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1 Alkyl Halides (Haloalkanes)

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Page 1: Alkyl Halides (Haloalkanes)apsacwestridge.edu.pk/assets/admin/upload/notes/AlkylandHalides.pdf · 5 5 Alkyl Halides: R-X The carbon center is sp3 hybridized in alkyl halides and the

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Alkyl Halides (Haloalkanes)

Page 2: Alkyl Halides (Haloalkanes)apsacwestridge.edu.pk/assets/admin/upload/notes/AlkylandHalides.pdf · 5 5 Alkyl Halides: R-X The carbon center is sp3 hybridized in alkyl halides and the

2

2

CH3 CH

Cl

CH CH3

CH3

Cl

CCl Cl

Cl

CH3 CH

Br

CH2 CH2

Cl

F

CH2CH3

BrCl

Cl

CCl F

Cl

F

CCl F

Cl

F

CF

F

C

F

H

H

Tetrachloromethaneor carbon tetrachloride

2-Chloro-3-methylbutane 3-Bromo-1-chlorobutane

1-Ethyl-2-fluorocyclohexane1-Bromobutane 2-Chloropropane or

Isopropyl chloride

Trichlorofluoromethane(Freon-11)

Dichlorodifluoromethane (Freon-12) 1,1,1, 2-Tetrafluoroethane

Structure of Alkyl Halides

Chlorofluorocarbons (CFCs) :Refrigerant Gases, Ozone Depletion

Page 3: Alkyl Halides (Haloalkanes)apsacwestridge.edu.pk/assets/admin/upload/notes/AlkylandHalides.pdf · 5 5 Alkyl Halides: R-X The carbon center is sp3 hybridized in alkyl halides and the

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3Halothane (Fluothane)

Page 4: Alkyl Halides (Haloalkanes)apsacwestridge.edu.pk/assets/admin/upload/notes/AlkylandHalides.pdf · 5 5 Alkyl Halides: R-X The carbon center is sp3 hybridized in alkyl halides and the

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• Most alkyl halides are liquids at room temperature.

• Liquid alkyl halides are insoluble in water and more dense than water.

Physical Properties of Alkyl Halides

Page 5: Alkyl Halides (Haloalkanes)apsacwestridge.edu.pk/assets/admin/upload/notes/AlkylandHalides.pdf · 5 5 Alkyl Halides: R-X The carbon center is sp3 hybridized in alkyl halides and the

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Alkyl Halides: R-X

The carbon center is sp3 hybridized in alkyl halides and the C-X bond is polarized as shown because of the greater electronegativity of the halogen.

Cδ+ δ−

X

Electronegativity is defined as the ability of atoms to attract shared electrons in a covalent bond ------------ leads to nucleophilic substitution in alkyl halides

Reactions of Alkyl halidesReactions of Alkyl halides

Page 6: Alkyl Halides (Haloalkanes)apsacwestridge.edu.pk/assets/admin/upload/notes/AlkylandHalides.pdf · 5 5 Alkyl Halides: R-X The carbon center is sp3 hybridized in alkyl halides and the

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Nucleophilic Substitution Reactions

Nu:- +

nucleophile

::

alkyl halideR-Nu + :

:

: -

substrateproduct halide ion

R-X X: :

A characteristic reaction of alkyl halides is nucleophilic substitutionwhere a nucleophile with an unshared pair of electrons replaces the halogen.

Nu:- ::

::

: -R X X

: :Substitution occurs by bond heterolysis:

+

bond heterolysis

Nu: +

electron pairfrom nucleophile

R:

Examples of Nucleophilic Substitution

::

- +:

:HO: CH3-Cl:

:: +

:::

-CH3-OH Cl:

:::

- :+

::I CH3CH2-Cl: :: +

::

: -CH3CH2-I Cl

::

Page 7: Alkyl Halides (Haloalkanes)apsacwestridge.edu.pk/assets/admin/upload/notes/AlkylandHalides.pdf · 5 5 Alkyl Halides: R-X The carbon center is sp3 hybridized in alkyl halides and the

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Nucleophiles

.A nucleophile has an unshared pair of electrons availablefor bonding to a positive center

Nucleophiles may be negatively charged:

HO , CH3O , I , NH2- - - -:

:

::

::

::::: :

or neutral:H2O , H3N, CH3OH

::

: ::

Nucleophiles attackelectropositive center.

