4. Substitution Reactions Flashcards
what happens in a substitution reaction and when does it occur
one group is exchanged for another
occurs when an electrophile is treated with a nucleophile
what is an electrophile known as and what must it contain
substrate
contain a good leaving group
what are 3 common leaving groups
Br, I, Cl
what are halogenated compounds are used as in substitution reactions
electrophiles
what is an alpha carbon
carbon atoms connected directly to halogen
what are beta carbons
carbons connected to the alpha carbon
what are the 2 mechanisms that occur during a substitution reaction
concerted process
stepwise process
what is a concerted process
nucleophilic attack and loss of leaving group occur at the same time
what is a stepwise process
leaving group is lost first and then the nucleophile attacks
what is the rate equation of an SN2 reactions
rate = k[substrate][nucleophile]
what order is the rate equation of SN2 reactions
why
second order
rate is linearly dependent on conc of 2 different compounds
is SN2 a concerted process
yes
when a SN2 reaction occurs what happens to the configuration
inversion of configuration
why does the configuration invert in a SN2 reaction
nucleophile can only attack from the side opposite the leaving group
what alkyl halide is most reactive
how does reactivity decrease
methyl alkyl halide most reactive
reactivity decreases when moving from primary to secondary to tertiary halides
what is steric hindrance responsible for in reactivity and reaction rate of SN2 reactions and why
from extra side chains
decrease in reactivity and reaction rate
why are SN2 reactions slow
steric hindrance
why does SN1 reactions likely to occur
as SN2 is very slow
what is the rate equation for SN1
rate = k[substrate]
what is the SN1 reaction rate dependent on
only the conc of the substrate
will a change in nucleophile conc affect the reaction rate of SN1
no
for SN1 reaction what are the 2 steps involved and which is the rate determining reaction
leaving group leaves first (rate determining step)
nucleophile attacks the carbocation formed
what process does SN1 reaction follow
stepwise process
what is the trend for SN1 compared to SN2
trend for SN1 and SN2 reverse of each other
what is the substrate that is most reactive for SN1 reactions
tertiary substrates most reactive and methyl substrates are least reactive
when the leaving group leaves in SN1 reaction what is formed
carbocation is formed
what kinds of carbocations are most stable
tertiary more stable than secondary > primary > methyl alkyl halide
what is a planar compound and when is it formed
when leaving group of SN1 is eliminated the carbocation (planar compound) is formed
what is a planar compound and when is it formed
when leaving group of SN1 is eliminated the carbocation (planar compound) is formed
does SN1 reactions result in inversion of configuration and why/why not
nucleophile has equal chance of attacking from either side of carbocation
some products show retention of configuration and some show inversion of configuration
what is a racemic mixture
show both R and S configurations
what substrates undergo SN1 substitution
tertiary substrates
what substrates undergo SN2 substitution
methyl and primary substrates
does secondary substrates undergo SN1 or SN2 reactions
what helps to identify the reation mechanisms
SN1 or SN2
look at nucleophiles to help identify reaction mechanisms
does strong nucleophiles and weak nucleophiles favor SN1 or SN2
strong = SN2
weak = SN1
what are the 9 strong nucleophiles
I-
Br-
HS-
HO-
H2S
RO-
Cl-
RSH
N(triple bond)C-
what are 3 weak nucleophiles
F-
H2O
ROH
what are protic solvents
contain Hydrogen connected to electronegative atom
what are polar aprotic solvents
contain no H atoms connected to an electronegative atom
do protic solvents favour SN1 or SN2
SN1 reaction
do polar aprotic solvents favour SN1 or SN2
SN2
what is the rate law for SN1 and SN2
SN1 = unimolecular (substrate only)
SN2 = bimolecular (substrate and nucleophile)
what is the barrier for SN1 and SN2 reactions
SN1 = carbocation stability
SN2 = steric hindrance
what is the order of reactivity of alkyl halides for SN1 and SN2 reactions
SN1 = 3 > 2»_space; 1
SN2 = 1 > 2»_space; 3
what is the nucleophile characteristics for SN1 and SN2 reactions in terms of strength
SN1 = weak (generally neutral)
SN2 = strong (generally bearing a negative charge)
what is the solvent characteristic like for SN1 and SN2 reactions in terms of protic
SN1 = protic
SN2 = polar aprotic
what is the stereochemistry of compounds resulting from SN1 and SN2 reactions
SN1 = retention and inversion
SN2 = inversion
