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Flow > Lecture 4 > Flashcards

Lecture 4 Flashcards

1
Q

What does a basic flow set up consist of?

A

At the basic level, a flow chemistry set-up will consist of a chip device/tubing, pumping system and heater or cooling system.

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2
Q

What may be included in more sophisticated set ups?

A
  • pressure and flow sensors
  • in-line optical detection
  • other analytical systems
  • automation for sample collection
  • feed back algorithms to optimise yields and flow rate.
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3
Q

Why is heat exchange better in micro reactors?

A
  • heat transfer coefficient is inversely proportional to channel diameter
  • much higher in microchannels
  • allows fast heating and cooling in reaction mixtures under isothermal conditions
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4
Q

What is hindered in microreactors?

A
  • development of hot spots
  • prevents fragmentation and undesirable side reactions
  • higher selectivity, yield and product quality
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5
Q

For what type of reactions is fast heat transfer most important?

A
  • strongly endothermic or exothermic
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6
Q

What is DIBAL used for?

A
  • reductions
  • converts esters and nitriles to aldehydes
  • effectively reduce unsaturated esters to alcohols
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7
Q

Why is DIBAL considered an electrophilic reducing agent?

A
  • reacts slowly with electron poor compounds

- reacts more quickly with electron rich ones

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8
Q

How are side products prevented when using DIBAL?

A
  • over reduction and then re oxidation

- e.g. ester is reduced to an alcohol and then oxidised to an aldehyde

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9
Q

How does flow chemistry improve DIBAL reduction?

A
  • improves product selectivity

- reduces rate down to milliseconds

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10
Q

Why is flow rate important in DIBAL reactions?

A
  • flow rate affects mixing and therefore conversion and selectivity
  • important when the reaction rate is high
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11
Q

What flow experiment was designed to investigate DIBAL retention time?

A
  • varied R1 volumes
  • varied flow rate
  • constant temperature
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12
Q

What happens when DIBAL flow rate is increased?

A
  • at a higher flow rate (at a constant R1 volume) higher conversion and yield of the desired aldehyde was observed, despite the shorter residence time.
  • indicates that the reaction is very fast and that the mixing heavily influences the outcome.
    • As the flow rate is increased, additional energy is provided for mixing, thus explaining the higher conversion observed at shorter residence times.
    • At very fast flow rates the outcome of the reaction was independent of residence time, indicating that mixing was very fast under these conditions.
    • at very short residence times (<50 ms), essentially full conversion and complete selectivity was obtained
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13
Q

What reagents are used in aromatic nitration?

A
  • nitric and sulphuric acids
  • form NO2(+) which is the active species
  • sulphuric acid is a catalyst and absorbent for water
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14
Q

Why is flow used for nitration reactions?

A
  • used to scale up exothermic and hazardous reactions

- prevents time delay in longer projects

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15
Q

Why is nitration dangerous?

A
  • heat generated can trigger nitric acid to degrade organic materials exothermically to gaseous products with explosive violence
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16
Q

Why is oxygen balance important

A
  • high oxygen balance indicates explosive/ potentially dangerous substances
  • nitration normally not explosive just dangerous
17
Q

How is oxygen balance calculated?

A
  • calculated from the empirical formula of a compound in percentage of oxygen required for complete conversion of carbon to carbon dioxide and hydrogen to water
18
Q

When is a reaction high risk?

A

When oxygen balance is above 200

19
Q

Why is the example reaction dangerous?

A
  • • during the course of the above reaction unstable N-nitro derivitives and a dinitropyrazole derivative are observed if temperature is not well controlled
  • not safe under batch conditions due to local hot spots
20
Q

What is the difference in temperature range in batch and reactors?
Why is this significant?

A
  • there is a much wider temperature range in batch reactors, with sufficient heat beng produced to form the dangerous side products
  • in microreactors the temperature range is much smaller so no dangerous side products are produced
21
Q

How can the flow system be adapted to make nitration safer?

