Transcript
Chapter 11: AlkenesChapter 11: Alkenes
CC CC Double Double bondbond
Names: Names: EndingEnding ananee
enenee
1. Find longest chain wit 1. Find longest chain wit bothboth C Cspsp22 s in s in it.it.
Rules:Rules:
An An octocteneene
11
2233
445566
7788
44
For example: Ethene, propene, butene, etc.For example: Ethene, propene, butene, etc.
Lingo: Double bond Lingo: Double bond positionposition
RR
RRInternalInternalTerminal Terminal
RR
RRCHCH22
3. Name and # substituents, in alphabetical 3. Name and # substituents, in alphabetical orderorder 4-Ethyl-3-methyl-4-Ethyl-3-methyl-3-3-
octocteneene
2. # Chain with 2. # Chain with C CC C close to close to terminusterminus
11
2233
445566
7788 AA 3-3-octoctene ene (only the (only the
first of the two Cfirst of the two Cspsp2 s 2 s is named by a #) is named by a #)
44
4. 4. CycloalkenesCycloalkenes
11
22
33 CHCH33
CC CC11 22
5. 5. Stereoisomers:Stereoisomers:
RR
RRRR RRciscis trantran
ssCis/trans used for 1,2-disubstituted ethenes.Cis/trans used for 1,2-disubstituted ethenes.
6. For tri- and tetrasubstituted alkenes: 6. For tri- and tetrasubstituted alkenes: E, ZE, Z naming. naming. Use R, S priority rules at each Use R, S priority rules at each spsp22-carbon-carbon separatelyseparately, to find higher priority groups at each end., to find higher priority groups at each end.
By definitionBy definition
EE--4-ethyl-3methyl-4-ethyl-3methyl-3-3-octocteneene
11
2233
445566
7788
44
Opposite sides: Opposite sides: EESame side: Same side: ZZ
3-Methyl3-Methylcyclohexcyclohexeneene
8. Substituents: 8. Substituents: Alkenyl Alkenyl
CHCH22 CHCH Ethenyl Ethenyl (vinyl)(vinyl)
CHCH22 CHCH CHCH22 2-Propenyl 2-Propenyl (allyl)(allyl)
9. Exocyclic alkenes: 9. Exocyclic alkenes: AlkylidenecycloalkanesAlkylidenecycloalkanes
MethylidenecyclohexanMethylidenecyclohexane e (Methylenecyclohexane)(Methylenecyclohexane)
OH ( SH)OH ( SH) >> enenee 11
22
33
OHOH2-Propen-1-2-Propen-1-olol
77..
Structure of Double Structure of Double BondBond
““Electron rich”Electron rich”
The The Sigma Sigma BondBond
The Pi The Pi BondBond
EtheneEthene
Bond is Bond is relatively weakrelatively weak
OrbitalOrbitalEnergieEnergie
ss
How Weak is the How Weak is the Bond?Bond?
EEaa = 65 kcal/mol = 65 kcal/mol
Bond Strengths Bond Strengths (kcal/mol)(kcal/mol)
Unusually strong because C uses Unusually strong because C uses spsp22 hybrids hybrids
Acidity: CC CC
HH
Alkenyl hydrogens Alkenyl hydrogens are relatively are relatively “acidic”“acidic”
ppKKaa ~ ~ 4444
Cf. Cf. CHCH33CHCH33 HH
++ CHCH33LiLi RCRCHH
CCHH
LiLiCHCH44++
~ ~ 50.50.
CC CCHH
HH
HH
RR
Therefore, in Therefore, in principle:principle:
Problems:Problems: Regio-, stereoselectivity. Better: Regio-, stereoselectivity. Better:
CHCH22 CCHH
BrBr++ LiLi CHCH22 CC
HH
LiLi
CC CCBrBr
HH
HH
RRMgMg++ CC CC
MgBrMgBr
HH
HH
RR
Useful:Useful: React React with with carbonylcarbonylss
Why are alkenyl hydrogens Why are alkenyl hydrogens acidic?acidic?
