Ch.6 Alkenes: Structure and Reactivity alkene = olefin
H2CCH2 CH3
Ethylene α-Pinene
β-Carotene (orange pigment and vitamin A precursor) Ch.6 Alkenes: Structure and Reactivity 6.1 Industrial Preparation and Use of Alkenes
Compounds derived industrially from ethylene
CH3CH2OH Ethanol CH3CHO Acetaldehyde CH3COOH Acetic acid HOCH2CH2OH Ethylene glycol ClCH2CH2Cl Ethylene dichloride H C=CHCl Vinyl chloride H2CCH2 2 O Ethylene oxide Ethylene (26 million tons / yr) O Vinyl acetate O
Polyethylene Ch.6 Alkenes: Structure and Reactivity
Compounds derived industrially from propylene
OH Isopropyl alcohol H3CCH3 O Propylene oxide CH3 H3CCH CH2 Propylene Cumene (14 million tons / yr)
CH3 CH3 Polypropylene 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).
850-900oC CH (CH ) CH H + CH + H C=CH + CH CH=CH 3 2 n 3 steam 2 4 2 2 3 2
+ CH3CH2CH=CH2
- the exact processes are complex; involve radical process
H 900oC CH3CH2 CH2CH3 22H2CCH 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
in general, each ring or double bond corresponds to a loss of two hydrogens from alkane formula
degree of unsaturation: the number of rings and/or multiple bonds 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
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
BrCH2CH=CHCH2Br HCH2CH=CHCH2H
C4H6Br2 = "C4H8" one unsaturation: one double bond or one cycle add Ch.6 Alkenes: Structure and Reactivity
■ 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
H2C=CHCH=CHCH2OH H2C=CHCH=CHCH2-H
C5H8O= "C5H8" two unsaturation: two double bonds Ch.6 Alkenes: Structure and Reactivity
■ 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
H H
NH2 H
C5H9N= "C5H8" two unsaturation: one double bond and one ring Ch.6 Alkenes: Structure and Reactivity 6.3 Naming Alkenes
Step 1 Name the parent hydrocarbon: Find the longest carbon chain containing the double bond and name the compound accordingly, using the suffix -ene:
NOT
pentene hexene Ch.6 Alkenes: Structure and Reactivity
Step 2 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: 2 6 3 1 3 4 1 NOT 6 2 5 4 6 3 1
2 3 6 NOT 1 2 5 3 1 4 6 Ch.6 Alkenes: Structure and Reactivity
Step 3 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... 2 3 1 1 2 3 2-Hexene 2-Methyl-3-hexene
2 1 2 4 1 3
2-Ethyl-1-pentene 2-Methyl-1,3-butadiene Ch.6 Alkenes: Structure and Reactivity 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) 1 CH3 5 1
2 4 2
1-Methylcyclohexene 1,4-Cyclohexadiene
CH3 5 CH3 1 3 2 CH3 CH3 2 1 1,5-Dimethylcyclopentene NOT Ch.6 Alkenes: Structure and Reactivity Common names IUPAC name Common name
Ethene Ethylene
Propene Propylene
2-Methylpropene Isobutylene
2-Methyl-1,3-butadiene Isoprene
1,3-Pentadiene Piperylene Ch.6 Alkenes: Structure and Reactivity
Substituent Names H H2C H2C C
A methylene group A vinyl group An allyl group
Br CH2 Br
Μethylenecyclopentane Vinyl bromide Allyl bromide 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 90o C rotation C
π-bond broken π-bond after rotation (p-orbitals are parallel) (p-orbitals are perpendicular) Ch.6 Alkenes: Structure and Reactivity 6.5 Cis-Trans Isomerism in Alkenes
H CCH H3CH 3 3 X HH HCH3
cis-2-Butene trans-2-Butene cis-trans isomerism: when both carbons are bonded to two different groups
BD AD these two compounds are identical; BD ADthey are not cis-trans isomers
AD BDthese two compounds are not identical; they are cis-trans isomers BE A E 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
H CCHCH CH H3CCH3 3 2 2 3 HCH HCH2CH2CH3 3
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 High Low
Low Low Low High
Z E the higher priority groups on the higher priority groups on each carbon are on the same each carbon are on the opposite side of the double bond side of the double bond Ch.6 Alkenes: Structure and Reactivity
Sequence Rule (Cahn-Ingold-Prelog rule; CIP rule) ; priority of substituents
Rule 1 Considering each of the double-bond carbons separately, identify the two atoms directly attached and rank them according to atomic number. 35 17 8 7 6 1 Br > Cl > O > N > C > H
Cl H Cl CH3 H C CH H3CH 3 3 (Z)-2-Chloro-2-butene (E)-2-Chloro-2-butene Ch.6 Alkenes: Structure and Reactivity
Rule 2 If a decision can't be reached by ranking the first atoms in the substituents, look at the second, third, or fourth atoms away from the double-bond carbons until the first difference is found.
