general Flashcards
Ray-Dutt Twist
- fac-delta to mer-lambda
- C2v transition state
Bailar Twist
- fac-delta to fac-lambda
- D3h transition state
sigma-hole interactions
- intermolecular bonding between electron deficient region of atoms with anisotropic electron density and along covalent bond axis (sigma holes) and electron rich molecules/regions
- halogen bonding, chalcogen bonding, pnictogen bonding, tetrel bonding
- increases in strength down group due to larger sigma holes/higher polarisability/less electronegative
- increases in strength with stronger EWG substituents
- either electrostatic interactions or orbital overlap with sigma*
- identified by short bonding distance
- comparable strength to halogen bonding
- highly directional
hydrogen bonding
- electrostatic interaction between highly polarised OH/NH/FH and nucleophile/anion/electron rich
- directional
- significantly stronger than normal dipole-dipole forces
Fast Exchange NMR
- chemical shifts become weighted average
- changes in equilibrium changes the chemical shift
Slow Exchange NMR
- seperate signals for each
- changes in equilibrium changes intensities of signals
- integration proportional to relative equilibrium concentrations/molar fraction
Van’t Hoff Analysis
- determines enthalpy/entropy of reaction
- plot lnK against 1/T (determine equilibrium constant at different temperatures)
- easier than calorimetry
-RTlnK = enthalpy - T*entropy lnK = -(enthalpy/RT) + entropy/R
Arrhenius Law
- determines activation energy (from rate constant)
- plot lnk against 1/T
- determine the rate constant by plotting the integrated rate law
- record rate constant at different temperatures
k = Ae^(-Ea/RT)
Eyring Equation
- determine activation parameters entropy/enthalpy (i.e. kinetic/thermodynamic stability)
- more general form of Arrhenius equation
enantiomeric excess calculation
the difference in the relative abundance of the two enantiomers/diastereomers
ee = %R - %S
Coalescence Method
determines activation energy/rate constant from the coalescence temperature
Lineshape analysis
- line shape is proportional to rate of exchange due to uncertainty principle
- hence can determine rate constant/activation energy
Time resolved NMR
- start from out of equilibrium
- measure changes in signals as changes in concentration
- determine rate constant/activation energy
How determine activation energy
- coalescence method
- lineshape analysis
- time resolved NMR
Diffusion NMR principle
- The attenuation of the signal as the gradient strength is increased is proportional to the diffusion constant
- faster diffusing species attenuate faster
- by Stejskal-Tanner equation
Diffusion NMR pitfalls and solutions
- convection (use high viscosity solvent, spin sample, narrower tubes)
- overlapping signals (use other signals or purify)
- varying temperature
Braggs Law
n*lambda = 2dsin(theta)
Hofmeister Series
- Orders anions based on hydrophilicity.
- Small, high pKa anions are hydrophilic e.g. F-, OH-
- large, low pKa anions are hydrophobic e.g. I-
Titration Experiment
- titrate fast binding guest into host while measuring property (that is linearly proportional to complex concentration)
- plot change in property against amount of guest added
- fit binding isotherm to determine equilibrium constant
Kinetic Stability
delta G double bar.
- Is the activation energy.
- Indicates how quickly reaction reaches equilibrium,
- small = kinetically labile, large = kinetically inert.
Thermodynamic Stability
delta G circle.
- Equal to difference in energy.
- Indicates position of equilibrium.
- If large positive = thermodyanamically stable, non spontaneous.
- If large negative = thermodynamically unstable, spontaneous.
- If small = mixture of reactant and product.
Quantum Numbers
The properties of atomic electrons are quantized. The four quantum numbers that describe atomic electrons are: principle quantum number (n), azimuthal quantum number (l), magnetic quantum number (ML), spin quantum number (Ms).
Hund’s Rule
Degenerate orbitals are singularly occupied with the same spin before they are paired
Aufbau Principle
Electrons fill orbitals in order of increasing energy.
Pauli Exclusion Principle
No two electrons can have the same set of quantum numbers in an atom
formal charge
Formal Charge = [# of valence electrons on atom] – [non-bonded electrons + number of bonds]
Reaction Rate Factors
- concentration (increases collisions and so effective collisions)
- temperature (molecules move faster, more than proportional increase in effective collisions).
- surface area (more reactant available to collide).
- partial pressure (concentration for gases)
- catalyst (lowers activation energy increasing proportion of effective collisions)
Factors of splitting parameter
- ligand field strength: strong field ligands increase energy difference
- oxidation state: higher oxidation metals have shorter ligand bonds and so higher repulsion.
amine basicity
depends on ability to donate lone pair to proton. EWG/conjugation reduces electron density on amine, decreases basicity. EDG increase amine nucleophilicity, increase basicity. Amide show no basicity due to lone pair in conjugation with carbonyl
Electron withdrawing groups
NO2, SO3H, CN, carbonyls, CF3, NR4+, halides
Electron Donating Groups
Amines, oxo groups, alcohols, alkoxides, amides/esters (not carbonyl side), alkanes/alkenes/aryls
Diazo
C=N+=N-
unconjugated sp2 carbon bonded to N2
neutral
diazo synthesis
- elimination from N-alkyl-N-nitroso compounds
- diazo transfer
diazo reactions
- as a carbon nucleophile
- carbene precursor
- cycloadditions to pyrazolines
Diazonium salts
C - N+ — N
cationic
only practically stable as aryl diazomiums
very strong leaving groups i.e. N2
Diazonium synthesis
Aniline react with NaNO2/HCl and Mineral acid (HCl, HBF4…)
Diazonium Reactions
- Nucleophilic substitution (Sandmeyer reaction)
- diazo coupling with electron rich benzenes to form azo
- reduction to benzene with H3PO2
- reduction to hydrazines with SnCl2
Azide Alkyne Cylcoaddition
reaction between azide and alkyne to form 1,2,3-triazoles
- Cu catalysed selectively forms 1,4 isomer
- ‘click’ reaction
Click Chemistry
Reactions that are: simple fast easy to purify tolerant efficient highly thermodynamically favoured
