1
11
Organic ChemistryOrganic Chemistry513 250513 250
KanokKanok--on on RayanilRayanil
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22
Part 1: Organic reactions and Part 1: Organic reactions and functional groupsfunctional groups
Introduction to organic molecules and functional Introduction to organic molecules and functional groupsgroupsAlkanesAlkanes, Alkenes and Alkynes, Alkenes and AlkynesStereochemistryStereochemistryAlkyl HalidesAlkyl HalidesAlcohols & EthersAlcohols & EthersBenzene & Aromatic compoundsBenzene & Aromatic compoundsCarbonyl compoundsCarbonyl compoundsCarboxylic acid derivativesCarboxylic acid derivativesAminesAmines
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TextsTextsJ. G. Smith, J. G. Smith, ““ Organic ChemistryOrganic Chemistry””, McGraw, McGraw--Hill Hill companies, New York, 2006companies, New York, 2006J. J. McMurryMcMurry, , ““ Organic ChemistryOrganic Chemistry””, 6, 6th th ed., Brooks/Cole, ed., Brooks/Cole, 20042004Organic chemistry text books by any authors.Organic chemistry text books by any authors.
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44
IntroductionIntroduction
What is Organic Chemistry?What is Organic Chemistry?The chemistry of carbon compounds.The chemistry of carbon compounds.
7 million Organic Compounds7 million Organic Compounds1.5 million Inorganic Compounds1.5 million Inorganic Compounds
Animal and plant matter, Foods, Pharmaceuticals, Cosmetics, Fertilizers, Plastics, Petrochemicals, Clothing
5
55
Organic ChemistryOrganic ChemistryOrganic chemistry is the study of carbon compounds. Although carbon is the principal element in organic compounds, most also contain hydrogen, and many contain nitrogen, oxygen, phosphorus, sulfur, chlorine, or other elements.
6
66
Examples of Organic CompoundsExamples of Organic Compounds
7
77
Morphine
HO
CH3
CH3
Cholesterol
8
88
9
99
CarbonCarbon
HCH HH
methane
Carbon has 4 valence electrons
Stable Octet required
C
H
H
H
H
H C
Covalent Bonding – Atoms Share Electrons
10
1010
Methane is TetrahedralMethane is Tetrahedral
109.50
Sp3 hybridized carbon 4 equivalent C-H bonds (σ−bonds)
All purely single bonds are called σ−bonds
HC
H HH
11
1111
Classification of Organic Classification of Organic CompoundsCompounds
12
1212
Functional GroupsFunctional Groups
13
1313
14
1414
15
1515
Organic Reactions and their Mechanisms
A reaction mechanism is a detailed description of the bonding changes asa reaction proceeds. The reaction mechanism also includes the many important principles of organic chemistry. A plausible reaction mechanism must be consistent with the principles of organic chemistry.
16
1616
Four General Categories of Organic Reactions
.Organic reactions tend to fall into four categories:substitutions, additions, eliminations, rearrangements
Substitutions
In a substitution reaction, one atom or group replaces another in astructure. This type of reaction is commonly observed in saturated hydrocarbons and aromatics.
an alkyl halide+ Na+ -OH + Na+ X-R-X R-OHH2O
an aromatic+ Br2 + HBrAr-H
FeAr-Br
The mechanisms of the above two substitution reactions are completely different.
17
1717
Additions
.
Addition reactions are found in organic compounds with multiple bonds:alkenes, alkynes, carbonyl-containing compounds. In this reaction, the π-component of the multiple bond is lost as new bonds are formed to the carbon (or other atomic) centers
+ Br2
Bromine adds to the alkene (ethene).
C CH
H
H
HC C
H
H
Br Br
H
H
18
1818
Eliminations
.
These reactions are the reverse of addition reactions. In an eliminationreaction, a molecule loses atoms or groups from adjoining carbon (or other atomic) centers, forming a multiple bond
C CH
H
H
H+ KOH + K+ Br- + H2OC C
H
H
H Br
H
H
The above reaction is a dehydrohalogenation, loss of HBr, of an alkylhalide to form an alkene.