H H C H < C CH3 O H < O CH3 H H
H CH3 CH3 H C CH3 < C CH3 C NH2 < C Cl H H H H Ch.6 Alkenes: Structure and Reactivity
Rule 3 Multiple-bonded atoms are equivalent to the same number of single- bonded atoms.
H H C O C O O C
H H H H C C C C H H C C
C C C C H C C H C C Ch.6 Alkenes: Structure and Reactivity
H (E)-3-Methyl-1,3-pentadiene H3C CH3
Br (E)-1-Bromo-2-isopropyl-1,3-butadiene H
O H C OH 3 (Z)-2-Hydroxymethyl-2-butenoic acid HOH Ch.6 Alkenes: Structure and Reactivity
6.7 Stability of Alkenes
Relative stability from equilibrium constant: - cis-trans isomers interconvert under strong acid condition
acid H3CCH3 H3C H
HH catalyst H CH3
cis (24 %) trans (76%)
Erel= + 2.8 kJ/mol (0.66 kcal/mol) Erel= 0.0 kcal/mol Ch.6 Alkenes: Structure and Reactivity
H H H H H H H H C H CCH H H HH C H H cis trans Ch.6 Alkenes: Structure and Reactivity
From heat of combustion
H3CCH3 H3CH
HH HCH3
o o ∆H combustion= -2685.5 kJ/mol ∆H combustion= -2682.2 kJ/mol E = +0.0 kJ/mol Erel = +3.3 kJ/mol rel Ch.6 Alkenes: Structure and Reactivity
From heat of hydrogenation
H3CH H3CCH3 H2 H2 CH3CH2CH2CH3 HH Pd Pd HCH3
o o ∆H hydro = -116 kJ/mol ∆H hydro = -120 kJ/mol 4 kJ/mol difference Ch.6 Alkenes: Structure and Reactivity
Energy profile for hydrogenation
Cis Energy Trans o o ∆G cis ∆G trans
Butane
Reaction progress Ch.6 Alkenes: Structure and Reactivity
Stabilities of alkenes: increasing the degree of substitution leads further stabilization
RR RR H R R H R H > > ~ > RR R H R H R H H H tetrasubstituted trisubstituted disubstituted monosubstituted Ch.6 Alkenes: Structure and Reactivity
Explanations of alkene stabilities
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.
bonding C-H σ orbital (filled)
π* H CC σ C
antibonding C-C π orbital (unfilled) 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
sp3-sp3
CH3 CH CH CH3 CH3 CH2 CH CH2
3 2 sp3-sp2 sp3-sp2 sp -sp 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 Br- H Br H Br H H3CH H C 3 H C H 3 H H C H H3C H3C H 3 H carbocation intermediate
The electrophile HBr is attacked by the The Br- donates an electron pair to the p-electrons of the double bond, and a positively charged carbon atom, new C-H σ-bond is formed. This leaves forming a C-Br σ-bond and yielding the other carbon atom with a + charge the neutral addition product. and a vacant p orbital Ch.6 Alkenes: Structure and Reactivity
Reaction energy diagram for the two-step electrophilic addition of HBr to 2-methylpropene.
carbocation intermediate
TS1 TS2 ∆G2 ∆G1 > ∆G2 H3C CCH3 H3C ∆G1 Br The first step is slower than the second step. Energy
reactants ∆Go H3C CCH2 + HBr H C Br 3 H3C CCH3 H3C Reaction progress Ch.6 Alkenes: Structure and Reactivity
Writing Organic Reactions
A + BC both reactants (A and B) are equally emphasized
Ether Cl + HCl o R 25 C R CH3
B A C
reactants A is of greater interest than B
HCl Cl R o Et2O, 25 C R CH3
solvents, temperature and other reaction conditions are written either above or below the reaction arrow Ch.6 Alkenes: Structure and Reactivity
Electrophilic addition of HX: HCl, HBr, HI (HI is generated in the reaction mixture)
Ether Cl + HCl
(94 %) 2-Methylpropene 2-Chloro-2-methylpropane
I KI
H3PO4 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
regioselective: one of two possible orientation of additions preferred
Cl Cl + HCl +
sole product NOT formed 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.
less substituted carbon
Cl H + HCl
more substituted carbon sole product
more substituted carbon
CH CH3 3 Ether + HBr Br H Ch.6 Alkenes: Structure and Reactivity
When both ends of the double bond have the same degree of substitution, a mixture of products formed.
