1

11

Organic ChemistryOrganic Chemistry513 250513 250

KanokKanok--on on RayanilRayanil

2

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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33

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

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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 + -

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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

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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

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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)

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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