Organic Chemistry

Question

I am stuck on these questions- I cant seem to wrap my head around stereoselective and stereospecific. I have to describe my answers to each and I have drawn out some of the reactions but I am struggling with understanding each of these. If anyone can help with explaining each that would be awesome. Thank you!

a)The addition of HBr to 3-methyl-2-pentene is stereoselective TRUE FALSE

b) The addition of Cl2 to 3-hexene is stereospecific TRUE FALSE

c) The alkene addition reaction of bromine in an excess of water results in syn addition TRUE FALSE

d) The hydrogenation of an alkene involves anti addition of each H atom TRUE FALSE

Answer 1

In a nutshell:

1. A stereospecific reaction yields 100% of only 1 isomer of a product (i.e., the reaction specifies which isomer will be formed exclusively).
2. A stereoselective reaction favors 1 isomer over the other, but both isomers will still be formed. In other words, a stereoselective reaction yields a major and a minor product.

As such, with regard to your questions:

a) The addition of HBr to 3-methyl-2-pentene will favor the Markovnikov addition, yielding 3-bromo-3-methylpentane as a major product. However, some of the anti-Markovnikov product (2-bromo-3-methylpentane) will be formed as a minor product. As such, this reaction is stereoselective (TRUE).

b) This one is a bit tricky, because the question does not indicate the geometry around the C=C double bond in 3-hexene. In the case of trans-3-hexene, because the two Cl atoms will add anti across the C=C double bond, the end result is a meso compound (the molecule has two stereocenters, but also has a plane of symmetry). As such, because only 1 final compound is formed, the reaction of Cl₂ with trans-3-hexene is stereospecific (TRUE).

In the case of cis-3-hexene, the two Cl atoms will still add anti to each other, but the result will be a pair of enantiomers in a racemic mixture. Therefore, because we end up with 2 different isomers in roughly a 1:1 ratio, the reaction of Cl₂ with cis-3-hexene is neither stereoselective nor stereospecific (FALSE).

(I have approached question b) as if the geometry of the C=C double bond was given to you but omitted in your write-up. If it was not given to you originally, then the overall statement would be FALSE, as different isomers of starting material yield different isomers of products.)

c) When adding Br₂ or Cl₂ across an alkene, the key intermediate is a three-membered ring with the Br/Cl atom bearing a positive charge; this ring is then attacked in an Sn2 fashion by the incoming nucleophile (in this case, H₂O). As such, the addition of bromine to an alkene with an excess of H2O will result in anti addition. (FALSE)

d) Keep in mind that the hydrogenation of an alkene requires a metal catalyst to work. As such, when the alkene binds to the metal surface, one face of the alkene is blocked, making anti addition impossible. Thus, the two H atoms must add in a syn configuration. (FALSE)

Answer 2

Stereospecific means the reaction produces only one stereoisomer. On the other hand, stereoselective means that one stereoisomer will be present in larger quantities than the other, but both are still produced.

Here are the products of the four reactions you are asking about: https://imgur.com/a/2ofLSRf

a) TRUE because only one product is produced.

b) TRUE because only one product is produced.

c) FALSE because the Br and OH groups are added anti to each other

d) FALSE because the H's are added syn to each other

It would be a great idea to review the mechanisms for these reactions to rationalize why the stereochemistry is the way it is!