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Alkenes Group

Alkenes Group. Naming of Alkenes. Hydrocarbons with double bonds make up the alkene family. Open chain alkenes with one double bond have the general formula, C n H2 n , where n equals the number of carbon atoms.

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Alkenes Group

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  1. Alkenes Group

  2. Naming of Alkenes Hydrocarbons with double bonds make up the alkene family. Open chain alkenes with one double bond have the general formula, CnH2n, where n equals the number of carbon atoms.

  3. Alkenes are named by dropping the -ane ending of the parent and adding -ene. The parent structure is the longest chain containing both carbon atoms of the double bond.Example:a. CH2=CH—CH2—CH3 (1-butene/ but-1-ene)b. CH3—CH=CH—CH3(2-butene/ but-2-ene)c. CH3—CH2—CH=CH2(??)d. CH3—CH2—C=CH2(2-methyl-1-butene/ 2-methyl-but-1-ene) CH3

  4. Try to name the following compounds... ________________________________ _______________________________________

  5. Try to draw structures for the following compounds...  2-pentene  3-heptene

  6. 2. Give the double bond the lowest possible numbers regardless of substituent placement.  · Try to name the following compound... ______________________________________ · Try to draw a structure for the following compound... 4-methyl-2-pentene answer\6.mht

  7. Common names that you should know are... vinyl substituent H2C=CH- allyl substituent H2C=CH-CH2- allene molecule H2C=C=CH2 isoprene

  8. 3. Endocyclic double bonds have both carbons in the ring and exocyclic double bonds have only one carbon as part of the ring. Cyclopentene is an example of an endocyclic double bond. Methylenecylopentane is an example of an exocyclic double bond.

  9. · Try to name the following compounds... answer\7.txt ___________________________________________________________ answer\8.txt _________________________________________

  10. 4. Double bonds can exist as geometric isomers and these isomers are designated by using the cis / trans designation or the modern E / Z designation. cis...The two largest groups are on the same side of the double bond. trans...The two largest groups are on opposite sides of the double bond. E/Z nomenclature E = entgegan ("trans") Z = zusamen ("cis")

  11. · Try to name the following compounds using both conventions... answer\9.txt _________________________________________ answer\10.txt _________________________________________

  12. Properties of Alkene • They are made from alkanes by cracking • Because they have a double bond, they are called unsaturated. (Alkanes have no double bonds, so are saturated) • They are much more reactive than alkanes C2H4 + H2 ---C3H6 The hydrogen just add on, so this is called an addition reaction • Alkenes also do an addition reaction with steam, to form compounds called alcohols • The alkenes are highly flammable, and burn readily in air C2H4 + 3O2 - 2CO2 + 2H2O • But note that an alkene has a higher % of carbon than the corresponding alkane does. So alkenes are more likely to undergo incomplete combustion. • Finally, alkene molecules can add on to each other to form compounds with very long carbon chains. These compounds are called polymers C2H4 - -(CH2—CH2)-n

  13. Physical Properties: The physical properties of alkenes are comparable with alkanes. The physical state depends on molecular mass. The simplest alkenes, ethylene, propylene and butylene are gases. Linear alkenes of approximately five to sixteen carbons are liquids, and higher alkenes are waxy solids. Chemical Properties: Alkenes are relatively stable compounds, but are more reactive than alkanes due to their double carbon-carbon bond. Although stronger than the single carbon-carbon bond in alkanes, the majority of the reactions of alkenes involve the rupture of this double bond, forming two new single bonds.

  14. Isomerism of Alkenes • Positional Isomers of Alkenes • The location of the double bond is unambiguous in propene (also known as propylene), but two butenes are possible.  These two isomers are POSITIONAL ISOMERS.  They have been called n-butene, CH2=CHCH2CH3, and isobutene, H3C-CH=CHCH3, but are more commonly and correctly called 1-butene and 2-butene.  Two butadienes, which each contain two double bonds, are known also: 1,2-butadiene, H2C=C=CHCH3, and 1,3-butadiene, H2C=CHCH=CH2.

  15. Geometrical Isomer • Geometric isomers are a result of the rigidity of a molecule, which prevents rotation of atoms about a bond.  This rigidity is usually due to a ring structure or a double bond. Such rigidity is usually not present in singly bonded, open-chained molecules. In those molecules in which the substituent groups are on the same side of the double bond, the name is preceded by the prefix cis-.  In those molecules in which the groups are on opposite side of the double bond, the name is preceded by the prefix trans-.

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