19
1919
Rearrangements
.In rearrangement reactions, there is a reorganization ofthe atoms or groups in a structure
H+
.In the presence of acid, the alkene on the left rearranges to the alkeneon the right
20
2020
Bond Making and Bond Breaking Processes: Heterolysis and Homolysis
.A covalent bond may break by either of two different processes:heterolysis or homolysis
A : B +ions
+ -A B .
Double-barbed arrowis used to show movementof an electron pair
Homolysis (Gr: homo-"the same" + lysis)
Heterolysis (Gr: hetero- "different" + lysis-"cleavage")
A : B . +radicalsA B. ..
Single-barbed arrowis used to show movementof a single electron
21
2121
Bond Heterolysis
.
Bond heterolysis typically is found with polar covalent bonds wherethe intrinsic electronegativities promote polarization of the bonding electrons
A : B ++ -A B
δ+ δ−
polar covalent bond
Assisted Bond Heterolysis
.Bond heterolysis may be assisted by a second molecule or chemicalspecies that can either donate or accept an electron pair.
A : Bδ+ δ− C +C A : B
δ+ δ−+ B
+ -C : A C donates an electron
pair in forming bond to Aas B departs as anion.
A : Bδ+ δ− D+
+A : Bδ+ δ−
D+ +A+ B : D D+ accepts an electron pairas it bonds to B releasing A+.
22
2222
Carbon Bond Heterolysis Processes:Carbocations and Carbanions
.When a bond to carbon is broken heterolytically, the carbon maycarry either a positive (carbocation) or negative (carbanion) charge.
Modes of Heterolytic Bond Cleavage
C:Xδ+ δ−
+ + X:-
a carbocation
C
δ+δ− - + Y+
a carbanionC:Y C:
23
2323
Formal Charges
.
Carbocations have only six electrons in the valence or bonding level andare electron-deficient. The charge on carbon can be determined by a simple calculation
For each pair of bonding electrons, one "belongs" to carbon.
All nonbonding electrons in the valence level "belong" to carbon.
Step One: Determine how many of the valence electrons "belong" to carbon.
+
a carbocation
In a carbocation, only 3 of the 6 bonding electrons "belong" to the carbon. There are no nonbonding electrons.
C
24
2424
Step Two: Compare the number of "owned" electrons in the valence level of the bonded state with the number in the atomic state.
.
Atomic carbon has 4 electrons in the valence level. Since in thecarbocation, only 3 electrons belong to carbon, there is a deficiency of one electron (a formal charge imbalance at carbon). Therefore, there is a formal charge of +1 on carbon
Carbanions
Carbanions have 6 bonding and two nonbonding electrons in the valencelevel, and have a formal charge of -1.
-
a carbanion
Three of the bonding electrons and the two nonbonding electrons belong to carbon.
C:
Calculation of Formal Charge
Since atomic carbon has 4 electrons in the valence level while 5 of the8 valence electrons in the carbanion belong to carbon, there is a surplus of one electron in the bonded state. The formal charge is -1.
25
2525
The Reactivity of Chemical Intermediates
Carbocations as Electrophiles
.
Since carbocations are electron-deficient in the valence level, they arestrong Lewis acids. Carbocations react rapidly with Lewis bases, species that are capable of donating electrons. Carbocations are called electrophiles
+
a carbocation(an electrophile)
+ :B-C
An Example
+ +C OH
H
Lewis Base
OH
HCH
HH
O HCH
HH
an alcohol
-H+
C B C B
26
2626
Carbanions as Nucleophiles
.
Carbanions are Lewis bases. They donate an electron pair to Lewisacids such as H+ and other electropositive atoms and groups. Carbanions are called nucleophiles
-C
carbanion(a nucleophile)
+ H-A
Lewis acid
δ+ δ−AC H + -
-C LC
carbanion(a nucleophile)
+
Lewis acid
δ+ δ−LC C + -
27
2727
The Curved Arrow Formalism
.
The above bonding changes are illustrated with a formalism calledcurved arrows where the arrow shows the direction of electron flow from nucleophile to electrophile
The curved arrow begins at an electron pair (bonding ornonbonding) and moves towards an electron-deficient atom or group (Lewis acid).
direction of electron flow
nucleophile electrophile
Examples
+δ−δ+
+
nucleophile electrophile
-H Cl ClC C C C H
= δ− δ++ :
: = :: : + :
nucleophileelectrophile
+-CH3C
OO H O H
HCH3C
OO O H
H
H
28
2828
Quiz Chapter 3 Section 4
In the reaction below, use the curved arrow formalism to show movement of an electron pair.
