lecture 6: substitution Flashcards
what is homolyssi
bond dissociateion and each atom takes one electron (forms radicals)
what is heterolyssi
bond breaks and 1 atom takes both electrons
forms ions
heterolysis can lead to what two types of carbon ions
carbocations
carboanions
if c is more electroneg what tpy of ion do you have
carbanon
if c is less electroneg what tpy of ion do you have
carbocation
carbocations are acids or basis
they are acids since they are electron deficient
carboanions are lewis acids or bases
bases (wants to lose e pair)
electrophiles are acids or bases
acids (seeking negative charge (
true or false: does the C need to have full postiive charge
no it can have partial charge due to bond polarity
nucleophiles are seeking positive or negative charges
position chair
are nucleophiles bases or acids
bases (seeking positive chair)
what are the 4 main raction types
sub
elim
addition
rearrange
in addition, do you form or lose bonds
go from double to single
in eliminattion are you adding or losing bonds
you are forming bonds
single to double
how can an organic compound act as a base
as long as it has an unshaired electron pair it can be a base
true or false: lone/unshared electron pairs are the only things available for basicity
false, pi bonds or alkenes can also be available for protenattion
what is an alkyl halide
it has a halogen bonded to an sp3 hybridized (tetrahedral) carbon aotm
alkyl halides make bonds polairized or neutral
polarizes since the halongen is more electrogenegative
what is the classification of carbons based on
how many c groups attached to the c directly bonded to the halogen
wwhatt happens to halogen size as we go down periodic table
atomic size increases
what happens to the carbon halogen bond length and strength as we go down the periodic table and why
the carbon halogen bond length increases and the strength decreases because larger attoms are more polarizable and have less overlap (weaker bond)
CI bond is weaker or stroenger than CF
weak
SN2 is concerted or multistep
concerted
sn1 is concerted or mult step
mult step
what is the rate equation for SN2
rate= K (NU)(SUB)
depemnds on conc of nu and sub
what is the preference for SN2 reactions in terms of substrates
methyl 1 2 3
why is methyl preffered for sn2 recations
beacuse less steric hinderance
wgat is the rate equation for sn1 reactions
rate= k (sub)
only depends on the concerntration of substrate
what is the rate limiting step in sn1 reactions
the first step (formation of the carbocations)
true or fals: sn2 reactions are solvolysis reactions
false , sn1 are
true ro false: sn2 can attack from any side
false, only backside attack
rtue or false: sn1 reactions can approach from either side and why
treu because the carbocation is trtigonal planar
what must you do with a neutral NU
3 steps, must deprotonate
what is the substrate preference for sn1
3 2 1 methyl
why is 3 degree more preffered in sn1 reactions
because it forms the best stable carbocations
rearragnements can occur in what type of reactions
sn1
in a nucelophilic sub rxn, what replaces what
the nu replaces the LG
the nucleophile is always a what
a lewis base (electron pair donor)
either negative or neutral
what is the main mechanism of substitution reactions
the nuclophile is a lewis base and donates the electron pair to the substrate
the bond b/w the C and LG breaks giving both electrons from thebbond to the LG (heterolotically)
the NU uses its electron pair to form a new covalent bodn wit hthe substraite
the LG gains the pair of electrons
in substaition reactions does the LG dissoacted heterolytically or homolytically
hetero
in sn1 reactions is there inversion of stereochem
no only sn2
how does the NU approach in an SN2 reaction
nu approaches the sub from the back side (180) into the anti bonding portion
what reaction results in inversion of sterochem
sn2
what is the reactivty of the alkyl halifes in sn2
methyl 1 2 3 because of steric hinderance
what are aryl halides
halogen bonded to sp2 carbons or to aromatic rings
is sn2 bimol or unimol
bimol (requires 2 steps in the rate limiting step)
the rate of rnx in sn2 is dependient on what two facors
the concetrnation of nucleo and electron (substates)
what is basicity
measure of how well a compound shres its lone pair (tendency of conj acid to lose a proton)
what is nucleophilicity
measure of how readily a copound is able to attach to e deficient atom
what are the 3 factors affecting NU reactiviety
periodic trends
basicicity for same element
steric hinderance decreases nucleo
what is the NU reactivity trend across the table
basicity of conj base increases from left to right NU gets stronger going to the left
what is the NU reactivity trend down the table
nuc increases down a group in POLAR PROTIC SOLVENTS
more polarizable = better nucleophile
more polarixable is a better or worse LG
better
wha tis NU trend in polar protic solvent
