Chapter 3. Alkenes And Alkynes
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1 Chapter 3 Alkenes And Alkynes
2 Alkenes ydrocarbons containing double bonds C C double bond the functional group center of reactivity
3 Molecular Formula of Alkene Acyclic alkene: C n 2n Cyclic alkene: C n 2n-2 C 3 C 2 =C 2
4 Systematic Nomenclature of Alkenes 1. Longest continuous chain containing the functional group
5 2. For a compound with two double bonds, the suffix is diene 3. When there are both a functional group and a substituent, the functional group gets the lowest number
6 4. Cite the substituents in alphabetical order 5. Name with the lowest functional group number and then the lowest substituent numbers
7 6. No numbering of the functional group is needed in a cyclic alkene.
8 Special Nomenclatures
9 Conjugated and Isolated Diene
10 Examples cyclohepta-1,3,5-triene 5 3 C C C 3 1,6,6-trimethylcylcohexa-1,4-diene
11 Structure of Alkene
12 Cis-Trans Isomerism in Alkenes 3 C C 3 3 C C 3 Cis-2-butene trans-2-butene
13 Dipole Moments of Alkene Isomers
14 Conversion of alkene isomers requires breaking of the π bond between the two sp 2 carbons
15 Examples cis-2-pentene trans-3-heptene
16 Cis-Trans Interconversion in Vision
17 Stability The stability of alkenes depends upon number of substituents R R R R R < < < R R R R R The more substituents, the more stable
18 Which of the following alkenes is the most stable one? A B C D
19 Addition And substitution Reactions Typical for unsaturated systems is the addition reaction: A+B C 2 C C C 2 C 3 C 3 Br(aq) 2 C C 2 C Br C 3 C 3 Substitution reactions: A-B + C-D A-D + C-B
20 Electrophilic Addition Reaction electrophile R R R R R +Y + - Z - R nucleophile Y Z R R Y-Z =( -X, -O, X-X, -OSO3)
21 Reactions Electrophiles (Lewis acid): molecules that contain atoms with empty orbitals, which can accommodate electrons. Typically, these are positively charged. Examples: C 3 C 2 BF 3 Boron has only 6 valence electrons. BF 3 is a Lewis acid.
22 Nucleophiles Nucleophiles (Lewis base): molecules that contain atoms with lone pairs, which can donate electrons. Often these are negatively charged. Almost all the time they contain elements from groups V VII of the periodic table, since those have lone pairs. Examples: O Cl C 3 N 2 2 O
23 Nucleophiles Organic molecules with double bonds (alkenes, alkynes) are also nucleophilic. Examples: 3 C C 3 3 C C C
24 One of the following reacts with AlCl3: A. BF 3 B. N 3 C. Cl D. 3 O +
25 Electrophilic Addition Reactions Br 2 2 O, + Br Br Br O Br alogenation ydration ydrohalogenation
26 Electrophilic Addition Reaction 1) additions of Acids ( X) Br cyclopentene Br bromo-cyclopentane
27 In case the olefin has different, or different number of substituents, two products are possible. Br A Br Is there a preference for one product? Which is the preferred product? + B Br 1-bromo-1-methylcyclopentane 1-bromomethylcyclopentane
28 Electrophilic Addition Experiment shows that (A) is preferred over (B) Br A Br + B Br The preference for (A) is explained by the mechanism
29 Mechanism Br slow A C 2 + B C 2 We could obtain two structurally different Carbocations; however, A is formed much faster. C 2 Br - fast Br
30 Comparing many reactions with different alkenes Leads to a classification of carbocation intermediates. R R R > > > R R R The more substituents, the more stable the cation
31 I I + I 1-methylcycohexene Major product 1-iodo-1-methylcyclohexane 2-iodo-1-methylcyclohexane Minor product
32 Markovnikov Rule The regioselectivity of this reaction was first observed by Markovnikov. e found that the hydrogen, which is the electrophile, adds to the carbon that contains the larger number of hydrogens.
33 3.11Reaction Equilibrium: What makes a reaction go?
