lecture 7: elimination Flashcards
explain why trans is more stable than cis
beacuse in cis isomers there is steric hinderance (more crowding)
the more subsituted an alkene is, the more or less stable is it
more stable
why are more subsititued alknes more stable
because added alkyl groups donate electrons to the sp2 carbon because of hyperconj and inductition
rings of cyclos contained 5 C or fewer can only exist in what form
cis form
why can cyclo alkenes 5c or less only excist in cis form
because of bond strain
in elimination do you make or remove bonds
make bonds
what two reactions compete
elimination and subsition
if the reactor is nucleophillic is it S or E
S
if the reactor is basic is it S or E
E
what is another name for dehydrohalogenation
beta elimination, 1-2 elimination
explain dehydrohalogenation
synthesiszing alkenes via the elimination of H-X bond from adjacent atoms of alkyl halides
the carbon atom that bears the LG is called what
the alpha carbon
the carbon atom adjacent to the C that bears the LG is called what
the beta carbon
is the e2 bimol or uni
bimol
explain tthe rate for e2
in the rate determining step, the reaction reatet is deermined by the concentrtatiton of substrtatte and of base
what tis the general mechniams of e2
in e2 rxn, a base removes a b hydrogen from b carbon as the double bond forms and a LG departs from alpha carbon
explain e2 mechamisn
1) a base regioselecttively removes an H+ from a carbon that is adjacent (beta) to the halogen carbon (alpha)
2) as the H+ is removed, e tthat is shared w carbon moves ttowards adjacent halogen bonded c atom
3) as the e moves to the alpha carbon, the halogen LG leaves and takes the e pair with it (hetero)
know the e2 mechanism
what is the difference between syn and anti eliminatiton
in anti, tthe base atttacks the b hydrogen on the oPPOSITE side of the LG
why is anti more sttable
since less steric hinderance
what is zaitzevs rule
an elimination that occurs to give the most stable and highly subsituted alkene
(removes H from the beta position thtat is most subsituted/least amount of carbons)
is a more internal or external bond more stable
internal
formation of more subsituted alkene is favoured with a large or small base
small
formation of a less substituted alkene is favoured with what type of base
a bulky base
explain the affectt of base size on formation of alekens
if you use a small base, the major product will be the highly subsituted alkene (because there is no steric hinderance)
if you use a bulky sterically hindered base, it has a hard time removing an internal hydrogen (crowding) therefore it will remove an external H to form a less subsituted
the e2 recation produces mostly what isomer and why
thee E isomer (because it does antti elim)
=steric interactions prefer bulky groups on opposite sides form the most stable bond)
in a cyclohexane, e2 rection can only occur when the groups are in what positions
can only occur when the LG is axial and the H is axial
true or false: for cyclohexane e2 reactions, the LG and the H mus tbe in the equitorial position
false, axial
why do the halogen and hydrogen need to be axial from each otther
they need to be in the antiperiplanar position from the leaving group
when NU attacks b hydrogen, what rxn occurs
elim
when NU attacks the c bearing the LG, what rxn happens
substitutuon
en2 and sn2 prefer what nucleophiles and solvents
strong nu
aprotic solvents
explain the affect of classificaiton of the alkyl halide on elim vs substituon
primary= favours sn2 due to less steric hinderance (can easily reach C)
ttertiary= favours e2 due to more stable tertiaty carbocation (hard to reach center C)
explain affect of termperate on what type of reaction
w/o heating= substituion
w/ heating= elim
true or false: increase temp faours subsitution
false, elimination
strong sterically hindered bases favour what rxn
elimination
give example of bulky base
tbut
why do small bases (ex: CH3O) favour sub
has easy time rection the C
why do bulky bases prefer elimination
they cannot get tclose to the center C due to steric hinderance
hydroxide, amides icnrease likelyhood or what
elimination
cl-, ch3co2- increase likelyhood or what
sub
if there is a negative charge on the oxygen that makes a good or bad base
strong base
if solvent acts as a BASE and removes a hydrogen, what rxn is it
elim
if solvent acts as a NU what rxn
sn
explain the e1 recation mechanism
the alkyl halide dissocates forming a carbocation (similar to sn1)
2) removal of h by the base
how is leaving group able to leave on its own in e1 reactions
it is aided by the polar solvent
e1 reactions are favoured by substrates that form stable carbocations true or false
true
list the relative reactivities of alkyl halides in e1 reaction
3 benzylic (most stable=resonance) 3 allylic 2 benxylic 2 allylic 3 1 benzylic 1 allylic 2 1 vinyl
what is the reaction of alcohols when heated with strong acid
most alcohols undergo dehydration b=rxn (loss of h20) to form an alkene when heated with a strong acid
2 and 3 degree alcohols favour what type of reaction
e1
primary alcs favour what type of recatiton
e2 with high temps
true or false: 2 and 3 degree alchols are most diffuclt to dehydrtea==ate
false, primayr==ry
what is the relative ease with which alcohols undergo dehydration
3 better than 2 than 1
know the mechanism for dehydration of alcohols (2.3)
what is the general mechanism for dehydration of 2 3 alchols in elimination
1) alcholol acceots a protton from the acid in the fast step
2) the protonated alcohol loses a molucule of water (LG) to become a carbocattion=rate determining
3) carbocation loses a porton to a base (base may be another alcohol, waater etc) and formation of alkene
e1 and sn1 reactons take place via carbocation intermediates which means what is possibor
rearragnements
sn2 prefers protic or aprotic
aprotic
sn2 reacvtity
me
1
2
sn2 nucleophile needed
good (ex: OH-)
sn2 tetmperated needed
low (25)
sterics of sn2
low (OH-)
what is the sterochem of sn2
incversion of configuration
what is the solvent of sn1
polar protic (h20, etoh)
what is the recatiivty for sn1
3
2
1
what is the nu for sn1 reaction
weak (h20)
what is the temperate for sn1
low (25)
what are the stterics for sn1
low
what is the sterochemistry of sn1
makes racemic mixtures
what is the solvent for e2
polar aprotic (DMSO, DMF)
what is the reacivtiy of e2
3
2
1
what is the nu for e2
good (OH-)
what is the teermpeaerted for e2
high (80)
what is the sterics of NU for E2
high
what is the sterochem of e2 recaetions
cis trans
internal bond favoured
what is the solvent for e1
polar protic
what is the reacivtiy for e1
3
2
1
what is the nu for e1
poor (h2)
what is the temperated for e1
high
what is the sterics for nu for e1
high (T\HotBU)
what are the sterochemi for e1
cis trans internal bond favoured
