Orgo Exam 2

addition of hydrogen halides (XH)

• markovnikov product

• mechanism:

  • alkene + H-Br

  • alkene looks for more positive = H

  • arrow from double bond to H, arrow from H-Br bond to Br

  • H adds to least subst. carbon in double bond b/c positive charge is more stable on more subst. carbon

  • Br- attacks the carbocation

• forming carbocation so rearrangement possible

free radical addition of HBr

• anti-markovnikov

• in presence of peroxide, HBr forms free radicals

• start with Br radical + alkene

• double bonds breaks, one arrow to each carbon, Br joins with least subst carbon bc radical more stable on more subst carbon

• H-Br attacks radical on carbon and H adds to radical

Addition of Water: Hydration of Alkenes

• alkene reacts with water in presence of strong acid to form alcohol

• reagents: —>(H+ on top, H2O on bottom)

• markovnikov products

• forming carbocation so can have markovnikov rearrangements

• H+ first reacts with alkene —> attaches to less subst carbon

• carbocation formed on more subst. carbon

• H2O attacks carbocation —> another H2O attacks to neutralize

Addition of Water/OH: Hydration by oxymercuration-demurcuration

1. no carbocation formed —> no rearrangements

2. follows markovnikov product

3. anti-addition = have to write both enantiomer

4. reagents:(1. Hg(OAc)+, H2O, 2.NaBH4)

5. Hg(OAc)+, H2O —> Hg(OAc) forms triangle ring with carbons, water attacks more subst. carbons

6. opens ring with Hg(OAc)+, H2O on more subst carbon, Hg(OAc)+ becomes neutral

7. another H2O molecule attacks oxonium to make it neutral

8. with NaBH4 —> replaces Hg(OAc) with H

Addition of Water/OH: alkoxymercuration-demercuration

• mercuration taking place in alcohol (R-OH) solvent, alcohol = nucleophile —> attacks mercurinium

• markovnikov product

• anti-addition

• reagents: —> (1. Hg(OAc)+, H2O, 2.NaBH4)

• alchohol attacks more subst carbon and opens Hg(OAc) ring —> alcohol atacks alochol attached again to make it neutral

• NaBH4 attacks to replace Hg(OAc) with H

• have to write both enantiomers bc anti addition

Addition of Water/OH: Hydroboration of Alkenes

• anti-markovnikov alcohol —> if want OH on least subst carbon

• reagents: —> (1. BH3 THF, 2. H2O2, OH-)

• BH3 breaks double bond —> BH2 attaches to least subst carbon, H attaches to more subst C

• H2O2, OH- —> OH replaces BH2 to least subst carbon

• if double bond is in cycloalkene/ring has alkyl subst (methyl, ethyl, etc) the alchohol will be trans of it —> have to write both produts

  • OH will be trans of alkyl

Addition of Halogens to Alkenes

• add to double bond to form vicinal dihalide

• usually anti-addition

• Mechanism:

  • alkene + X2 (ex. Br2)

  • One of the Br’s forms bond with double bond to make a ring, other Br in bond expelled

  • Br in ring is positive, Br- that was expelled does backside attack —> results in anti-sterochemistry

  • Br- attacks more subst carbon

• can’t be asked stereochemistry in non-ring, Br just has to be on opposite side, in ring Br are trans (one wedge on dash) —> have to draw both products

Formation of Halohydrins

• halohydrin: compound that contains a halogen atom (such as chlorine, bromine, or iodine) and a hydroxyl group (–OH) attached to adjacent (neighboring) carbon atoms

• halogen added in the presence of water

• markovnikov product

• anti-addition —> have to write both products

  • OH and halogen will always be opposite (one wedge other dash)

• reagents: —> (Halogen on top, H2O bottom)

• halogen attaches to least subst carbon in double bond, H2O to most subst carbon

• have to write both enantiomers

Catalytic Hydrogenation

• adding H2 across double bond in presence of catalyst (Cd, Ni, Pd, Pt, Rh)

• reagents: —> (H2 on top, catalyst on bottom)

• syn addition

• alkene —> (H2 on top, catalyst on bottom) then double bonds breaks and H added to each carbon in double bond, H will be syn —> draw both products of both wedge and both dash

Addition of Carbenes: Simmons Smith Reaction

• reagent: CH2ZnI2

• —> (CH2ZnI2 on top; Zn, CuCl on bottom)

• 3 reagents togethers form 3 carbon ring

Addition of Carbenes: Formation of Carbenes by Alpha Elimination

• forms carbene ring with two halogens

• reagents: —> (CHBr3 on top; KOH/H2O on buttom)

• retains any cis/trans stereochemistry of the alkenes it had —> if it was cis then write both products of both groups in wedge and then both in dash

Epoxidation of Alkenes

• alkene reacts with peroxyacid in one step to form epoxide and and carboxylic acid

• peroxyacid has extra oxygen it can donate —> most common = mCPBA

• regants: (mCPBA on top; Ch2Cl3 on bottom)

• alkene —> (mCPBA on top; Ch2Cl3 on bottom) makes ring with oxygen (epoxide)

• retains stereochemistry of the alkene —> have to write both products

Acid Opening of the Epoxides

• makes anti-diol/anti-addition

• have epoxide react with H3O+ —> H+ adds to Oxygen in epoxide —> oxygen now positive

• backside attach of H2O on more subst carbon in the ring —> opens the ring

• another H2O attacks to neutralize the previous H2O

• both OH are anti

Syn Dihydroxylation of Alkenes

• want cis diol

• two types of reagents

  • OsO4

  • cold, dilute solution of KMnO4 in base

• —> (OsO4 or KMnO4, H2O2)

• —> (KMnO4 on top; cold or diluted on bottom)

• syn addition of OH, breaks carbene

Oxidative Cleavage of Alkenes: Cleavage by KMnO4

• reagent: warm, acidic, or concentrated KMnO4 produces oxidative cleavage: C=C cleaved into two C=O

• reagent: —> (KMnO4 on top; concen. or warm or dilute on bottom)

• break C=C , add double bond to Oxygen to both carbons

  • if ketone (no hydrogen attached to C) —> can leave as is

  • if aldehyde (hydrogen attached to C) —> have to oxidize to OH (whole thing is COOH)

  • if CH2 (terminal carbon, 2 hydrogens attached to C), make that a double bond to O and get CO2 (losing a carbon basically)

Oxidative Cleavage of Alkenes: Ozonolysis

• ozone cleaves double bond to make ketones and aldehydes —> don’t get oxidized into acids

• same as Cleavage by KMnO4 except no extra oxidation step can leave H as is if aldehyde

• if terminal carbon, still make it into a double bond with O, but get H2C=O instead of CO2