Difficulty: Intermediate to Advanced | Prerequisites: Nomenclature, structure/bonding/reactivity, familiarity with functional groups
The synthesis roadmap tests whether you can connect reagents to products across a web of transformations, while the mechanisms section tests whether you understand how each reaction happens at the electron level. Together these two sections accounted for 69 of 150 points (46%) on the exam. If you can push curly arrows through a mechanism and match reagents to functional-group changes, you are nearly halfway to passing.
Curly arrow (curved arrow) mechanism
A notation showing the movement of electron pairs during a reaction. The tail of the arrow starts at the electron source (a lone pair or a bond) and the head points to the electron sink (an atom or a bond).
In simple terms, curly arrows are the "paper trail" that tracks where electrons go in each step.
Carbocation
A carbon atom bearing a formal positive charge and only six electrons in its valence shell. Carbocations are electrophilic intermediates stabilised by hyperconjugation, induction and resonance.
Think of it as a carbon that desperately wants electrons and will accept them from anything nearby.
Resonance structures
Alternative Lewis structures for the same molecule that differ only in the placement of electrons (not atoms). The true structure is a hybrid, a weighted average of all valid resonance contributors.
In simple terms, resonance structures are like different camera angles of the same scene: none is "real" alone, but together they show you the full picture.
Markovnikov addition
In the addition of HX to an alkene, the hydrogen adds to the carbon with more hydrogens (the less substituted carbon) and the halide adds to the more substituted carbon, forming the more stable carbocation intermediate.
Think of it as "the rich get richer": the carbon that already has more hydrogens gets another one.
Anti-Markovnikov addition
The opposite regiochemistry, where the hydrogen adds to the more substituted carbon. This occurs with radical conditions (e.g. HBr with peroxides) or with hydroboration-oxidation (BH3-THF followed by NaOH/H2O2).
Kinetic vs thermodynamic product
The kinetic product forms faster (lower activation energy) and predominates at lower temperatures or shorter reaction times. The thermodynamic product is more stable (lower energy) and predominates at higher temperatures or longer reaction times, where equilibrium is reached.
In simple terms, kinetic = quick to form, thermodynamic = most stable once everything settles.
Benzylic position
The carbon directly attached to a benzene ring. Benzylic carbocations, radicals and anions are stabilised by resonance with the aromatic ring.
NBS (N-bromosuccinimide)
A reagent used for allylic and benzylic bromination via a radical mechanism when heated or irradiated with light.
Tosylate (OTs)
A tosylate group (p-toluenesulphonate) converts a poor leaving group (OH) into an excellent one. Formed by treating an alcohol with TsCl and pyridine.
In simple terms, tosylation is like giving a bad leaving group a new identity so it can actually leave.
The roadmap section gave a multistep synthesis diagram and asked you to fill in either the product or the reagents at each step. Every reagent set and every product appeared exactly once, so process of elimination is a legitimate strategy after you have identified the ones you know.
TsCl, pyridine: converts an alcohol (OH) to a tosylate (OTs), turning a poor leaving group into an excellent one.
NBS, heat: radical benzylic or allylic bromination. Puts a Br at the benzylic or allylic position.
HBr: Markovnikov addition of HBr to an alkene (Br on the more substituted carbon). With peroxides, anti-Markovnikov (Br on the less substituted carbon).
NaCN: nucleophilic substitution, replaces a leaving group with CN (cyanide). Works well on primary and secondary substrates.
NaOH, H2O: hydrolysis or substitution, replaces a leaving group with OH.
H2SO4, water: acid-catalysed hydration of an alkene (Markovnikov, OH on the more substituted carbon).
HgSO4, H2SO4, H2O: oxymercuration-type hydration of an alkene (Markovnikov selectivity).
1. O3, 2. Zn/H2O: ozonolysis with reductive workup. Cleaves a double bond to give two carbonyl fragments (aldehydes and/or ketones).
SOCl2, pyridine: converts an alcohol (OH) to an alkyl chloride (Cl) with inversion of configuration.
H2, Pd (or Pt or Ni): catalytic hydrogenation. Reduces a double bond to a single bond (syn addition of H2).
H2, Lindlar's catalyst: partial reduction of an alkyne to a cis (Z) alkene. Stops at the alkene stage.
