Post on 06-Sep-2018
Ch.6 Alkenes: Structure and Reactivity
H2C CH2
Ethylene
CH3
α-Pinene
β-Carotene(orange pigment and vitamin A precursor)
alkene = olefin
Ch.6 Alkenes: Structure and Reactivity
6.1 Industrial Preparation and Use of Alkenes
H2C CH2
Ethylene(26 million tons / yr)
CH3CH2OHCH3CHOCH3COOHHOCH2CH2OHClCH2CH2ClH2C=CHCl
EthanolAcetaldehydeAcetic acidEthylene glycolEthylene dichlorideVinyl chloride
O
O
OEthylene oxide
Vinyl acetate
Polyethylene
Compounds derived industrially from ethylene
Ch.6 Alkenes: Structure and Reactivity
Compounds derived industrially from propylene
CH CH2
Propylene(14 million tons / yr)
O
Cumene
Polypropylene
H3C
CH3
H3C CH3
OH
CH3
CH3
Isopropyl alcohol
Propylene oxide
Ch.6 Alkenes: Structure and Reactivity
• Ethylene, propylene, and butene are synthesized industrially by thermal cracking of natural gas (C1-C4 alkanes) and straight-run gasoline (C4-C8
alkanes).
CH3(CH2)nCH3850-900oC
steamH2 + CH4 + H2C=CH2 + CH3CH=CH2
+ CH3CH2CH=CH2
- the exact processes are complex; involve radical process
900oCCH3CH2 CH2CH3 H2C CH
H2 2 H2C=CH2 + H2
Ch.6 Alkenes: Structure and Reactivity
• Thermal cracking is an example of a reaction whose energetics are dominated by entropy (∆So) rather than enthalpy (∆Ho) in the free-energy equation (∆Go = ∆Ho - T∆So) .; C-C bond cleavage (positive ∆Ho) ; high T and increased number of molecules → larger T∆So
Ch.6 Alkenes: Structure and Reactivity
6.2 Calculating Degree of Unsaturation
unsaturated: formula of alkene CnH2n
; formula of alkane CnH2n+2
degree of unsaturation: the number of rings and/or multiple bonds
in general, each ring or double bond corresponds to a loss of two hydrogens from alkane formula
Ch.6 Alkenes: Structure and Reactivity
unknown hydrocarbon with molecular weight 82; C6H10
corresponding alkane; C6H14
H14-H10 = H4 = 2H2
therefore, degree of unsaturation= 2
possible structures:
Ch.6 Alkenes: Structure and Reactivity
BrCH2CH=CHCH2Br HCH2CH=CHCH2H
C4H6Br2 = "C4H8" one unsaturation: one double bond or one cycle
add
degree of unsaturation: containing elements other than just C, H
■Organohalogen compounds (C, H, X, X= F, Cl, Br, I)Add the number of halogens to the number of hydrogens; a halogen is simply a replacement of hydrogen
Ch.6 Alkenes: Structure and Reactivity
H2C=CHCH=CHCH2OH
C5H8O= "C5H8" two unsaturation: two double bonds
H2C=CHCH=CHCH2-H
■Organooxygen compounds (C, H, O)Ignore the number of oxygens; oxygen forms two bonds; C-C vs C-O-C or C-H vs C-O-H
Ch.6 Alkenes: Structure and Reactivity
C5H9N= "C5H8" two unsaturation: one double bond and one ring
H
NH2
H
H
■Organonitrogen compounds (C, H, N)Subtract the number of nitrogens from the number of hydrogens; nitrogen forms three bonds; C-C vs C-NH-C or C-H vs C-NH2
Ch.6 Alkenes: Structure and Reactivity
6.3 Naming Alkenes
pentene hexene
NOT
Name the parent hydrocarbon: Find the longest carbon chain containing the double bond and name the compound accordingly, using the suffix -ene:
Step 1
Ch.6 Alkenes: Structure and Reactivity
1
2
36
Numbering: Begin at the end nearer the double bond or, if the double bond is equivalent from the two ends, begin at the end nearer the first branch point. This rule ensures that the double bond carbons receive the lowest possible numbers:
Step 2
NOT64
31
1
4
36
2 5
NOT64
31
1
2
36
25
Ch.6 Alkenes: Structure and Reactivity
2-Hexene
1 212
3
3
2-Methyl-3-hexene
12
2-Ethyl-1-pentene
12
34
2-Methyl-1,3-butadiene
Write the full name: list substituents alphabetically; indicate the position of double bond (the number of the first alkene carbon) immediately before the parent name; more than one double bonds: -diene, triene...
