Chapter 3 Alkenes And Alkynes
Alkenes ydrocarbons containing double bonds C C double bond the functional group center of reactivity
Molecular Formula of Alkene Acyclic alkene: C n 2n Cyclic alkene: C n 2n-2 C 3 C 2 =C 2
Systematic Nomenclature of Alkenes 1. Longest continuous chain containing the functional group
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
4. Cite the substituents in alphabetical order 5. Name with the lowest functional group number and then the lowest substituent numbers
6. No numbering of the functional group is needed in a cyclic alkene.
Special Nomenclatures
Conjugated and Isolated Diene
Examples 2 3 1 4 7 6 cyclohepta-1,3,5-triene 5 3 C 5 4 6 3 C 3 1 2 C 3 1,6,6-trimethylcylcohexa-1,4-diene
Structure of Alkene
Cis-Trans Isomerism in Alkenes 3 C C 3 3 C C 3 Cis-2-butene trans-2-butene
Dipole Moments of Alkene Isomers
Conversion of alkene isomers requires breaking of the π bond between the two sp 2 carbons
Examples 5 3 2 4 1 cis-2-pentene 6 5 4 3 2 1 7 trans-3-heptene
Cis-Trans Interconversion in Vision
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
Which of the following alkenes is the most stable one? A B C D
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
Electrophilic Addition Reaction electrophile R R R R R +Y + - Z - R nucleophile Y Z R R Y-Z =( -X, -O, X-X, -OSO3)
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.
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
Nucleophiles Organic molecules with double bonds (alkenes, alkynes) are also nucleophilic. Examples: 3 C C 3 3 C C C
One of the following reacts with AlCl3: A. BF 3 B. N 3 C. Cl D. 3 O +
Electrophilic Addition Reactions Br 2 2 O, + Br Br Br O Br alogenation ydration ydrohalogenation
Electrophilic Addition Reaction 1) additions of Acids ( X) Br cyclopentene Br bromo-cyclopentane
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
Electrophilic Addition Experiment shows that (A) is preferred over (B) Br A Br + B Br The preference for (A) is explained by the mechanism
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
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
I I + I 1-methylcycohexene Major product 1-iodo-1-methylcyclohexane 2-iodo-1-methylcyclohexane Minor product
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.
3.11Reaction Equilibrium: What makes a reaction go?
A Reaction Coordinate Diagram Describes the Reaction Pathway Transition states have partially formed bonds
Exothermic Reaction -ΔG o Endothermic Reaction +ΔG o
Transition State Versus Intermediate intermediate intermediate Transition states have partially formed bonds Intermediates have fully formed bonds
Electrophilic Addition of Br to 2-Butene The rate-limiting step controls the overall rates of the reaction
ENERGY PROFILE 1 st step energy maximum activation energy
ENERGY PROFILE 2 nd step energy maximum activation energy
Electrophilic Addition of Br to 2-Butene The rate-limiting step controls the overall rates of the reaction
Carbocation Stability
d f E a b c e g Reaction Progress
ydration 3 CC 2 C C 2 + 2 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
ydration 3 CC 2 C C 2 + + slow C 2 3 C C C 2 1 st step is electrophilic attack of +
ydration 3 CC 2 C C 2 + 2 O fast C 2 3 C C C 2 O addition of nucleophile protonated alcohol 2 nd step is nucleophilic attack of 2 O
ydration 3 C 2 C C 2 C O protonated alcohol + + 3 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
Addition of Borane ydroboration-oxidation Anti-Markovnikov s rule in product formation
Addition of Water to Alkenes: ydroboration Borane adds to an alkene to give an organoborane This is oxidized to the alcohol by hydrogen peroxide
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
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
Two Possible Orientations: + / 2 O
Addition of ydrogen to Alkenes
Catalytic ydrogenation of an Alkene
ydrogenation 2 / Pt cat
Addition of a halogen to an alkene Mechanism for the addition of bromine to an alkene
Br Br 2 Br Br Br Or Br Br
Addition to Conjugated Dienes A conjugated diene undergoes both 1,2- and 1,4-addition
f the conjugated diene is not symmetrical
Oxidation of Alkene with KMnO4 (ydroxylation) (Purple) (brown-black)
Oxidative Cleavage of Alkenes by Ozonolysis
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
Examples of Ozonolysis of Alkenes Used in determination of structure of an unknown alkene
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 180 0 C C C p x R
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
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 2 5 3 2 C C C C 2 3 4
A substituent receives the lowest number if the same number for the functional group suffix is obtained in both directions
f the two functional groups are a double bond and a triple ond,
If there is a tie between a double bond and a triple bond,
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.
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
he halogens Cl 2 and Br 2 also add to alkynes
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.
Alkynes: Reactions 3 C O C 2 C C O 3 CC 2 The ENOL readily tautomerizes into a ketone.
Examples of Water Addition
Addition of ydrogen Formation of Cis Alkene
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 25 44 >60
Arrange according to Polarity,Acidity,pKa 1 2 3
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