Addition of halogens These occur between alkenes and electrophiles, often halogens as in
halogen addition reactions. Common reactions include use of bromine water to
titrate against a sample to deduce the number of double bonds present. For example,
ethene +
bromine →
1,2-dibromoethane: :C2H4 + Br2 → BrCH2CH2Br This takes the form of 3 main steps shown below; : •
Forming of a π-complex • : The electrophilic Br-Br molecule interacts with electron-rich alkene molecule to form a
π-complex 1. •
Forming of a three-membered bromonium ion • : The alkene is working as an electron donor and bromine as an electrophile. The three-membered
bromonium ion 2 consisted of two carbon atoms and a bromine atom forms with a release of Br−. •
Attacking of bromide ion • : The bromonium ion is opened by the attack of Br− from the back side. This yields the
vicinal dibromide with an
antiperiplanar configuration. When other nucleophiles such as water or alcohol are existing, these may attack
2 to give an alcohol or an ether. This process is called
AdE2 mechanism ("addition, electrophilic, second-order").
Iodine (I2),
chlorine (Cl2),
sulfenyl ion (RS+),
mercury cation (Hg2+), and
dichlorocarbene (:CCl2) also react through similar pathways. The direct conversion of
1 to
3 will appear when the Br− is large excess in the reaction medium. A β-bromo
carbenium ion intermediate may be predominant instead of
3 if the alkene has a cation-stabilizing substituent like phenyl group. There is an example of the isolation of the bromonium ion
2.
Addition of hydrogen halides 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
AdE2 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 AdE2 mechanism is generally competitive with the
AdE3 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 AdE2 pathway, while a more nucleophilic bromide ion favors the AdE3 pathway to a greater extent compared to reactions involving the chloride ion. 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
AdE2ip ("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.
Hydration One of the more complex
hydration reactions utilises
sulfuric acid as a
catalyst. This reaction occurs in a similar way to the addition reaction but has an extra step in which the OSO3H group is replaced by an OH group, forming an alcohol: :C2H4 + H2O → C2H5OH As can be seen, the H2SO4 does take part in the overall reaction, however it remains unchanged so is classified as a catalyst. This is the reaction in more detail: : • The H–OSO3H molecule has a δ+ charge on the initial H atom. This is attracted to and reacts with the double bond in the same way as before. • The remaining (negatively charged) −OSO3H ion then attaches to the
carbocation, forming ethyl hydrogensulphate (upper way on the above scheme). • When water (H2O) is added and the mixture heated, ethanol (C2H5OH) is produced. The "spare" hydrogen atom from the water goes into "replacing" the "lost" hydrogen and, thus, reproduces sulfuric acid. Another pathway in which water molecule combines directly to the intermediate carbocation (lower way) is also possible. This pathway become predominant when aqueous sulfuric acid is used. Overall, this process adds a molecule of water to a molecule of ethene. This is an important reaction in industry, as it produces
ethanol, whose purposes include fuels and starting material for other chemicals.
Chiral derivatives Many electrophiles are
chiral and optically stable. Typically chiral electrophiles are also optically pure. One such
reagent is the
fructose-derived organocatalyst used in the
Shi epoxidation. The catalyst can accomplish highly enantioselective epoxidations of
trans-disubstituted and trisubstituted
alkenes. The Shi catalyst, a
ketone, is oxidized by stoichiometric
oxone to the active
dioxirane form before proceeding in the catalytic cycle.
Oxaziridines such as chiral
N-sulfonyloxaziridines effect enantioselective ketone alpha oxidation en route to the AB-ring segments of various
natural products, including γ-rhodomycionone and α-citromycinone. Polymer-bound chiral
selenium electrophiles effect asymmetric selenenylation reactions. The reagents are aryl selenenyl bromides, and they were first developed for solution phase chemistry and then modified for solid phase bead attachment via an aryloxy moiety. The solid-phase reagents were applied toward the selenenylation of various alkenes with good enantioselectivities. The products can be cleaved from the solid support using
organotin hydride reducing agents. Solid-supported reagents offers advantages over solution phase chemistry due to the ease of workup and purification. ==Electrophilicity scale==