Alkynes Unsaturated hydrocarbons that

Alkynes Unsaturated hydrocarbons that contain a carbon-carbon triple bond. 

Another analogous series of unsaturated hydrocarbons that contain just one multiple bond, but, instead of being a double bond, it is a triple bond is the alkynes. The names of all the compounds end in -yne. The only compound m this series that is at all common happens to be an extremely hazardous material. It is a highly unstable (to heat, shock, and pressure), highly flammable gas that is the first compound in the series. This two-carbon unsaturated hydrocarbon with a triple bond between its two carbon atoms is called ethyne, and indeed this is its proper name. It is, however, known by its common name, acetylene. 

Alkynes are hydrocarbons that contain a carbon-carbon triple bond. A triple bond consists of a cr bond and two tt bonds. The general formula for the alkynes is C li2n-2- The triple bond possesses two elements of unsaturation. Alkynes are commonly named as substituted acetylenes. Compounds with triple bonds at the end of a molecule are called terminal alkynes. Terminal —CH groups are called acetylenic hydrogens. If the triple bond has two alkyl groups on both sides, it is called an internal alkyne. 

Another analogous series of unsaturated hydrocarbons that contain just one multiple bond, but, instead of being a double bond, it is a triple bond is the alkynes. The names of all die compounds... 

In contrast to alkanes, which have only single bonds, alkenes and alkynes have multiple bonds Alkenes are hydrocarbons that contain a carbon-carbon double bond, and alkynes are hydrocarbons that contain a carbon-carbon triple bond. Both groups of compounds are unsaturated, meaning that they have fewer hydrogens per carbon than the related alkanes. Ethylene (H2C = CH2), for example, has the formula C2H4, whereas ethane (CH3CH3) has the formula C2H6. 

Carbon is tetravalent (forming four bonds) and can form single bonds, double bonds, and triple bonds. As seen in Table 12.1, the four types of hydrocarbons are alkanes (single bonds), alkenes (double bonds), alkynes (triple bonds), and aromatic. Aromatics are unsaturated hydrocarbons that have cyclic structures. A common and representative compound for aromatic is benzene. 

Unsaturated hydrocarbons that contain one or more triple bonds between carbon atoms in a chain are called alkynes. Triple bonds involve the sharing of three pairs of electrons. The simplest and most commonly used alkyne is ethyne (C2H2), which is widely known by its common name acetylene. Study the models of ethyne in the following diagram. 

Alkynes are unsaturated hydrocarbons that contain a carbon-carbon triple bond, i.e., two adjacent carbon atoms are joined by three bonds. Alkynes have the general formula C H2 2 where n is an integer greater than 1. The simplest member of the alkyne family is C2H2 (lUPAC name ethyne common name acetylene) ... 

Alkyne (Section 7.1) An unsaturated hydrocarbon that contains one or more carbon-carbon triple bonds. 

Alkynes An unsaturated hydrocarbon that contains one or more triple bonds between carbon atoms is called an alkyne. Alkynes with one triple bond have the general formula C H2 2. The simplest alkyne is ethyne (C2H2), which is commonly called acetylene. The next simplest alkyne is propyne (C3H4). Their condensed structural formulas are shown below. 

The other hydrocarbon functional group is a carbon-to-carbon triple bond. Hydrocarbons that contain a triple bond are called alkynes. Alkynes use the -yne ending on the base hydrocarbon. The presence of a double or triple bond make these hydrocarbons unsaturated. 

The above discussions have concentrated on hydrocarbons, both saturated and unsaturated, with the unsaturated hydrocarbons containing only one multiple bond. The unsaturated hydrocarbons are the alkenes with one double bond and the alkynes with one triple bond. There are other straight-chain hydrocarbons that are unsaturated containing more than one multiple bond, some with more than one double bond, and some with a mixture of double bonds and triple bonds. The combinations and permutations are endless, but there are only a few of the highly unstable materials. 

Unsaturated hydrocarbons (alkenes, dienes) react with carbon monoxide and a proton source (H20, alcohols, amines, acids) under strong acidic conditions to form carboxylic acids or carboxylic acid derivatives. Since a carbocationic mechanism is operative, not only alkenes but also other compounds that can serve as the carbocation source (alcohols, saturated hydrocarbons) can be carboxylated. Metal catalysts can also effect the carboxylation of alkenes, dienes, alkynes, and alcohols. 

The metal-catalysed hydrogenation of multiply unsaturated hydrocarbons is, of necessity, more complex than that of monoolefins. The problems encountered in alkyne and alkadiene hydrogenation are essentially similar and it is appropriate, therefore, that the two systems are considered together. 

Another means of classification depends on the type of bonding that exists between carbons. Hydrocarbons which contain only carbon-to-carbon single bonds are called alkanes. These are also referred to as saturated molecules. Hydrocarbons containing at least one carbon-to-carbon double bond are called alkenes, and those compounds with at least one carbon-to-carbon triple bond are called alkynes. These are compounds that are referred to as unsaturated molecules. Finally, a class of cyclic hydrocarbons that contain a closed loop (sextet) of electrons are called aromatic (see Chapter 14 in your text for further details). Table 28.1 distinguishes between the families of hydrocarbons. 

Both double and triple bonds are multiple bonds. Therefore alkynes are unsaturated hydrocarbons, just as alkenes are. To name alkynes and draw their structures, you follow the same rules that you used for alkenes. The only difference is the suffix -yne, which you need to use when naming alkyne compounds. Also, remember to count the number of bonds for each carbon. An alkyne bond counts as three bonds. 

Cycloaddition with unsaturated hydrocarbons such as alkenes and alkynes have been explored (68-74). Schrauzer and Mayweg (74) first reported that M(sdt)2 (M = Ni, Pd, Pt) reacts with alkynes and alkenes via cycloaddition to the dithiolene ligand. With alkynes, dithiane is formed via decomposition of the adduct (Scheme 7). 

Hydrocarbons that contain the carbon-carbon triple bond — C = C — are called alkynes. Each triple bond contains one sigma (o) and two pi (n) bonds. Because of the n bonds in their structure a%nes are unsaturated hydrocarbons. 

Because the unsaturated hydrocarbon has to bind to rhodium in the presence of bulky PPh3 groups, the catalyst favours unsubstituted double bonds (RCH=CH2 rather than RR C=CR r ). Since the alkyl intermediate is shortlived, there is little tendency to / -elimination with concomitant alkene isomerization. Although both alkene and alkyne functions are reduced, in general carbonyl or carboxylic groups and benzene rings are not, though aldehydes are frequently decarbonylated. Peroxides tend to oxidize and thus destroy the catalyst, so that substrates need to be purified carefully before use. 

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