Chemical Bonds Flashcards
Quantum Mechanics
- particles = waves, ie. they can be described as a wave function
- wave functions of electrons in atoms can be described by PSI x,y,z
Electron Density
electron distribution in space for an orbital
Coulomb’s Law
- electrostatic potential energy
- closer an electron is to the nuclear the greater energy it has (greater attraction)
Aufbau Principle
electrons placed in orbitals starting with lowest energy and working up
Pauli Exclusion Principle
2 electrons per orbital with spins paired
Hunds Rule
when multiple orbitals of the same energy are available electrons are distributed amongst them
Molecular Orbitals
combining atomic orbitals to form molecular orbitals (sigma and pi)
bonding orbitals: wave functions together. high probability electron is between nucleus to give a favorable electrostatic attraction
- favorable coloumb interaction with both nuclei
anti bonding orbitals: wave functions opposite no electron density between nuclei, ie. energetically unfavorable
Bonding and Antibonding Orbitals
- bonding orbitals filled first
- when anti bonding orbitals are filled, the energy comes from putting electron in bonding orbital
- this cancels out gain in energy and determines whether two atoms will bond spontaneously
Hybridization
the idea that atomic orbitals fuse to form newly hybridised orbitals, which in turn, influences molecular geometry and bonding properties
- sigma: hybridized
- pi: unhybridized
repositioning of orbitals so the electrons are in optimal locations for being shared in covalent bond formation
Configuration
conversion between isomers needs a single bond to break
Enantiomers
Non superimposable mirror images of each other
Diastereoisomers
Not mirror images but are non superimposable
Glucose Polymers
Amylose vs Cellulose
- amylose has alpha linkages with bends and twists
- celulose has beta linkages and is flat with straight chains
Features Defining Molecular Shape and Structure
- bond length
- bond angles (VSEPR theory)
- bond rotation
Conformation
how atoms are joined, is fixed
molecules will adopt a configuration minimizing repulsion forces
- Eclipsed: same orientation of groups
- Staggered: opposite orientations of groups (preferred because of steric hindrance)
Isomers
Same molecular formula but different structural formulas
- structural: different bonding
- constitutional: geometrical isomerism
Constitutional isomers
You can’t interconvert them without breaking a covalent bond. Configurational change needed. Cis Trans nomenclature needed
Types of staggered conformation
Gauche: R groups next to each other
Anti (favored): R groups opposite
Conjugated Double bonds
Different types of double bonds
The single and double bonds alternate. These enables the electrons to be delocalised over the whole system and so be shared by many atoms. This means that the delocalised electrons may move around the whole system.
Aromatic Compounds
Hydrocarbons which contain benzene, or some other related ring structure. These pi-bonds are delocalized around the ring, leading to an unusual stability for the benzene ring compared to other alkenes.
Heterocyclic Aromatic Systems
Pyridine
Pyrimidine
Pyrrole
- carbon in ring replaced by one or more nitrogens
Wavelength and Frequency
increase in frequency = energy increase
smaller wavelength = energy increase
Electron Orbital Movement
Energy goes into moving electrons between orbitals
Electrons use energy to move to antibonding orbitals
sigma to antisigma = highest energy
pi to anti pi = lowest energy
Molecular Vibrations
Electrons vibrate within an orbital/energy level
Stretching, rocking, scissoring, wobbling, etc
Absorbance vs wavelength graph gives a broad peak causes by this vibration
Beer Lambert Law
A = E x c x l
Absorbance proportional to concentration
Fluoresence
When going back to the ground state, electrons move down to lower energy molecular vibrator levels releasing heat
When going back to bonding orbitals they drop from lowest level of anti sigma to highest sigma level
We see this lowest energy difference
Electronic Properties of Phosphate
- hydridize orbitals to make 4 sp3 orbitals and one 3d electron
- tetrahedral geometry with all oxygens being equivalent and having partial negative charge
- phosphate ion is very stable and exists in different resonance forms
- this stability means it can pull reaction forwards of ATP hydrolysis
Delocalisation
- resonance is a type of delocalisation where electrons move around
- conjugation is another type of delocalisation where multiple double bonds are allowing electrons to move around in molecular orbitals beyond the 2 atoms
Electronic Properties of Oxygen
- 2p orbitals form bond
- 1s/2s orbitals non bonding
- filled sigma, pi, and anti pi orbitals
- anti pi bond cancels out one of the bonding pairs
Bond Order
electrons in bonding orbitals - electrons in anti bonding orbitals / 2
Gives net number of shared electron pairs
Oxygen Paramegnetism
- electrons in antibonding orbitals line up their spins with each other so have paramagnetism
- explains oxygen’s reactivity
Electronegativity
- attraction between electron and nucleus and the ability of the nucleus to pull electrons
- increases across periods (increased nuclear charge)
- decreases down groups (electron shielding)
- leads to partial charges on atoms
Steric Clash
Two atoms closer than the sum of their VDW radii
VDW Attraction
Little more than sum of VDW radii
- transient dipoles created by random change
- weak interactions
Hydrogen Bonds
- linear bonds
- dipoles in H bonds align (directionality)
- essentially a form of dipole dipole interaction only between H and C,N,O,F
Hydrophobic Effect
- water molecules form organised clathrates when in contact with non polar structures
- this organised structure is energetically unfavorable
- bringing non polar faces together relaxes the organised water
Bond Energies
Covalent bonds: 350 kj H bond: 5-20 kj VDW: 0.2-2 kj hydrophobic: entropy rotational conformations: 10 kj
