(8) addition reactions of alkenes

addition reactions

• the opposite of elimination reactions

• a carbon pi bond is converted to two sigma bonds

• favored by enthalpy but not favored by entropy

addition vs elimination reactions

both are equilibrating reactions

high temperatures: elimination is preferred

low temperatures: addition is preferred

hydrohalogenation

the addition to H—X to an alkene

HCl, HBr, HI

markovnikov addition

the halogen is installed at the more substituted carbon

the hydrogen is installed at the less substituted carbon

antimarkovnikov addition

a peroxide must be used (ROOR)

the halogen is installed at the less substituted carbon

the hydrogen is installed at the more substituted carbon

chiral centers formed from hydrohalogenation

2 markovnikov products are formed: (R) enantiomer and (S) enantiomer

acid catalyzed hydration

• components of water (OH) are added across the pi bond following markovnikov addition

• sulfuric acid is typically the acid catalyst used

• the last step is a proton transfer to remove the excess hydrogen from the OH

• dilute H₂SO₄ (more H₂O) installs the alcohol from a pi bond

• concentrated H₂SO₄ (less H₂O) installs the pi from an alcohol

• if a chiral center is present, both (R) and (S) configurations are formed