Our Guide for How to Tackle a Technical Question in Your Chemistry / Natural Sciences Oxbridge Interview 2024
Science interviews often require far more rigorous testing of a candidate’s technical knowledge than an Arts student can expect. This is because courses such as Chemistry, or Natural Sciences at Cambridge, are all evident extensions of the A-Level / IB syllabus. You will need to prove that you are of the right academic standard, and that you can apply your existing knowledge to new intellectual arenas. This blog will take you through a technical question that was put in front of a former Natural Sciences candidate at Cambridge. U2 mentor, George (Physical Natural Sciences, University of Cambridge), provides tips on how to go about answering the question, discussion points to bring up in interview, as well as full worked solution.
The question: “Can you predict the site of attack of the bromide ion in the image below?”
Tips to solve the question:
The structure in the diagram has been simplified to remove unnecessary information i.e. the end groups have been labelled as R or R’ instead of drawing the whole thing out (this both saves time but also makes diagram clearer) . If you were asked to draw the diagram yourself, you could also suggest this simplified form to your interviewer
The Br- ion can attack at the two sites marked a & b. The purpose of the question is to choose which site will be preferred
Factors to consider are sterics, electronics and leaving group ability
The answer/further discussion:
SITE B is preferred! Here’s why:
Both sites a & b are secondary carbon sites, so there should not be too much difference in terms of steric hindrance
The reaction occurring here is SN2 (nucleophilic substitution involving two species in the rate determining step) where Br- is the nucleophile
The reaction is not SN1 because the primary carbocation formed here would be rather unstable
If the bromide ion attacks at site a, the leaving group will be RO-. If the bromide ion instead attacks at site b, the leaving group will be NR’3. Of course, NR’3 is a far better leaving group because it is more stable than RO-, notably because it is uncharged
There is also a secondary consideration which favours attack at site b; there is less steric hindrance from the bulky NR’3 cage-like structure.
The Mechanism (George has provided his rough working below!):
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