1. BH3-THF, 2. NaOH/H2O2: hydroboration-oxidation. Anti-Markovnikov, syn addition of OH to an alkene.
1. H2O2, 2. NaOH: oxidative workup (often part of a hydroboration or epoxide-opening sequence).
Cl2, H2O: halohydrin formation. Adds Cl and OH across a double bond (anti addition, Markovnikov-like regioselectivity for the halide on the more substituted carbon).
Sodium phenoxide/alkoxide (ONa with OH): Williamson ether synthesis or ring opening, depending on context.
Start with transformations you are most confident about.
Look at the functional group change at each arrow: what is gained, what is lost?
If a leaving group appears where there was none, a reagent like TsCl or SOCl2 probably installed it.
If a C-C bond was broken (two carbonyl products from one alkene), think ozonolysis.
If an OH appeared anti-Markovnikov, think hydroboration-oxidation.
Use process of elimination for the last few boxes.
This mechanism involves converting one acetal into another by exchanging the alkoxy group, using acid catalysis and a new alcohol (methanol in the exam question).
Step 1: Protonation of one of the acetal oxygens by H2SO4.
Step 2: Loss of the protonated alkoxy group to form an oxocarbenium ion (a resonance-stabilised carbocation where positive charge is shared between carbon and oxygen).
Step 3: Nucleophilic attack by methanol on the oxocarbenium ion.
Step 4: Deprotonation to give the new hemiacetal/acetal.
Repeat the sequence for the second alkoxy exchange if a full transacetalisation is required.
Key points for scoring: each curly arrow was worth 2 points, each intermediate was worth 2 points, and a missing formal charge cost 1 point. Draw every arrow carefully with the tail on the electron source and the head on the electron sink.
The benzylic carbon forms the most stable carbocation intermediate because the positive charge is delocalised into the aromatic ring through resonance. Nucleophilic attack at that carbon is therefore favoured because the intermediate leading to it is lowest in energy.
HCl adds across a conjugated diene to give two constitutional isomers: a 1,2-addition product and a 1,4-addition product.
The 1,2-addition product is the kinetic product (forms faster, lower activation energy).
The 1,4-addition product is the thermodynamic product (more stable, more substituted double bond).
At high temperature ("heat" in the exam), the thermodynamic product predominates because there is enough energy to reach equilibrium.
The minor product (1,2-addition) has the chlorine on the carbon adjacent to where the proton added, with the double bond in its original position.
For the addition of HCl to a cyclic alkene:
Step 1: Protonation of the alkene pi bond by HCl to form the more stable carbocation (Markovnikov). Draw the curly arrow from the pi bond to the H of HCl, and a second arrow from the H-Cl bond to Cl.
Step 2: The carbocation intermediate may be stabilised by resonance if it is allylic or benzylic.
Step 3: Chloride ion attacks the carbocation from either face (if there is no steric or stereoelectronic bias), giving a racemic product ("plus enantiomer").
When a tertiary benzylic alcohol is treated with strong acid (H2SO4):
The OH is protonated, then water leaves to form a tertiary carbocation.
Because the carbocation is adjacent to the aromatic ring, you can draw resonance structures that delocalise the positive charge into the ring.
Two additional resonance structures (not related by symmetry) place the positive charge on ring carbons at the ortho and para positions.
The most stable resonance structure is the one that maintains an intact aromatic ring (the one where the positive charge sits on the exocyclic carbon, keeping all six ring carbons in a complete aromatic sextet).
If you could not identify the most stable structure, the key reasoning is: the carbocation is stabilised by resonance with (conjugation into) the benzene ring.
Acetal chemistry is central to carbohydrate biochemistry: sugars exist as cyclic acetals, and the acid-catalysed ring-opening and closing of these acetals is how your body processes glucose. Markovnikov selectivity matters in polymer chemistry, where the regiochemistry of monomer addition determines the polymer's properties.
Students often draw curly arrows backwards, starting at the electrophile and pointing toward the nucleophile. The arrow tail always starts at the electron-rich species (the nucleophile or the bond being broken) and the head always points toward the electron-poor species or atom.
