Sn2 Flashcards
What is the one step substitution reaction called?
Sn2
Substitution, nucleophilic, 2nd rate
How many steps in sn2?
One step
substitution, nucleophilic, 2nd rate
What does 2nd rate reaction mean?
Both reagents are involved in the rate determining step
Where does the nucleophile attack from in sn2?
180 degrees, from behind
What substitution reaction mechanism attacks from behind? 180 degrees
sn2
what undergoes sn2 reactions?
primary and secondary, aren’t sterically hindered
tertiary, aromatic and double bond to halogen (vinylic halide) don’t do sn2
What doesn’t undergo sn2?
Tertiary
aromatic
double bond to halogen (vinylic halide)
all too sterically hindered
Why do tertiary, aromatic and vinylic halides not undergo sn2 reactions?
They’re too sterically hindered
For a reaction to proceed is gibbs free energy of products above or below zero?
below, releases energy
What is transition state?
the combination of reagents, exists at energy maxima and extremely unstable
Where is the transition state on an energy diagram?
at energy maxima
what is transition state represented by?
double dagger (two plusses on-top of each other (‡ ))
What is the rate of reaction determined by?
The activation energy
What is the activation energy?
sn2
Energy required for a reaction to happen,
energy difference between starting and transition state
How can you control rate of reaction?
Changing activation energy by:
Changing energy level of reactant
Changing energy level of transition state ( ‡ )
What are solvent effects for sn2 reaction?
Adding polar solvent slows reaction.
Since sn2 involves anionic nucleophiles the polar solvent can stabilise these, lowering the starting materials energy, but not the transition state so more energy is needed to reach the transition state
What does adding a polar solvent do to a sn2 reaction?
Slows it down since the polar solvent stabilises anionic nucleophiles lowering starting material energy, so more energy is needed to move it to transition state
What are the steric effects of sn2 reactions?
The larger the substituents the slower the reaction.
Since transition state has ‘5 bonds’ its very sterically hindered, the more hindered it is the higher in energy it becomes, so more energy is needed to reach it.
For an sn2 reaction what is faster a carbon with lots of substituents or a small one?
A small one is faster, since the transition state will be less sterically hindered, so lower in energy
What materials does sn2 not happen with?
Tertiary carbons
Vinylic halides (double bond with halogen to one of the c’s)
Aromatic compounds
ALL TOO STERICALLY HINDERED cant approach from the back
Order of reaction speed for sn2 steric effects
Fastest to slowest
Methyl> Primary> secondary
tertiary doesn’t happen too sterically hindered
sn2 doesn’t occur on…
SP2 carbons
What angle does nucleophile attack from in sn2?
180 degrees
What are the electronic effects of sn2 reactions?
Stabilising transition state lowers it in energy, making it easier to reach. Conjugated systems can do this.