Step 3
Ch.6 Alkenes: Structure and Reactivity
CH31
2
1
24
5
1-Methylcyclohexene 1,4-Cyclohexadiene
cycloalkanes are named similarly, but double bond is between C1 and C2 and the first substituent has as low a number as possible; it's not necessary to indicate the position of the double bond in the name (always C1 and C2)
CH3
CH31
2
5
1,5-Dimethylcyclopentene
CH3
CH32
1
3
NOT
Ch.6 Alkenes: Structure and Reactivity
Ethene Ethylene
Propene Propylene
2-Methylpropene Isobutylene
2-Methyl-1,3-butadiene Isoprene
1,3-Pentadiene Piperylene
IUPAC name Common nameCommon names
Ch.6 Alkenes: Structure and Reactivity
Substituent Names
H2C H2CHC
A methylene group A vinyl group An allyl group
Br
Vinyl bromide
Br
Allyl bromide
CH2
Μethylenecyclopentane
Ch.6 Alkenes: Structure and Reactivity
6.4 Electronic Structure of Alkenes
• Rotation around double bond is restricted: The π-bond must break for rotation to take place around a C=C double bond - 268 kJ/mol (64 kcal/mol) is required to break the π-bond- rotational energy barrier for ethane: only 12 kJ/mol
C
C
C
C90o
rotation
π-bond(p-orbitals are parallel)
broken π-bond after rotation(p-orbitals are perpendicular)
Ch.6 Alkenes: Structure and Reactivity
6.5 Cis-Trans Isomerism in Alkenes
cis-trans isomerism: when both carbons are bonded to two different groups
H3C CH3
H H
H3C H
H CH3
cis-2-Butene trans-2-Butene
X
A D
B D
B D
A Dthese two compounds are identical;they are not cis-trans isomers
A D
B E
B D
A E
these two compounds are not identical;they are cis-trans isomers
Ch.6 Alkenes: Structure and Reactivity
6.6 Sequence Rules: The E,Z Designation
cis-trans isomerism: describe the disubstituted double bond geometries; tri-and tetrasubstituted double bonds- a general method is needed
H3C CH3
H CH2CH2CH3
H3C CH2CH2CH3
H CH3
cis or trans ? cis or trans ?
Ch.6 Alkenes: Structure and Reactivity
E, Z isomerism: a more general method for describing double-bond geometry; E (entgegen, "opposite"); Z (zusammen, "together")
High High
Low Low
Z
High Low
Low High
E
the higher priority groups on each carbon are on the same side of the double bond
the higher priority groups on each carbon are on the oppositeside of the double bond
Ch.6 Alkenes: Structure and Reactivity
Sequence Rule (Cahn-Ingold-Prelog rule; CIP rule); priority of substituents
Br > Cl > O > N > C > H
35 17 8 7 6 1
Considering each of the double-bond carbons separately, identify the two atoms directly attached and rank them according to atomic number.
Cl CH3
H3C H
(Z)-2-Chloro-2-butene
Cl H
H3C CH3
(E)-2-Chloro-2-butene
Rule 1
Ch.6 Alkenes: Structure and Reactivity
If a decision can't be reached by ranking the first atoms in thesubstituents, look at the second, third, or fourth atoms away from the double-bond carbons until the first difference is found.
CH
HH C
H
HCH3< O H O CH3<
CH
HCH3 C
CH3
HCH3< C NH2 C Cl<
H H
CH3 H
Rule 2
Ch.6 Alkenes: Structure and Reactivity
Multiple-bonded atoms are equivalent to the same number of single-bonded atoms.
