Orbitals and Mechanism Flashcards
(37 cards)
What type of solvents stabilise ions?
Polar solvents stabilise cations and anions
Polar protic solvents stabilise anions via hydrogen bonding
What type of compound is the best for solvation and why?
Where the charge is spread out, e.g lots of EWGs
This is because solvation is entropically unfavourable, so spreading out the charges makes it easier for the substance to solvated and stabilised
What is PkAH and how can it be useful?
pkA of a conjugate acid, used to assess base strength
e.g pkAH= NH4+ ⇌ NH3 + H+
High pkAH suggests a weaker conjugate acid, so a stronger base
Why is 2-methyl-N,N-dimethylphenyl amine more basic than expected?
Steric clash between the methyl groups of the amines and the methyl on the carbon adjacent
Therefore, sp3 nitrogen to reduce steric hindrance, prevents delocalisation of the lone pair, increasing electron density on the nitrogen and so increases basicity
What does the Gibbs/reaction coordinate graph look like for Sn2? Describe all details
Reactants at higher energy than the products
Curve starting from reactants to products with peak halfway
Peak=Transition state
From A to B = ∆ G, driving force
From A to TS= ∆G‡, free energy of activation
How does the transition state link to rate of reaction?
Molecules must have sufficient energy to overcome the free energy of activation
Lower energy transition states will have a greater rate of reaction
What does the Gibbs/reaction coordinate graph look like for Sn1? Describe all details
Reactants at higher energy than the products
Intermediate at higher energy than reactants and products
Curve starting from reactants to intermediate with peak halfway
Curve starting from intermediate to products with peak halfway
Peak=Transition state
From A to B = ∆ G, driving force
From A to TS1= ∆G‡1
From intermediate to TS2= ∆G‡2
Whichever ∆G‡ is larger is the rate determining step
What is Hammond’s postulate? Why is it useful?
For consecutive species, closer in energy implies close in structure
This means for most reactions, the transition states are closer in energy to the intermediate than reactants or products, so closer in structure to these
Factors which stabilise the intermediate will tend to stabilise the TS, increasing the rate of reaction
What does the Gibbs/reaction coordinate graph look like for a divergent reaction?
From the reactants, two paths, each with a different product
Two products- B higher in energy than C
2 curves starting from reactants to B or C, the peak for C is larger
so ∆G‡AB<∆G‡AC
but also ∆G AB < ∆G AC
How can the products of a divergent be controlled?
Kinetic Control- typically using lower temperatures to give the higher energy (less stable) , but lower activation energy product which is formed faster, irreversible
Thermodynamic Control- typically using higher temperatures to give the lower energy (more stable), but higher activation energy product which is formed more slowly, reversible
What are nucleophiles and give examples of each type?
Electron rich, electron pair donors
Either as lone pairs or filled orbitals
LP= NH3, OH-
π bond= Alkenes
σ bond= BH4-
What are electrophiles and give examples of each type?
Electron deficient, electron pair acceptors, containing unoccupied orbitals
P0= BH3, CH3+
π= carbonyls
σ=Br2, alkyl halides
Why types of HOMO/LUMO interaction is best and why? What is the order of energy for each type of orbital?
Where there is a small energy gap between the HOMO of the nucleophile and LUMO of the electrophile
Leads to the greatest stabilisation
σ>π>non-bonding lp>π >σ
What are the best types of electrophiles/nucleophiles?
Why do you need to take care with formal charges?
Low energy LUMO of electrophiles, and high energy HOMO of nucleophile to minimise the energy gap
Formal charges do not align with empty orbtials
e.g H3O+, but nucleophiles cannot attack the oxygen as already has a full octet, instead a base
How is the π nucleophile similar to the empty p orbital electrophile?
Both can have attacks from above or below as planar
How does the nucleophile attack a σ* and π*?
σ*= attacks from behind the leaving group so their is maximum HOMO/LUMO overlap, leading to inversion of stereochemistry
π*= attacks at the burgi-dunitz angle of 105-107 degrees between trajectory and C=O. Also to maximise HOMO-LUMO overlap whilst minimising steric repulsion
How are carbocations and carbanions stabilised?
Carbocations- more alkyl groups and electron donating groups , inductive effects
Carbanions- less alkyl groups and more EWGs via inductive
Both are stabilised by conjugation
What is the shape of carbon-based radicals? How can these radicals be stabilised?
CH3· = trigonal planar like the carbocation, but CF3·= pyramidal, so depends
Stabilised by alkyls like cation, and can also exhibit resonance
What are carbenes?
R2C:
Either exist as a diradical where 1 unpaired electron is in an Sp2 and the other p = triplet
Or as a lone pair in an Sp2 with an empty p orbital= singlet
How does a polar solvent affect a compound?
Increases the stability of ions of the compound
e.g stabilises a charged conjugate base
promotes ionisation of an alkyl halide
What is the mechanism for Sn1? And the rate equation?
- leaving group leaves, forming a carbocation
- nucleophile attacks the carbocation, from above or below, racemic mixture
rate= k [alkyl halide]
not dependent on the nucleophile
What conditions support Sn1 and why?
Polar, protic solvent to stabilise the carbocation by solvation
Tertiary alkyls so the carbocation can be stabilised via hyperconjugation
Must be planar to enable hyperconjugation, so no bridgeheads
Generally weak nucleophiles
What is the mechanism for Sn2 and the rate equation?
- nucleophilic attack behind the leaving group to maximise overlap with σ*
- TS with a trigonal bipyramidal structure
-LG leaves, inversion of stereochemistry
rate= k [alkyl halide] [nucleophile]
What does the TS for Sn2 look like?
Trigonal bipyramidal with R groups planar around the centre
Partial (dashed bonds) between nu and LG
Partial negatives on LG and nu, partial positive on carbon
Double dagger sign around square brackets