C6H5CH2+ + CH3OH:: :+
C6H5CH2-OCH3
H
Identify the electrophile and nucleophile in the reaction.
Identify the Lewis acid and Lewis base in the reaction.
electrophile nucleophile
Lewis acid Lewis base
29
2929
30
3030
ALKANES & CYCLOALKANESALKANES & CYCLOALKANES
31
3131
Secretion of undecane by a cockroach causes other members of the species to aggregate.
Structure of Structure of AlkanesAlkanesconsist of only carbon and hydrogen bonded by single covalent bonds single
Cyclohexane is one component of the mango.
32
3232
propaneC3H8
33
3333
Fossil Fuels Fossil Fuels Many alkanes occur in nature, primarily in natural gas and petroleum. Petroleum is a complex mixture of compounds, most of which are hydrocarbons containing one to forty carbon atoms. Distilling crude petroleum, a process called refining, separates it into usable fractions that differ in boiling point.
♦ natural gas: C1H4-C4H10♦ gasoline: C5H12-C12H26♦ kerosene: C12H26-C16H34♦ diesel fuel: C15H32-C18H38
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3434
Petroleum provides more than fuel. About 3% of crude oil is used to make plastics and other synthetic compounds including drugs, fabrics, dyes, and pesticides. These products are responsible for many of the comforts we now take for granted in industrialized countries.
Products obtained from a barrel of crude oil1 barrel = 42 gallons = 115.6 litters crude oil 19% gasoline!
35
3535
Petroleum Technologies
Cracking is a process for breaking down larger alkanes intosmaller alkanes by heating. The mixture of larger alkanes is heated in the absence of oxygen at high temperatures (~500oC) for only a few minutes in the presence of catalysts. By this method, alkanes of size C12 and larger can be turned into gasoline-size alkanes (C5 to C10).
Isomerization
Since the 1920s, it has been known that highly branched alkanesperform better in the internal combustion engine than straight-chain alkanes. Catalytic isomerization changes straight-chain alkanes into the more useful branched alkanes.
hexane
acid catalyst+
branched alkanes
CH3CH2CH2CH2CH2CH3CH3CH2CH2CHCH3
CH3
CH3CH2CHCH2CH3CH3
36
3636
Physical Properties of Physical Properties of AlkanesAlkanes
Non-polar molecules, which are less dense than water. Alkanes are immiscible with water making two layers.
Alkanes show regular increases in b.p. and m.p. as molecular weight increases down the homolgousseries
37
3737
Chemical Reactions of Alkanes
The Limited Chemical Reactivity of Alkanes
.
As nonpolar organic structures, alkanes have limited chemicalreactivity compared with other classes of organic compounds. Neither acids nor bases react readily with alkanes. One notable and important reaction of alkanes is combustion, the combination with oxygen, discussed earlier
Synthesis of Alkanes and Cycloalkanes
It is important to distinguish between large scale industrial production ofsome alkanes, often carried out under carefully controlled catalytic methods, and standard laboratory procedures.
These standard procedures are the synthetic methods covered in detail inthis course. The study of these general syntheses is closely connected with, if not inseparable from, the investigation of reaction mechanism theory in organic chemistry.
38
3838
Hydrogenation of Alkenes and Cycloalkenes
.