CH
O CH
OOC
Rule 3
CH
C CH
CC
H
H
H
CH
C C CC
CCH
C
CH
Ch.6 Alkenes: Structure and Reactivity
CH3H3C
H
H3C
H OH
OOH
(E)-3-Methyl-1,3-pentadiene
(E)-1-Bromo-2-isopropyl-1,3-butadieneBr
H
(Z)-2-Hydroxymethyl-2-butenoic acid
Ch.6 Alkenes: Structure and Reactivity
6.7 Stability of Alkenes
Relative stability from equilibrium constant:- cis-trans isomers interconvert under strong acid condition
H3C CH3
H H
H3C H
H CH3
cis (24 %) trans (76%)
acid
catalyst
Erel= + 2.8 kJ/mol (0.66 kcal/mol) Erel= 0.0 kcal/mol
Ch.6 Alkenes: Structure and Reactivity
C C
H H
cis trans
H
HH H H
H C
CH
HH
HH
HH
H
Ch.6 Alkenes: Structure and Reactivity
From heat of combustion
H3C CH3
H H
H3C H
H CH3
∆Hocombustion= -2685.5 kJ/mol ∆Ho
combustion= -2682.2 kJ/mol
Erel = +3.3 kJ/mol Erel = +0.0 kJ/mol
Ch.6 Alkenes: Structure and Reactivity
From heat of hydrogenation
H3C CH3
H H
H3C H
H CH3
∆Hohydro = -120 kJ/mol
Pd
H2CH3CH2CH2CH3 Pd
H2
∆Hohydro = -116 kJ/mol
4 kJ/mol difference
Ch.6 Alkenes: Structure and Reactivity
Cis
Trans
Butane
∆Gotrans
∆Gocis
Ener
gy
Reaction progress
Energy profile for hydrogenation
Ch.6 Alkenes: Structure and Reactivity
Stabilities of alkenes: increasing the degree of substitution leads further stabilization
R R
R R>
R R
R H>
H R
R H~
R H
R H>
R H
H H
tetrasubstituted trisubstituted disubstituted monosubstituted
Ch.6 Alkenes: Structure and Reactivity
1. Hyperconjugation: a stabilizing interaction between the unfilled antibonding C=C p bond and a filled C-H s bond orbital on a neighboring substituent. The more substutuents that are present, the more opportunities exist for hyperconjugation, and the more stable the alkene.
C CC
H
bonding C-H σ orbital (filled)
antibonding C-C π orbital (unfilled)
Explanations of alkene stabilities
π*σ
Ch.6 Alkenes: Structure and Reactivity
2. Bond strength: sp2-sp3 C-C bond is stronger than sp3-sp3 C-C bond; more highly substituted alkenes always have a higher ratio of sp2-sp3
bonds to sp3-sp3 bonds
CH CH CH3CH3 CH2 CH CH2CH3
sp3-sp2 sp3-sp2 sp3-sp2
sp3-sp3
Ch.6 Alkenes: Structure and Reactivity
6.8 Electrophilic Addition of HX to Alkenes
• alkenes: electron rich, nucleophilic
Electrophilic addition reaction: addition of electrophiles to nucleophilic alkenes
H3C
H3C H
H H3CH3C
HH
H
Br-
H3CH3C H
H
HBr
carbocation intermediate
H Br
The electrophile HBr is attacked by the p-electrons of the double bond, and a new C-H σ-bond is formed. This leaves the other carbon atom with a + charge and a vacant p orbital
The Br- donates an electron pair to the positively charged carbon atom, forming a C-Br σ-bond and yielding the neutral addition product.
Ch.6 Alkenes: Structure and Reactivity
Reaction energy diagram for the two-step electrophilic addition of HBr to 2-methylpropene.
Br
C CH2
Ener
gy
Reaction progress
reactants ∆Go
∆G1
+ HBr
∆G2
TS1TS2
H3CH3C
C CH3H3CH3C
C CH3H3CH3C
Br
carbocation intermediate
∆G1 > ∆G2
The first step is slower than the second step.
Ch.6 Alkenes: Structure and Reactivity
Writing Organic Reactions
A + B C
both reactants (A and B) are equally emphasized
R CH3R
ClHCl
Et2O, 25oC
solvents, temperature and other reaction conditions are written either above or belowthe reaction arrow
R CH3R
Cl+ HCl
Ether
25oC
AB
C
reactants A is of greater interest than B
Ch.6 Alkenes: Structure and Reactivity
Electrophilic addition of HX: HCl, HBr, HI (HI is generated in the reaction mixture)
Cl+ HCl
Ether
2-Methylpropene 2-Chloro-2-methylpropane(94 %)
KIH3PO4
I
1-Pentene 2-Iodopentane
Ch.6 Alkenes: Structure and Reactivity
6.9 Orientation of Electrophilic Addition: Markovnikov's Rule
regiospeccific: only one of two possible orietation of additions occurs
Cl+ HCl
Cl+
NOT formedsole product
regioselective: one of two possible orientation of additions preferred
Ch.6 Alkenes: Structure and Reactivity
Markovnikov's rule: In the addition of HX to an alkene, the H attaches to the carbon with fewer alkyl substituents and the X attaches to the carbon with more alkyl substituents.
