Fundamental organic reactions Flashcards
nucleophile definition
a negative ion or electron rich species which has an electron pair in its HOMO which it will donate to form dative covalent bonds.
electrophile
a positive ion or electron deficient species which will have a low energy antibonding or unoccupied P orbital, which will allow it to accept non bonding electron pairs to form dative covalent bonds.
curly arrow starting point
nucleophiles electron pair
curly arrow end point
electrophiles LUMO
strength of nucleophiles
n
π
σ
strength of electrophiles
P
π*
σ*
types of nucleophile
lone pair
negative charge (lone pair)
double bonded (π electron pair will act as the HOMO)
types of electrophile
positive ions
neutral atoms with a delta positive dipole from a polar covalent bond
carbonyls and imines
checking mechanisms
a reaction mechanism must have the same overall charge and ratio of species on both sides of the reaction.
SN1 rate order explanation
SN1 is 1st order because only the concentration of the molecule containing the leaving group involved will affect the rate of reaction.
SN2 rate order explanation
SN2 is 2nd order because both the nucleophile and the molecule containing the leaving groups concentrations will alter the rate of reaction.
SN1 rate determining step
the leaving group cleaving the bond and retaining the electrons.
SN2 rate determining step
the nucleophile attacking the σ* orbital of the C - X bond followed by the leaving group cleaving off and retaining an electron pair.
SN1 energy diagram
reactants
large activation energy
intermediate
small activation energy
products
SN2 energy diagram
reactants
activation energy
products
primary haloalkane mechanism
SN2
secondary alkyl halide mechanism
SN2 or SN1
tertiary alkyl halide mechanism
SN1
methyl halide mechanism
SN2
reason for a tertiary alkyl halide being an SN1mechanism
the alkyl halide has sufficient steric hinderance to allow a carbocation (tertiary) to form due to hyperconjugation, and also to prevent the nucleophile from directly attacking the carbon halogen bond.
reason for primary alkyl halides SN2 mechanism
the alkyl halide will not have enough steric hinderance to form a carbocation, due to no hyperconjugation, and the lack of bulk around the molecules will mean that the nucleophile can directly attack the carbon halogen bond.
SN1 mechanism
check book
SN2 mechanism
check book
which orbital does the nucleophile attack in SN2
σ* orbital of the C - X bond
which orbital does the nucleophile attack in SN1
unoccupied 2P orbital in the carbocation
what is formed halfway through an SN1 mechanism
an intermediate
what is formed halfway through an SN2 mechanism
a transition state
methods of speeding up an SN1 mechanism
hyperconjugation/positive inductive effect
P - π conjugation
P - n conjugation
methods of speeding up SN1 mechanisms explanation
the method of speeding up the reaction mechanism is by stabilizing the intermediate carbocation so it will form more easily.
hyperconjugation effect on SN1
parallel σ bonds to the positive unoccupied 2P orbital will donate electron density which will more evenly distribute the charge around the molecule and stabilize the carbocation.
p - n conjugation effect on SN1
the non bonding electron pair on the heteroatom will be donated to the unoccupied P orbital of the bond adjacent, which will allow for the charge from the carbocation to be transferred between the carbon and the heteroatom, which will spread the charge out and stabilize the carbocation.
P - π resonance effect on SN1
electron pairs from the π orbital will be donated to the adjacent bond which will redistribute the charge to a different carbon, allowing the charge to be spread out and stabilizing the carbocation.
types of carbon atom which can cause
p - π conjugation
allylic carbons and benzylic carbons
the benzylic carbon can only receive a nucleophile at the benzylic carbon, however an allylic molecule can receive a nucleophile at any +ve charge location.
SN1 molecules
tertiary alkyl halides - hyperconjugation
secondary alkyl halides - hyperconjugation
allylic halides - p - π resonance
benzylic halides - p - π resonance
heteroatom one bond away from carbocation - p - n resonance.
speeding up SN2 mechanisms methods
carbon double bond or carbon oxygen double bond
carbon double bond and carbon oxygen double bond method of speeding up SN2
the carbon double bonds will have a π orbital - need to finish this when I understand it.
leaving group strength order
RSO3 > I- > Br- > Cl- > H2O
OH- leaving group strength solution 1
OH- is a very weak leaving group and wont cleave by itself, so it should be protonated to form a water leaving group which will cleave easily due to high electronegativity and its positive charge.
this is done by using a strong acid.
