OCHEM: Reaction Mechanisms

Dehydrohalogenation reaction

Reactants: KOH / ch3ch2oh

Elimination reaction, single step

What happens: a strong base, such as KOH or NaOH removes a proton (H+) and a halogen is removed, creating a pi bond

hydrohalogenation reaction

Reactants: H-Br or H-Cl

Carbocation rearrangement reaction

What happens:

1. Hydrophilic attack on alkene

2. Secondary carbocation remains and has a positive formal charge (+)

3. Hydride shift to second carbocation, becomes more stable tertiary carbon, leaving a positive formal charge (+) on the carbon that provided the hydrogen

4. Halogen attaches to more substituted carbon (the carbon mentioned in the previous step)

dehydration reaction

Reactants: H2SO4, H2O / THF (solvent), 50 Celsius (heat)

Condensation reaction, multi-step

What happens:

a water molecule is eliminated from the reactant molecule using a strong acid catalyst (H2SO4)

Halogenation reaction

Reactants: X2 (x= br or cl)

What happens:

alkene is treated with halogen which leads to the formation of two new C-halogen bonds on opposite faces of the compound

halohydrin reaction

Reactants: X2 / H2O (x=cl,br)

Anti-addition reaction

What happens:

1. Nucleophilic pi bond attacks halogen and breaks the bond

2. Halogen lone pair attacks carbon and forms halogen bridge

3. Second halogen breaks off, another water molecule acts as nucleophile and breaks the halide bridge

4. water attacks at the more substituted carbon and product is halohydrin with halogen on one carbon and OH group on adjacent carbon

oxymercuration reaction

1. Hg(OAc)2, H2o

2. NaBH4

What happens:

• a mercury acting as a reagent attacking the alkene double bond to form a Mercurinium Ion Bridge (weird triangle)

• water molecule attacks the most substituted carbon —> opens the mercurium ion bridge, dashes followed by proton transfer to solvent water molecule.

hydroboration reaction

Reactants:

1. BH3/THF (solvent, not important)

2.H2O2 / NaOH

two-step, Anti-Markovnikov

What happens:

1. alkene is treated with BH₃ breaking the C=C pi bond and forms a C-H and C-B bond.

2. H₂O₂ (oxidant) is added in the presence of NaOH or KOH (strong base). A rearrangement occurs where the C-B bond is broken and a new C-OH bond is formed.

C-OH bond attaches to the less substituted carbon of the alkene with the C-H bond being formed on the more substituted carbon

hydrogenation reaction

Reactants: H2 & catalyst (Pd/C, Pt)

Syn addition, one-step

What happens: hydrogens are added to the same side of the double bond

Markovnikov’s Rule

In the addition reaction of HX to an unsymmetrical alkene, the H adds to the carbon that already has the greater number of hydrogen atoms.

• “H goes to more Hs”.

epoxidation reaction

Reactants: mCPBA / CH2Cl2 (solvent)

syn-addition, one-step

What happens:

• The C=C pi bond breaks

• Two new C–O single bonds form

• The (weak) O–OH bond breaks

• A carboxylic acid is transferred, a new C–O pi bond forms, while the existing C-O pi bond acts as a base to remove a proton from oxygen.

• cis alkene gives cis epoxide, trans alkene gives trans epoxide

NEVER gives anti addition products

epoxides from halohydrins reaction

Reactants: 1. br2 / h2o 2. NaOH / H2O

two-step

What happens:

• Preparation of a halohydrin by electrophilic addition of
  HO-X to an alkene

• Treatment of the halohydrin with a base (NaOH)

dihydroxylation of alkenes (two step)

1. mCPBA / ch2cl2 2. H3O+

two-step

What happens:

• epoxidation followed by the acid catalyzed ring opening
  reaction with water

dihydroxylation of alkenes reaction (one step)

Reactants:1. OsO4, pyridine 2. NaHSO3, H2O

What happens:

• treatment of the alkene directly with osmium tetraoxide
  (OsO4)

• epoxides undergo an acid catalyzed reaction with water to give
  the corresponding 1,2-diol
  

ozonolysis reaction

Reactants: 1. O3 2. Zn, ch3co2H

What happens:

The C=C bond is broken and two new C=O bonds are formed. The resulting C=O groups are known as carbonyl functional groups.

KMnO4 oxidation reaction

Reactants: KMnO4 / H3O+

What happens:

Alkene is cleaved down the middle splitting off into two products, one section being the left side of the alkene with a terminal oxygen

the right side:

• if it is a terminal alkene, second product is carbon dioxide, if not, product is carboxylic acid

dihydroxylation / oxidation reaction

1. OsO4 / pyridine|NaHSO3 2. HIO

syn-addition

What happens:

• always gives 1,2-diols (vicinal diols)

• A C-C (pi) bond is broken

• Two C-O bonds form on adjacent carbons

• The two new C-O bonds are attached 

cleavage of 1,2-diols reaction

HIO4 / H2O, THF(solvent)

What happens: 

1,2- or vicinal diols are cleaved by periodic acid, HIO₄, into two carbonyl compounds.

the simmons-smith reaction compounds

Reactants: CH2I2/ Zn(Cu) ether

What happens: The iodomethyl zinc iodide reacts with an alkene to give a cyclopropane. (creates CH2 triangle)

Anti-Markovnikov

addition reactions to unsymmetrical alkenes where the hydrogen atom adds to the carbon with less hydrogen atoms, and the other substituent (like a halogen or hydroxyl group) adds to the carbon with more hydrogen atoms

• anti-Markovnikov's rule is observed only in H-Br

Dichlorocarbene reaction

Reactants: CHCl3, KOH

Deprotonation of CHCl₃ gives the trichloromethanide anion, −:CCl3, which spontaneously expels a Cl^– ion.