A standard way of preparing alkanes and cycloalkanes is by thecatalytic hydrogenation of the corresponding alkenes and cycloalkenes. Metals such as Pt, Ni, and Pd provide active surfaces for the reactants to rapidly interact
+ H-H Pt or Nisolvent
an alkene an alkane
R
R'
R
R'
H H
R' R'RR
Examples
+ H2ethanol, 25oC, 50 atm1-butene
CH3CH2CH=CH2Ni
butane
CH3CH2CH2CH3
+ H2ethanol, 25oC, 1 atm
cyclohexene
Pt
cyclohexane
39
3939
CH4O2 CO2 H2O energy+ 2 2+ +
RCH2 CH2R RHC CHR H H+alkene
High Temp.catalyst
Reactions of Reactions of AlkanesAlkanesCombustion
Dehydrogenation
Halogenation – radical substitution reactions
Br2
Br+ + HBr
light or heat
40
4040
Halogenation
H
CH
H
C
H
H
H + Br2
H
CH
H
C
H
H
Br
Heat or Light ∆ or hv
Ethane Bromoethane
HBr+
C
H
H
H + Cl2
Heat or Light ∆ or hv
Methane chloromethane
HCl+H C
H
H
ClH
CH2Cl2 and CHCl3may be observed
41
4141
MechanismsInitiation
Two highly reactive Chlorine radicals formed
Cl Cl Cl + Clhv or ∆
+Cl
H
C H
H
HH
C HH
H Cl
Hydrogen abstraction to form a methyl radical
Propagation
Cl ClCl+
H
C Cl
H
HH
C HH
Chlorine atom is abstracted to form a chlorine radicalPropagation are the product forming stepsChain Reaction – thousands of radical forming cycles
42
4242
Cl ClCl
CH3H3C CH3CH3
Cl
Cl CH3 CH3Cl
Termination Radicals Couple Product forming Chains are broken
As the reaction progresses chloromethane accumulates and its hydrogen atoms can be abstracted.
43
4343
Fluorine is the most reactive halogen – mixtures of fluorine and methane can be explosive. Fluorine radical is very reactive. The reaction is controlled with the addition of an inert gas to dilute the reaction.
Chlorine is next most reactive, followed by bromine. Cl2 and Br2 require heat or light. Iodine does not react with methane easily. Iodine radical is disperse and large - unreactive
ClCl ClCl
CH
H
H
CH
Cl
Cl
Dichloromethane (DCM)
+
44
4444
ALKENESALKENES
45
4545
Structure of AlkenesStructure of AlkenesAlkenes are compounds that contain a carbon-carbondouble bond. Cycloalkenes contain a double bond in a ring.
46
4646
47
4747
FragrantFragrant AlkenesAlkenes
48
4848
cis- trans-H
R R
H R
R H
H
H
H3C CH3
H CH3
H3C H
Hcis-(Z)-2-butene trans-(E)-2-butene
Z-E system, we take the group with higher priority (here higher molecular weight), and compare it with the group with higher priority on the other carbon
Geometric isomers have different chemical & physical properties
Cis-Trans Isomerism
49
4949
Cl Br
HF
Cl > F Br > H
(Z)-2-Bromo-1-chloro-1-fluoroethene
Cl H
BrF
Cl > F Br > H
(E)-2-Bromo-1-chloro-1-fluoroethene
CH3 H
CH3F
F > CH3 CH3 > H(Z)-2-fluorobutene
1 2
50
5050
Syntheses of Alkenes by Elimination Reactions
Dehydrohalogenation of Alkyl Halides(1)
Base
(-HX)
H
X
Dehydration of Alcohols(2)
H+, heat
(-H2O)
H
OH
Debromination of Vicinal Dibromides(3)
Zn/Acetic Acid
(-ZnBr2)Br
Brvicinal dibromide (-Br2)
NaI/Acetone
51
5151
π−bond lobes represent areas of high electron density
Therefore, the π−bond is susceptible to attack by electron deficient molecules, called electrophiles, E+
E+
Reaction of Alkenes
52
5252
ADDITION REACTIONSC C + A B CA C B
C C
H X CH C X
CX C X
CH C OHH OHH2SO4
X = Cl, Br, I
X = Cl, Br
X2, H2OX = Cl, Br
X2
CX C OH
1. BH32. H2O2, HO-
CH C OH
Alkyl Halides(hydrohalogenation)
Alcohols(hydration)
Dihaloalkanes(halogenation)
hydrohydrins(halohydrin formation)
Alcohols(hydroboration-oxidation)
53
5353
Hydrohalogenation : the addition of hydrogen halide HX to alkenesto form alkyl halides
The electrophile is a Lewis acid, its accepted a pair of electrons, the simplest Lewis acid is H+