CC33% 33% s s character.character.
In contrast:In contrast:
spsp33 has 25% has 25% s s charactercharacter
Net effect: relativelyNet effect: relatively e-withdrawing e-withdrawing
HH
::
CCspsp22 HasHas
11H NMRH NMR CC CCHH
δδ ~ 4.5-6 ppm: ~ 4.5-6 ppm: deshielded!deshielded!
Why?Why? 1.1. spsp22 2.2. e-Flow of e-Flow of cloud cloud
Strengthens Strengthens HH00
Coupling ConstantsCoupling Constants
CC CCHH
HH HH
RRJJHHH H transtrans= 11-18 Hz; = 11-18 Hz; JJHHH H ciscis= 6-14 Hz= 6-14 Hz
JJHHHH geminalgeminal ~ 0-3 Hz ~ 0-3 Hz
For cis/trans isomers: For cis/trans isomers: JJtranstrans always always JJciscis..
Double bond “transmits” Double bond “transmits” long rangelong range (over 3- (over 3-4 C) 4 C) coupling coupling (1-3 Hz).(1-3 Hz).
>>
Depend on Depend on stereochemistry.stereochemistry.
““Vicinal” coupling:Vicinal” coupling:
““Geminal” coupling:Geminal” coupling:
1313C C NMRNMR
CCspsp22 deshielded (reasons are deshielded (reasons are complex)complex)δδ = 110 – 150 ppm “left half” of total spectral = 110 – 150 ppm “left half” of total spectral
windowwindow
CC CCCCHH
33
CCHH
33
HH33
CC
HH33
CC
CC CC
HH
CHCH22CHCH33
HH
HH33
CC
132.7132.7
122.8122.8
18.18.99
12.12.33
123.123.77
20.20.55
14.14.00
AlkeneAlkeness
HHCC CCHH
CCHH
33
CCHH
33
HH33
CC
HH33
CC
34.034.0
19.19.22
CCHH33CCHH22CCHH22CHCH22CHCH
33
13.13.55
34.34.11
22.22.22
AlkaneAlkaness
((CHCH33))44SiSi
((CHCH33))44SiSi
Vibrations in Vibrations in Molecules: Infrared (IR) Molecules: Infrared (IR)
SpectroscopySpectroscopy
Compounds resemble a mechanical frame: Compounds resemble a mechanical frame: they “rattle”. they “rattle”. Rattling is quantized.Rattling is quantized.
ΔΔEE = = hhνν ~ 1-10 kcal ~ 1-10 kcal molmol-1-1in in λλ or 1/ or 1/λλ = = υυ “wave “wave
numbers”numbers”
~~
Range: Range: 600-4000 cm600-4000 cm-1-1
EE
Excited stateExcited state
Ground stateGround state
AA BB stretching stretching υυ : Determined by Hooke’s : Determined by Hooke’s LawLaw
~~
υυ = = kk~~ √√ ff mmAA+m+mBB
mmAAmmBB
f = force constant m = f = force constant m = mass (reflects bond mass (reflects bond strength)strength)
υυ goes up with larger f, smaller goes up with larger f, smaller
mm
~~
Not only stretching: also bending and coupled Not only stretching: also bending and coupled modes modes Complex patterns 600-1500 cmComplex patterns 600-1500 cm-1-1 : The : The fingerprintfingerprint
regionregion
Infrared ModesInfrared Modes
The Infrared The Infrared SpectrometerSpectrometer
Fingerprint regionFingerprint region
Most Most useful:useful:
υυC HC H~~
--1.1. Alkyl Alkyl = 2900cm = 2900cm--
11
υυCCspsp22 H H
~~-- = 3080 cm= 3080 cm-1-1,, υυC CC C
~~---- = 1640 cm= 1640 cm--
11,,
CC CCHH
RR
RR
HH
transtrans
υυ = 970 = 970 cmcm-1-1
~~
R O HR O H
2.2. AlkenesAlkenes
33..
