5.1 Alkene Nomenclature

Transcription

1 5.1 Alkene Nomenclature

2 Alkenes Alkenes are hydrocarbons that contain a carbon-carbon double bond also called "olefins" characterized by molecular formula n 2n said to be "unsaturated"

3 Alkene Nomenclature Ethene or Ethylene (both are acceptable IUPA names) Propene (Propylene is sometimes used but is not an acceptable IUPA name)

4 Alkene Nomenclature ) Find the longest continuous chain that includes the double bond. 2) Replace the -ane ending of the unbranched alkane having the same number of carbons by -ene. 3) Number the chain in the direction that gives the lowest number to the doubly bonded carbon.

5 Alkene Nomenclature Butene 1) Find the longest continuous chain that includes the double bond. 2) Replace the -ane ending of the unbranched alkane having the same number of carbons by -ene. 3) Number the chain in the direction that gives the lowest number to the doubly bonded carbon.

6 Alkene Nomenclature 2 2 Br 3 4) If a substituent is present, identify its position by number. The double bond takes precedence over alkyl groups and halogens when the chain is numbered. The compound shown above is 4-bromo-3-methyl-1-butene.

7 Alkene Nomenclature 2 2 O 3 4) If a substituent is present, identify its position by number. ydroxyl groups take precedence over the double bond when the chain is numbered. The compound shown above is 2-methyl-3-buten-1-ol.

8 Alkenyl Groups methylene 2 vinyl 2 allyl 2 2 isopropenyl 2 3

9 ycloalkene Nomenclature yclohexene 1) Replace the -ane ending of the cycloalkane having the same number of carbons by -ene.

10 ycloalkene Nomenclature ) Replace the -ane ending of the cycloalkane having the same number of carbons by -ene. 2) Number through the double bond in the direction that gives the lower number to the first-appearing substituent.

11 ycloalkene Nomenclature 3 6-Ethyl-1-methylcyclohexene 2 3 1) Replace the -ane ending of the cycloalkane having the same number of carbons by -ene. 2) Number through the double bond in the direction that gives the lower number to the first-appearing substituent.

12 5.12 Structure and Bonding in Alkenes

13 Structure of Ethylene bond angles: -- = 117 bond distances: -- = 121 = 110 pm = = 134 pm planar

14 Bonding in Ethylene σ σ σ σ σ Framework of σ bonds Each carbon is sp 2 hybridized

15 Bonding in Ethylene Each carbon has a half-filled filled p orbital

16 Bonding in Ethylene Side-by by-side overlap of half- filled p orbitals gives a π bond

17 5.3 Isomerism in Alkenes

18 Isomers Isomers are different compounds that have the same molecular formula.

19 Isomers onstitutional isomers Stereoisomers

20 Isomers onstitutional isomers different connectivity Stereoisomers same connectivity; different arrangement of atoms in space

21 Isomers onstitutional isomers Stereoisomers consider the isomeric alkenes of molecular formula 4 8

22 Butene 2-Methylpropene cis-2-butene trans-2-butene

23 Butene 2-Methylpropene 3 3 onstitutional isomers cis-2-butene

24 Butene 2-Methylpropene 3 onstitutional isomers 3 trans-2-butene

25 Stereoisomers cis-2-butene trans-2-butene

26 Stereochemical Notation cis (identical or analogous substitutents on same side) trans (identical or analogous substituents on opposite sides)

27 Figure 5.2 Interconversion of stereoisomeric alkenes does not normally occur. Requires that π component of double bond be broken. cis trans

28 Figure 5.2 cis trans

29 5.4 Naming Steroisomeric Alkenes by the E-Z E Z Notational System

30 Stereochemical Notation 3 ( 2 ) ( 2 ) 6 O 2 Oleic acid cis and trans are useful when substituents are identical or analogous (oleic acid has a cis double bond) cis and trans are ambiguous when analogies are not obvious

31 Example l Br F What is needed: 1) systematic body of rules for ranking substituents 2) new set of stereochemical symbols other than cis and trans

32 The E-Z E Z Notational System E : Z : higher ranked substituents on opposite sides higher ranked substituents on same side higher lower

33 The E-Z E Z Notational System E : Z : higher ranked substituents on opposite sides higher ranked substituents on same side lower higher

34 The E-Z E Z Notational System E : Z : higher ranked substituents on opposite sides higher ranked substituents on same side higher lower lower higher Entgegen

35 The E-Z E Z Notational System E : Z : higher ranked substituents on opposite sides higher ranked substituents on same side higher lower higher higher lower higher lower lower Entgegen Zusammen

36 Answer: They are ranked in order of decreasing atomic number. higher The E-Z E Z Notational System Question: ow are substituents ranked? lower higher higher lower higher lower lower Entgegen Zusammen

37 The ahn-ingold Ingold-Prelog (IP) System The system that we use was devised by R. S. ahn Sir hristopher Ingold Vladimir Prelog Their rules for ranking groups were devised in connection with a different kind of stereochemistry one one that we will discuss in hapter 7 but 7 have been adapted to alkene stereochemistry.

38 Table 5.1 IP Rules (1) igher atomic number outranks lower atomic number Br > F l > higher Br l higher lower F lower

39 Table 5.1 IP Rules (1) igher atomic number outranks lower atomic number Br > F l > higher Br l higher lower F lower (Z )-1-Bromo-2-chloro-1-fluoroethene

40 Table 5.1 IP Rules (2) When two atoms are identical, compare the atoms attached to them on the basis of their atomic numbers. Precedence is established at the first point of difference. 2 3 outranks 3 (,,) (,,)

41 Table 5.1 IP Rules (3) Work outward from the point of attachment, comparing all the atoms attached to a particular atom before proceeding further along the chain. ( ( 3 ) 2 outranks 2 2 O (,,),) (,,)

42 Table 5.1 IP Rules (4) Evaluate substituents one by one. Don't add atomic numbers within groups. 2 O outranks ( 3 ) 3 (O,,) (,,)

43 Table 5.1 IP Rules (5) An atom that is multiply bonded to another atom is considered to be replicated as a substituent on that atom. = =O outranks 2 O (O,O,),) (O,,)

44 Table 5.1 IP Rules A table of commonly encountered substituents ranked according to precedence is given on the inside back cover of the text.