OH- leaving group strength solution 2
alternatively you could remove the hydrogen from the OH- and bond the O to a new element such as P or S. This will allow the leaving group strength to increase.
SN1 stereoisomerism
will produce a racemic mixture if the carbon of the C - X bond was a chiral centre
SN1 racemic mixture explanation
the nucleophile can attack the carbocation from above or below the unoccupied 2P orbital, which will result in the formation of two enantiomers of equal proportions.
SN2 stereoisomerism explanation
the nucleophile can only attack the substrate at one orientation, at the σ* orbital of the C - X bond, which will result in only one enantiomer being produced by the reaction. This is called stereospecific
stereospecific definition
where one stereoisomer substrate will form a specific stereoisomer product exclusively.
stereoselective definition
where one stereoisomer substrate will produce a mixture of two stereoisomer products with one being the major product and one being the minor product.
molecules acting by and SN2 mechanism
methyl halides - low steric hinderance
primary alkyl halides - low steric hinderance
α carbonyl molecules - further knowledge needed
factors promoting SN1 over SN2
polar solvent - stabilize carbocation
steric hindrance - stabilize carbocation
factors promoting SN2 over SN1
less polar solvents
and lower steric hindrance
steric hindrance effect on reaction rate of nucleophilic substitution
the large electron density of the steric hindrance will be more stable the further apart each of the branches gets, which will mean that the SN1 carbocation will be more stable and the SN2 transition state will be less stable.
slows down SN2 and speeds up SN1.
Elimination reaction types
E1, E2 and E1cB
elimination reaction definition
where a nucleophile will remove a proton and cause the formation of a carbon to carbo n double bond, forcing off a leaving group as well.
E1 mechanism
check book
E2 mechanism
check book
E1cB mechanism
check book
elimination reaction reagent
a nucleophile which acts as a base
rule for nucleophile behavior and reaction mechanism
more basic nucleophiles will cause E reactions to occur
‘nucleophilic’ nucleophiles will cause nucleophilic substitutions.
this is because in elimination the base removes a hydrogen, whereas in nucleophilic substitution the nucleophile attacks the delta positive carbon.
why are elimination reactions competing reactions with nucleophilic substitutions
because both reactions require a nucleophile as its reagent and the reaction mechanism depends on how the nucleophile acts.
E1 rate equation
E1 rate = k[ leaving group ]
E2 rate equation
rate = k [ leaving group] x [ base ]
E1cB rate equation
rate = k [ conjugate base]
factors promoting Elimination over nucleophilic substitution
more basic nucleophile
greater steric bulk of nucleophile
increasing temperature
pKa relationship with basicity
the higher the pKa the stronger the base
how to identify an E1cB reaction
if it contains a carbonyl 2 carbons over from the leaving group its an E1cB
how does a more basic nucleophile promote elimination
the nucleophile will act to remove the hydrogen from the adjacent carbon instead of attacking the carbocation/delta positive carbon, which will promote the elimination pathway.
how does temperature promote elimination reactions
need more info
how does greater steric hinderance promote elimination reactions
the steric hinderance will reduce the ability of the nucleophile to attack the delta positive carbon/ carbocation, resulting in the elimination mechanism being promoted due to the nucleophilic substitution not being possible.
factors promoting E2 over E1
less polar solvent - stabilize the transition state
stronger bases - will be able to remove the hydrogen 1st - which is a key part of the mechanism
more bulky base - need more info
factors promoting E1 over E2
more polar solvent - stabilize the intermediate carbocation
weaker base - will mean the hydrogen will be removed after the carbocation is formed.
less bulky base - need more information
bulky nucleophile for elimination
DBU
N - C = N
with two different rings attached
each nitrogen will have a non bonding electron pair which it will donate to form a mesomeric effect.
tosylate structure and purpose
benzene ring attached to SO3-
used to improve the leaving group of OH-
mesylate structure and purpose
methyl group attached to SO3-
used to improve the leaving group of OH-
stereoisomerism requirement in elimination
for stereoisomers to form in elimination the carbon bonded to the leaving group must be a stereogenic center.
regioisomers requirement in elimination
for regioisomers to form there must be two distinct hydrogens available in different positions which can be lost during the elimination mechanism.