Halide ionis the leavinggroup.

C Xδ+

δ−The polarity of thecarbon-halogen bond determines the reactivity pattern:

Page 8: Alkyl Halides (Haloalkanes)apsacwestridge.edu.pk/assets/admin/upload/notes/AlkylandHalides.pdf · 5 5 Alkyl Halides: R-X The carbon center is sp3 hybridized in alkyl halides and the

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Examples

(1) -+

nucleophile substrate

HOC Cl

H3CH

H

product leaving group

C OH

H3CH

H

+ Cl

(2) +

nucleophile substrate

C Cl

H3CH

H

HO

H

ethyloxonium ion leaving group

C O

H3CH

H

+ Cl

H

H

product

C OH

H3CH

H

+ H3O

H2O

Page 9: Alkyl Halides (Haloalkanes)apsacwestridge.edu.pk/assets/admin/upload/notes/AlkylandHalides.pdf · 5 5 Alkyl Halides: R-X The carbon center is sp3 hybridized in alkyl halides and the

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Leaving Groups

:The halogen is only one of many leaving groups, "L". A more generaldescription of nucleophilic substitution is

Nu: + R-L- R-Nu + L:-

leaving group

.

A good leaving group produces a stable anion or neutral molecule.Generally, the anions (conjugate bases) of strong acids are good leaving groups

A good leaving group in R-A .

+ H2O + A:+ -strong acid anion

very stable

H-A H3O

Page 10: Alkyl Halides (Haloalkanes)apsacwestridge.edu.pk/assets/admin/upload/notes/AlkylandHalides.pdf · 5 5 Alkyl Halides: R-X The carbon center is sp3 hybridized in alkyl halides and the

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Neutral Molecules as Leaving Groups

.

Poor leaving groups canbe turned into good leaving groups by protonation

Hydroxide ion is apoor leaving groupbecause it is the anion of a weak acid, H2O.

CH3-OH

:a nucleophilic substitution reaction occurs

+CH3OH

H+ H2O

leaving groupnucleophile+

good leaving group

CH3OH+

CH3OCH3H

:: +

+ +CH3OH H2SO4 CH3OHH

HSO4

In the presence of a strong acid,

Page 11: Alkyl Halides (Haloalkanes)apsacwestridge.edu.pk/assets/admin/upload/notes/AlkylandHalides.pdf · 5 5 Alkyl Halides: R-X The carbon center is sp3 hybridized in alkyl halides and the

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Page 12: Alkyl Halides (Haloalkanes)apsacwestridge.edu.pk/assets/admin/upload/notes/AlkylandHalides.pdf · 5 5 Alkyl Halides: R-X The carbon center is sp3 hybridized in alkyl halides and the

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Page 13: Alkyl Halides (Haloalkanes)apsacwestridge.edu.pk/assets/admin/upload/notes/AlkylandHalides.pdf · 5 5 Alkyl Halides: R-X The carbon center is sp3 hybridized in alkyl halides and the

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A Mechanism for the SN2 Reaction

The Hughes-Ingold Mechanism for the SN2 Reaction

In their mechanism, the nucleophile attacks the carbon center on the sideopposite the leaving group. As overlap develops between the orbital with the electron pair of the nucleophile and the antibonding orbital of the substrate, the bond between the carbon and the leaving group weakens.

In 1937 Edward Hughes and Sir Christopher Ingold proposed amechanism to explain the second order kinetics and other important features of this nucleophilic substitution reaction that were known at that time.

C ClH

HH

OH- δ− δ−CHO Cl

HH

H

+ -ClCHO

H

HH

TS

Page 14: Alkyl Halides (Haloalkanes)apsacwestridge.edu.pk/assets/admin/upload/notes/AlkylandHalides.pdf · 5 5 Alkyl Halides: R-X The carbon center is sp3 hybridized in alkyl halides and the

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SN2 reaction

All SN2 reactions proceed with backside attack of the nucleophile, resulting ininversion of configuration at the stereogenic center.

