Organic Chemistry I

1. Example
   
2. Mechanism
   

Addition of HX follows Markovnikov's Rule: the carbon with the most hydrogen atoms attached gets the hydrogen. This rule is followed because the most stable carbocation intermediate will be formed. The carbocation is now sp² hybridized with an empty p-orbital which can accept electrons from a nucleophile (Cl^- in this case).

Note: Attack of the nucleophile can come from above or below with the concomitant stereochemical consequences.

1. Example
   
2. Mechanism
   

Addition of halogen to an alkene is possible because of the polarizability of the halogen. As the halogen approaches the site of high electron density (the double bond), the bromine molecule is polarized such that the nearest bromine atom now has some partial positive charge. The p electrons attack this bromine atom breaking the Br-Br bond. The carbocation intermediate formed is stabilized by non-bonding electrons donated by the adjacent bromine atom. This non-classical carbocation is referred to as a bromonium ion intermediate (the generic term is halonium ion, and the halogen name is used for specific a particular halogen). The bromonium ion shields the one face of the molecule from attack by a nucleophile; attack must come from the side opposite of the bromonium ion. The net result is anti-addition of halogen to the double bond.

Note: The nucleophile will attack the carbon atom of the bromonium ion intermediate most able to bear positive charge. In this case, that is the carbon atom with the methyl substituent (it is a 3^o carbon atom).

1. Example
   
2. Mechanism
   

Halohydrin formation is analogous to the addition of halogen to an alkene with two changes: the nucleophile in this case is a water molecule, and a final deprotonation is required to make the neutral alcohol.

Note: Since a bromonium ion intermediate is formed, we again find anti-stereochemistry in the product. Also, the nucleophile (water) will again attack the more highly-substituted carbon atom of the bromonium ion intermediate.

1. Example
   
2. Mechanism
   

Hydration of an alkene with aqueous acid follows Markovnikov's Rule. The mechanism is very similar to the addition of hydrogen halide (see above) with two exceptions: the nucleophile is water, and an additional deprotonation step is required to make the neutral alcohol.

Note: This reaction requires the use of strong acid and high heat. It is an industrially important reaction, but not of much synthetic use in the lab as many organic compounds are adversely affected by these conditions.

1. Example
   
2. Mechanism
   

Oxymercuration/demercuration involves mercuric acetate as the electrophile in the reaction. An intermediate mercurinium ion reminiscent of the halonium ion forms, and then goes on to react with the nucleophile water.

Note: The nucleophile (water) will attack the more highly-substituted carbon atom of the mercurinium ion intermediate. Deprotonation yields the neutral alcohol - organomercury product. Reduction with sodium borohydride yields the alcohol. Sodium borohydride effectively replaces the mercury atom where it is in the molecule with hydrogen. You are not responsible for this part of the mechanism. This can be thought of as the Markovnikov product, because the carbon with the most hydrogens will ultimately get the hydrogen from the sodium borohydride reduction.

1. Example
   
2. Mechanism
   

Hydroboration of an alkene involves the reaction of an alkene with borane. The boron atom has an empty p-orbital which acts as an electrophile toward the double bond. Hydride transfer from borane to the other carbon atom of the double bond occurs simultaneously with bond formation to boron. A four-atom transition state closely describes the bond-breaking and bond-forming interactions. Because borane has three hydrogen atoms attached, each borane molecule reacts with three alkenes to form a trialkyl borane. Hydrogen peroxide oxidizes the boron atom, and the carbon-boron bonds are replaced with carbon-oxygen bonds. You are not responsible for this portion of the mechanism.

Note: The product exhibits anti-Markovnikov addition since the more highly-substituted carbon atom gets the hydrogen. Remember, that the boron and hydrogen add from the same side of the alkene, and the boron is replaced where it is in the molecule with oxygen. Therefore, the -H and -OH are added syn to each other.

1. Example
   
2. Mechanism
   

Hydrogen gas adsorbs onto the surface of the catalyst. The hydrogen adds to the same face of the alkene producing an alkane.

1. Example
   
2. Mechanism
   

Dihydroxylation occurs via the reaction of an alkene with osmium tetroxide. A cyclic osmate intermediate is formed which is cleaved in a second separate step with sodium bisulfite.

Note: You are not responsible for this mechanism. However, be aware that the two hydroxyl groups are added to the same side of the alkene resulting in syn stereochemistry.

1. Example
   
2. Mechanism
   

You are not responsible for this mechanism. However, realize that a 1,2-diols are oxidatively cleaved to form a dicarbonyl compound.

Note: If the 1,2-diol is part of an open chain molecule, then two carbonyl compounds are produced. If the diol is part of a ring system (as in this example, then a single dicarbonyl compound is produced.

1. Example
   
2. Mechanism
   

Ozonolysis is very complicated mechanistically, but the key points involve the formation of a molozonide intermediate which rearranges to an ozonide. Ozonides are very explosive, and are never isolated. Instead, they are treated with a reducing agent (Zn) to convert them to carbonyl compounds. You are not responsible for this mechanism, however, be aware of the result: an alkene is converted into a dicarbonyl compound.

Note: If the alkene is part of an open chain molecule, then two carbonyl compounds are produced. If the alkene is part of a ring system (as in this example, then a single dicarbonyl compound is produced. The net result of this reaction is the same as for the oxidative cleavage of 1,2-diols.

1. Example
   
2. Mechanism
   
   No mechanism is required for this reaction.
   

Potassium permanganate oxidative cleavage of an alkene is a very complicated chemical reaction. However, you must be aware of the result: the carbon-carbon double bond is replaced with a carbon-oxygen double bond. Additionally, all vinylic hydrogen atoms are replaced with -OH groups

Note: In the case of a terminal alkene, the terminal carbon atom is removed as carbon dioxide. This is a chain-shortening reaction (see top reaction above). This reaction works best for symmetrical alkenes, because, in this case, only one product is produced (see middle example above). For unsymmetrical alkenes mixtures are formed (see bottom example above).

The Simmons-Smith reaction is related to the dichlorocarbene reaction. We will show an example of each and study the mechanism of the dichlorocarbene reaction. The mechanism for the Simmons-Smith reaction is not as straight forward, but the end result is the same as for the dichlorocarbene reaction.

1. Example
   
2. Mechanism
   

The dichlorocarbene reaction involves the decomposition of chloroform with base. The hydrogen in chloroform is fairly acidic and is abstracted by hydroxide ion. The carbanion initially formed decomposes to form a carbene (a neutral carbon species with an unshared pair of electrons in an sp³ hybrid orbital and an unoccupied p-orbital). The carbene is both nucleophilic (lone pair) and electrophilic (empty p-orbital) and can react with alkenes to form dichlorocyclopropane rings.

Note: The Simmons-Smith reaction is very similar, but produces cyclopropane rings, which are more synthetically useful than the dichloro counterpart produced in the dichlorocarbene reaction.