Alkynes Hydrocarbons with Triple Bonds

Hydrocarbons that contain at least one C=C bond are called alkynes (general formula C H2 2). Because a carbon in a C=C bond can bond to only one other atom, the geometry around each C atom is linear (180°) each C is ip hybridized. Alkynes are named in the same way as alkenes, except that the suffix is -yne. Because of their localized it electrons, C=C and C=C bonds are electron rich and act as functional groups. Thus, alkenes and alkynes are much more reactive than alkanes, as we ll discuss in Section 15.4. 

CHAPTER 15 Organic Compounds and the Atomic Properties of Carbon 

SAMPLE PROBLEM 15.2 Naming Alkanes, Alkenes, and Alkynes 

Problem Give the systematic name for each of the following, indicate the chiral center in 

Plan For (a) to (c), we refer to Table 15.2. We first name the longest chain [root- + -am). Then we find the lowest branch numbers by counting C atoms from the end closer to a branch. Finally, we name each branch root- + -yt) and put the names alphabetically before the chain name. For (d) and (e), the longest chain that includes the multiple bond is numbered from the end closer to it. For (d), the chiral center is the C atom bonded to four different groups. In (e), the cis isomer has larger groups on the same side of the double bond, and the trans isomer has them on opposite sides. 

Problem Give the systematic name for each of the following, indicate the chiral center in part (d), and draw two geometric isomers for part (e)  

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Now consider the alkynes, hydrocarbons with triple bonds. The Lewis structure of the linear molecule ethyne (acetylene) is H—C=C—H. To describe the bonding in a linear molecule, we need a hybridization scheme that produces two equivalent orbitals at 180° from each other this is sp hybridization. Each C atom has one electron in each of its two sp hybrid orbitals and one electron in each of its two perpendicular unhybridized 2p-orbitals (47). The electrons in an sp hybrid orbital on each C atom pair and form a carbon-carbon tr-bond. The electrons in the remaining sp hybrid orbitals pair with hydrogen ls-electrons to form two carbon-hydrogen o-bonds. The electrons in the perpendicular 2p-orbitals pair with a side-by-side overlap, forming two ir-bonds at 90° to each other. As in the N2 molecule, the electron density in the ir-bonds form a cylinder about the C—C bond axis. The resulting bonding pattern is shown in Fig. 3.27. 

Alkenes Hydrocarbons with Double Bonds 469 Alkynes Hydrocarbons with Triple Bonds 470... 

Diynes and triynes refer to alkynes containing two or three triple bonds poly-ynes contain multiple triple bonds. A conjugated triyne is a straight-chain hydrocarbon with triple bonds alternating with single bonds. An examples is... 

Unsaturated hydrocarbons are formed by double and triple bonds between adjacent C-atoms. Hydrocarbons with double bonds derived from alkanes are called alkenes hydrocarbons with triple bonds derived from alkanes are called alkynes. The same holds for the cyclo-alkanes, where double bonds lead to c-alkenes. Benzene is the basic molecule for the class of aromatic compounds. Examples are... 

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. 

Our discussion has centered around hydrocarbon molecules that consist of carbon atoms bonded to each other in long chains. These chains are called normal, straight, or unbranched. The chains may have only single bonds (alkanes), double bonds (alkenes), or triple bonds (alkynes). Hydrocarbons with only single bonds are called saturated hydrocarbons with double or triple bonds are called unsaturated. Not all hydrocarbons want to form straight chains. After all, who among us wants to be straight and serious all the time Some of us will always be comedians. In the world of hydrocarbon molecules, the comedians are the branched and cyclic molecules. 

We classify hydrocarbons according to their bonding (Section 2-12), as shown in Table 3-1. Alkanes have only single bonds. A hydrocarbon with a carbon-carbon double bond (such as ethylene) is an alkene. If a hydrocarbon has a carbon-carbon triple bond (like acetylene), it is an alkyne. Hydrocarbons with aromatic rings (resembling benzene) are called aromatic hydrocarbons. 

Alkynes are hydrocarbons that contain a carbon-carbon triple bond. A triple bond can be thought of as a cylinder of electrons wrapped around the a bond. The functional group suffix of an alkyne is yne. A terminal alkyne has the triple bond at the end of the chain an internal alkyne has the triple bond located elsewhere along the chain. Internal alkynes, with two alkyl substituents bonded to the sp carbons, are more stable than terminal alkynes. We now have seen that alkyl groups stabilize alkenes, alkynes, carbo-cations, and alkyl radicals. 

