Reactions of Alkenes: Addition Reactions. Addition Reactions. Addition is the opposite of elimination. Here are some of the many addition reactions that will be considered. Addition of Halogens to Alkenes. C. C. C. C. General features. electrophilic addition to double bond
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C C C C General features • electrophilic addition to double bond • forms a vicinal dihalide +X2 X X
Example Br2 CH3CHCHCH(CH3)2 CH3CH CHCH(CH3)2 CHCl3 0°C Br Br (100%)
Scope limited to Cl2 and Br2 • F2 addition proceeds with explosive violence • I2 addition is endothermic: vicinal diiodidesdissociate to an alkene and I2
Mechanism is electrophilic addition • Br2 is not polar, but it is polarizable • two steps (1) formation of bromonium ion (2) nucleophilic attack on bromonium ion by bromide
Relative Rates of Bromination • ethylene H2C=CH2 1 • propene CH3CH=CH2 61 • 2-methylpropene (CH3)2C=CH2 5400 • 2,3-dimethyl-2-butene (CH3)2C=C(CH3)2 920,000 • More highly substituted double bonds react faster.Alkyl groups on the double bond make itmore “electron rich.”
Question • Arrange the alkenes in order of decreasing rate of reaction toward bromine addition: • 2-methyl-1-butene, 2-methyl-2-butene, and 3-methyl-1-butene • A) 2-methyl-1-butene > 3-methyl-1-butene > 2-methyl-2-butene • B) 3-methyl-1-butene > 2-methyl-1-butene > 2-methyl-2-butene • C) 2-methyl-2-butene > 2-methyl-1-butene > 3-methyl-1-butene • D) 2-methyl-2-butene > 3-methyl-1-butene > 2-methyl-1-butene
H H Br Br H H Example Br2 trans-1,2-dibromocyclopentane80% yield; 2 asymmetric carbon atoms, Chiral product,Optically inactive, Racemic mixture, only products formed
Diastereomers • In general, diastereomers fall into two categories: • geometric isomers • cis-trans • stereoisomers containing two or more asymmetric atoms; (that are not enantiomers) (S) (R) diastereomers (R) (R)
Diastereomers • Diastereomers have different physical properties: • BP, MP, density, refractive index, solubility • Can be separated through conventional means (distillation, recrystallization, chromatography) MP = 158oC MP = 256oC
Diastereomers • A compound with “n” asymmetric carbon atoms can have a maximum of 2nstereoisomers. 2,3-dibromobutane (S) (R) (R) (S) s (S) (R) (S) (R) Enantiomers Same compound (meso) diastereomers Only 3stereoisomers for 2,3-dibromobutane.
CH3 CH3 CH3 H Br Br Br H H Br Br H Br H H CH3 CH3 CH3 Three stereoisomers of 2,3-dibromobutane 2R,3R 2S,3S 2R,3S chiral chiral achiral
Question • The addition of bromine to cis-2-butene produces: • A single enantiomer • A product with one asymmetric carbon atom • An optically inactive meso product • A racemic mixture • 4 different stereoisomers each with 2 chiral carbon atoms
Question • The addition of bromine to trans-2-butene produces: • A single enantiomer • A product with one asymmetric carbon atom • An optically inactive meso product • A racemic mixture • 4 different stereoisomers each with 2 chiral carbon atoms
Diastereomers • When naming compounds containing multiple chiral atoms, you must give the configuration around each chiral atom: • position number and configuration of each chiral atom, separated by commas, all in ( ) at the start of the compound name (S) (2S, 3S)-2-bromo-3-chlorobutane (S)
Diastereomers • Which ones are chiral? Name each one. Give the stereochemical relationship between them. All of them are chiral.
Diastereomers A B C D A and B: enantiomers C and D: enantiomers A and C: diastereomers A and D: diastereomers B and C: diastereomers B and D: diastereomers
A B C C C C Reactions of Alkenes • The characteristic reaction of alkenes is addition to the double bond. + A—B
H H H H C C H H C C H H H H Hydrogenation of Ethylene • exergonic H° = –136 kJ/mol • catalyzed by finely divided (heterogeneous) metals • (Insoluble) Pt, Pd, Rh, Ni + H—H
H2, Pt Problem • What three alkenes yield 2-methylbutane on catalytic hydrogenation?
Question • Which one of the following terms best applies to the hydrogenation of an alkene in the • presence of finely divided platinum? (in ethanol as solvent) • A) anti addition • B) concerted reaction • C) heterogeneous catalysis • D) endothermic reaction
Catalytic Hydrogenation • If catalysis takes place on the surface of a solid surrounded by solution, the catalyst is HETEROGENEOUS. • HOMOGENEOUS catalysts also exist. • What advantage might a homogeneous catalyst have?
Asymmetric Hydrogenation • In 1968, Knowles modified Wilkinson’s catalyst by using a chiral phosphine ligand. • A chiral catalyst can produce one desired enantiomer over another.
Asymmetric Hydrogenation • A chiral catalyst allows one enantiomer to be formed. • Some chiral catalysts give better enantioselectivity than others.
Asymmetric Hydrogenation • BINAP is a chiral ligand that gives very pronounced enantioselectivity. • For any reaction, stereoselectivity can only be occur if at least one reagent (reactant or catalyst) ischiral.
Question • Which of the alkenes below will produce 2-methylbutane on catalytic hydrogenation? • A) 1 and 3 • B) 1, 2 and 3 • C) 2 and 4 • D) 2, 3 and 4
Heats of Hydrogenation • can be used to measure relative stability of isomeric alkenes • correlation with structure is same as when heats of combustion are measured
Question • Rank the following alkenes in order of decreasing heat of hydrogenation. • A) 1 > 3 > 2 • B) 3 > 2 > 1 • C) 2 > 3 > 1 • D) 1 > 2 > 3
highest heat ofhydrogenation;least stable isomer 126 kJ/mol 118 kJ/mol lowest heat ofhydrogenation;most stable isomer 112 kJ/mol Problem Match each alkene with its correctheat of hydrogenation.
Question • Which alkene has the lowest heat of hydrogenation? • A) B) • C) D)
Two Spatial (stereochemical) Aspects ofAlkene Hydrogenation • (1) syn addition of both H atoms to double bond (adds from the same side) • (2) hydrogenation is stereoselective, corresponding to addition to less crowded face of double bond
