dehydration

Question 1

what are the key features of a dehydration

Answer 1

protonation of oh creates a good leaving group

Question 2

what is the key features of an E1 mechanism

Answer 2

loss of water produces carbocation intermediate

alcohols leading to more stable carbocation will react faster

carbocations may undergo rearrangement

most stable alkene forms as the major product

Question 3

what is the key requirement of an E2 mechanism

Answer 3

it requires an anti-periplanar geometry

Question 3

what makes a good catalyst?

Answer 3

must be strong enough to protonate alchohol

must not be nucleophillic or it will undergo a substitution pathway

Question 4

alcohols that form more stable carbocations react

Answer 4

faster

Question 5

Benzylic, allylicm and tertiary alcohols undergo what type of mechanism and whats the rate

Answer 5

they undergo rapid elimination

Question 6

what type of conditions are required for secondary and primary alcohols to undergo E1

Answer 6

heat and extreme condtions

Question 7

what is the rate limiting step in an E1

Answer 7

the loss of water leading to a carbocation

Question 8

what is zaitsev’s rule

Answer 8

it is the formation of the most stable alkene that is thermodynamically favorable

Question 9

what do the relative percentages of major and minor products depend on

Answer 9

the substrate and the reaction temperature

Question 10

what is a rearrangement

Answer 10

less stable carbocations can readily rearrange to more stable carbocations through 1,2 hydride or methyl shifts

Question 11

when will a ring expansion occur

Answer 11

when a carbocation of equal stability is rearranged to form a more stable overall compound

Question 12

when do rearrangements to less stable carbocations occur

Answer 12

when there is a subsequent carbocation rearrangement that leads to a more stable one

Question 13

when will an E2 occur

Answer 13

when the substrate cannot form a stable intermediate

Question 14

why do we wash the fractions with h2O, NaHCO3, and dry

Answer 14

to remove traces of acid

Question 15

Why might you expect a poor yield of alkene products if the dehydration reaction
was carried out using hydrochloric acid as the catalys6?

Answer 15

if you use HCl you are using a base that can also act as a nucleophile, so it could undergo a substitution pathway

Question 16

What is the purpose of the calcium chloride used in this experiment?

Answer 16

to prevent water from reversing the reaction and to isolate product also to remove traces of acid

Question 17

why might you not see all 4 products in the GC analysis

Answer 17

-similar compounds may not be identified as separate
- e1 mechanism
e2 mechanism may only allow 1-2 depending on the isomer used

Question 18

hat is the purpose of extracting the product mixture with water before extracting
with sodium bicarbonate solution

Answer 18

remove the acid and add CaCl as a drying agent to absorb the wsolution

Question 19

why did you set up a fractional distillation

Answer 19

fractional distillation helps enhance separation of the components in the mixture, this is more efficient than simple distillation since it can only cleanly distill with temperdifferences less than 100

Question 20

What would be the problem with heating the reaction mixture above 190 Celsius?

Answer 20

everything would distill

Question 21

What is the purpose of the calcium chloride used in this experiment

Answer 21

The calcium chloride helps to dry the mixture by removing any excess base (water) in the mixture. The base is water soluble.

Question 23

what are commonly used acids for dehydration

Answer 23

H3PO4 and H2SO4

Question 24

what are the steps to an E1 mechanism

Answer 24

1. acid catalyst protonates alcohol
2. loss of water leads to carbocation
3. carbocation eliminates to alkene (may rearrange)
4. acid catalyst regenerated

Question 25

more stable carbocations take _______ (more or less) energy to form and form _______ (faster or slower)

Answer 25

less; faster

Question 26

what do the percentages of major and minor products depend on

Answer 26

substrate and reaction temperatures