Question. Chapter 5 Structure and Preparation of Alkenes (C n H 2n ): Elimination Reactions

Transcription

1 hapter 5 Structure and Preparation of Alkenes ( n 2n ): Elimination Reactions The molecular formula of β-arotene is 40 On catalytic hydrogenation, β-carotene is converted to a saturated hydrocarbon of molecular formula ow many rings does β-carotene contain? A) none B) one ) two D) three Butene or But-1-ene 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 with -ene. 3) Number the chain in the direction that gives the lowest number to the doubly bonded carbon. 2 2 Br 2 2 O 3 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. (or 4-bromo-3-methylbut-1-ene) 4) If a substituent is present, identify its position by number. Alcohol groups take precedence over the double bond when the chain is numbered. The compound shown above is 2-methyl-3-buten-1-ol. (or 2-methyl-but-3-en-1-ol)

2 methylene vinyl ommon Alkenyl Groups ycloalkene Nomenclature Ethyl-1-methylcyclohexene allyl 2 2 1) Replace the -ane ending of the cycloalkane having the same number of carbons with -ene. 2) Number through the double bond in the direction that gives the number to the first-appearing substituent. Name the alkene below according to the IUPA system. 5.2 Structure and Bonding in Alkenes A) 2-bromo-5-heptane B) 6-bromo-2-heptene ) 2-bromo-5-heptene D) 5-bromo-2-heptene Bonding in Ethylene Bonding in Ethylene Framework of bonds Each carbon is sp 2 hybridized Each carbon has a half-filled p orbital.

3 Bonding in Ethylene 5.3 Isomerism in Alkenes Side-by-side overlap of half- filled p orbitals gives a π bond. Isomers Isomers onstitutional Isomers Stereoisomers onstitutional Isomers Stereoisomers different connectivity same connectivity; different arrangement of atoms in space consider the isomeric alkenes of molecular formula Butene 2-Methylpropene 1-Butene 2-Methylpropene onstitutional isomers 3 cis-2-butene trans-2-butene cis-2-butene

4 1-Butene Methylpropene Stereoisomers onstitutional isomers 3 3 trans-2-butene cis-2-butene trans-2-butene Stereochemical Notation Which alkene can have cis/trans stereoisomers? A) B) cis (identical or analogous substituents on same side) or Z trans (identical or analogous substituents on opposite sides) or E ) D) Of the compounds with the formula 3 l, 5 which one has cis/trans isomers? A) 1-chloropropene B) 2-chloropropene ) 3-chloropropene D) all of them 5.4 Naming Steroisomeric Alkenes by the E-Z Notational System

5 Stereochemical Notation 3 ( 2 ) ( 2 ) 6 O 2 Oleic acid E : Z : ranked substituents on opposite sides ranked substituents on same side 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 E : Z : ranked substituents on opposite sides ranked substituents on same side E : Z : ranked substituents on opposite sides ranked substituents on same side Entgegen (E) Zusammen (Z) : ow are substituents ranked? Answer: They are ranked in order of decreasing atomic number. The ahn-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 are most important in relation to stereochemistry and most be mastered. E Z

6 Table 5.1 IP Rules (1) igher atomic number outranks atomic number Table 5.1 IP Rules (1) igher atomic number outranks atomic number Br > F l > Br > F l > Br l Br l F F (Z )-1-Bromo-2-chloro-1-fluoroethene 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 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 (,,) (,,) (,,),) (,,) Table 5.1 IP Rules (4) Evaluate substituents one by one. Don't add atomic numbers within groups. 2 O outranks ( 3 ) 3 (O,,) (,,) 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,,) See the table on the inside back cover of the text.

7 Which one of the following groups has the highest rank when precedence is assigned according to the ahn-ingold Ingold-Prelog system? A) - l 2 B) -=O ) -( 3 ) 3 D) N 2 2 Assign the configuration of the alkenes below as Z or E, respectively. A) E,E B) E,Z ) Z,Z D) Z,E