A
  • corrosive acids can be pumped using special polymeric tubing
  • pressure detector systems can be used to automatically switch off the reaction
  • concentrations can be optimised in batch reactions and then used in flow chem to determine the optimum temperature and residence time to use
22
Q

Describe the batch process for the nitration of pyrazole-5-carboxylic acid

A
  • The reaction temperature and heat formation in the quench process has to be maintained exactly in a narrow range to facilitate high chemoselectivity and to avoid decomposition reactions.
    • batch is slow and unsafe
    •Calorimetric investigations indicated the liberation of 249 kJ/mol and, under strong acidic conditions, an exothermic decarboxylation
    •In large scale the amount of carbon dioxide derived from the decarboxylation will lead to excessive frothing and dangerous pressure increase
    • nitrating reagent was added in small portions and the com-position of the mixture was monitored accurately by (time-consuming) HPLC analysis after each treatment.
    •Under safe conditions at large scale the nitration takes about 10 hours at 50 °C to complete.
  • yield of 75%.
23
Q

What are the advantages of the flow process for

the nitration of pyrazole-5-carboxylic acid

A
  • no side reactions
  • 73% yield
  • NO ADDITION TIME, just reacting time
  • improve heat exchange so the ideal temp of 90 degrees was maintained with out the decarboxylation limit of 100 degrees
24
Q

How can DSC be used in Flow?

A
  • the amount of heat required to increase the temperature of a sample is measured as a function of temperature
  • when the sample undergoes a physical transformation, more or less heat will need to flow to it than the reference to maintain both at the same temperature.
  • Whether less or more heat must flow to the sample depends on whether the process is exothermic or endothermic.
  • By observing the difference in heat flow between the sample and reference, differential scanning calorimeters are able to measure the amount of heat absorbed or released during such transitions.
25
Q

What is flash chemistry?

A
  • Conducting extremely fast reactions in a controlled manner in flow reactors
  • millisecond to second range
26
Q

Why are extremely fast reactions better in a flow chemistry regime?

A
  • In a flask the concentration of products will increase and the concentrations of reactants will decrease rapidly, with uniform concentration throughout the flask
  • In flow the reaction proceeds as the reagent travels through the reactor, with reagent concentration decreases as distance from the inlet increases
27
Q

How are flash reaction times determined in bulk and flow?

A
  • reaction time is related to the length of the flow reactor, although it can also change with flow rate
  • In bulk the reaction time is often defined as the time between the first mixing and the mixing with a quench
  • Flow reactions offer a more precise way of calculating and controlling the reaction time
  • For fast reaction flow chemistry is much more efficient at controlling reaction time
28
Q

Why are RLi compounds important and why are they hard to synthesise?

A
  • Aryl lithium reagents are often used in organic synthesis due the their high reactivity but they have low functional group compatibility
  • Difficult to prepare aryl lithium compounds containing other electrophilic groups due to side reactions occurring
  • To overcome this problem the halogen-lithium exchange reactions often occur at low temperatures
    however, it is still difficult to prepare aryllithium compounds having highly reactive functional groups
29
Q

What is the alternative method of RLi production?

A
  • The second approach is the use of less reactive, hence more stable, organometallic compounds, such as arylmagnesium and arylzinc compounds.
  • such organometallic com- pounds are often prepared by a metal-exchange reaction from aryllithium compounds and this method suffers from the same problem.
  • organometallic compounds can also be pre- pared directly from aryl halides without using organolithium reagents.
  • However, direct preparation often requires the use of highly reactive precursors such as aryl iodides, which are usually more difficult to prepare than the aryl bromides
30
Q

What are typical batch conditions of the RLi reaction?

A
  • The Br/Li exchange reaction of alkyl o-bromobenzoates (BrC6H4CO2R) followed by reaction with ROH in a conventional macro- batch reactor.
  • A solution of sBuLi in hexane/cyclohexane was added dropwise to a solution of o-bromobenzoates 1 in THF at -78 oC. After stirring for 10 min at -78 oC, an alcohol was added as an electrophile to give the product
31
Q

What are the RLi reaction conditions in a batch reaction?

A
  • two simple T-shaped micromixers (M1 and M2)
  • two microtube reactors (R1 and R2).
  • 1 was mixed with sec-BuLi to generate the corresponding aryl- lithium species (2) in R1. and treated with ethanol in M2 to give the protonated product 3.
32
Q

How does the bromine species concentration vary in the RLi reaction?

A

The amount of 1 decreased rapidly with an increase in the residence time (tR) in R1, indicating that the Br–Li exchange reaction is very fast. It was complete within 0.06 s.

33
Q

How does residence time affect the yield of the reaction?

A
  • The yield of 3 increased rapidly with tR and became a maximum at tR = 0.06 s.
  • the yield of 3 decreased with tR and became less than 50% when tR = 6.3 s. This is presumably because of the decomposition of aryllithium intermediate 2.

Flow (4 decks)

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