Page 15: Alkyl Halides (Haloalkanes)apsacwestridge.edu.pk/assets/admin/upload/notes/AlkylandHalides.pdf · 5 5 Alkyl Halides: R-X The carbon center is sp3 hybridized in alkyl halides and the

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Examples of inversion of configuration in the SN2 reaction

Page 16: Alkyl Halides (Haloalkanes)apsacwestridge.edu.pk/assets/admin/upload/notes/AlkylandHalides.pdf · 5 5 Alkyl Halides: R-X The carbon center is sp3 hybridized in alkyl halides and the

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Reaction of t-Butyl Chloride with Hydroxide: SN1 Reaction

:

The reaction of t-butyl chloride with sodium hydroxide in a mixture ofwater and acetone (to help dissolve the RCl) shows the following rate expression

+ HO-acetone

+ Cl-CH3-C-ClCH3

CH3

H2OCH3-C-OH

CH3

CH3

.The reaction rate depends on the concentration of t-butyl chloride, butshows no dependence on the concentration of hydroxide ion

A reaction rate that depends on the concentration of only one reactant (to the first power) is called first-order or unimolecular.

The symbol is SN1.

Page 17: Alkyl Halides (Haloalkanes)apsacwestridge.edu.pk/assets/admin/upload/notes/AlkylandHalides.pdf · 5 5 Alkyl Halides: R-X The carbon center is sp3 hybridized in alkyl halides and the

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SN1 reaction

+ + + +

3o 2o 1o methyl

> > >

most stable least stable

CR

RR

CR

RH

CH

RH

CH

HH

Relative stabilities of carbocations

The key features1. The mechanism has two steps.2. Carbocations are formed as

reactive intermediates.3. Reactions proceed with

racemization at a single stereogenic center.

Page 18: Alkyl Halides (Haloalkanes)apsacwestridge.edu.pk/assets/admin/upload/notes/AlkylandHalides.pdf · 5 5 Alkyl Halides: R-X The carbon center is sp3 hybridized in alkyl halides and the

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Use of the SN2 Reaction in Organic Synthesis

.

The conversion of one compound into another through a chemicalreaction is called synthesis. The SN2 reaction is often used to convert alkyl halides into other functional groups

for R= CH3, 1o, 2o

X = Cl,Br, I

Nucleophiles

R'O-

-

R'C

R'-C-O-=

R'3N:

N3-

R-X

HO-

HS-

:CN

C:-

O

alcohols

R-OR' ethers

thiols

nitriles

RC CR' alkynes

R-O-C-R'

=

esters

R-NR'3ammonium ion

azides

R-OH

R-SH

R-CN

O

R-N3

+

Page 19: Alkyl Halides (Haloalkanes)apsacwestridge.edu.pk/assets/admin/upload/notes/AlkylandHalides.pdf · 5 5 Alkyl Halides: R-X The carbon center is sp3 hybridized in alkyl halides and the

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Stereochemistry of SN2 Synthetic Reactions

.As in all SN2 reactions, these syntheses proceedwith inversion at a stereocenter

N C + CH3C

Br

H3CH2CH

(R)-2-bromobutane

CCH3

CCH2CH3

HN

(S)-2-methylbutanenitrile

Page 20: Alkyl Halides (Haloalkanes)apsacwestridge.edu.pk/assets/admin/upload/notes/AlkylandHalides.pdf · 5 5 Alkyl Halides: R-X The carbon center is sp3 hybridized in alkyl halides and the

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Elimination Reactions of Alkyl Halides

In an elimination reaction, the atoms or groups X and Y are lost fromadjacent carbons forming a multiple bond.

CX

CY

C C(-XY)

The Dehydrohalogenation Reaction

A standard synthesis ofalkenes is the dehydrohalogenationreaction of alkyl halides.

