Molecular structure, molecular orbitals, stereochemistry Flashcards
What is empirical formula
A simple formula which shows the ratio of atoms in a molecule
How do you obtain the empirical formulae
Through elemental microanalysis
How do you obtain molecular formula
You need the empirical formula and the relative molecular mass
How can you obtain the relative molecular mass
Through spectrometry
What is skeletal structural formula
An abbreviation which represents carbon-to-carbon framework through lines
What does the molecular orbital model indicate
Electrons in atoms occupy the atomic orbitals and electrons in molecules occupy molecular orbitals
When is an orbital full
When it contains 2 electrons of opposite spin
How are molecular orbitals made
By combining atomic orbitals (the number of molecular orbitals formed is equal to the number of atomic orbitals that combine)
What is a bonding molecular oribital
A molecular orbital that has less energy than the 2 atomic orbitals that made it
What is an antibonding molecular orbital
A molecular orbital with a higher energy level than the 2 atomic orbitals that combined to produce it.
Whats the difference between bonding/antibonding molecular orbits
Bonding molecular orbits embrace both nuclei whereas antibonding molecular orbital doesn’t. Antibonding molecular orbitals have more energy.
How is a sigma bond produced
When the atomic orbitals overlap along the axis of a covalent bond.
What can a sigma bond be referred to as
‘end-on’ overlap
How do two parallel p orbitals bond
Bond laterally - ‘side-on’ and form a Pi bond
How do Pi bonds arise
When Atoms form multiple bonds
Whats the difference between ‘side-on’ and ‘end-on’ overlaps
‘side-on’ overlaps are less efficient and which means Pi bonds are weaker than sigma bonds
Define hybridisation
The process of mixing atomic orbitals within an atom to generate a set of new atomic orbitals called hybrid orbitals.
What is an sp*3 hybrid orbital
when the one ‘2s’ orbital mixes with the 3 ‘2p’ orbitals to form 4 hybrid orbitals.
Why do degenerate sp*3 orbitals point towards the corners of a tetrahedron
To minimise repulsion
How can you tell how many hybrid orbitals were formed
It is equal to the number of atomic orbitals that were mixed
How do alkanes form bonds
The carbons 4 ‘sp3’ orbitals overlap ‘end-on’ with other hydrogens (1s orbitals) and carbons (sp3 orbitals) to make 4 sigma bonds which will adopt a tetrahedral arrangement.
How does bonding in alkenes occur with reference to orbitals
2 p orbitals mix with the s orbital to make 3 sp*2 orbitals while the remaining 2p orbital is left unhybridised
Why do sp*2 orbitals lie in a trigonal pyramidal arrangement
To minimise repulsions
How do carbons bond in alkenes
The 3 sp*2 (from 1 2s mixing with 2 2p) hybrid orbitals make sigma bonds with two hydrogens and the other carbon while the unhybridised 2p orbital (on the carbon) overlap ‘side-on’ to form a pi bond
Whats the difference between carbons that are in a double bond and carbons that aren’t
Carbons in the double bond form sp2 orbitals and a 2p orbital while single carbons have 4 sp3 orbitals
What are the three types of bonds that exist between atoms
non-polar covalent
polar covalent
ionic
What does the slight charge of a polar covalent bond indicate
The more polar the bond, the greater the ionic character
Describe the electron orbital of a non-polar covalent bond
Completely symmetrical.
Describe the electron orbital of a polar covalent bond
Asymmetrical. The orbit skews towards the atom with the greater electronegativity.
Describe the electron orbital of an ionic bond
The orbital embraces only one atom in the bond which causes extreme asymmetry where an atom has effectively been stripped off its electrons.
Why are transition metals coloured
They absorb visible light which moves electrons to a higher energy state
Why are most organic compounds colourless
The wavelength of the light absorbed doesn’t lay in the visible range of the electromagnetic spectrum (the energy gap between the homo and lumo orbitals is too large)
What is the orbital which contains electrons with the highest energy called
Highest occupied molecular orbital (homo)
What is the orbital which contains electrons with the lowest energy called
Lowest unoccupied molecular orbit (lumo)
What type of energy does the homo - lumo gap correspond to
UV
Describe the structure of most coloured molecules
Relatively large numbers of carbons, multiple double bonds/benzene rings
What is the importance of a coloured molecules structure
It allows the molecule to have delocalised electrons across carbon atoms
What happens if a molecule has a large degree of conjugation
It is likely to be coloured
How can a molecule have a conjugated system
It must have a chain of alternating sigma and pi bonds or benzene rings that allow electrons to be delocalised across a number of carbon atoms.
How does the length of the carbon chain in the conjugated system affect the colour of the molecule
The colour arises as the electrons absorb energy and move from HOMO to LUMO, the colour of the molecule is the colour complementary to that absorbed
Explain the HOMO - LUMO gap with regards to degree of conjugation
The greater the degree of conjugation, the smaller the HOMO -LUMO gap and therefore less absorbed energy
Define chromophore
The group of atoms in a molecule responsible for the absorption of light in the visible region of the em spectrum
What is an isomer
Molecules that have the same molecular formula but differ in the way their atoms are arranged
Describe the ways atoms can be arranged differently in isomers
The isomers bond together in a different order
(structural isomers)
The order of bonding is the same but the arrangement of atoms in the space is different (stereoisomers)
What are the 2 types of stereoisomerism
Geometric and optical
What is geometrical stereoisomerism
There is a carbon to carbon double bond and the bonds are fixed so it is impossible to rotate one end of the molecule to create the other isomer.
Can also occur in saturated rings where rotation around the c-c single bond is restricted
What is optical stereoisomerism
The molecule is trigonal planar and an atom will join on one side or the other to create two different tetrahedral molecules (the isomers are mirror imaged and are non-superimposable).
What are the two names given to geometric isomers
Cis
Trans
What is another name for an optical isomer
An enantiomer
What does non-superimposable mean
The molecules cannot be placed on top of one another to make the same molecule
What type of molecules are optical isomers
Chiral molecules
In what way are optical isomers non-identical
Identical in every physical property except their effect on plane polarised light
How does a polariser turn light into plane-polarised light
Cuts out all vibrations of a light wave except for those on a particular plane
How does geometric isomerism arise
Lack of free rotation around a bond (usually a carbon to carbon double bond).
Explain the differences of optical isomers when plane polarised light passes through them
A wave passing through one isomer will rotate in the opposite direction at the same angle as another wave coming through the other isomer
What would be the effect of plane-polarised light through an equimolar solution of both isomers? Why?
There would be no effect as the rotational effect of the molecule would be cancelled out by the anti-rotational effect of the other isomer.
What is the term used to describe a mixture of both isomers that has no effect on plane-polarised light
Optically inactive
What is the name given to an optically inactive, equimolar mixture
Racemic