HH
HH
TransTrans-2--2-hexenehexene
4.4.
OO
CC 1740 cm1740 cm-1-1
3350 3350 cmcm-1 -1
(broad)(broad)
Degrees of Degrees of UnsaturationUnsaturation
Molecular formula Molecular formula tells us how many rings tells us how many rings
and/or and/or bonds are present in a molecule. bonds are present in a molecule. Reference is a Reference is a saturated acyclic saturated acyclic hydrocarbon: hydrocarbon: CCnnHH22nn+2+2..
Simple examples:Simple examples:
We need to determine the deviation of the We need to determine the deviation of the molecular formula from Cmolecular formula from CnnHH22nn+2+2 (in increments (in increments of 2H). Every ring or double bond takes away of 2H). Every ring or double bond takes away 2H, triple bond 4H from C2H, triple bond 4H from CnnHH22nn+2+2..
CC66HH1212,, not Cnot C66HH14.14. CC66HH1010,, not Cnot C66HH14.14.
HalogeHalogen:n:
-1-1;; CC HH CC XX
NitrogenNitrogen::
+1+1;; CC HH CC NN RRHH
Effect of Presence of Effect of Presence of Heteroatoms Heteroatoms on on CCnnHH22nn+2+2
CC HH CC OO HH
S, OS, O no effect on count (still C no effect on count (still CnnHH22nn+2 +2 + S+ Sx x or Oor Oyy))
Depends on valency of element:Depends on valency of element:
Steps:Steps:
1.1. Calculate H Calculate Hsatsat = 2 = 2n n C + 2 – C + 2 – nnX X + + nnN N n = “number of”n = “number of”
2.2. Count H Count Hactual actual in given in given molecular molecular formulaformula..
3.3. Degree of UnsaturationDegree of Unsaturation: (H: (Hsatsat – – HHactualactual)/2)/2
CC55HH55NN
ExamplesExamples::
CC1010HH1616 1. H1. Hsatsat = (2x10) + 2 = 22 = (2x10) + 2 = 22
2. Degree of unsaturation: (22-16)/2 2. Degree of unsaturation: (22-16)/2 = 3= 3
etc.etc.oror
1.1. HHsatsat= 10 + 2 + 1 = 13= 10 + 2 + 1 = 13
2. (13 - 5)/2 = 4 degrees of 2. (13 - 5)/2 = 4 degrees of unsaturation:unsaturation:
NN
PyridinPyridinee
CC NN Or?Or?OrOr
ProblemProblem
CC33HN: How many degrees of unsaturation?HN: How many degrees of unsaturation?
HHsatsat = 2 = 2n n C + 2 – C + 2 – nnX X + + nnNN
Degree of Unsaturation: (HDegree of Unsaturation: (Hsatsat – H – Hactualactual)/2)/2
A. 2A. 2B. 3B. 3C. 4C. 4
Relative Stability of Relative Stability of AlkenesAlkenes
Measure heat of hydrogenation Measure heat of hydrogenation ΔΔHHH2 H2 of of isomers, e.g., buteneisomers, e.g., butene
++ HH22
++ HH22
++ HH22
ΔΔHHH2H2 (kcal mol (kcal mol-1-1))
-28.6-28.6
-27.6-27.6
-30.3-30.3
InternalInternal terminaterminall
transtrans ciscisStability:Stability: >> >>,,
cat.cat.
cat.cat.
cat.cat.
Why?Why? 1. 1. Hyperconjugation:Hyperconjugation:
2. Steric hindrance 2. Steric hindrance (strain)(strain)
CCHH
CCCC
::
CisCis is is less stableless stable than than transtrans because of because ofsteric hindrancesteric hindrance
GeneralGeneral order of stability:order of stability:
CHCH22 CHCH22
RCH RCH CHCH22
RCH RCH CHRCHR
ciscis<< <<RCH RCH CHRCHR
trantranss
tritri tetrasubstitutedtetrasubstituted<< <<<<
Synthesis of AlkenesSynthesis of Alkenes
E revisitedE revisited. Best: E. Best: E22 on RX. on RX. Regioselectivity?Regioselectivity?
