SN1 vs SN2 vs E1 vs E2 Comparison Chart (Free Printable)

Ask any organic chemistry student what finally made substitution and elimination click, and odds are they'll point to a chart like this one. Four mechanisms, two questions, one page. This is the classic Chemistery comparison chart — the one that's lived inside orgo binders since 2015 — redrawn cleaner, tighter, and free to print.

The short answer: every one of the four mechanisms is named by two questions. Does a nucleophile attack the carbon (substitution, the "SN") or does a base remove a β-hydrogen (elimination, the "E")? And does it all happen in one concerted step (bimolecular — the "2") or in two steps through a carbocation (unimolecular — the "1")? Answer both and the mechanism names itself: SN1, SN2, E1 or E2.

SN1 vs SN2 vs E1 vs E2 comparison chart — substitution and elimination mechanisms

The remastered SN1 · SN2 · E1 · E2 chart. Click it for full size — or grab the printable PDF below and tape it inside your binder.

⏰ Take the chart with you. The remastered chart is a free, printable five-minute sheet — US Letter and A4, prints clean in black & white.
Get the free PDF →

The 2×2 map

Hold those two questions in mind and the whole topic collapses into a 2×2 grid:

SUBSTITUTION — Nu attacks the carbonELIMINATION — base takes a β-H, alkene forms
One concerted step (bimolecular · the "2")SN2E2
Two steps, via a carbocation (unimolecular · the "1")SN1E1

The master table

This is the heart of the chart — every column is one mechanism, every row is a fact examiners test:

SN1SN2E1E2
the storyLG leaves first → flat carbocation → Nu strolls inone backside hit — attack and exit togetherLG leaves first → carbocation → β-H lostbase plucks the β-H as the LG leaves
rate lawk[RX]k[RX][Nu]k[RX]k[RX][base]
substrate3° > 2° — 1°/CH₃ neverCH₃ > 1° > 2° — 3° never3° > 2°3° > 2°
what it needsweak Nu — often the solvent itselfstrong, unhindered nucleophileweak base · heat helpsstrong base (bulky is fine)
solventpolar protic (H₂O, ROH)polar aprotic (DMSO, acetone, DMF)polar proticless critical — the base decides
stereochemistryracemization — both faces of the flat cationinversion of configuration (Walden)Zaitsev alkeneanti-periplanar β-H required · Zaitsev (Hofmann with a bulky base)
carbocation?YES — rearrangements possibleneverYES — rearrangements possiblenever

Which one wins? The five quick calls

  1. CH₃ or primary + strong nucleophile → SN2. A bulky base flips it to E2.
  2. Tertiary + weak nucleophile in a protic solvent → SN1 + E1 mix. Heat pushes it toward E1.
  3. Tertiary + strong base → E2, cleanly.
  4. Secondary is the battleground: strong Nu in aprotic → SN2 · strong bulky base → E2 · weak Nu in protic → SN1/E1.
  5. Heat favors elimination — warm it up and the alkene share climbs, for both E1 and E2.

Try it: call the mechanism

Commit to a call before you peek at the answers.

  1. CH₃CH₂Br + NaOCH₃ in DMSO.
  2. (CH₃)₃CBr warmed in ethanol, nothing else added.
  3. 2-bromobutane + potassium tert-butoxide.

Answers: 1) SN2 — primary substrate, strong small nucleophile, polar aprotic solvent. 2) SN1 + E1 mix — tertiary substrate, the weak nucleophile is the solvent itself, polar protic, and the warmth leans it toward E1. 3) E2 — secondary substrate + a strong, bulky base, so expect the Hofmann-leaning alkene.

Common mistakes to avoid

  • Ignoring the two vetoes. Tertiary substrates never do SN2 (the backside is blocked) and primary substrates never do SN1 (the cation is too unstable). Those two facts alone kill half the wrong answer choices.
  • Forgetting rearrangements. Whenever a carbocation forms — SN1 or E1 — check for a hydride or methyl shift before drawing the product. Graders hide one there constantly.
  • Confusing a strong base with a strong nucleophile. tert-Butoxide is a strong base but a poor nucleophile — too bulky to reach the carbon, so it grabs the β-H instead. That combination is exactly how exams force E2.

FAQ

How do I decide between SN1 and SN2?
Substrate first: CH₃ or primary points to SN2; tertiary points to SN1. Then confirm with the nucleophile (strong and small → SN2; weak → SN1) and the solvent (polar aprotic → SN2; polar protic → SN1).

Does heat favor elimination?
Yes — elimination creates more particles, so it wins on entropy, and heat makes that term count for more. Warming the flask shifts the outcome toward the alkene for both E1 and E2.

Why can't a tertiary substrate do SN2?
SN2 requires a backside attack on the carbon carrying the leaving group. Three alkyl groups physically block that approach, so the SN2 rate on a tertiary carbon is effectively zero.

Is the chart really free?
Yes. The printable PDF comes in US Letter and A4, free forever — it is the remastered version of the chart this blog has been known for since 2015.

The takeaway

Two questions name the mechanism: who attacks (nucleophile on carbon → substitution; base on a β-H → elimination) and how many steps (one concerted step → the "2"s; a carbocation first → the "1"s). Substrate, nucleophile or base strength, solvent, and heat then decide which quadrant wins. Print the chart, drill the five quick calls, and the exam question becomes a ten-second read. Download the free PDF here.

Go deeper into each mechanism: Intro to the 4 Basic Types of Reaction · SN1 SN2 Comparison · E1 E2 Comparison · Factors affecting rate of SN1 · Factors affecting rate of SN2

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