[JMaria]

I have a couple questions about regioselectivity and stereochemistry in certain reactions of alkenes.

Firstly, can any of the types of reactions have stereochemistry if there's a chiral center? Also, what's the rule about an a chiral starting molecule creating a racemic mixture of the enantiomers and a chiral starting molecule creating a pair of diastereomers?

For hydrohalogenation, if the starting molecule is CH3CH=CHCH3, according to Markovnikov's rule, can the H add to either C of the double bond?

Does halogenation always lead to an anti configuration even if no chiral center is created (ex. methylenecyclohexene and Cl2)? And are only the cycloalkenes anti or are the alkenes anti as well?

Thanks!

[izchief360]

To address your questions in order,

First, stereochemistry is not a property inherent in reactions, it is a property of molecules. All molecules (not just chiral ones) have stereochemistry. However, in cases without a chiral center, we choose not to draw the molecule such that stereochemistry is depicted (we draw it without wedges and dashes). So, it doesn't matter how you react a molecule - it has stereochemistry. However, you only have to draw wedges and dashed is there is a chiral center.

A racemic mixture is created when the products of a reaction produce EQUAL left and right hand (d/l) enantiomers at a chiral center. Diastereomers, however, are stereoisomers that aren't enantiomers. For instance, if you have a molecule with multiple chiral centers, and you perform a reaction that reacts at and inverts one chiral center, do you get a racemic mixture? No, you don't, because EVERY chiral center is not inverted. Instead, you get a diastereomer.

In the example you presented, the H can add to either carbon 2 or carbon 3 because the molecule is a meso compound (symmetrical). Also, there is no opportunity for hydride shifts to occur, so you're good on that front.

The reason halogenation of alkenes leads to an anti configuration is because that is the configuration with the least amount of steric hindrance (strain) between the two halogens. The point is, if halogenation produces two adjacent chiral centers, then we well indicate the chirality (wedges/dashes). If it doesn't produce a chiral center, then the chirality doesn't have to be indicated, but know that the two halogens will have a configuration that minimizes the interactions between them.

Hopefully, I was able to shed light on some of your questions.

[JMaria]

Thank you so much! You explained everything very thoroughly.

[JMaria]

Actually, if you could answer one more question... In the reaction of 2-butene with Br2/CH2Cl2, aren't there 4 molecules created? 2 pairs of enantiomers? I know there's a maximum number of 4 because there are 2 chiral centers but are all of them created?

[izchief360]

You've just described a stereospecific reaction. The problem is that you haven't indicated if we're reacting cis-2-butene or trans-2-butere. Bromination of each produces different products.

Bromination of the trans isomer produces a racemic mixture in which the bromines are in an anti arrangement (remember, this minimizes steric strain). The carbons on either end of the molecule are in different directions, so we place the bromines in different directions.

Bromination of the cis isomer product yields a syn addition and a single product in which the bromines are both either wedged or dashed. Remember, since this is cis, we already have the carbons on either end of the molecule in the same direction, so we place both bromines in the other direction.

Here's a link for more info (see Observation #2): http://www.masterorganicchemistry.com/2013/03/06/bromination-of-alkenes-how-does-it-work/

Just rememeber, when you're dealing with a chiral center, visualize it as a tetrahedral structure and don't forget to take sterics into account!

[JMaria]

Thank you for all your *delete me* =)