The nucleophile is a Lewis base, its donated a pair of electrons
X- is the nucleophile, and the carbocation is the electrophile
C C
H X
C C
H
+Slow
X
δ+ δ−
X = Cl, Br, I
C C
H X
C C
H
Xfast
Mechanism
54
5454
Markovnikov’s Rule
H atom adds to the carbon atom which already has the most H atoms
H2CHC CH3 H2C
HC CH3
H Br Markovnikov additionProduct
H Br
Unsymmetrical Alkene
H
C H
H
CH3
C H
H
CH3
C CH3
H
CH3
C CH3
CH3
+ + + +
INCREASING STABILITY OF CARBOCATIONS
55
5555
C CH
H
H
CH3
H Br
C C
H
H
HCH3
H
Slow
Br
C C
H
H
HCH3
HBr
C CH
H
H
CH3
H Br
C CSlow
CH3
H
HH
HBr
C C
CH3
H
HH
HBr
2o Carbocation prefered
1o Carbocation
3o > 2o > 1o Carbocation stability
2-Bromopropane(main product)
1-Bromopropane (little formed)
The Slow Step is the Rate Determining Step
56
5656
Hydration : the addition of water to an alkene to form an alcohol
Mechanism
57
5757
Examples
Markovnikov’s Rule
58
5858
Halogenation: The addition of halogen X2 (X=Cl or Br) to an alkene, forming a vicinal halide.
1. Bromine molecule becomes polarised
3. Formation of Bromonium cation and bromide anion
2. Bromine bond breaks heterolytically
4. Back-side nucleophilic attack – opening of three membered ring5. Stereospecific product
C CBrBr
δ+δ−
C CBr
+ Br
Br
C CBr
C CBr
Brcolourless
Mechanism
59
5959
Test for DB or TBDecolorization of Br2/CCl4
60
6060
Halohydrin Formation: treatment of an alkene with a halogen X2
(X=Cl or Br) and H2O forms a hydrohydrin by addition of X and OH to the double bond.
colourless
61
6161
Mechanism
62
6262
As in the opening of an epoxide ring, nucleophilic attack occurs at the more substituted carbon end of the bridged halonium ion because that carbon is better able to accommodate a partial positive charge in the transition state.
63
6363
Hydroboration-Oxidation: a two-step reaction sequence that converts an alkene to an alcohol
Examples
64
6464
Mechanism
Oxidation replaces the C - B bond with a C - O bond, forming a new OH group with retention of configuration
65
6565
Unsymmtrical alkenes
Anti-Markovnikov product
66
6666
Problem
Solution
67
6767
Oxymercuration:
.
An alkene reacts with water in the presence of mercuric acetate,Hg(OAc)2, (an electrophile) in a mixture of THF and water. The role of theTHF is to help dissolve the alkene
=
alkene
O
THF
H2O, Hg(OCCH3)2
acetic acidoxymercuration product
HO HgO
O
CH3+
H3C OH
O
oxymercuration product THFH2O, NaBH4HO Hg
O
O
CH3alcohol
HO + Hg + CH3CO2-
(i) Hg(OAc)2,/THF, H2O
(ii) NaBH4, HO-
1-methylcyclopentene 1-methylcyclopentanol
OH
(Markovnikov product)
Examples
68
6868
A Mechanism for the Oxymercuration-Demercuration Reaction
(1) Hg(OAC)2+ + -Hg-OAc OAc
(2) + +an electrophile
Hg-OAc +Hg OAc
H
H
H
A bridging mercurinium ion is believed to form rather than a freecarbocation since typical carbocation rearrangements do not occur.
There is general agreement the reaction begins with electrophilic attackof mercury (II), possibly as +Hg(OAc), on the π-bond of the alkene.
69
6969
+Hg OAc
H
H
H
+ H2O(3)+
Hg OAc
H
H
H
OH2
+ H2O(4) +Hg OAc
H
H
H
HO H fast
++ H3OHg OAc
H
H
H
OH
oxymercuration product
(5) Hg OAc
H
H
H
OHBH H
H
H
-
Na+
sodium borohydride
+ Hgo + CH3CO2
H
H
H
OHH3,3-dimethyl-2-butanol
-
(Markovnikov Addition Product)
70
7070
CC
HH
C HC H
Pt or Pd - catalystsolvent, pressure
Example Pt ethanol, 1 atm
cyclohexene cyclohexane
H2
Hydrogenation – “Reduction”
71
7171
mechanism
72
7272
The stereospecific formation of 1,2-diols (or glycols) fromalkenes may be carried out in two ways:
KMnO4, HO-
cold H2O HO OH
(i) OsO4, pyridine(ii) Na2SO3/H2O or NaHSO3/H2O HO OH
Oxidation of Alkenes
73
7373
.
