File Name: reactions of alkenes and alkynes .zip
Summary Sheet 8: The Oxidation Ladder. Summary Sheet — Introduction to Reactivity and Nomenclature. Do you have an exam on alkene and alkynes coming up?
Reactions of Alkynes. A carbon-carbon triple bond may be located at any unbranched site within a carbon chain or at the end of a chain, in which case it is called terminal. 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 discussed earlier also take place with alkynes, and with similar regio- and stereoselectivity.
The principal reaction of the alkynes is addition across the triple bond to form alkanes. These addition reactions are analogous to those of the alkenes. Alkynes undergo catalytic hydrogenation with the same catalysts used in alkene hydrogenation: platinum, palladium, nickel, and rhodium. Hydrogenation proceeds in a stepwise fashion, forming an alkene first, which undergoes further hydrogenation to an alkane.
This reaction proceeds so smoothly that it is difficult, if not impossible, to stop the reaction at the alkene stage, although by using palladium or nickel for the catalyst, the reaction can be used to isolate some alkenes. Greater yields of alkenes are possible with the use of poisoned catalysts.
One such catalyst, the Lindlar catalyst , is composed of finely divided palladium coated with quinoline and absorbed on calcium carbonate. This treatment makes the palladium less receptive to hydrogen, so fewer hydrogen atoms are available to react. When a catalyst is deactivated in such a manner, it is referred to as being poisoned.
The mechanism of alkyne hydrogenation is identical to that of the alkenes. Because the hydrogen is absorbed on the catalyst surface, it is supplied to the triple bond in a syn manner. Alkynes can also be hydrogenated with sodium in liquid ammonia at low temperatures. This reaction is a chemical reduction rather than a catalytic reaction, so the hydrogen atoms are not attached to a surface, and they may approach an alkene from different directions, leading to the formation of trans alkenes.
The addition of halogens to an alkyne proceeds in the same manner as halogen addition to alkenes. The halogen atoms add to an alkyne molecule in a stepwise fashion, leading to the formation of the corresponding alkene, which undergoes further reaction to a tetrahaloalkane.
Hydrogen halides react with alkynes in the same manner as they do with alkenes. Both steps in the above addition follow the Markovnikov rule. The addition of the elements of water across the triple bond of an alkyne leads to the formation of aldehydes and ketones. Water addition to terminal alkynes leads to the generation of aldehydes , while nonterminal alkynes and water generate ketones. These products are produced by rearrangement of an unstable enol vinyl alcohol intermediate.
A vinyl group is. Water adds across the triple bond of an alkyne via a carbocation mechanism. Dilute mineral acid and mercury II ions are needed for the reaction to occur. A molecule of water is attracted to the carbocation to form an oxonium ion. The oxonium ion loses a proton to stabilize itself. Vinyl alcohols enols are unstable intermediates, and they undergo rapid isomerization to form ketones.
Alkynes are oxidized by the same reagents that oxidize alkenes. Ozonolysis of an alkyne also leads to carboxylic acid formation. The reactions and mechanisms are identical with those of the alkenes.
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Unit: Alkenes and alkynes
In chemistry , an alkene is a hydrocarbon that contains a carbon —carbon double bond. The term is often used as synonym of olefin , that is, any hydrocarbon containing one or more double bonds. Acyclic alkenes, with only one double bond and no other functional groups also known as mono-enes form a homologous series of hydrocarbons with the general formula C n H 2 n with n being 2 or more which is two hydrogens less than the corresponding alkane. When n is four or more, there are multiple isomers with this formula, distinguished by the position and conformation of the double bond. Alkenes are generally colorless apolar compounds, somewhat similar to alkanes but more reactive. The first few members of the series are gases or liquids at room temperature. The simplest alkene, ethylene C 2 H 4 or "ethene" in the IUPAC nomenclature is the organic compound produced on the largest scale industrially.
Summary Sheet: Reactions of Alkenes
The principal reaction of the alkynes is addition across the triple bond to form alkanes. These addition reactions are analogous to those of the alkenes. Alkynes undergo catalytic hydrogenation with the same catalysts used in alkene hydrogenation: platinum, palladium, nickel, and rhodium.
Addition of strong acids to alkynes is quite similar to the addition of strong acids to alkenes. Addition to alkenes is usually not stereospecific, whereas alkynes usually undergo anti addition. Strong acids usually protonate alkenes on their less substituted carbon thus developing a positive charge on the more substituted carbon of the reacting double bond. This intermediate carbocation may rearrange or simply pick up a nucleophile to form the expected addition product.
Just as alkenes, alkynes can be hydrated by two different methods. The direct addition of water catalyzed by mercury II salts yields the Markovnikov product. In contrast, the indirect hydration by the reaction sequence of hydroboration, oxidation and hydrolysis results in the anti-Markovnikov product.
Alkynes: Addition Reactions
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