Elimination Reactions

Question 1

What is true about eliminations and substitution reactions?

Answer 1

they often compete with one another since they have the same starting material

Question 2

in an elimination pathway, how does the reagent/nucleophile act?

Answer 2

it acts as a base rather than a nucleophile

Question 3

In an elimination reaction, what are the three products?

Answer 3

an alkene, the leaving group anion, and the formation of a conjugate acid

Question 4

what is the rate law for an E2 reaction

Answer 4

R=K[subs][base]

Question 5

how many arrows should you draw in an E2 mechanism and what are the steps?

Answer 5

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

Question 6

what defines a good nucleophile?

Answer 6

it has a full negative charge and polarizable

but it varies in the solvent

Question 7

what is the trend of nucleophilicty in all solvents across a row

Answer 7

the nucleophile is better across a row from right to left

Ch3>NH2>OH>F

Question 8

what is the trend of nucleophilicity in protic solutions

Answer 8

nucleophilicity increases down a column (in protic solvents, the best nucleophiles have larger atoms)

Question 9

what’s the trend of nucleophilicity in aprotic solutions?

Answer 9

nucleophilicity increases up a column (the electronegativity does not attract the aprotic solvents)

Question 10

if you have a methyl substrate, what reaction will proceed?

Answer 10

an SN2

Question 11

If you have a primary substrate and a strong base bulky (TBuO, ) what reaction will proceed

Answer 11

E2

Question 12

If you have a primary substrate and a good nucleophile (CN-, NH2) ), what reaction will proceed

Answer 12

SN2

Question 13

If you have a secondary alkyl halide and a weak base (Cl-, I-, Br-) what reaction will proceed?

Answer 13

SN2

Question 14

if you have a secondary alkyl halide and a strong base (tBUO, MeO, EtO)

Answer 14

E2

Question 15

what reaction does an allylic halide and a benzyllic halide favor and why?

Answer 15

it favors an SN1 And E1 this is because it is highly resonance stabilized

Question 16

if you have a tertiary alkyl halide, what reaction is favored if you have a weak base

Answer 16

it will favor an SN1 and an E1 in a 50:50 mixture

Question 17

if you have a tertiary alkyl halide, what reaction will be favored if you have a strong base

Answer 17

it will favor an E2 and have 100% alkene formation

Question 18

an E2 reaction will not occur unless

Answer 18

the leaving group and the Beta proton are anti-periplanar to one another

Question 19

what is another way to describe an elimination reaction

Answer 19

you could call it a Beta-Elimination reaction

Question 20

what does dehydrohalogenation mean?

Answer 20

it means you removed an H+ and a halogen while forming an alkene

Question 21

what always occur in company with an Sn1

Answer 21

an E1 will always occur with an Sn1 in a 50:50 mix

Question 22

what type of reaction is favored with a hindered strong base?

Answer 22

an E2, it is too big to get in so it prefers acid base chemistry

Question 23

what type of reaction is favored with a small unhindered strong base?

Answer 23

an SN2, it can fit into the alkane as a substitution

Question 24

when can we use cis and trans nomenclature

Answer 24

only with disubstituted alkenes

Question 25

what is the z nomenclature

Answer 25

when the groups with higher priority (atomic #) are on the same side

Question 26

what is the e nomenclature

Answer 26

when the priority groups are on opposite sides

Question 27

why does SN2 not occur in a tertiary reaction?

Answer 27

due to steric strain

Question 28

What reactions favor solvolysis reactions?

Answer 28

S1 and E1

Question 29

what are some common solvolysis solvents and why do they favor S1 and E1

Answer 29

H20, MeOH, EtOH, these are all protic solvents which are good for SN1 + E2

Question 30

Why do strong bases prefer Elimination

Answer 30

they prefer to react in acid base chemistry rather than substitution

Question 31

Which alkene is more stable and why? Cis or Trans?

Answer 31

trans is more stable since it has less steric strain so it is lower in energy and produces less heat

Question 32

what is hydrogenation?

Answer 32

it is the addition of a hydrogen to an alkene

Question 33

what is the heat of hydrogenation

Answer 33

it describes the enthalpy change that comes fro the exothermic release of a hydrogenation reaction

Question 34

if you have a more negative/ larger enthalpy change in a hydrogenation reaction, what does it mean?

Answer 34

it means that the alkene was less stable and higher in energy

Question 35

what alkene is the most stable

Answer 35

the one with the most substitutions

(tetra> tri>di>mono>ethene)

Question 36

what is hyper-conjugation

Answer 36

greater number # of alkyl groups = greater alkene stability

Question 37

how does hyper conjugation work

Answer 37

the more alkyl halides you have, the more electron donating groups to donate electron density into empty Π* orbital

Question 38

if you have two possible beta hydrogens for an E2 reaction, which will be the major product and which will be the minor product

Answer 38

the major product will have more substitutions

Question 39

what is zaitsevs rule?

Answer 39

Zaitsev’s rule states that an elimination reaction will give a more stable, highly substituted alkene

Question 40

if you use a bulky base and have two possible beta hydrogens to take, which way will the reaction proceed?

Answer 40

the bulky base will take the more accessible hydrogen, which will result in a less stable, less substituted alkene

Question 41

what is hoffmans rule

Answer 41

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

Question 42

why is cyclopropene less stable than cyclopropane

Answer 42

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

Question 43

can a cyclohexene be trans

Answer 43

no, it is always cis