The mechanisms of the two hydroxylation reactions arevery similar. The initial step is oxidative addition to the alkene to produce cyclic ester-type intermediates
The Mechanism of the Hydroxylation Reaction
two electron reduction
MnO
O
O
Opermanganate ion
+7 -O
manganate ester
+5
-O
MnO O
two electron reduction
OsO
O
O
Oosmium tetroxide
+8
O
osmate ester
+6 OOs
O O
74
7474
Conversion to 1,2-Diols
:
The syn stereochemistry of the hydroxylation reactions isset with formation of the cyclic intermediates. The cyclic esters are converted to the diols as follows
O
manganate ester
+5
-O
MnO O
alkaline hydrolysis
OH-
H2O HO OH+ MnO2
Hydrolysis of the manganate ester occurs under the aqueous alkaline conditions.
O
osmate ester
+6 OOs
O O
reductive cleavage
NaHSO3
H2O HO OH+ Os
Reductive cleavage of the osmate ester occurs in a second step when the reducing agent is added.
75
7575
Examples of Syn-Hydroxylation
MnO4-
cold O
stereochemistry is set
+5
-O
MnO O
H2OOH-
cis-1,2-cyclopentanediolOHHO
OsO4pyridine, 25oC
O +6 OOs
O Ostereochemistry is set
H2ONaHSO3
cis-1,2-cyclopentanediolOHHO
76
7676
Ozonolysis of Alkenes
77
7777
The Ozonolysis Reactions--Preparation of Ozone
Ozone is produced in the laboratory by flowing a stream of oxygen gas (O2) through an electric discharge in an apparatus called an ozonator. The exiting stream of gas contains ozone in the range 4-8% by weight.
electric discharge3O2 2O3
This gas stream is bubbled into a solution of the alkene inCH2Cl2 (dichloromethane) at -78o C.
78
7878
Quiz Chapter 8 Section 17
Provide the products of the following ozonolysis reactions.
(i) O3, CH2Cl2, -78oC(ii) Zn, H2O
(i) O3, CH2Cl2, -78oC
(ii) Zn, H2O
H
O+
H H
O
H
O+
O
H
OO
79
7979
Ozonolysis is mainly used for determining the structure of an unknown alkene
80
8080
ALKYNES
81
8181
H C C H H3C C C HEthyne(acetylene)
Propyne
180oLinear Molecule
Structure of AlkynesAlkynes contain a carbon-carbon triple bond.
82
8282
Alkynes are high energy compounds
C CH H + 2.5 O2 2 CO2 + H2O
Welding gas
Combustion
83
8383
Alkynes by Elimination Reactions
Alkynes may be prepared from alkenes in a two-step syntheticsequence of bromination-debromination.
General Scheme
Bromination
vicinal dibromidealkene
Br2CH
CH
R RHC
HCR R
Br Br
Debromination
2 B- 2 HBr
alkyne
C CR R
The base must be sufficientlystrong to affect two successivedehydrobromination reactions.
Synthesis of Alkynes
84
8484
vicinal dibromide
HC
HCR R
Br Br
Debromination
2 B- 2 HBr
alkyne
C CR R
A base sufficiently strong for the two successive dehydrobrominationreactions is sodium amide (Na+NH2
- ) in liquid ammonia (-33o C) or in a hydrocarbon solvent at elevated temperature.
