Hydrogenation of an alkene with a metal catalyst, such as platinum, occurs via syn addition.

It is important to note the three main types of reactions for alkenes. The first type of reaction is a 2-step mechanism in which the electrophile attacks the carbocation nucleophile. This can yield syn or anti products. The second type of reaction is a 2-step mechanism that forms a bridged carbocation as the intermediate. This can yield only anti products. The third and last type of reaction is a 1-step addition. This can only yield syn products.

An example of the third type of reaction is the addition of a hydrogen with palladium, platinum, or nickel as demonstrated in the picture.

Molecular bromine reacts with an alkene in the presence of water to yield a halohydrin product. Below is the mechanism for this reaction:

Reaction of an alkene with osmium tetroxide results in syn addition of to hydroxyl groups across the double bond, resulting in a 1,2-diol. The correct answer is the option in which hydroxide groups are added to the previously double-bonded carbons on the same side of the ring. The correct answer is compound II.

The reaction given is called a Markovnikov's addition reaction. This is a regiospecific reaction because only one orientation of the reaction occurs. It is an electrophilic addition reaction that involves the reaction of a halogenic acid such as or with an alkene. The hydrogen bonds to the carbon with the less alkyl groups bonded to the organic compound and the halogen ion bonds to the carbon with more alkyl substituents. Therefore the answer is:

Reactions that proceed by syn addition include hydrogenation, epoxidation, and hydroboration-oxidation. Reactions that proceed by anti addition include halogenation, halohydrin addition and epoxidation followed by ring opening. Syn addition occurs because reagents must approach from only one side of the double bond.

The reaction is a catalytic hydrogenation of an alkene using a palladium (Pd) catalyst. This reaction can be used to add a hydrogen atom to each carbon involved in a pi bond. The resulting molecule will be a an alkane.

Hydroxylation of alkene compounds is called a hydration reaction. The reaction occurs in the presence of a strong acid catalyst to protonate the organic compound similar to Markovnikov's addition forming a carbocation as an intermediate. A water molecule nucleophilicly bonds to the carbocation intermediate. A proton is lost from the positively charged oxygen to a chloride ion yielding the alcohol product and regenerating the acid.

Alkenes react with NBS (N-bromosuccinimide) in the presence of light to giving products in the position next to the double called the allylic position. Below is the mechanism for this reaction:

This is an alkene electrophilic addition reaction. The alkene first attacks the hydrogen on the . The hydrogen always attaches to the last carbon, never the third. This form of regioselectivity is referred to as the Markovnikov rule. This occurs because it creates a secondary carbocation, rather than a primary carbocation intermediate, which is far more stable. The bromine is then free to attack the third, now positively charged carbon.

N-bromosuccinimide (NBS) brominates at allylic positions. Br2 will not complete this reaction with the presence of the double bond.