dehydration Flashcards
what are the key features of a dehydration
protonation of oh creates a good leaving group
what is the key features of an E1 mechanism
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
what is the key requirement of an E2 mechanism
it requires an anti-periplanar geometry
what makes a good catalyst?
must be strong enough to protonate alchohol
must not be nucleophillic or it will undergo a substitution pathway
alcohols that form more stable carbocations react
faster
Benzylic, allylicm and tertiary alcohols undergo what type of mechanism and whats the rate
they undergo rapid elimination
what type of conditions are required for secondary and primary alcohols to undergo E1
heat and extreme condtions
what is the rate limiting step in an E1
the loss of water leading to a carbocation
what is zaitsev’s rule
it is the formation of the most stable alkene that is thermodynamically favorable
what do the relative percentages of major and minor products depend on
the substrate and the reaction temperature
what is a rearrangement
less stable carbocations can readily rearrange to more stable carbocations through 1,2 hydride or methyl shifts
when will a ring expansion occur
when a carbocation of equal stability is rearranged to form a more stable overall compound
when do rearrangements to less stable carbocations occur
when there is a subsequent carbocation rearrangement that leads to a more stable one
when will an E2 occur
when the substrate cannot form a stable intermediate
why do we wash the fractions with h2O, NaHCO3, and dry
to remove traces of acid
Why might you expect a poor yield of alkene products if the dehydration reaction
was carried out using hydrochloric acid as the catalys6?
if you use HCl you are using a base that can also act as a nucleophile, so it could undergo a substitution pathway
What is the purpose of the calcium chloride used in this experiment?
to prevent water from reversing the reaction and to isolate product also to remove traces of acid
why might you not see all 4 products in the GC analysis
-similar compounds may not be identified as separate
- e1 mechanism
e2 mechanism may only allow 1-2 depending on the isomer used
hat is the purpose of extracting the product mixture with water before extracting
with sodium bicarbonate solution
remove the acid and add CaCl as a drying agent to absorb the wsolution
why did you set up a fractional distillation
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
What would be the problem with heating the reaction mixture above 190 Celsius?
everything would distill
What is the purpose of the calcium chloride used in this experiment
The calcium chloride helps to dry the mixture by removing any excess base (water) in the mixture. The base is water soluble.
what are commonly used acids for dehydration
H3PO4 and H2SO4
what are the steps to an E1 mechanism
- acid catalyst protonates alcohol
- loss of water leads to carbocation
- carbocation eliminates to alkene (may rearrange)
- acid catalyst regenerated
more stable carbocations take _______ (more or less) energy to form and form _______ (faster or slower)
less; faster
what do the percentages of major and minor products depend on
substrate and reaction temperatures