Cl+ HCl
sole productmore substituted carbon
less substituted carbon
H
more substituted carbon
CH3
+ HBr
CH3
Br
H
Ether
Ch.6 Alkenes: Structure and Reactivity
When both ends of the double bond have the same degree of substitution, a mixture of products formed.
+ HBr
Br
Br+
Ch.6 Alkenes: Structure and Reactivity
Interpretation of Markovnikov's rule: In the addition of HX to an alkene, the more highly substituted carbocation is formed as the intermediate
+ HCl
H
H
H
H
Cl-
Cl-
Cl
Cl
3o carbocation
1o carbocationNOT formed
X
Ch.6 Alkenes: Structure and Reactivity
Why should this be?
+
Br-
Br-
3o carbocation
2o carbocation NOT formed
CH3HBr
CH3
H
CH3H
CH3
H
CH3H
Br
Br
X
Ch.6 Alkenes: Structure and Reactivity
6.10 Carbocation Structure and Stability
Carbocation: planar, sp2-hybridized, electron deficient
C RR
R
sp2
120o
vacant p orbital
Ch.6 Alkenes: Structure and Reactivity
Stability of carbocation: measure the amount of energy required to form the carbocation from its alkyl halide: R-X → R+ + :X-
CH3Cl CH3 + Cl
CH3 + e-CH3
Cl-Cl + e-
D = 351 kJ/mol (Bond dissociation energy)
Ei = 948 kJ/mol (Ionization energy)
Eea= -348 kJ/mol (Electron Affinity)
CH3Cl +CH3 Cl- 951 kJ/mol (Dissociation enthalpy)
Ch.6 Alkenes: Structure and Reactivity
→ tertiary halides dissociate to give carbocation much more easily than secondary or primary halides; tertiary carbocations are more stable than secondary or primary ones
Dissociation enthalpy:
1o alkyl halide > 2o alkyl halide > 3o alkyl halideMethyl halide >
Carbocation stability:
< <<CH
HH C
H
HR C
R
HR C
R
RR
1o 2o 3o
Ch.6 Alkenes: Structure and Reactivity
Why?
1. Inductive effect: result from the shifting of electrons in a σ-bond in response to the electronegativity of nearby atom; Electrons from a relatively large and polarizable alkyl group can shift toward a neighboring positive charge more easily than the electron from a hydrogen. ; alkyl grpups donates electrons inductively and stabilize carbocations
inductive effect of alkyl groups:
1o 2o 3o
C+
R
R
R
C+
H
R
R
C+
H
R
H
C+
H
H
H
methyl
Ch.6 Alkenes: Structure and Reactivity
2. Hyperconjugation: interaction of nearby C-H σ-rebital with the vacant carbocation p orbital stabilizes the cation and lowers its energy
C C
bonding C-H σ orbital (filled)
empty p orbital (unfilled)
stabilization carbocation through hyperconjugation
H
Ch.6 Alkenes: Structure and Reactivity
Two different C-H bonds in t-Butyl cation:
C CH
H
H
H3C
H3C
2 C-H σ orbitals are in plane:perpendicular to the cation p orbital(no hyperconjugation)
(CH3)3C+
6 C-H σ orbitals above and below the plane:nearly parallel to the cation p orbital(hyperconjugation)
H
H HH
HHH
H
H
H
HH
staggered conformation eclipsed conformation
Ch.6 Alkenes: Structure and Reactivity
H
H H
H
HH
σ
σ*
H
H
H
H
HH
stabilizing hyperconjugation
destabilizing torsional strain
Ch.6 Alkenes: Structure and Reactivity
6.11 The Hammond Postulate
Summary about electrophilic addition:
■ Electrophilic addition to an unsymmetrically substituted alkene gives the more highly substituted carbocation. A more highly substituted carbocationforms faster than a less highly substituted one and, once formed, rapidly goes to give the final product.
■ A more highly substituted carbocation is more stable than a less highly substituted one.
- How are these two points (stability and rate) related?- Why does the stability of the carbocation intermediate affect the rate at which it's formed and thereby determine the structure of the final product?
