After completing this section, you should be able to
- explain the reactivity of alkynes based on the known strengths of carbon-carbon single, double and triple bonds.
- write equations for the reaction of an alkyne with one or two equivalents of halogen (chlorine or bromine) or halogen acid (HCl, HBr or HI).
- draw the structure of the product formed when an alkyne reacts with one equivalent of the halogens and halogen acids
- identify the alkyne which must have been used in an addition reaction with a halogen or halogen acid, given the product of such a reaction.
Addition by Electrophilic Reagents
Since the most common chemical transformation of a carbon-carbon double bond is an addition reaction, we might expect the same to be true for carbon-carbon triple bonds. Indeed, most of the alkene addition reactions also take place with alkynes, and with similar regio- and stereoselectivity.
Although these electrophilic additions to alkynes are sluggish, they do take place and generally display Markovnikov's Rule regioselectivity. One problem, of course, is that the products of these additions are themselves substituted alkenes and can therefore undergo further addition. Because of their high electronegativity, halogen substituents on a double bond act to reduce its nucleophilicity, and thereby decrease the rate of electrophilic addition reactions. Consequently, there is a delicate balance as to whether the product of an initial addition to an alkyne will suffer further addition to a saturated product. Although the initial alkene products can often be isolated and identified, they are commonly present in mixtures of products and may not be obtained in high yield. The following reactions illustrate many of these features. In the last example, 1,2-diodoethene does not suffer further addition inasmuch as vicinal-diiodoalkanes are relatively unstable.
Addition of Hydrogen Halide to an Alkyne
Summary: Reactivity order of hydrogen halides: HI > HB r> HCl > HF.
Follows Markovnikov’s rule:
- Hydrogen adds to the carbon with the greatest number of hydrogens, the halogen adds to the carbon with fewest hydrogens.
- Protination occurs on the more stable carbocation. With the addition of HX, haloalkenes form.
- With the addition of excess HX, you get anti addition forming a geminal dihaloalkane.
Addition of HX to an Alkyne
For example, HBr herefore attacks the double bond in 2-butyne to generate 2-bromobutene, as shown below.
Now, what if you have excess HBr? Due to Markovnivov's rule, the addition of HX in excess yields a geminal dihaloalkane
Addition of HX to Terminal Alkyne
- Here is an addition of HBr to an asymmetric molecule. Now, what if you have excess HBr? Due to Markovnivov's rule, the addition of HX in excess yields a geminal dihaloalkane
Most Hydrogen halide reactions with alkynes occur in a Markovnikov-manner in which the halide attaches to the most substituted carbon since it is the most positively polarized.
Reaction: Addition of Halogens (Cl2, Br2, I2)
The addition of halogen to an alkyne is analogous to adding to an alkene. The trans-alkene is obtained first. The use of halogen in excess results in a tetrahalogened alkane.
Draw the structure and give the IUPAC name of the product formed in each of the reactions listed below:
Prof. Steven Farmer (Sonoma State University)
Jim Clark (Chemguide.co.uk)
- Layne Morsch (University of Illinois Springfield)