Saytzev-RuleSaytzev-Rule
Non-bulky base: Non-bulky base: More stable More stable product.product.
CHCH33 CH CH22 C C CHCH33
CHCH
33
XX
basebase
more more stablestable
less less stablestable
CC CCHH
CHCH
33
CHCH
33
HH33
CC CC CHCH22
HH33CC
HH22CCHCCH33
++
Hofmann RuleHofmann RuleBulky base: Less stable, terminal product is Bulky base: Less stable, terminal product is majormajor
51%51% 18%18% 31%31%
++ ++
BrBrNa OCHNa OCH33
CHCH33OHOH
--++
Trans predominatesTrans predominates (not (not much)much)Stereospecificity?Stereospecificity? YesYes..
E or Z from respective E or Z from respective diastereomeric haloalkanes:diastereomeric haloalkanes: C C
CCHH XX
****
IsIs elimination stereoselectiveelimination stereoselective? ? I.e., will it make preferentially I.e., will it make preferentially ciscis or or transtrans product? Yes, but not completely. product? Yes, but not completely.
Stereospecificity:Stereospecificity:
Good! Each Good! Each diastereomerdiastereomergives gives only oneonly one stereoisomer of stereoisomer of alkene productalkene product
Alkenes from ROH by Alkenes from ROH by Dehydration:Dehydration:Often Often MessyMessy
RRprimprim OHOH
::::
CHCH22 CH CH22 OO
+ H+ H22SOSO4 4 conc., goes by Econc., goes by E2, 2, requires requires heat:heat:
++
HH
HH
::
HH
++ HSOHSO44 CHCH22 CHCH22 HH22SOSO44
HH22OO
::::++
++
--++
RRsecsec, R, Rterttert OH : E OH : E11 + + rearrangementsrearrangements
CHCH33CHCH22OOH + H + HH
Acid-Mediated Dehydration of Acid-Mediated Dehydration of AlcoholsAlcohols
CC CC
HH OHOH
Acid, Acid, ΔΔCC CC ++ HHOHOH
Relative Reactivity Relative Reactivity of Alcohols (ROH) of Alcohols (ROH) in Dehydration Reactionsin Dehydration Reactions
R = primary R = primary << secondary secondary << tertiarytertiary
CHCH33CHCH22OHOH CHCH22 CHCH22
CHCH33CC CCHCCH
33HH HH
HOHO HH
conc. Hconc. H22SOSO44, , 170°C170°CHOHHOH
50% H50% H22SOSO44, , 100°C100°C
HOHHOHCHCH33CHCH CHCHCHCH33
CHCH22 CHCHCHCH22CHCH33
80%80%
TraceTrace
++
HH22CC CC(CH(CH33))33COHCOHCHCH33
CHCH33
100100%%
Dilute HDilute H22SOSO44, , 50°C50°C
HOHHOH
CHCH33CC CCHCCH
33HH HH
OHOHCHCH33
CHCH22
HH22SOSO44, , ΔΔ
HOHHOH
HH33CCCC CC
HH
HH33CC CHCH22CHCH33
5454%%
8%8%
CHCHCHCH
33
CHCH33CCCCHHHH
CHCH33
other minor other minor isomersisomers
++ ++
Dehydration with Dehydration with RearrangementRearrangement
OHOH
αα--TerpineolTerpineol
++
TerpinolenTerpinolenee
1515%% LimonenLimonen
ee
9%9%
33% H33% H22SOSO44, 1 h, , 1 h, 100°C100°C HH22OO
++ ++
28.528.5%% IsoterpinolenIsoterpinolen
ee
18.518.5%% γγ-Terpinene-Terpinene
1515%%
++
αα--TerpineneTerpinene
Acid-catalyzedAcid-catalyzeddehydrations givedehydrations givemixturesmixtures
Terpenes: The scentTerpenes: The scent of soapof soap
top related