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Chem 219 Unit 5 Test with Questions Solved 100% Correct | Latest Update
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Chem 219 Unit 5 Test with Questions Solved100% Correct | Latest Update
What is the product of the protonation of an alcohol called? - ✔✔alkyloxonium ion
What is a Dehydration reaction for alcohols?
What happens during a dehydration reaction for alcohols? - ✔✔- A specific type of
elimination reaction. In a dehydration reaction, an alcohol molecule will lose H2O to
form an alkene. Alcohols undergo a reaction known as dehydration when they are
heated with strong mineral acids (like sulfuric or phosphoric).
- the alcohol molecule loses the hydroxyl (OH) from one carbon (the α-carbon) and a
hydrogen atom (H) from an adjacent (β) carbon.
What is the function of an acid during a dehydration reactions? Why does it do what it
does? - ✔✔To protonate the the OH group
It does this to make it a better (more stable) leaving group because H2O is a neutral
while hydroxide ion is strongly basic.
What type of mechanism is a dehydration reaction for a sterically bulky tertiary alcohol?
- ✔✔E1 mechanism
Steps of a dehydration reaction for a tertiary alcohol - ✔✔- the OH group is pronated
(the alcohol acts as a base and accepts a proton from the acid solvent onto its OH group)
- the OH group disassociates, forming a carbocation
- then water, acting as a base, removes a β-hydrogen to form the alkene.
, - The net result is that H and OH are removed (eliminated) from the original alcohol
substrate.
- products: an alkene and H3O+
What type of mechanism is a dehydration reaction for a sterically non-bulky primary
alcohol? - ✔✔E2 mechanism
Why do primary alcohols dehydrate through the E2 mechanism? - ✔✔Because these
substrates do not form stable carbocation intermediates.
Steps of a dehydration reaction for a primary alcohol - ✔✔- hydroxyl (OH) group is
pronated by the solvent
- a water molecule (from the solvent?) removes a β-hydrogen. - As the β-hydrogen is
removed the bond H-C bond is broken and then there is a C=C bond formed between
the β-carbon and the α-carbon.
- At the same time. the double bond kicks off the H2O molecule from the α-carbon
- The net result is that H and OH are removed (eliminated) from the original alcohol
substrate.
- products: an alkene and H3O+
What is the order of ease of dehydration of alcohol substrates? - ✔✔3° > 2° > 1°
Tertiary alcohols require the least harsh reaction conditions (typically 25% aqueous acid
and 60-80°C), whereas 1° alcohols require fully concentrated acid and temperatures
approaching 200°C.
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