot explain the number of
bonds formed by carbon atoms in molecules. The bonding and shape of molecules of carbon can be explained by hybridisation.
Hybridisation
Hybridisation is the process of mixing atomic orbitals within an atom to generate a set of new atomic orbitals called hybrid orbitals. These hybrid orbitals are degenerate.
Hybridisation in alkanes
the 2s orbital and the three 2p orbitals of carbon hybridise to form four degenerate sp3 hybrid orbitals. These adopt a tetrahedral arrangement. The sp3 hybrid
orbitals overlap end-on with other atomic orbitals to form σ bonds.
Hybridisation in alkenes
The bonding in alkenes can be described in terms of sp2 hybridisation. The 2s orbital and
two of the 2p orbitals hybridise to form three degenerate sp2 hybrid orbitals. These adopt a trigonal planar arrangement. The hybrid sp2 orbitals overlap end-on to form σ bonds. The
remaining 2p orbital on each carbon atom of the double bond is unhybridised and lies
perpendicular to the axis of the σ bond. The unhybridised p orbitals overlap side-on to
form π bonds.
Hybridisation in benzene
The bonding in benzene and other aromatic systems can be described in terms of sp2
hybridisation. The six carbon atoms in benzene are arranged in a cyclic structure with σ
bonds between the carbon atoms. The unhybridised p orbitals on each carbon atom
overlap side-on to form a π molecular system, perpendicular to the plane of the σ bonds.
This π molecular system extends across all six carbon atoms. The electrons in this
system are delocalised.
Hybridisation in alkynes
The bonding in alkynes can be described in terms of sp hybridisation. The 2s orbital and
one 2p orbital of carbon hybridise to form two degenerate hybrid orbitals. These adopt a
linear arrangement. The hybrid sp orbitals overlap end-on to form σ bonds. The remaining
two 2p orbitals on each carbon atom lie perpendicular to each other and to the axis of the
σ bond. The unhybridised p orbitals overlap side-on to form two π bonds.
HOMO
The highest bonding molecular orbital containing electrons is called the highest occupied molecular orbital
LUMO
The lowest antibonding molecular orbital is
called the lowest unoccupied molecular orbital (LUMO).
What happens to electrons when radiation is absorbed?
Absorption of electromagnetic energy can cause electrons to be promoted from HOMO to
LUMO.
Why do most organic molecules appear colourless?
Most organic molecules appear colourless because the energy difference between HOMO
and LUMO is relatively large. This results in absorption of light from the ultraviolet region of
the spectrum.
What is a chromophore and where do they exist?
A chromophore is a group of atoms within
a molecule that is responsible for absorption of light in the visible region of the spectrum.
Light can be absorbed when electrons in a chromophore are promoted from the HOMO to
the LUMO. Chromophores exist in molecules containing a conjugated system
Conjugated system
a system of adjacent
unhybridised p orbitals that overlap side-on to form a molecular orbital across a number of
carbon atoms. Electrons within this conjugated system are delocalised. Molecules with
alternating single and double bonds, and aromatic molecules have conjugated systems.
The more atoms in a conjugated system the smaller…?
The more atoms in the conjugated system the smaller the energy gap between HOMO and
LUMO. A lower frequency of light (longer wavelength, lower energy) is absorbed by the
compound. When the wavelength of light absorbed is in the visible region, the compound
will exhibit the complementary colour.
What is bond fission?
When an organic reaction takes place, bonds in the reactant molecules are broken and
bonds in the product molecules are made. The process of bond breaking is known as bond
fission.
name the two types of bond fission
homolytic and heterolytic
What is homolytic fission?
♦ results in the formation of two neutral radicals
♦ occurs when each atom retains one electron from the σ covalent bond and the bond
breaks evenly
♦ normally occurs when non-polar covalent bonds are broken
is homolytic fission appropriate for organic synthesis?
no, Reactions involving homolytic fission tend to result in the formation of very complex
mixtures of products, making them unsuitable for organic synthesis.
What is heterolytic fission?
Heterolytic fission:
♦ results in the formation of two oppositely charged ions
♦ occurs when one atom retains both electrons from the σ covalent bond and the bond
breaks unevenly
♦ normally occurs when polar covalent bonds are broken
