The short answer
How electrons flow in organic reactions, and the SN1-vs-SN2 decision the GAMSAT loves.
Written and checked by GAMSAT tutors — not AI-generated.
Try the reasoning style
We treat forgetting as a failure — a lapse to be patched with reminders and records. Yet a mind that kept everything could not think; it would drown in the undifferentiated noise of every moment it had ever lived. To forget is not so much to lose information as to decide, mostly without our noticing, what was never worth keeping.
The author's argument relies most directly on which unstated assumption?
Pick an option to see how the tutor reasons to the answer — not just whether you were right.
Not quite — the answer is B.
Work backwards from the conclusion: a mind that ‘kept everything’ supposedly ‘could not think.’ That only follows if thinking means leaving most of experience out — so B is the premise the argument quietly rests on. A raises reliability, which the passage never weighs; C contradicts ‘mostly without our noticing’; D smuggles in a claim about intellect the passage never makes. The question rewards finding the hidden premise, not recalling a fact.
Organic mechanisms look intimidating but follow one rule: electrons flow from electron-rich to electron-poor. An electron-rich nucleophile attacks an electron-poor electrophile. The classic GAMSAT example is nucleophilic substitution — and whether it goes by the SN1 or SN2 route.
Nucleophiles and electrophiles
A nucleophile ("nucleus-loving") is electron-rich and donates a pair of electrons. An electrophile ("electron-loving") is electron-poor and accepts them. Curly arrows in a mechanism always show electrons moving from the nucleophile toward the electrophile.
SN2 vs SN1
SN2
- One concerted step (backside attack)
- Rate depends on BOTH substrate and nucleophile
- Favoured by primary carbons + strong nucleophile
- Inverts stereochemistry (like an umbrella flipping)
SN1
- Two steps via a carbocation intermediate
- Rate depends ONLY on the substrate
- Favoured by tertiary carbons (stable carbocation)
- Gives a racemic mixture (attack from either side)
Let the substrate decide
Tertiary substrates can't do SN2 (too crowded for backside attack) but form stable carbocations, so they go SN1. Primary substrates can't form stable carbocations, so they go SN2. The degree of substitution at the carbon is your biggest clue.
Worked example
A tertiary alkyl halide is reacted with a weak nucleophile in a polar protic solvent. Is the reaction more likely SN1 or SN2, and what does the rate depend on?
Check yourself
An SN2 reaction's rate depends on:
Key takeaways
- Electrons flow from electron-rich (nucleophile) to electron-poor (electrophile).
- SN2: one step, backside attack, rate depends on both, inverts stereochemistry.
- SN1: two steps via a carbocation, rate depends only on the substrate, racemises.
- Primary substrate ⇒ SN2; tertiary substrate ⇒ SN1.
- Strong nucleophile favours SN2; stable carbocation favours SN1.
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