07 Alkenes and Alkynes Free Online FlashCards

What is the usual Markovnikov orientation of HX addition?

H adds to the alkene carbon that already bears more hydrogens, while X bonds to the more substituted carbon.

What is the effect of peroxides on HBr addition to an alkene?

Peroxides promote radical addition of HBr, giving the anti-Markovnikov product. This peroxide effect is characteristic of HBr, not HCl or HI in standard introductory reactions.

Why does Br₂ or Cl₂ add anti to an alkene?

The alkene forms a bridged halonium ion, then halide attacks from the opposite face, producing anti addition.

Where does OH attach in alkene halohydrin formation?

OH usually attaches to the more substituted carbon, and the addition is anti because water opens the bridged halonium ion from the opposite face.

What is the regioselectivity and stereochemistry of alkene hydroboration–oxidation?

Hydroboration–oxidation gives syn, anti-Markovnikov hydration: OH ends up on the less substituted carbon. Its concerted pathway has no carbocation intermediate, so carbocation rearrangements do not occur.

What advantage does oxymercuration–demercuration have over acid-catalyzed hydration?

Oxymercuration–demercuration gives a Markovnikov alcohol while avoiding rearrangements associated with a free carbocation.

How does the amount of HX affect alkyne addition?

One equivalent usually gives a vinyl halide; excess HX can add again to form a geminal dihalide, with both halogens on the same carbon.

What product does mercury-catalyzed hydration give from a terminal alkyne?

The enol formed by hydration rapidly tautomerizes to a carbonyl compound; a terminal alkyne gives a methyl ketone.

What product does alkyne hydroboration–oxidation give from a terminal alkyne?

With a bulky borane, hydroboration–oxidation of a terminal alkyne gives an aldehyde after the initial enol tautomerizes.

What product does an alkyne form with H₂ and Lindlar catalyst?

Lindlar catalyst partially reduces an alkyne to a cis alkene by syn addition of hydrogen.

What product does an alkyne form with Na or Li in liquid NH₃?

Sodium or lithium in liquid ammonia partially reduces an alkyne to a trans alkene.

How do the stereochemical outcomes of these two alkene dihydroxylation routes differ?

OsO₄ adds two OH groups to the same face, giving a syn 1,2-diol. Epoxidation followed by acid-catalyzed ring opening instead gives an anti (trans) 1,2-diol.