Difficulty: Intermediate | Prerequisites: E2 elimination notes, Chapter 3 (carbocation stability, SN1)
E1 elimination is the stepwise counterpart to E2. Where E2 happens in one concerted step with a strong base, E1 proceeds through a carbocation intermediate and does not require a strong base. Acid-catalysed dehydration of alcohols is the most common example of E1 in practice: treat an alcohol with concentrated H₂SO₄ and heat, and you get an alkene. This set of notes covers E1 mechanism, dehydration reactions, carbocation rearrangements, and product prediction. You should be comfortable with E2 and with carbocation stability (tertiary > secondary > primary) before working through this material.
E1 elimination is a two-step process: the leaving group departs first to form a carbocation, then a base removes a proton to form the alkene. Dehydration reactions (alcohol + H₂SO₄) follow the same logic, with an extra protonation step at the start. Carbocation rearrangements (hydride and methyl shifts) are common and frequently tested.
E1 (Elimination, Unimolecular)
A two-step elimination in which the leaving group departs first to form a carbocation intermediate, then a base removes a proton from a beta-carbon to generate the alkene. The rate depends only on the substrate concentration (rate = k[substrate]). Think of it as the leaving group walking away on its own, leaving behind a positively charged carbon that then loses a proton.
Carbocation
A carbon atom bearing a formal positive charge, with only three bonds and an empty p-orbital. Stability order: tertiary > secondary > primary > methyl. Carbocations are the key intermediate in E1 and SN1 reactions.
Dehydration
The removal of water from an alcohol under acidic conditions (typically concentrated H₂SO₄ with heat) to form an alkene. This is an E1 process. In simple terms, the OH group leaves as water and takes a neighbouring hydrogen with it, producing a double bond.
Proton transfer
The first step in any acid-catalysed dehydration. The acid protonates the OH group of the alcohol, converting the poor leaving group (OH⁻) into a good one (H₂O).
Carbocation rearrangement
A 1,2-hydride shift or 1,2-methyl shift that converts a less stable carbocation into a more stable one. Rearrangements occur whenever a more stable carbocation is accessible by moving one H or one CH₃ from an adjacent carbon. In simple terms, the positive charge jumps to a neighbouring carbon if doing so makes the carbocation more stable.
1,2-hydride shift
A hydrogen atom (with its bonding electrons) migrates from an adjacent carbon to the carbocation centre, moving the positive charge one carbon over.
1,2-methyl shift
A methyl group (with its bonding electrons) migrates from an adjacent carbon to the carbocation centre. This is common when a hydride shift alone would not increase stability, but moving a methyl group would (e.g. expanding a strained ring or reaching a tertiary centre).
Step 1: The leaving group departs on its own, forming a carbocation intermediate. This is the rate-determining (slow) step.
Step 2: A base (often the solvent) removes a proton from a beta-carbon, and the electrons from the C–H bond form the new C=C double bond.
The energy diagram for E1 shows two energy maxima separated by a local minimum (the carbocation intermediate). Compare this to E2, which has a single maximum.
E1 is favoured by weak bases, polar protic solvents, and tertiary or secondary substrates that can form stable carbocations.
Dehydration converts an alcohol into an alkene using concentrated H₂SO₄ and heat. The mechanism has three (sometimes four) arrow-pushing steps:
Step 1: Proton transfer. The acid protonates the OH group, turning it into a good leaving group (H₂O). This is always the first step of any dehydration. The arrow-pushing pattern for this step is proton transfer.
Step 2: Loss of leaving group. Water departs, generating a carbocation. This is the slow step.
Step 3 (if applicable): Rearrangement. If a more stable carbocation is available via a 1,2-hydride shift or 1,2-methyl shift, the rearrangement occurs here.
Final step: Proton transfer. A base (often water or bisulphate) removes a beta-proton, forming the alkene.
The overall sequence for a simple dehydration is therefore: proton transfer, loss of leaving group, proton transfer. When rearrangement occurs, insert the rearrangement step between loss of leaving group and the final proton transfer.
Rearrangements happen when a shift can produce a more stable carbocation (secondary to tertiary, or primary to secondary/tertiary).
A secondary carbocation adjacent to a tertiary carbon will rearrange via a 1,2-hydride shift.
A secondary carbocation adjacent to a quaternary carbon (no H to shift) may rearrange via a 1,2-methyl shift, which can also expand a strained ring (e.g. four-membered to five-membered).
Rearrangements do NOT occur if the carbocation is already at maximum stability for its surroundings (e.g. a tertiary carbocation next to secondary or primary carbons).
The major alkene product follows Zaitsev's rule: the more substituted alkene is preferred.
If a rearrangement occurs, draw the rearranged carbocation first, then identify which beta-protons can be lost to give which alkene(s). The most substituted alkene from the rearranged carbocation is the major product.