Br + HBr + 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
Cl H Cl- H
3o carbocation + HCl
X H H Cl Cl-
NOT formed 1o carbocation Ch.6 Alkenes: Structure and Reactivity
CH3 CH3 Br- Br H H o CH3 3 carbocation + HBr
X CH CH3 3 - H Br H Br
2o carbocation NOT formed
Why should this be? Ch.6 Alkenes: Structure and Reactivity
6.10 Carbocation Structure and Stability
Carbocation: planar, sp2-hybridized, electron deficient
vacant p orbital R CR R 120o
sp2 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 D = 351 kJ/mol (Bond dissociation energy)
- CH3 CH3 +e Ei = 948 kJ/mol (Ionization energy)
Cl + - - e Cl 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:
o o Methyl halide > 1o alkyl halide > 2 alkyl halide > 3 alkyl halide
Carbocation stability:
H H R R H C < R C < R C < R C H H H R o 1o 2o 3 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:
H H H R
+ + + + H C R C R C R C
H H R R
o methyl 1o 2o 3 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
stabilization carbocation through hyperconjugation
H bonding C-H σ orbital (filled)
CC
empty p orbital (unfilled) Ch.6 Alkenes: Structure and Reactivity
Two different C-H bonds in t-Butyl cation:
(CH3)3C+
H 6 C-H σ orbitals above and below the plane: H3C CC nearly parallel to the cation p orbital H C H 3 H (hyperconjugation)
2 C-H σ orbitals are in plane: perpendicular to the cation p orbital (no hyperconjugation) Ch.6 Alkenes: Structure and Reactivity
H HH HH H H H H HH H staggered conformation eclipsed conformation
σ* H H H H H σ H H H H H HH stabilizing destabilizing hyperconjugation 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 carbocation forms 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
reaction rate ~ activation energy (∆G‡) stability ~ ∆Go
slower slower reaction reaction less stable intermediate faster faster eaction eaction less stable intermediate
more stable more stable intermediate intermediate 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.”
transition state transition state
product reactant reactant product
product-like TS reactant-like TS Ch.6 Alkenes: Structure and Reactivity
The hypothetical structure of a transition state for alkene protonation.
carbocation intermediate TS1
alkene
The transition state is closer in both energy and structure to the carbocation than 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
- δ+ δ H Br H R R HBr R + R R R δ R R R R R R alkene carbocation product-like transition state Ch.6 Alkenes: Structure and Reactivity
More stable carbocations form faster because their stability is reflected in the transition state leading to them.
primary TS
+ - (CH3)2CHCH2 Cl tertiary TS
∆Gprim + - (CH3)3C Cl
Energy ∆Gtert
∆Go
(CH3)2CCH2
+ HCl (CH3)2CHCH2Cl
(CH3)3CCl
Reaction progress 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
CH3 CH3 CH3 + H H H3C HCl H C + H C H 3 H 3 Cl Cl 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)
Hydride CH3 CH CH3 3 shift + H H C HCl H C H H3C 3 H 3 H H 2o carbocation 3o carbocation - Cl Cl-
CH 3 CH3 H H3C H C H H 3 Cl Cl H
- 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
methyl CH3 CH CH3 3 shift + H H3C HCl H3C H H3C CH3 H C 3 CH3 2o carbocation 3o carbocation - Cl Cl-
CH 3 CH3 H C 3 H H H C H3C 3 Cl Cl CH3 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 Chemistry @ Work Terpenes: Naturally Occuring Alkenes
Essential oils: fragrant mixtures of liquids from plant materials Terpenes: plant essential oils consist largely of mixtures of compounds
O
Myrcene α-Pinene Carvone (oil of bay) (turpentine) (spearmint oil)
Isoprene rule: head-to-tail joining of five-carbon isoprene
Tail
Head Isoprene Myrecene Chemistry @ Work Terpenes: Naturally Occuring Alkenes
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.
CH3 H CH3 Lanosterol, a triterpene (C30) CH3 - precusors for steroid hormons HO H Ture biological precursor of terpenes is not isoprene itselt but iosprene equivalents made from acetic acid. OO P P Isopentenyl diphosphate O O OH O O- O- C H3C OH OO P P Dimethylallyl diphosphate Acetic acid O O OH O- O-