CX

CH

C C(-HX)

alkyl halide alkene

Example: The Dehydrobromination of tert-Butyl Bromide

tert-butyl bromide

+ NaOCH2CH3CH3C-BrCH3

CH3

CH2=C + HOCH2CH3

+ Na+ Br-isobutene

CH3

CH3

Page 21: Alkyl Halides (Haloalkanes)apsacwestridge.edu.pk/assets/admin/upload/notes/AlkylandHalides.pdf · 5 5 Alkyl Halides: R-X The carbon center is sp3 hybridized in alkyl halides and the

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A Beta- or 1,2-Elimination Reaction

C

H

H

H

C

CH3

CH3

Br(-HBr)

β position α position

The α or 1 position is the carbon with the halogen leaving group.

CH2 CCH3

CH3

β αCH2 C

CH3

CH3

2 1or

The Role of Base in Dehydrohalogenation Reactions

This reaction is described as a beta-elimination or 1,2-eliminationindicating the positions of the lost atoms or groups.

A number of different bases may be used in the dehydrohalogenationreaction. Typical bases are potassium hydroxide in ethanol (to solubilize the alkyl halide) or sodium ethoxide in ethanol. Potassium tert-butoxide is another oxygen base that is often used in dehydrohalogenation reactions.

KOBu-t

Some Oxygen Bases

KOH NaOEt

Page 22: Alkyl Halides (Haloalkanes)apsacwestridge.edu.pk/assets/admin/upload/notes/AlkylandHalides.pdf · 5 5 Alkyl Halides: R-X The carbon center is sp3 hybridized in alkyl halides and the

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Mechanism of Dehydrohalogenation: The E2 Reaction

The reaction of isopropyl bromide with sodium ethoxide in ethanol togive propene:

isopropyl bromide+

sodium ethoxide ethanolCH3CHCH3

BrNaOCH2CH3

CH3CH=CH2 +

HOCH2CH3 + Na+Br-propene

Page 23: Alkyl Halides (Haloalkanes)apsacwestridge.edu.pk/assets/admin/upload/notes/AlkylandHalides.pdf · 5 5 Alkyl Halides: R-X The carbon center is sp3 hybridized in alkyl halides and the

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A Mechanism for the E2 Reaction

The alkyl bromide reactsfrom a conformation wherethe leaving groups are anti-coplanar.

CH3CH2O-H

C CBrH

CH3

H

H

++ CH3CH2OHBr C CH H

H CH3

.

The mechanism proposed for the E2 reaction is based on the observedsecond order rate expression, as well as the stereochemical outcome observed in alkyl halides with multiple stereocenters. The E2 like the SN2 is a single step mechanism

HC C

BrH

CH3

H

H

As the base removes the H+,the double bond begins to develop and the Br- begins to depart.

CH3CH2Oδ−

δ−

Page 24: Alkyl Halides (Haloalkanes)apsacwestridge.edu.pk/assets/admin/upload/notes/AlkylandHalides.pdf · 5 5 Alkyl Halides: R-X The carbon center is sp3 hybridized in alkyl halides and the

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The E1 Reaction

The reaction of tert-butyl chloride in the mixed solvent of 80%ethanol-20% water at 25o C yields a product mixture from two competing reaction paths: substitution and elimination.

83%CH3C-OR R = H, CH3CH2

-

17%CH2=C

substitution

elimination

CH3CClCH3

CH3

CH3

CH3CH3

CH3

Page 25: Alkyl Halides (Haloalkanes)apsacwestridge.edu.pk/assets/admin/upload/notes/AlkylandHalides.pdf · 5 5 Alkyl Halides: R-X The carbon center is sp3 hybridized in alkyl halides and the

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A Mechanism for the Competing E1/SN1 Reactions

RDSslow step

+ + Cl-

tert-butyl carbocation

bond heterolysis

CH3CClCH3

CH3

CH3CCH3

CH3

:These two competing reactions have the same rate-determining step:bond heterolysis to produce the tert-butyl carbocation

After the rate-determining step, two competing modes of reactionbetween the tert-butyl carbocation and water/ethanol (acting as nucleophile/base) lead to substitution and elimination products.

+

::+

ethanolor water

fastSN1

E1

CH3CCH3

CH4

R-O-H

CH3C-ORCH3

CH3

CH2=CCH3

CH3

ROH asnucleophile

ROH asbase

Page 26: Alkyl Halides (Haloalkanes)apsacwestridge.edu.pk/assets/admin/upload/notes/AlkylandHalides.pdf · 5 5 Alkyl Halides: R-X The carbon center is sp3 hybridized in alkyl halides and the

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The E1 Path: Deprotonation Step

.