Conjugation with a phenyl ring (Ph) or other pi system further stabilises the alkene, making conjugated products especially favoured.
Students often forget that the first step of every dehydration is proton transfer. The acid protonates the OH before anything else happens. Jumping straight to "loss of leaving group" without protonation is a common exam error.
Rearrangements are frequently missed. Any time you draw a carbocation, stop and ask: is there a more stable carbocation one shift away? If yes, the rearrangement happens.
Students sometimes rearrange when they should not. A tertiary carbocation next to only secondary or primary centres will not rearrange, because the shift would produce a less stable carbocation.
E1 and SN1 share the same first step (forming the carbocation). Students mix up the second step: in E1, a proton is lost to form an alkene; in SN1, a nucleophile attacks the carbocation.
Acid-catalysed dehydration is used industrially to produce ethylene and propylene from ethanol and isopropanol, both important feedstocks for plastics. Carbocation rearrangements also explain the skeletal rearrangements observed in terpene biosynthesis and petroleum refining (catalytic cracking).
⚠️ Drawing the full dehydration mechanism (proton transfer, loss of leaving group, optional rearrangement, proton transfer) is a near-certain exam question. Know the arrow-pushing steps cold.
⚠️ "Will this alcohol undergo rearrangement during dehydration?" is a classic short-answer format. Check whether the initial carbocation is adjacent to a carbon that would yield a more stable cation.
⚠️ Drawing the rearranged carbocation intermediate is frequently tested. You must show the shift (hydride or methyl) and the resulting cation structure.
⚠️ Energy diagram interpretation (one hump = E2, two humps = E1) appears regularly as a quick identification question.
True or false: E1 is a concerted, one-step mechanism. (False. E1 is two steps: leaving group departs, then a proton is removed.)
Fill in the blank: The first step of any acid-catalysed dehydration is ______. (Proton transfer / protonation of the OH group.)
True or false: A secondary carbocation next to a tertiary carbon will rearrange. (True, via a 1,2-hydride shift to give the more stable tertiary carbocation.)
Fill in the blank: An energy diagram with two maxima and a local minimum in between indicates an ______ mechanism. (E1.)
True or false: In a dehydration reaction, the OH group leaves directly without being protonated. (False. OH⁻ is a poor leaving group; it must be protonated to H₂O first.)
Q: What are the arrow-pushing steps in the dehydration of a secondary alcohol with H₂SO₄, where a carbocation rearrangement occurs?
A: Proton transfer (acid protonates the OH), loss of leaving group (water departs, forming a carbocation), rearrangement (1,2-hydride or methyl shift to a more stable carbocation), proton transfer (base removes a beta-proton to form the alkene).
Q: Will a tertiary alcohol with no adjacent quaternary or tertiary carbons undergo rearrangement during dehydration?
A: No. The carbocation formed is already tertiary, and a shift to a secondary or primary centre would decrease stability.
Q: A secondary alcohol on a cyclohexane ring is treated with concentrated H₂SO₄ and heat. The ring has a tertiary carbon adjacent to the OH-bearing carbon. What happens?
A: Protonation of OH, loss of water to form a secondary carbocation, then a 1,2-hydride shift to give the more stable tertiary carbocation. The final proton loss produces the most substituted alkene.
Q: A cyclobutane derivative bearing an OH group undergoes dehydration. A rearranged carbocation that does not have a four-membered ring is requested. What type of shift occurs?
A: A 1,2-alkyl (methyl) shift that expands the four-membered ring to a five-membered ring, relieving ring strain and producing a more stable carbocation.
Q: How do you distinguish E1 from E2 on an energy diagram?
A: E1 has two energy maxima with a valley between them (the carbocation intermediate). E2 has a single energy maximum (one transition state, no intermediate).
Q: What is the major organic product of the dehydration of 1-phenylethanol with H₂SO₄?
A: Styrene (phenylethene). Loss of water and a proton produces the alkene conjugated with the phenyl ring, which is especially stable.
E1 shares its first step (carbocation formation) with SN1 substitution, so understanding one helps with the other. The decision between E1 and E2 depends on the base strength and substrate, which feeds directly into the substitution-vs-elimination framework (covered in the companion study notes). Carbocation rearrangements also appear in electrophilic addition to alkenes (Chapter 5+), so the skill of spotting possible shifts will pay off repeatedly.
E1 elimination, unimolecular elimination, acid-catalysed dehydration, dehydration of alcohols, H₂SO₄ dehydration, carbocation intermediate, carbocation rearrangement, 1,2-hydride shift, 1,2-methyl shift, ring expansion, proton transfer, loss of leaving group, energy diagram E1 vs E2, Zaitsev product dehydration, secondary carbocation, tertiary carbocation, organic chemistry chapter 4, Purdue organic chemistry