Difficulty: Intermediate | Prerequisites: Parts 1 and 2 of the Dehydration study notes, basic understanding of boiling points and intermolecular forces
This set of notes covers two topics from the additional questions on the dehydration worksheet: (1) distillation theory, including when to choose simple vs fractional distillation and what theoretical plates mean for separation efficiency, and (2) green chemistry, specifically atom economy calculations comparing the dehydration reaction to a dehydrohalogenation. These are conceptual topics that help you understand why the lab is designed the way it is, and they show up regularly on exams.
Fractional distillation uses a fractionating column to achieve multiple theoretical plates, letting you separate liquids with close boiling points. More theoretical plates means sharper separation. Atom economy measures how much of the reactants' mass ends up in the desired product; the dehydration reaction has better atom economy (~80%) than the dehydrohalogenation (~52%) because water is a smaller by-product than tert-butanol plus potassium iodide.
Simple distillation
A distillation with no fractionating column, providing roughly 1 theoretical plate. Suitable for separating liquids whose boiling points differ by at least 60 to 70 °C, or for removing a solvent from a dissolved solid.
Fractional distillation
A distillation that uses a packed or unpacked fractionating column to achieve multiple vaporisation-condensation cycles. Suitable for separating liquids with boiling points as close as 10 to 20 °C apart. In simple terms, the column acts like many simple distillations stacked on top of each other.
Theoretical plate
One complete vaporisation-condensation equilibrium cycle. Each theoretical plate enriches the vapour in the more volatile component. The more theoretical plates, the better the separation. Think of it as one "round" of purification.
HETP (height equivalent to a theoretical plate)
The length of column needed to achieve one theoretical plate. A smaller HETP means a more efficient column.
Atom economy
A green chemistry metric: the percentage of the total molecular weight of all reactants that ends up in the desired product. Atom economy = (MW of desired product / sum of MW of all products) × 100. A high atom economy means less waste. Note: the catalyst is not included in the calculation because it is regenerated.
Dehydrohalogenation
An elimination reaction in which a hydrogen halide (HX) is removed from a substrate to form an alkene. Typically uses a strong base (e.g., potassium tert-butoxide). Proceeds by an E2 mechanism when a strong, bulky base is used.
Hofmann product
The less substituted alkene product of an elimination reaction. Favoured when a bulky base is used (steric control) or in E2 reactions with certain substrates, as opposed to the Zaitsev product.
Simple distillation is appropriate when the boiling points of two liquids differ by more than about 60 to 70 °C, or when you are distilling a solvent away from a non-volatile solute. It provides only about 1 theoretical plate.
Fractional distillation is needed when the boiling points are closer together (within about 10 to 60 °C). The fractionating column provides additional theoretical plates, giving better separation.
A theoretical plate represents one ideal vaporisation-condensation equilibrium. Each time the vapour condenses and re-vaporises as it moves up the column, the vapour becomes enriched in the lower-boiling component. More plates means more enrichment steps, and therefore cleaner separation.
Technique | Approximate Theoretical Plates |
|---|---|
Simple distillation | ~1 |
Fractional distillation (packed column) | 3 to 20 (depends on column and packing) |
Capillary GC column | 10,000 to 100,000+ |
This is why GC can separate compounds with extremely similar boiling points (like 2-methyl-1-butene at 31 °C and 2-methyl-2-butene at 38 °C) that cannot be fully resolved by bench distillation.
Atom economy tells you what fraction of the atoms in your reactants end up in the desired product. A reaction with high atom economy produces less waste per molecule of product. The catalyst (H₂SO₄ in the dehydration) is not included because it is regenerated.
Reaction: C₅H₁₂O → C₅H₁₀ + H₂O
Atom economy = MW of desired product / (MW of desired product + MW of by-products) × 100
= 70.13 / (70.13 + 18.02) × 100 = 70.13 / 88.15 × 100 = 79.6%
The worksheet's dehydrohalogenation reaction: 2-iodo-2-methylbutane + potassium tert-butoxide → alkene + tert-butanol + KI
Reactant MW total: C₅H₁₁I (198.04) + KOC₄H₉ (112.21) = 310.25
Desired product (alkene): C₅H₁₀ = 70.13
By-products: tert-butanol (74.12) + KI (166.00) = 240.12
Atom economy = 70.13 / (70.13 + 74.12 + 166.00) × 100 = 70.13 / 310.25 × 100 = 22.6%
Alternatively, atom economy = MW desired product / total MW all products × 100 = 70.13 / 310.25 × 100 = 22.6%
The dehydration reaction has a much higher atom economy (79.6%) than the dehydrohalogenation (22.6%). This is because dehydration's only by-product is water (MW 18), whereas dehydrohalogenation produces two heavier by-products (tert-butanol and KI). From a green chemistry standpoint, the dehydration reaction is far more atom-efficient.
The dehydrohalogenation uses potassium tert-butoxide, which is a strong, bulky base. Bulky bases have difficulty accessing the more hindered protons needed to form the Zaitsev product. Instead, they preferentially remove the less hindered proton, giving the less substituted alkene (2-methyl-1-butene, the Hofmann product).
