Chapter 8

Question 1

What happens in an elimination rxn?

Answer 1

base attacks the H on a beta C, double bond is formed, halide is eliminated

Question 2

E2 is _____ and ______.

Answer 2

bimolecular, concerted

Question 3

E1 is _____ and _______.

Answer 3

unimolecular, stepwise

Question 4

Dehydrohalogenation?

Answer 4

• remove H, halide

- bases used: OH, OCH3, OCH2CH3

Question 5

How many membered rings do you need to make a trans isomer in cycloalkenes?

Answer 5

8, anything less = no trans

Question 6

Why would cis-2-butene be higher in energy than trans-2-butene?

Answer 6

steric hindrance

Question 7

What alkenes are more stable?

Answer 7

most substd

Question 8

E2 is ____ order kinetics.

Answer 8

2nd

Question 9

What is the rate of E2 rxns?

Answer 9

rate = k[reactant][base]

-bimolecular

Question 10

In E2, what is more likely, endo or exo?

Answer 10

• about the same
• reactants/products are about the same energy
• TS in middle

Question 11

What is the geometry of the transition state in E2?

Answer 11

in between sp2 and sp3, flatter than tetrahedral

Question 12

What are some strong bases that are good for elimination?

Answer 12

DBN

DBU

Question 13

Strong bases for elimination = _____ rate.

Answer 13

faster

Question 14

Good LGs for E2?

Answer 14

I is better than F

-E2 rxns are faster with better LGs

Question 15

What solvent is best for E2?

Answer 15

polar aprotic

-dont want it solvated

Question 16

E2 rates for alkyl halides?

Answer 16

3 is fastest, Me is slowest

-more R groups = more substd double bond in TS, lower energy

Question 17

Zaitsev’s rule?

Answer 17

major product is the more substd double bond

• deprotonate C with least number of beta H’s
• TS is lower in energy to more substd, forms faster

Question 18

Regioselective?

Answer 18

• more than constitutional isomer is possible, but one predominates or is formed exclusively
• E2

Question 19

Stereoselective?

Answer 19

• more than stereoisomer is possible, but one predominates or is formed exclusively
• cis and trans
• trans- lower e, planar geometry, major
• cis- steric hindrance, minor

Question 20

E1 is ___ order kinetics.

Answer 20

1st

Question 21

Rate law of E1?

Answer 21

rate = k[reactant]

• unimolecular
• stepwise, base not in rate law
• tertiary (3) and weak base in E1

Question 22

T/F

Hammonds postulate works for E1.

Answer 22

True

Question 23

E1 rates for alkyl halides?

Answer 23

3 is fastest, 1 is slowest (rarely happens)

Question 24

What type of base is used for E1?

Answer 24

weak

Question 25

When does E1 happen?

Answer 25

• when E2 is not possible
• weak base instead of strong base
• base sits around until carbocation forms
• regioselective

Question 26

Why are E1/Sn1 rxns bad for the lab?

Answer 26

they occur simultaneously many times

Question 27

What are the possible geometries for E2 rxns?

Answer 27

• synperiplanar- H, X eclipsed

- antiperiplanar- preferred, less torsional strain

Question 28

Where does the halide need to be positioned to perform E2 in a ring?

Answer 28

axial

-does not work in equatorial

Question 29

How does an alkyne form in E2?

Answer 29

double dehydrohalogenation

-need two halides, two beta H’s

Question 30

Vicinal dihalide?

Answer 30

halo on adjacent C’s

Question 31

Geminal dihalide?

Answer 31

halo on same C

Question 32

What are the best bases to use?

Answer 32

• NH2

- tertbutoxide in DMSO

Question 33

What are some strong Nucs that are weak bases? what rxn do they favor?

Answer 33

• Cl, Br, I, N3, CH3COO, SH, CN

- substn favored

Question 34

Bulky bases favor what type of rxn?

Answer 34

elimination

• tertbutoxide, DBU, DBN
• exception: methyl halide

Question 35

tertiary alkyl halides are prone to ______.

Answer 35

elimination

• mixture of Sn1, E1
• on test, do E1