Ch.6 Alkenes: Structure and Reactivity
slowerreaction
less stableintermediate
more stableintermediate
faster eaction
slowerreaction less stable
intermediate
more stableintermediate
faster eaction
reaction rate ~ activation energy (∆G‡)stability ~ ∆Go
Ch.6 Alkenes: Structure and Reactivity
Hammond Postulate
“The geometry of the transition state for a step most closely resembles the side (i.e. reactant or product) to which it is closer in energy.”
reactant
product
transition state
product-like TS
reactant
product
transition state
reactant-like TS
Ch.6 Alkenes: Structure and Reactivity
The hypothetical structure of a transition state for alkene protonation.
TS1carbocation intermediate
alkene
The transition state is closer in both energy and structure to the carbocationthan to the alkene. Thus, an increase in carbocation stability (lower ∆Go) also causes an increase in transition state stability (lower ∆G‡), therefore, increase the reaction rate.
Ch.6 Alkenes: Structure and Reactivity
RR
RR
H Br
product-like transition state
δ+
δ+ δ-
R RR R HBr
RR
RR
H
carbocationalkene
Ch.6 Alkenes: Structure and Reactivity
(CH3)2C CH2
Ener
gy
Reaction progress
∆Go
∆Gprim
+ HCl
tertiary TS
(CH3)3C+Cl-
(CH3)2CHCH2+Cl-
(CH3)3CCl
(CH3)2CHCH2Cl
∆Gtert
primary TS
More stable carbocations form faster because their stability is reflected in the transition state leading to them.
Ch.6 Alkenes: Structure and Reactivity
6.12 Evidence for the Mechanism of Electrophilic Addition: Carbocation Rearrangement
How do we know that the carbocation mechanism for addition of HX to alkenes is correct?
The answer is we never know the correct mechanism entirely proven. The best we can do is to show that a proposed mechanism is consistent with all known facts.
Ch.6 Alkenes: Structure and Reactivity
The evidence of two step, carbocation mechanism: structural rearrangements
H3C
CH3
H+ HCl H3C
CH3
HCl
H H3C
CH3
ClH
H+
~50 % ~50 %How is it formed ?
It is difficult to explain it with one step mechanism.
Ch.6 Alkenes: Structure and Reactivity
• two step mechanism can explain the rearrangements:( F.C. Whitmore, 1930s)
H3C
CH3
H+ HCl
H3C
CH3
HCl
H H3C
CH3
ClH
H
H3C
CH3
HH H3C
CH3
H
H
Hydrideshift
2o carbocation 3o carbocation
Cl- Cl-
- involve hydride shift: rearrangement of adjacent hydride ion (H-) to form more stable carbocation
Ch.6 Alkenes: Structure and Reactivity
• alkyl group can also rearrange with its electron pair
H3C
CH3
CH3+ HCl
H3CCH3
H3CCl
H H3C
CH3
ClCH3
H
H3C
CH3
H3CH H3C
CH3
CH3
H
methylshift
2o carbocation 3o carbocation
Cl- Cl-
Rearrangements of carbocations are common.a group (:H- or :CH3
-) moves to an adjacent positively charged carbon;- taking its bonding electron pair with it- a less stable carbocation rearranges to a more stable ion
Myrcene(oil of bay)
α-Pinene(turpentine)
O
Carvone(spearmint oil)
Terpenes: Naturally Occuring AlkenesChemistry @ Work
Essential oils: fragrant mixtures of liquids from plant materialsTerpenes: plant essential oils consist largely of mixtures of compounds
Isoprene rule: head-to-tail joining of five-carbon isoprene
IsopreneHead
Tail
Myrecene
Lanosterol, a triterpene (C30)- precusors for steroid hormons
CH3
CH3
HOH
CH3
H
Terpenes: Naturally Occuring AlkenesChemistry @ Work
Monoterpene: 10 carbons (two isoprene units)Sesquiterpene: 15 carbons (three isoprene units)Diterpene: 20 carbons (two isoprene units)Monoterpenes and sesquiterpenes are found primarily in plants, but the higher terpenes occur in both plants and animals.
Isopentenyl diphosphateO
PO
POH
O-
O O
O-
Dimethylallyl diphosphateO
PO
POH
O-
O O
O-
H3CC
OH
O
Acetic acid
Ture biological precursor of terpenes is not isoprene itselt but iospreneequivalents made from acetic acid.