NU increases going down table (incrase side)
what is NU trend going down table in polar APROTIC
NU decreases as you go down
what is the trend of basiciity for the same element in terms of NU reactivity
nucleo directly correlates with incrase lewis base strength when all compounds have the same attacking atom
list the relative nuclophilicity from streong to weak
RS- I- CN- CH3O- Br- NH3 Cl- F- ch3oh
what is the trend bwetween NU and steric hinderance
steric hinderance decrease nucleohpilicity
rate of reaction is not affected by leaveing group (true or F)
false, it is dependant on relative stability of LG
why does I have faster reaction rate than F
beacuse it is more stable (larger therefore can distrubute negative charge better)
rank these as least to most reactive (RI, RCL, RF, RBR)
RF
RCL
RBr
RI
why is HI tthe most stable conj base
most reactive due to long and weak bond
what determines whether or not something will be a good LG
the stability of the conjugate base determines
a LG that is a stronger base than the approaching group will do what
a LG that is a stronger base than the approaching NU will not be displaced
in sn2 reactions, which substrate have no measurable rate of reaction
3 beacuse of steric hinderance
in sn1 reactions, which substrate have no measurable rate of reaction
1
3 is the highest because 3 carbocations are the most stable because of hyperconj and inductive effect
true or false: sn1 is bimol
uni
alkyl halide reactivity is the same or oppsoite for sn1 and sn2
same beacuse weaker base is held less tightly to the carbon atoms
a carbocation is what hybridiization
sp2 (planar
what is the relative stability of carbocations related to
number of alkyl groups attached to the positive carbon (tertiaty most stable)
explain why more alkyl groups makes a more stable carbocations
1) inductive effect: alkyl groups donate electrong density to stabilize the positive charge)
2) hyperconjugation (dumping e density from sigma bonds into empty p orbitals of carbocations)
=negative charge paritally dispersed to the postive C bnecause more overlap with empty p orbitals
what is the facial preference for sn1 reactions
due to trigonal planar sym of carbocations in sn1 there is no facial preference
sn1 will lead to racemic mis=xuteres?
true
what is racemization
takes place when the rnx causes a chral molecule to be converted to achiral
what is solvolysis
sibstituion where the nucleophile is a molecule of the solvent
rate of sn1 rxn is unaffected by what and why
unaffected by NU
sicne bu does not particapte in rate limiting step (formation of carbocation)
the strenght of the nu depends on what structural features
1) a neg charge nu is always streonger than conj acid (ho stronger than h20)
2) in a group of nucleo where the atom is the same, nucleo corrleates with basicity
sn2 reactions are favoured by what solvents and gvie examples
favoured by polar aprotic (acetone, DMF, DMSO)
sn1 reactions are favoured by what solvents and give examples
polar protic solvenets
ETOH, MEOH, H20)
explain why polar protic solvents are bad for sn2
the sovlent will sovlate the NU (stabilizing and making it less reactive)
strong hydrogen bonding will encumber the NU and the NU must shed it before raecting )
in the same column, what is the relationship between atom size and solvent nu interaction
smaller atoms have more solvent nu interanctions
what is halide nucleophilicity in protic solvents
I bettwe than br, cl, F
why is I a better nucleophile than F
it is more polarizble and less electronegative therefore there is lesss solvent nu interactions
what is the relationship between polar protic solvents and sn1
sn1 prefers protic solvents because it forms H bodns with the carbos =stabilizing it (lowering energy of transition state)
rate of sn2 reactions genereally increase or decreases with polar aprotic
increase
why does sn2 favour polar aprotic
because they do not have any H bonded to highly electromneg atoms
nucleos are only weakely attracted to the solvent
nu do not need to break any strtong solvent intcatoions
what is the order of halide reactivity in APROTIC SOLVENTS
F
CL
BR
I
compare substrate factor for sn1 vs sn2
sn1= prefer 3 sn2= prefer methyl
compare nu factor for sn1 vs sn2
sn= weak nucleophile or neutral (solvolysis9 sn2= strong lowis base
compare solvent factor for sn1 vs sn2
sn1= polar protic (eg: water, alcohol) sn2= polar aprotic (DMSO, acetomo, DMF)
compare LG factor for sn1 vs sn2
same for both
I better than Br, Cl , F
true or false: vinylic and aryl halidies are unreactive in sn1 and sn2
true
what is an alkenyl halide / vinylic halide
a halogen bonded to 1 carbon of a couble bond
what is a aryl/phenyl halide
halogen attached to a benzyne ring
what are aryl/vinyl halides unreactive in sn1
due to instability of their carbocations
what are aryl/vinyl halides unreactive in sn2
beacuse the electrons of the ring or double bond repele the NU
