Elimination Reactions Flashcards
What is true about eliminations and substitution reactions?
they often compete with one another since they have the same starting material
in an elimination pathway, how does the reagent/nucleophile act?
it acts as a base rather than a nucleophile
In an elimination reaction, what are the three products?
an alkene, the leaving group anion, and the formation of a conjugate acid
what is the rate law for an E2 reaction
R=K[subs][base]
how many arrows should you draw in an E2 mechanism and what are the steps?
Three arrows
The first arrow shows the base taking a proton from a Beta carbon
the second arrow shows the electrons from the deprotonated proton moving into the C-C bond to form a double bond
in the third step, which is the third arrow, there are temporarily 10 electrons, which causes the leaving group to leave
what defines a good nucleophile?
it has a full negative charge and polarizable
but it varies in the solvent
what is the trend of nucleophilicty in all solvents across a row
the nucleophile is better across a row from right to left
Ch3>NH2>OH>F
what is the trend of nucleophilicity in protic solutions
nucleophilicity increases down a column (in protic solvents, the best nucleophiles have larger atoms)
what’s the trend of nucleophilicity in aprotic solutions?
nucleophilicity increases up a column (the electronegativity does not attract the aprotic solvents)
if you have a methyl substrate, what reaction will proceed?
an SN2
If you have a primary substrate and a strong base bulky (TBuO, ) what reaction will proceed
E2
If you have a primary substrate and a good nucleophile (CN-, NH2) ), what reaction will proceed
SN2
If you have a secondary alkyl halide and a weak base (Cl-, I-, Br-) what reaction will proceed?
SN2
if you have a secondary alkyl halide and a strong base (tBUO, MeO, EtO)
E2
what reaction does an allylic halide and a benzyllic halide favor and why?
it favors an SN1 And E1 this is because it is highly resonance stabilized
if you have a tertiary alkyl halide, what reaction is favored if you have a weak base
it will favor an SN1 and an E1 in a 50:50 mixture
if you have a tertiary alkyl halide, what reaction will be favored if you have a strong base
it will favor an E2 and have 100% alkene formation
an E2 reaction will not occur unless
the leaving group and the Beta proton are anti-periplanar to one another
what is another way to describe an elimination reaction
you could call it a Beta-Elimination reaction
what does dehydrohalogenation mean?
it means you removed an H+ and a halogen while forming an alkene
what always occur in company with an Sn1
an E1 will always occur with an Sn1 in a 50:50 mix
what type of reaction is favored with a hindered strong base?
an E2, it is too big to get in so it prefers acid base chemistry
what type of reaction is favored with a small unhindered strong base?
an SN2, it can fit into the alkane as a substitution
when can we use cis and trans nomenclature
only with disubstituted alkenes
what is the z nomenclature
when the groups with higher priority (atomic #) are on the same side
what is the e nomenclature
when the priority groups are on opposite sides
why does SN2 not occur in a tertiary reaction?
due to steric strain
What reactions favor solvolysis reactions?
S1 and E1
what are some common solvolysis solvents and why do they favor S1 and E1
H20, MeOH, EtOH, these are all protic solvents which are good for SN1 + E2
Why do strong bases prefer Elimination
they prefer to react in acid base chemistry rather than substitution
Which alkene is more stable and why? Cis or Trans?
trans is more stable since it has less steric strain so it is lower in energy and produces less heat
what is hydrogenation?
it is the addition of a hydrogen to an alkene
what is the heat of hydrogenation
it describes the enthalpy change that comes fro the exothermic release of a hydrogenation reaction
if you have a more negative/ larger enthalpy change in a hydrogenation reaction, what does it mean?
it means that the alkene was less stable and higher in energy
what alkene is the most stable
the one with the most substitutions
(tetra> tri>di>mono>ethene)
what is hyper-conjugation
greater number # of alkyl groups = greater alkene stability
how does hyper conjugation work
the more alkyl halides you have, the more electron donating groups to donate electron density into empty Π* orbital
if you have two possible beta hydrogens for an E2 reaction, which will be the major product and which will be the minor product
the major product will have more substitutions
what is zaitsevs rule?
Zaitsev’s rule states that an elimination reaction will give a more stable, highly substituted alkene
if you use a bulky base and have two possible beta hydrogens to take, which way will the reaction proceed?
the bulky base will take the more accessible hydrogen, which will result in a less stable, less substituted alkene
what is hoffmans rule
Hoffman’s rule is that when we use a strong bulky base, we need to take the most accessible hydrogen which may result in a less stable/ less substituted product as the major product
it is the reverse of zaitsev’s rule
why is cyclopropene less stable than cyclopropane
in cyclopropene, the Π bond creates an sp2 carbon which has a ideal 120° bond angle. In cyclopropene, the bond angle is 60° which is a larger deviation than the bond angle in a cyclopropane so it is higher energy and less stable
can a cyclohexene be trans
no, it is always cis
