Alkynes and dienes compounds

Alkynes and dienes compounds

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  ALKYNES AND DIENES
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  Higher alkynes. Nomenclature Likethe alkanes and alkenes, the alkynes form a homologous series, the increment again being CH2 . The alkynes are named according to two systems. In one, they are considered o be derived from acetylene by replacement of one or both hydrogen atoms by alkyl groups. Alkynes Industrial source of acetylene Preparation of acetylene The simplest member of the alkyne family is acetylene, C2H2 . The general formula for this family is C n H 2n -2
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  For more complicatedalkynes the IUPAC names are used. The rules are exactly the same as for the naming of alkenes, except that the ending -yne replaces-ene. The parent structure is the longest continuous chain that contains the triple bond, and the positions both of substituents and of the triple bond are indicated by numbers. The triple bond is given the number of Ihe first triply-bonded carbon encountered, starting from the end of the chain nearest the triple bond. Physical properties of alkynes
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  Preparation of alkynes Acarbon-carbon triple bond is formed in the same way as a double bond: elimination of atoms or groups from two adjacent carbons. The groups eliminated and the reagents used are essentially the same as in the preparations of alkenes. 1. Dehydrohalogenation of alkyl dihalides: 2. Reaction of sodium acetylides with primary alkyl halides:
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  3. Dehalogenation oftetrahalides: REACTIONS OF ALKYNES 1. Addition of hydrogen Addition Reactions
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  2. Addition ofhalogens 3. Addition of hydrogen halides
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  4. Addition ofwater Hydration 5. Formation of heavy metal acetylides 6. Formation of alkali metal acetylides
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  DIENES Structure and nomenclatureof dienes Dienes are simply alkenes that contain two carbon-carbon double bonds. They therefore have essentially the same properties as the alkenes we have already studied. For certain of the dienes, these alkene properties are modified in important ways; we shall focus our attention on these modifications. Although we shall consider chiefly dienes in this section, what we shall say applies equally well to compounds with more than two double bonds. Dienes are named by the IUPAC system in the same way as alkenes, except that the ending -diene is used, with two numbers to indicate the positions of the two double bonds. This system is easily extended to compounds containing any number of double bonds. Dienes are divided into two important classes according to the arrangement of the double bonds, Double bonds that alternate with single bonds are said to be conjugated; double bonds that are separated by more than one single bond are said to be isolated.
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  A third classof dienes, of increasing interest to organic chemists, contain cumulated double bonds; these compounds are known as allenes: Preparation and properties of dienes Dienes are usually prepared by adaptations of the methods used to make simple alkenes. For example, the most important diene, 1,3-butadiene (used to make synthetic rubber), has been made in this country by a cracking process, and in Germany by dehydration of an alcohol containing two OH groups:
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  Resonance in conjugateddienes Let us focus our attention on the four key carbon atoms of any conjugated diene system. We ordinarily write the C1—C2 and C3—C4 bonds as double, and the C2—C3 bond as single: Using the language of conventional valence-bond structures, we say that a conjugated diene is a resonance hybrid of I and II. The dotted line in II represents Electrophilic addition to conjugated dienes. 1,4-Addition When 1,4-pentadiene is treated with bromine under conditions (what are they?) that favor formation of the dihalide, there is obtained the expected product, 4,5- dibromo-1-pentene. Addition of more bromine yields the 1,2,4,5-tetrabromo- pentane.
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  This is typicalof the behavior of dienes containing isolated double bonds: the double bonds react independently, as though they were in different molecules. ‘ When 1,3- butadiene is treated with bromine under similar conditions, there is obtained not only the expected 3,4-dibromo-l-butene, but also l,4-dibromo-2- butene. Treatment with HC1 yields not only 3-chloro-l-butene, but also l-chloro-2-butene. Hydrogenation yields not only 1-butene but also 2-butene. Study of many conjugated dienes and many reagents shows that such behavior is typical: in additions to conjugated dienes, a reagent may attach itself not only to a pair of adjacent carbons (1,2-addition), but also to the carbons at the two ends of the conjugated system (1,4-addition). Very often the 1,4-addition product is the major one.