Dextra Laboratories interview questions

Two interviews: 1) First was a 25 min HR interview where they asked me to tell them about myself, what my strengths are, how my colleagues would describe me, what my greatest weakness was, where I see myself in 5 years, the longest synthetic sequence I had done, the largest scale chemistry I had worked on and about some hazards I had dealt with. 2) Technical interview lasting 1 h. I was asked to tell them about myself again and had to give a 15 minute presentation on my PhD research. They asked me about some of the hazards associated with the reagents I had used. They wanted to know about the most hazardous reagent I had used and the measures I had put in place to mitigate the risks associated. Then they showed a slide of a generic alcohol going to an aldehyde, carboxylic acid and methyl ester. I had to provide reagents for each transformation. Second slide was again transformations. They showed me a molecule that had an ester on one end and an enone on the other. And they asked me for reagents I would use. In one the product was where both the ester and enone carbonyl were OHs, so I mentioned LiAlH4 should reduce both. In another only the enone carbonyl was an OH and I said NaBH4 should selectively reduce that and not touch the ester. In another the ester had been transformed to an aldehyde but can’t remember whether the enone had been reduced. In any case, I could reduce the ester to the alcohol with LiAlH4 and then oxidise to aldehyde using PCC/Swern/Dess Martin/Pinnick etc. The was also one were the enone double bond had been reduced and I suggested H2 and Pd/C. Third slide was a complex scheme of various transformations and reagents and they just wanted me to talk about them, any hazards associated with them, what I’d do differently if doing these reactions on big scale etc. There were low temperatures and high temperatures in there so I spoke about how it would be important to make sure the big scale plants were able to cope with that and how they’d regulate heating and cooling. With the low temperature ones I said if they use things like liquid nitrogen or acetone/dry ice baths then they have to be aware of things like asfixiant hazards and so on. They also asked me why diethyl ether in one of the schemes wouldn’t be ideal for scale-up and I answered that it’s volatile, flammable and explosive. They asked what else could happen with diethyl ether and I correctly answered that it could potentially form peroxides. They also asked what ether is more likely to be used on the bigger scale. I didn’t know so they said it’s MTBE, and as it’s asymmetric it’s less prone to oxidation apparently. One of the schemes was a Swern and they asked me if I knew the gaseous products formed which I answered correctly as carbon dioxide and carbon monoxide. They also asked what else and hinted at the sulfur so I correctly said dimethyl sulphide and explained the hazards related to that (smelly etc.). There was also a cobalt catalyst in one which would need to potentially be recycled. And there were also some peroxide reagents. In final arrow scheme it was a Witting ([MePh3PI]+I- and I spoke about how this one forms Ph3P=O as a product which is horrible to get rid of as it comes out on the column. So I suggested doing a Horner-Wadsworth-Emmons modification so that the phosphorus product formed would be water soluble and thus easily removable in an aqueous work-up. They seemed to be pleased with most of my answers. Then they gave me a what would you do type question. Thhey said imagine I was doing a reaction on 50 g scale that needed to be anhydrous and was refluxing in toluene. And then I found out the reagent had 1 eq. of water in it, how would I ensure it gets/remains dry? After waffling a bit about potentially burning off the water by heating I remembered the Dean Stark apparatus which is the answer they were looking for. And they asked me to describe the set up to make sure I understood it. They talked a bit about how toluene is a good azeotrope for water. The also asked me to describe 3 things to think about when scaling up. The first answer I gave was heat and how it differs on scale up, need to make sure heating is properly regulated and that the reaction doesn’t turn into a runaway etc. I then spoke about solvents and how generally on the larger scale there’s a 20% relative reduction in solvent use. I also talked about how the solvent could be recycled and distilled, to which they asked when would it be cost effective to do so? And I talked about how if the cost of recycling and installing and managing distillation columns was less than the money for an incineration permit/storage of waste before it goes to incineration. And the final example I gave was the material the bigger scale reactors are made from, as lab bench scale is done in glass which won’t have the same volume to surface area ration as a larger container. And bigger ones made from steel or metal will have completely different heat capacity.