E1 orientation for mechanism to occur
The E1 mechanism requires a hydrogen to be facing in the same plane as the unoccupied 2P orbital of the carbocation, meaning a hydrogen rotates to the down position
E2 orientation for mechanism to occur.
for the E2 mechanism to proceed the molecule must be anti periplanar with the C -H σ orbital and the C - X σ* orbital being parallel.
elimination reactions type of stereoisomerism for enantiomers
stereoselective producing the E geometric isomer as the major product.
elimination reactions type of stereoisomers for diasterioisomers
stereospecific producing one corresponding diasterioisomer
electrophilic addition basic process
the π electrons of the carbon double bond will act as the nucleophile - donating electrons to a σ* orbital.
why does benzene not undergo electrophilic addition
the molecule contains a conjugated system which will allow for delocalized π electrons which will travel around the ring structure freely and therefore wont be donated to the σ* orbital of an electrophile.
types of electrophilic addition
halogenation
hydrohalogenation
epoxidation
halogenation mechanism
check book
rate determining step in electrophilic addition
the 1st step where the bond between the electrophile cleaves to the delta negative atom and the nucleophile attacks the electrophiles σ* orbital.
how to determine a rate determining step
the rate determining step will be the step with the greatest activation energy to achieve a transition state/ intermediate, which means that it will go the slowest due to the transition state/ intermediates low stability.
halogenation reaction profile
same as SN1
Halogenation in a solvent
the solvent molecule will act as the nucleophile during the second step due to the high ratio of solvent molecules compared to the number of anion halogens in the reaction mixture.
hydrohalogenation mechanism
check book
markovnikovs rule strict definition
during electrophilic addition the reaction mechanism will proceed by the most stable carbocation intermediate with the lowest activation energy.
markovnikovs rule for hydrogen when being lazy
hydrogen will be added to the side with more hydrogens/ less alkyl groups.
markovnikovs rule major product rule
electrophilic addition will proceed by the lowest activation energy reaction mechanism for a large majority of the reactants, however it is possible for a small percentage of reactants to form the higher energy transition state, hence why we have a major and minor product.
dienes electrophilic addition
they will go by the most stable carbocation - but if dienes are one bond apart this means they have a conjugated system so remember to consider stabilization by delocalization.
stereochemistry of ring structures in electrophilic addition
ring structures will prevent the rotation of bonds within the molecule which will mean the different electrophilic groups will bond on different planes, which will result in a racemate of the trans isomer, since the groups must be on opposite sides but can be bonded to either of the carbons in the original carbon to carbon double bond
what is the diasterioisomer product of electrophilic addition of a Z alkene with no ring
the anti diasterioisomer will be produced
in racemate
what is the diasterioisomer product of the E alkene with no ring
will produce the the syn diasterioisomer product.
concerted reaction definition
a reaction where many bonds break and form during the same step.
epoxidation reaction mechanism
check book
epoxidation electrophile
meta - chlroro per benzioc acid
m -cpba
acidic mechanism for epoxide opening product
non bonding electron pair on the oxygen will attack the hydrogen which makes the conditions acidic.
the nucleophile will attack the ring from the more substituted carbon.
The nucleophile will attack the carbon in an electrophilic addition fashion.
basic mechanism for epoxide opening
the nucleophile will attack the carbon at the less sterically hindered carbon in the epoxide ring.
epoxide opening with acidic water
hydrogen will be added to the oxygen using the oxygens lone pairs
the water molecule will attack the sigma* of the more substituted carbon atom, which will result hydroxyls bonding on opposite planes
producing a cis diol
carry out dihydroxylation - a form of electrophilic addition
electrophile - osmium tetroxide
nucleophile - an alkene
will be stereo selective due to the oxygens bonding on the same plane.
producing a trans diol
use a cycling epoxide and open the epoxide using acidic water, the water will open the ring on the more substituted carbon by attacking the sigma* orbital, which will mean the OH groups will form on different planes.
