substitution reactions Flashcards
what makes a good leaving group
neutral molecules or stable anions
a good leaving group readily accepts …
a pair of electrons
is OH a good or bad leaving group
bad
is H2O a good or bad leaving group
good
in an Sn2 reaction, the nucleophile attacks the carbon atom from _____
behind
in an Sn2 reaction, the nucleophile forms a new bond to the carbon _____ as the C-X bond is broken
at the same time
why does the carbon attack from the back in a Sn2 reaction
it maximises the overlap of the orbitals.
its the sigma anti bonding orbital of the C-X bond that overlaps in an Sn2 as
it has the lowest energy of the empty orbitals so
in Sn2 if the C in the C-X bond is chiral then …
it leads to an inversion of configuration (walden inversion).
the size of the substituents affects …
the rate of the Sn2 reaction, the larger the substituent, the slower the reaction.
the larger the size of the substituents,
the greater the steric hinderance and the higher the energy of the transition state.
what is step one of Sn1
arrow from bond of C-X to X, leaving C+
what is step 2 of Sn1
nucleophile attacks the C+ and bonds on
which Sn1 step is the rate determining step
1
in Sn1 the nucleophile can attack from above and below this means…
the product will be racemic (both R and S)
for Sn1, the greater the number of substituents …
why?
the faster the reaction
because alkyl groups are electron donating so the stabilise the carbocation intermediate .
a substituent that can stabilise a carbocation intermediate via mesmeric effects will …
promote Sn1 reactions.
changing to a nucleophile with greater strength or increasing the concentration of nucleophile …
why?
increases the rate of Sn2
the nucleophile takes place in the rate determining step of Sn2 , but not Sn1.
forSn1, the strength of a nucleophile …
does not affect the rate, but does affect the products that are formed.
a stronger nucleophile will produce a major product whereas a weaker one would produce the minor .
strong nucleophile tends to favour the …
Sn2
weak nucleophile tends to favour the …
Sn1
weak nucleophiles tend to be ….
uncharged
what is a protic solvent
they have O-H or N-H bonds
why are protic solvents good
So they take part in hydrogen bonding, they also serve as a source of protons (H+)
what a aprotic solvent
lack O-H or N-H bonds
the rates of Sn2 are fastest in ____ solvents
aprotic
the rates of Sn1 reactions are fastest in ____ solvents
protic
what are the first 2 steps of predicting Sn1 vs Sn2
identify the nucleophile and the leaving group, next intentify the halogenalkane as either primary, secondary or tertiary.
what is step 3 of predicting Sn1 vs Sn2
consider the strength of the nucleophile and the nature of the solvent
what should you remember when when drawing the products of either Sn1 or Sn2
Sn1 reaction of a single enantiomer of a halogenalkane gives a racemic mixture whereas Sn2 gives an inverted configuration.
what is the hammond postulate
for two elementary steps that are of the same type, the one with the more negative deltaG tends to be faster and have a smaller energy barrier