OBJECTIVES To learn to name hydrocarbons with double bonds (alkenes) and triple bonds (alkynes). To understand addition reactions. 

Hydrocarbons with triple covalent bonds are alkynes. Like the double bond of alkenes, the triple bond of alkynes requires that the simplest alkyne have two carbon atoms. 

A straight- or branched-chain alkane, with formula C H2 +2, has the maximum number of H atoms possible for its number of C atoms. In other classes of hydrocarbons, compounds with the same number of C atoms but fewer H atoms, the C atoms must join into rings, form carbon-to-carbon multiple bonds, or do both to ensure that each C atom forms a total of four bonds. We have already discussed some aspects of ring structures (in Section 26-3). In this section, we focus on hydrocarbons whose molecules contain some double or triple bonds between C atoms. Such molecules are said to be unsaturated. If the molecule has one double bond, the hydrocarbons are the simple alkenes, or olefins they have the general formula C H2 . Simple alkynes have one triple bond in their molecules and have the general formula C H2n-2-... 

Carbon can also form multiple bonds with other carbon atoms. This results in unsaturated hydrocarbons such as olefins (alkenes), containing a carbon-carbon double bond, or acetylenes (alkynes), containing a carbon-carbon triple bond. Dienes and polyenes contain two or more unsaturated bonds. 

Hydrocarbons that contain a carbon-carbon triple bond are called alkynes Non cyclic alkynes have the molecular formula C H2 -2 Acetylene (HC=CH) is the simplest alkyne We call compounds that have their triple bond at the end of a carbon chain (RC=CH) monosubstituted or terminal alkynes Disubstituted alkynes (RC=CR ) have internal triple bonds You will see m this chapter that a carbon-carbon triple bond is a functional group reacting with many of the same reagents that react with the double bonds of alkenes... 

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. 

An alkyne is a hydrocarbon that contains a carbon-carbon triple bond. Alkyne carbon atoms are sp-hybridized, and the triple bond consists of one sp-sp a bond and two p-p tt bonds. There are relatively few general methods of alkyne synthesis. Two good ones are the alkylation of an acetylide anion with a primary-alkyl halide and the twofold elimination of HX from a vicinal dihalide. 

The alkynes are hydrocarbons that have at least one carbon-carbon triple bond. The simplest is ethyne, FIO CH, which is commonly called acetylene (20). Alkynes are named like the alkenes but with the suffix -yne. 

The alkyne series of hydrocarbons is characterized by having molecules with one triple bond each. They have the general formula C H2 2. Like other unsaturated hydrocarbons, the alkynes are quite... 

Alkanes, with the general formula C H2 +2, are saturated hydrocarbons, in which each carbon atom is singly bonded to four other atoms. These atoms are either carbon atoms or hydrogen atoms. Alkenes, C H2 , and alkynes, C H2 2, are unsaturated hydrocarbons in which there is a carbon-carbon double bond or a carbon-carbon triple bond, respectively. 

On the basis of these results we embarked on a systematic study on the synthesis of vinyl cations by intramolecular addition of transient silylium ions to C=C-triple bonds using alkynyl substituted disila alkanes 6 as precursors.(35-37) In a hydride transfer reaction with trityl cation the alkynes 6 are transformed into the reactive silylium ions 7. Under essentially nonHnucleophilic reaction conditions, i.e. in the presence of only weakly coordinating anions and using aromatic hydrocarbons as solvents, the preferred reaction channel for cations 7 is the intramolecular addition of the positively charged silicon atom to the C=C triple bond which results in the formation of vinyl cations 8-10 (Scheme 1). 

A hydrocarbon with a triple bond between carbons is an alkyne, and the simplest compound in this class is acetylene, C2H2, as shown in Figure 6-8. 

Hydrocarbons that contain one or more carbon-to-carbon triple bonds are called alkynes. The names of alkynes end in -yne. Ethyne, commonly known as acetylene, is the simplest alkyne. It consists of two carbon atoms with a triple bond between them, with each carbon also bonded to one hydrogen atom. The chemical formula for ethyne is C2H2. 

There are no other alkynes that are of commercial importance, and so acetylene will be the only member of this series that is considered in fire discussions. There are other alkynes, however, along with hydrocarbons that might have one double bond and a triple bond present in the molecule. 

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. 

Alkynes An analogous series of unsaturated hydrocarbons with the general formula C,HM the alkynes all contain just one triple bond between carbon atoms. 

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