This is the opposite of the dehydration reaction, where E1 conditions and no bulky base mean Zaitsev's rule applies and the more substituted alkene dominates.
Potassium tert-butoxide is a strong base but a poor nucleophile (too bulky to attack carbon in an SN2 reaction). The substrate is also tertiary, which blocks SN2 (backside attack is sterically impossible at a tertiary carbon). SN1 is unlikely under these strongly basic conditions because the base would deprotonate the carbocation faster than a nucleophile could attack. So elimination (E2) wins cleanly over substitution.
Atom economy is a core principle of green chemistry, used by pharmaceutical and chemical companies to evaluate how wasteful a synthesis route is before scaling it up. Fractional distillation is the basis of petroleum refining: crude oil is separated into fractions (gasoline, kerosene, diesel, etc.) in massive fractionating towers with many theoretical plates.
Students often include the catalyst in the atom economy calculation. The catalyst is not included because it is regenerated and is not part of the stoichiometric equation.
A common mistake is confusing atom economy with percent yield. Atom economy is a theoretical metric based on the balanced equation alone; it does not depend on how well the reaction actually worked in the lab. Percent yield measures what you actually obtained vs what was theoretically possible.
Students sometimes think simple distillation with a very tall flask neck acts like fractional distillation. It does not. Without packing or a proper fractionating column, you get only about 1 theoretical plate regardless of neck length.
Some students think the Hofmann product is always the minor product. It is not. With a bulky base like potassium tert-butoxide, the Hofmann (less substituted) product can be the major product.
⚠️ Be able to calculate atom economy for any reaction given the balanced equation and molecular weights.
⚠️ Know when to use simple vs fractional distillation, and explain why in terms of boiling-point difference and theoretical plates.
⚠️ Be able to explain why a bulky base favours the Hofmann product while acid-catalysed E1 conditions favour the Zaitsev product.
⚠️ Understand why SN2 is impossible at a tertiary carbon and why SN1 is suppressed by a strong base.
True or false: a simple distillation provides about 10 theoretical plates. (False: about 1.)
Fill in the blank: atom economy = (MW of desired product / ______) × 100. (sum of MW of all products, or total MW of reactants)
True or false: the catalyst should be included when calculating atom economy. (False: it is regenerated.)
Fill in the blank: a bulky base like potassium tert-butoxide favours the ______ (Zaitsev / Hofmann) product. (Hofmann)
True or false: SN2 substitution is likely at a tertiary carbon. (False: steric hindrance blocks backside attack.)
Q: When separating a mixture of two liquids, when would you choose simple distillation over fractional distillation?
A: Use simple distillation when the boiling points differ by more than about 60 to 70 °C, or when separating a volatile liquid from a non-volatile dissolved solid. Use fractional distillation when the boiling points are closer together (within about 10 to 60 °C).
Q: What is a theoretical plate and how does it relate to separation efficiency?
A: A theoretical plate is one complete vaporisation-condensation equilibrium cycle. Each plate enriches the vapour in the more volatile component. More theoretical plates means better separation. Simple distillation gives about 1 plate, fractional distillation gives roughly 3 to 20, and a capillary GC column can provide tens of thousands.
Q: Calculate the atom economy for the dehydration of 2-methyl-2-butanol.
A: Atom economy = MW of alkene / (MW of alkene + MW of water) × 100 = 70.13 / (70.13 + 18.02) × 100 = 79.6%. The catalyst (H₂SO₄) is not included because it is regenerated.
Q: Why would the preferred product be the 1-butene in the dehydrohalogenation, in contrast to the dehydration reaction?
A: The dehydrohalogenation uses potassium tert-butoxide, a bulky base that preferentially removes the less sterically hindered proton, giving the less substituted (Hofmann) product. The dehydration uses E1 conditions with no bulky base, so Zaitsev's rule applies and the more substituted alkene is major.
Q: Why would the substitution product not be expected in the dehydrohalogenation reaction?
A: Potassium tert-butoxide is too bulky to act as a nucleophile in SN2 (and the tertiary carbon blocks SN2 sterically anyway). SN1 is suppressed because the strong base removes a proton from the carbocation faster than a nucleophile can attack. Elimination (E2) dominates.
Atom economy ties into the broader green chemistry principles covered in CHEM 2311 and beyond, where reaction design aims to minimise waste. The distillation theory here connects to every separation experiment you will do in the course. The Zaitsev vs Hofmann selectivity and the E1 vs E2 distinction are central to the elimination chapter and appear on virtually every organic chemistry exam.
Simple distillation, fractional distillation, fractionating column, theoretical plate, HETP, separation efficiency, boiling point, atom economy, green chemistry, reaction efficiency, percent yield, dehydration, dehydrohalogenation, E1 vs E2, Zaitsev's rule, Hofmann product, bulky base, potassium tert-butoxide, KOtBu, SN1, SN2, substitution vs elimination, CHEM 2311, organic chemistry lab, Mohrig