Chapter11烯烃

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