Mechanism:

• An electrophile bonds to one carbon of an electron rich Pi bond of an alkene or alkyne leaving the other carbon of the Pi bond electron deficient.
• The electrophile may be a cation, a neutral but electron deficient atom, or a radical.
• A nucleophile then bonds to the electron deficient carbon completing the net addition reaction.
• If the bonding of the electrophile and the nucleophile occur close to each other then the electron deficient carbon is a transition state and not a carbocation intermediate.

Regiochemistry:

• The electrophile bonds to the Pi-bond carbon with most hydrogens so as to make a more stable carbocation intermediate or transition state.
• If in the net addition a hydrogen is added to the Pi-bond carbon with most hydrogens, this is called a net Markovnikov addition, "the rich get richer".
• If in the net addition a hydrogen is added to the Pi-bond carbon with least hydrogens, this is called a net anti-Markovnikov addition, "the poor get richer".
• You cannot apply the term Markovnikov to all additions.

Stereochemistry:

• If the electrophile and the nucleophile bond to the same face of the Pi-bond then the addition is referred to as being "syn".
• In syn addition to cyclic alkenes, the electrophile and nucleophile end up cis to each other on the saturated ring.
• For acyclic alkenes and alkynes, syn addition may be inferred from the relative stereochemistry of the product.
• Syn addition occurs when both the nucleophile and electrophile come from the same reagent and are never not bonded in some way or when one atom is bonded to both Pi-bond carbons.
• If the electrophile and the nucleophile bond to opposite faces of the Pi-bond then the addition is referred to as being "anti" (different that anti-Markovnikov!).
• In anti addition to cyclic alkenes, the electrophile and nucleophile end up trans to each other on the saturated ring.
• For acyclic alkenes and alkynes, anti addition may be inferred from the relative stereochemistry of the product.
• Anti addition occurs when the electrophile blocks the approach of the nucleophile by the neighboring group effect.
• You cannot always tell if a reaction is a net syn or anti addition.
• Whether the addition is syn or anti, a mixture of enantiomers is formed from symmetric starting materials.
• Either enantiomer may be used to represent the relative stereochemistry of the reaction.