Nucleophilic subs and eliminations Flashcards
Which nucleophiles react faster in subs reactions and why?
CH3O-, CH3NH2, Ch3OH
CH3O- reacts fastest b/c is has a negative charge, CH3NH2 is second b/c it is more basic b/c N is less electronegative.
Leaving group ability of halides increases with…
decreasing base strength/electronegativity (I>Br>Cl>F)
How does atom size affect nucleophilicity?
as atom size increases (moving down a column) nucleophilicity increases.
How does a reaction via SN2 mechanism affect optical activity of a molecule?
optical polarization is equal and opposite, b/c the stereochemistry is inverted.
Rank these molecules in terms of fastest reactant for an SN2 reaction: CH3CH2CH2-Br, CH2=CHCH2Br, Benzyl bromide
Benzyl bromide > CH2=CHCH2Br > CH3CH2CH2-Br. Being next to a P-system enhances reactivity of halide (resonance donates e- density.)
Ph-O-R + conc HBr and heat
Ph-OH + Br-R
Why don’t phenols and and phenyl halides undergo substitution reactions?
Cant undergo SN2 b/c they are too bulky, and can’t undergo SN1 b/c there is no H to abstract.
Which mechanism does ring-opening of epoxides involve and what is the result at the reaction site?
SN2 - reaction site is inverted, while alcohol site retains stereochemistry.
Where does the nucleophile add to an epoxide if the epoxide is protonated first? if it isn’t?
If it is - adds to more subs carbon, if it isn’t protonated - adds to less subs carbon (nl SN2)
Which will undergo SN1 faster, vinylic or allylic halides?
allylic - b/c resonance stabilized cation, vinylic will not undergo SN1 b/c they are too unstable.
What conformation does a cyclohexane halide need to be in to undergo E2?
trans-diaxial (H and X)
What is the major product in a mixture formed by an E2 reaction?
The product with the more substituted alkene
What is the preferred conformation to undergo E2 and why?
Anti-coplanar (more common) or syn-coplanar, because the orbitals of the H and X are aligned, allowing formation of pi overlap when atoms rehybridize to form the double bond.
What happens to product distribution of an E2 reaction when a bulky base is used?
The less hindered product predominates, because the steric factors that affect transition state outweigh the electronic factor of having a more stable alkene
What are some examples of good leaving groups for SN2 reactions?
Halide ions, sulfonate/sulfate ions, phosphate, NH3, alcohol, water
