Hydrogen halides such as hydrogen chloride (HCl) adds to alkenes to give alkyl halides in hydrohalogenation. For example, the reaction of HCl with ethylene furnishes chloroethane. The reaction proceeds with a cation intermediate, being different from the above halogen addition. An example is shown below:
#Proton (H) adds (by working as an electrophile) to one of the carbon atoms on the alkene to form cation 1.
#Chloride ion (Cl) combines with the cation 1 to form the adducts 2 and 3.
In this manner, the stereoselectivity of the product, that is, from which side Cl will attack relies on the types of alkenes applied and conditions of the reaction. At least, which of the two carbon atoms will be attacked by H is usually decided by Markovnikov's rule. Thus, H attacks the carbon atom that carries fewer substituents so as the more stabilized carbocation (with the more stabilizing substituents) will form.
This is another example of an Ad2 mechanism. Hydrogen fluoride (HF) and hydrogen iodide (HI) react with alkenes in a similar manner, and Markovnikov-type products will be given. Hydrogen bromide (HBr) also takes this pathway, but sometimes a radical process competes and a mixture of isomers may form.  Although introductory textbooks seldom mentions this alternative, the Ad2 mechanism is generally competitive with the Ad3 mechanism (described in more detail for alkynes, below), in which transfer of the proton and nucleophilic addition occur in a concerted manner.  The extent to which each pathway contributes depends on the several factors like the nature of the solvent (e.g., polarity), nucleophilicity of the halide ion, stability of the carbocation, and steric effects.  As brief examples, the formation of a sterically unencumbered, stabilized carbocation favors the Ad2 pathway, while a more nucleophilic bromide ion favors the Ad3 pathway to a greater extent compared to reactions involving the chloride ion.  
In the case of dialkyl-substituted alkynes (e.g., 3-hexyne), the intermediate vinyl cation that would result from this process is highly unstable.  In such cases, the simultaneous protonation (by HCl) and attack of the alkyne by the nucleophile (Cl) is believed to take place.  This mechanistic pathway is known by the Ingold label Ad3 ("addition, electrophilic, third-order").  Because the simultaneous collision of three chemical species in a reactive orientation is improbable, the termolecular transition state is believed to be reached when the nucleophile attacks a reversibly-formed weak association of the alkyne and HCl.  Such a mechanism is consistent with the predominantly anti addition (>15:1 anti:syn for the example shown)  of the hydrochlorination product and the termolecular rate law, Rate = k[alkyne][HCl].  In support of the proposed alkyne-HCl association, a T-shaped complex of an alkyne and HCl has been characterized crystallographically.
In contrast, phenylpropyne reacts by the Ad2 ("addition, electrophilic, second-order, ion pair") mechanism to give predominantly the syn product (~10:1 syn:anti).  In this case, the intermediate vinyl cation is formed by addition of HCl because it is resonance-stabilized by the phenyl group.  Nevertheless, the lifetime of this high energy species is short, and the resulting vinyl cation-chloride anion ion pair immediately collapses, before the chloride ion has a chance to leave the solvent shell, to give the vinyl chloride.  The proximity of the anion to the side of the vinyl cation where the proton was added is used to rationalize the observed predominance of syn addition.
<br />