Two Successive Dehydrobrominations
bromoalkene(vinyl bromide)
R
Br
H
R+ NH3 + Br-
+ NH3 + Br-RRR
Br
H
R
N HH+
: -
amide aniona strong base
+H-N:H C C
H
BrR
Br
RH
85
8585
The Acidity of Terminal Alkynes
Among a broader range of compounds, the relative acidity ofterminal alkynes is:
> >pKa 15.7 16-17
H-O-H RO-H RC >25
C-H38
>H2-N-H >44
RCH=CH-H50>~
RCH2-H
pKa 25 43 44 > 50
Acidities of Hydrocarbons
HC CH
H H
H H
H
CH
H
H
C
H
H
H
least acidicmost acidic
86
8686
Replacement of the Alkynyl Hydrogen: Alkylation
.The acidity of the alkynyl hydrogen is the basis of an alkylationreaction wherein an alkyl group is introduced
RC
terminal alkynepKa ~ 25
+ NaNH2
sodium amideC-H RC + NH3
pKa = 38sodium alkynideC:-Na+
Alkylation
A Two-Step Synthesis
SN2RCa good nucleophile
+ R'Xalkyl halide
C:-Na+ RC C-R' + NaX
Reaction of Alkynes
87
8787
Examples of Alkylation Reactions
+ CH3CH2Br liquid NH3 -33o C
CH3CH2C C:- Na+
3-hexyneCH3CH2C CCH2CH3
+ C6H5CH2Brbenzyl bromide
liquid NH3 -33o C
HC C:- Na+
3-phenyl-1-propyneHC CCH2C6H5
A Limitation: With 2o and 3o alkyl halides, the E2 dominates.
RC +R'
C:- Na+ C CR
HBr
HH
RC
R'CH=CHR
CH4 +
88
8888
Quiz Chapter 7 Section 11
Provide a synthesis of pentane from propene using any needed reagents and alkyl halides.
solutionCH3CH=CH2 CH3CH2CH2CH2CH3
Retrosynthetic analysis suggests the following steps to couple the twohydrocarbon units:
CH3C C-CH2CH3
CH3C C:- + XCH2CH3
CH3CH2CH2CH2CH3
CH3CH=CH2
proposed synthesis
CH3CH=CH2
Br2 CH3CHBrCH2Br(i) 3 eq NaNH2
(ii) workup NH4ClCH3C CH
(i) NaNH2
(ii) CH3CH2Br
CH3C CCH2CH3excess H2
PtCH3CH2CH2CH2CH3
89
8989
Hydrogenation of Alkynes
Catalytic hydrogenation of the triple bond occurs step-wise and may be controlled to yield alkenes:
2-butyne 2-butene butaneCH3C CCH3
H2
PtCH3CH=CHCH3 CH3CH2CH2CH3
H2
Pt
Example: Hydrogenation of Diphenylethyne
diphenylethyne+ 2H2
Pd/C(no poison)
Pd/C is palladium metal finely dispersed on powdered charcoal.
1,2-diphenylethane (95%)C6H5C CC6H5 C6H5CH2CH2C6H5
90
9090
StereochemistryStereochemistry
91
9191
Isomers : different compounds that have the same molecular formula
Structural isomers are isomers that differ because their atoms are connected in a different order
Cl H
Cl H
Cl H
H Cl
cis-1,2-Dichloroethene trans-1,2-Dichloroethene
C2H2Cl2
Geometric Isomers
Stereoisomers differ only in the arrangement of their atoms in space
92
9292
The Representation of Molecules in The Representation of Molecules in 33--DD
Br
HClI
ในระนาบ
เหนือระนาบ
ใตระนาบ
Bromochloroiodomethane
93
9393
Enantiomers are stereoisomers whose molecules are nonsuperimposable mirror images of one another
94
9494
CH3 COH
CH2 CH3
2-Butanol
H
Chiral Centre
Objects that are nonsuperimposable on their mirror images are said to be chiral
ChiralChiral moleculesmolecules
95
9595
If any of the groups attached to the tetrahedral atom are the same, the centre is achiral.
AchiralAchiral moleculesmolecules
96
9696
A molecule will not be chiral if it possess acentre or plane of Symmetry
2-Propanol
CH3
C OHH
CH3
CCH3
HHOCH3
AchiralAchiral moleculesmolecules
97
9797
RR, , S S --configurationconfiguration
98
9898
Examples
99
9999
Two enantiomers can have different odors
100
100100
Chiral Drugs
COOH
H3C HO CF3
H CH2CH2NHCH3
(S)-ibuprofenanti-flammatory agent
(R)-fluoxetineantidepressant
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