May Organic Chemistry Show Mercy

Alkene + Br₂ or Cl₂ in an inert solvent: product?

Vicinal dihalide: one halogen on each former double-bond carbon. Anti addition. No rearrangements. Common solvents: CH₂Cl₂, CCl₄.

Alkene + Br₂/H₂O or Cl₂/H₂O: product?

Halohydrin: OH and halogen on adjacent carbons. OH favors the more substituted carbon; halogen goes on the other. Anti addition. No rearrangements.

Alkene + Br₂ in an alcohol solvent, ROH: product?

Add OR and Br across C=C. OR favors the more substituted carbon. Anti addition. The solvent acts as the nucleophile.

Alkene → Markovnikov alcohol without rearrangement: reagents?

1. Hg(OAc)₂, H₂O, THF 2. NaBH₄, NaOH. Called oxymercuration–reduction. OH goes on the more substituted carbon; H goes on the other.

What happens during oxymercuration, before NaBH₄?

Add OH and HgOAc across C=C. OH goes on the more substituted carbon. Typically anti addition, through a mercurinium ion.

What does NaBH₄ do in oxymercuration–reduction?

Sodium borohydride replaces C–HgOAc with C–H. OH stays attached. The overall sequence is not reliably syn or anti.

Alkene → anti-Markovnikov alcohol: reagents?

1. BH₃·THF 2. H₂O₂, NaOH. Called hydroboration–oxidation. OH goes on the less substituted carbon. Net syn addition of H/OH. No rearrangements.

Where do B and H attach during hydroboration?

B → less substituted carbon. H → more substituted carbon. They add syn, in one concerted step, without a carbocation.

What happens during oxidation of an organoborane?

H₂O₂/NaOH replaces C–B with C–OH, with retention of configuration. The H previously added to carbon stays there.

How many alkene molecules can one BH₃ react with?

Up to three, because BH₃ has three B–H bonds. This forms R₃B, a trialkylborane. Crowding can limit further additions

Alkene + 1. O₃ 2. DMS: product?

Reductive ozonolysis. Cut C=C and put C=O on each original double-bond carbon. A carbon originally bearing H gives an aldehyde; one bearing two carbon groups gives a ketone.

Ozonolysis with oxidative workup: what changes?

Use O₃ followed by H₂O₂/H₂O; your course also shows water workup generating peroxide. Aldehydes become carboxylic acids. Ketones remain ketones.

What happens when a cycloalkene undergoes ozonolysis?

Cut the ring at C=C and put C=O on both cut carbons. A ring containing one double bond usually opens into one molecule with two carbonyl groups.

Terminal alkyne R–C≡CH + HgSO₄, H₂SO₄, H₂O: product?

Markovnikov hydration → enol → methyl ketone: R–C(=O)–CH₃. Exception: acetylene, HC≡CH, gives ethanal.

Terminal alkyne + 1. bulky borane 2. H₂O₂, NaOH: product

Anti-Markovnikov hydration → enol → aldehyde: R–CH₂–CHO. Common bulky boranes: disiamylborane or 9-BBN.

Do internal alkynes give aldehydes after hydroboration–oxidation?

They give ketones. Unsymmetrical internal alkynes can give regioisomer mixtures; symmetrical ones give one constitutional carbonyl product

Why use a bulky borane for an alkyne?

To favor one addition and limit a second addition to the remaining double bond

How do I convert an enol into its carbonyl tautomer?

The OH-bearing carbon becomes C=O; the neighboring alkene carbon gains H; C=C becomes C–C. Keep the carbon skeleton connected.

Is enol → carbonyl conversion resonance?

No. It is tautomerization: proton transfers and bond changes produce a different constitutional isomer. Most simple examples favor the carbonyl form.

What is the key intermediate in halogenation/halohydrin formation?

A halonium ion, such as a bromonium ion. A nucleophile opens it from the opposite face, explaining anti addition.

What is the key intermediate in oxymercuration?

A mercurinium ion. There is no free carbocation, so ordinary hydride and alkyl shifts do not occur.

What is the difference between an inert solvent and a nucleophilic solvent?

An inert solvent mainly dissolves the reactants. A nucleophilic solvent, such as water or an alcohol, can attack the intermediate and become part of the product.

What does regioselective mean?

Preference for where groups attach. Example: OH favors one of the two former double-bond carbons.

What does stereospecific mean?

Different starting stereoisomers give different stereoisomeric products because of the reaction’s mechanism.

What do syn and anti describe?

The newly added groups: syn = same face; anti = opposite faces. Always identify which two groups you are comparing.