34 A Reaction Coordinate Diagram Describes the Reaction Pathway Transition states have partially formed bonds
35 Exothermic Reaction -ΔG o Endothermic Reaction +ΔG o
36 Transition State Versus Intermediate intermediate intermediate Transition states have partially formed bonds Intermediates have fully formed bonds
37 Electrophilic Addition of Br to 2-Butene The rate-limiting step controls the overall rates of the reaction
38 ENERGY PROFILE 1 st step energy maximum activation energy
39 ENERGY PROFILE 2 nd step energy maximum activation energy
40 Electrophilic Addition of Br to 2-Butene The rate-limiting step controls the overall rates of the reaction
41 Carbocation Stability
42 d f E a b c e g Reaction Progress
43 ydration 3 CC 2 C C O Cl, trace 3 CC 2 C C 2 1-butene 2-butanol When small amounts of acid are present, hydration of the double bond takes place. O
44 ydration 3 CC 2 C C slow C 2 3 C C C 2 1 st step is electrophilic attack of +
45 ydration 3 CC 2 C C O fast C 2 3 C C C 2 O addition of nucleophile protonated alcohol 2 nd step is nucleophilic attack of 2 O
46 ydration 3 C 2 C C 2 C O protonated alcohol rd step is deprotonation to generate the alcohol. + is regenerated, 2 C 3 C C 2 C so that only catalytic amounts of acid are necessary. O
47 Addition of Borane ydroboration-oxidation Anti-Markovnikov s rule in product formation
48 Addition of Water to Alkenes: ydroboration Borane adds to an alkene to give an organoborane This is oxidized to the alcohol by hydrogen peroxide
49 ydroboration-oxidation Forms an Alcohol from an Alkene Addition of -B 2 (from B 3 -TF complex) to three alkenes gives a trialkylborane Oxidation with alkaline hydrogen peroxide in water produces the alcohol derived from the alkene
50 Orientation in ydration via ydroboration Regiochemistry is opposite to Markovnikov orientation O is added to carbon with most s and O add with syn stereochemistry, to the same face of the alkene (opposite of anti addition) 50
51 Two Possible Orientations: + / 2 O
52 Addition of ydrogen to Alkenes
53 Catalytic ydrogenation of an Alkene
54 ydrogenation 2 / Pt cat
55 Addition of a halogen to an alkene Mechanism for the addition of bromine to an alkene
56 Br Br 2 Br Br Br Or Br Br
57 Addition to Conjugated Dienes A conjugated diene undergoes both 1,2- and 1,4-addition
58
59 f the conjugated diene is not symmetrical
60 Oxidation of Alkene with KMnO4 (ydroxylation) (Purple) (brown-black)
61 Oxidative Cleavage of Alkenes by Ozonolysis
62 The alkene and ozone undergo a concerted cycloaddition Mechanism of ozonide formation The molozonide is unstable because it has two O O bonds Ozonide is stable
63 Examples of Ozonolysis of Alkenes Used in determination of structure of an unknown alkene
64 Alkynes Alkynes are molecules containing a triple bond: R The triple bond consists of one σ bond R R p y sp-hybridized carbon R and two π bonds: p y R C C C C p x R
65 Alkynes: Nomenclature Systematic names of alkynes are derived from alkanes by replacing the ending ane with yne. C C 3 C C 2 C C C 3 3 C C C C 2 ethyne 1-butyne 2-pentyne
66 Alkynes: Nomenclature What is the name of the following compound? C 3 3 C 2 C C C C 2 1. Find longest chain. 2. Enumerate such that substituents obtain lowest number. 3. Determine name of substituents. 3 C methyl 1-bromo-2-methyl-3-hexyne C 2 Br 1 Bromo C 2 Br 6 C C C C C 2 3 4
67 A substituent receives the lowest number if the same number for the functional group suffix is obtained in both directions
68 f the two functional groups are a double bond and a triple ond,
69 If there is a tie between a double bond and a triple bond,
70 Alkynes: Reactions Alkynes are electron-rich molecules, thus also electrophilic addition takes place. δ+ δ Br 3 C C C C 3 2-butyne 3 C C 3 Br After the addition of the nucleophile we obtain an alkene that still can be attacked by an electrophile.
71 Alkynes: Reactions Terminal alkynes follow Markovnikov s rule: 3 C C 2 C C Cl excess 3 C Cl C 2 C C Cl Cl C 3 CC 2 C The stability of the resulting vinylic cations differs. 3 C C 2 C C > 3 C C 2 C C
72 he halogens Cl 2 and Br 2 also add to alkynes
73 Alkynes: Reactions Yet another situation arises during the addition of water. 3 C C 2 C C 2 O 3 C O C 2 C C After the addition of 1 equivalent of 2 O an ENOL is formed.
74 Alkynes: Reactions 3 C O C 2 C C O 3 CC 2 The ENOL readily tautomerizes into a ketone.
75 Examples of Water Addition
76 Addition of ydrogen Formation of Cis Alkene
77 Acidity The electronegativity of carbon varies with the level of ybridization. sp > sp 2 > sp 3 C C 2 C C 2 3 C C 3 pk a >60
78 Arrange according to Polarity,Acidity,pKa 1 2 3
79 Acidity In aqueous solutions none of the protons can be removed, pk a ( 2 O) = 14. C C + 2 O C C + 3 O owever, with strong bases such as N 2, a proton can be removed from ethyne. C C + N 2 C C + 3 N
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