Students often confuse 1,2-addition with 1,4-addition in conjugated dienes. 1,2-addition places both new groups on adjacent carbons. 1,4-addition places them at the 1 and 4 positions, moving the double bond to the 2,3 position. Both products form through the same allylic carbocation intermediate.
Students often assume the thermodynamic product is always the major product. It is only major at higher temperatures or longer reaction times where equilibrium is reached. At low temperature, the kinetic product dominates.
Students often forget to show formal charges on intermediates. Every curly arrow mechanism should show the formal charge that results from each electron-pair movement. Missing charges cost marks.
True or false: In an acid-catalysed mechanism, the first step is typically protonation of an electronegative atom (O or N).
Answer: True.
Fill in the blank: NBS with heat performs radical bromination at the ____ or ____ position.
Answer: benzylic, allylic
True or false: Ozonolysis (O3, then Zn/H2O) of a terminal alkene gives one aldehyde and one formaldehyde (methanal).
Answer: True.
Fill in the blank: The thermodynamic product of HCl addition to 1,3-butadiene is the ____ addition product.
Answer: 1,4
True or false: SOCl2 with pyridine converts an alcohol to an alkyl chloride with retention of configuration.
Answer: False. It proceeds with inversion of configuration.
Q: You treat a primary alcohol with TsCl and pyridine, then with NaCN. What is the overall transformation?
A: The alcohol is converted to a tosylate (good leaving group), then the cyanide ion displaces the tosylate via SN2 to give a nitrile. Overall: R-OH becomes R-CN.
Q: Draw the products of ozonolysis (1. O3, 2. Zn/H2O) of 2-methylbut-2-ene.
A: The double bond between C2 and C3 is cleaved. You get acetone (propan-2-one) from the more substituted side and acetaldehyde (ethanal) from the less substituted side.
Q: What reagents convert an alkene to an anti-Markovnikov alcohol in one synthetic step?
A: 1. BH3-THF, 2. NaOH/H2O2 (hydroboration-oxidation). The OH ends up on the less substituted carbon with syn stereochemistry.
Q: In the addition of HCl to a conjugated diene at high temperature, which product predominates and why?
A: The 1,4-addition (thermodynamic) product predominates because it has the more substituted, more stable alkene. At high temperature the reaction reaches equilibrium and the more stable product accumulates.
Q: A tertiary benzylic alcohol is treated with H2SO4. Draw the carbocation intermediate and explain why it is especially stable.
A: Loss of water gives a tertiary carbocation adjacent to the benzene ring. The positive charge delocalises into the ring via resonance (you can draw contributors placing the positive charge at the ortho and para positions). The combination of tertiary substitution and aromatic conjugation makes this carbocation unusually stable.
The synthesis roadmap was worth 33 points (22%). Mechanisms were worth 36 points (24%). Combined, nearly half the exam.
Curly arrow mechanisms: expect to draw full mechanisms with every arrow and every intermediate shown. Points are awarded per arrow and per intermediate, so partial credit is generous if you show your working.
Rationalisation questions ("explain why...") appear after mechanism drawings. A one- or two-sentence answer invoking resonance stabilisation, carbocation stability or Markovnikov selectivity is usually sufficient.
The roadmap is a matching exercise under time pressure. Drill the reagent-to-transformation pairs above until they are automatic.
Every mechanism in this section builds on the nucleophile/electrophile and acidity concepts from the structure/bonding/reactivity section. The reagent-product pairs from the roadmap are the building blocks of multistep synthesis (the next section). If you can match reagents to transformations here, you can reverse-engineer a synthesis pathway. Carbocation stability and resonance connect directly to electrophilic aromatic substitution and rearrangements in Organic Chemistry II.
curly arrow mechanism, curved arrow, electron-pair movement, carbocation, oxocarbenium ion, resonance structures, Markovnikov addition, anti-Markovnikov, hydroboration oxidation, ozonolysis, NBS benzylic bromination, allylic bromination, tosylate TsCl, SOCl2 pyridine, Lindlar catalyst, catalytic hydrogenation, acid-catalysed hydration, halohydrin formation, kinetic product, thermodynamic product, 1 2 addition, 1 4 addition, conjugated diene, synthesis roadmap, reagent matching, CHEM 2301, organic chemistry I, Pomerantz, University of Minnesota