Oxbridge interviews are always very specific and technical, but what are the professors looking for in your answers? Let’s dissect an example Chemistry interview question from Oxford:
Please rank the following seven molecules in order of acidity.
Right away, this is already shaping up to be quite a weird question. A lot of the compounds listed are not traditionally considered acids. How can we rank them in terms of acidity if they are not acids?
Let’s take the quantitative approach and start with the definition of acids. Under the Bronsted-Lowry definition, an acid is a proton (H+) donor.
HA ⇌ H+ + A–
Now that we have a formal definition, we can see that the strength of an acid is dependent on the degree of dissociation of the generic acid (HA). Since one of the products (H+) never changes, the degree of dissociation is solely dependent on the other product, the conjugate base (A–).
Let’s investigate the various conjugate bases.
In this step, picking the right hydrogen to deprotonate is important. Oxygens are more electronegative than carbons, and that’s why they hold electron densities better, and the subsequent conjugate bases are generally more stable. Now we can separate the conjugate bases into two groups: one with oxygen holding the negative charge, the other one with carbon holding the negative charge.
Among the three oxygen-based A–, how should we compare their stability and, thus, their acidity? The stability is highly based on the ability of the molecule to spread the electron density around the molecule.
Since most RHS structure has resonance structures, they can share the electron density with the other oxygen present in the ethanoate ion, stablising the entire structure. Therefore, this is the strongest acid!
Among the other two, they both have methyl groups (CH3). The methoxide ion (RHS) has one, and the t-butyl alkoxide (LHS) has three. Since carbon is more electronegative than hydrogen, the electron density is dragged toward carbon, making CH3 an electron-donating group (EDG). With more EDGs donating electron density to the already negative oxygen, it destabilises the structure and makes the 2-methylpropan-2-ol a weaker acid.
Now that we have sorted out the hierarchy amongst the oxygen-based acids, let’s have a look at the carbanions!
The right-most structure deviates the most from the rest, so let’s investigate it first! The ester with two carbonyl groups has its proton removed from the middle carbon, forming a semi-stable carbanion. Since oxygen is more electronegative than carbon, the carbonyl groups pull electron density away from the carbanion, stabilising it. The carbonyl groups also resonate with the carbanion, further improving its thermodynamic stability, making it the strongest acid among the three molecules.
Keto-enol tautomerisation
Now all that’s left are ethane, ethene, and ethyne. Finding the distinction between them is quite tricky because they are all carbon-holding-electron species, but we can look into the specific orbitals that are holding the electron.
This is the time for us to bring out the hybridisation theory! If we look at each carbon’s hybridization, ethane is sp3 hybridised; ethene is sp2; ethyne is sp. Out of all of them, sp orbital has the highest s character (50%) and experiences a stronger effective nuclear charge, making it a stronger acid. The order becomes easier to crack once we realise the trick:
sp3 (25%) < sp2 (33.3%) < sp (50%)
This makes ‘ethyne’ the strongest acid! The final order is as follows:
This question is quite challenging, but the key is to utilise a systematic way to analyse the problem. Remember that during interviews, Oxbridge professors care more about your thought process than the actual answer!
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By Anson Chung, Admissions Consulting Partner
Published 22-05-2024