The substitution pathway follows the usual course for an SN1reaction. In a fast step, the carbocation intermediate reacts with a nucleophile (water or ethanol) to yield the substitution product

::+

nucleophile substitution product

fast fastCH3C

CH3

CH3

ROH(-H+)

CH3CORCH3

CH3

Along the elimination pathway, in a fast step, a base (water orethanol) removes a beta proton from the carbocation to produce the alkene product.

R-O

: :

+

β+fastC C

HH

HH

CH3

CH3 ROH2 C CHH

CH3

CH3

In the above conformation, the bonding orbital of the beta-H is aligned with the empty p-orbital. This stereoelectronic requirement allows immediate overlap of the developing p-orbitals of the π bonding molecular orbital.

Page 27: Alkyl Halides (Haloalkanes)apsacwestridge.edu.pk/assets/admin/upload/notes/AlkylandHalides.pdf · 5 5 Alkyl Halides: R-X The carbon center is sp3 hybridized in alkyl halides and the

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Summary of Reactions

Page 28: Alkyl Halides (Haloalkanes)apsacwestridge.edu.pk/assets/admin/upload/notes/AlkylandHalides.pdf · 5 5 Alkyl Halides: R-X The carbon center is sp3 hybridized in alkyl halides and the

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Summary of Reactions

Page 29: Alkyl Halides (Haloalkanes)apsacwestridge.edu.pk/assets/admin/upload/notes/AlkylandHalides.pdf · 5 5 Alkyl Halides: R-X The carbon center is sp3 hybridized in alkyl halides and the

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ALCOHOLS

Page 30: Alkyl Halides (Haloalkanes)apsacwestridge.edu.pk/assets/admin/upload/notes/AlkylandHalides.pdf · 5 5 Alkyl Halides: R-X The carbon center is sp3 hybridized in alkyl halides and the

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AlcoholsAlcohols can be regarded as derivatives of water in which one or two of the H atoms has been replaced by an alkyl group

OH H

104.5o

0.96 AoWater, H2O

OC H

108.5o

0.96 AoMethanol, CH3OH

HHH

1.43 Ao

Page 31: Alkyl Halides (Haloalkanes)apsacwestridge.edu.pk/assets/admin/upload/notes/AlkylandHalides.pdf · 5 5 Alkyl Halides: R-X The carbon center is sp3 hybridized in alkyl halides and the

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Methanol

Electronegativity of oxygen causes an unsymmetrical distribution of charge

OH3C H

- I (net dipole)

δ +δ −

Page 32: Alkyl Halides (Haloalkanes)apsacwestridge.edu.pk/assets/admin/upload/notes/AlkylandHalides.pdf · 5 5 Alkyl Halides: R-X The carbon center is sp3 hybridized in alkyl halides and the

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Ethanol content

Beer, 3-9% Wine, 11-13%

Whisky, 40-45% Vanilla Extracts, 35%

Listerine, 25%

Page 33: Alkyl Halides (Haloalkanes)apsacwestridge.edu.pk/assets/admin/upload/notes/AlkylandHalides.pdf · 5 5 Alkyl Halides: R-X The carbon center is sp3 hybridized in alkyl halides and the

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Classification of Alcohols

HC OHH3CH

CH3

C OHH3CH

CH3

C OHH3CCH3

Primary (1o) Alcohol Secondary (2o) AlcoholTertiary (3o) Alcohol

Page 34: Alkyl Halides (Haloalkanes)apsacwestridge.edu.pk/assets/admin/upload/notes/AlkylandHalides.pdf · 5 5 Alkyl Halides: R-X The carbon center is sp3 hybridized in alkyl halides and the

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Alcohols are very weak Acids

R O HHO H R O +

HO HH

Alcohol Alkoxideδ +δ + δ −

Relative Acidity ; H2O > ROH > C CR H > RH

CH3CH2OH + Na CH3CH2 O Na + H2

Vigorous Reaction

2 2 2

Acidity of Alcohols

sodium ethoxide

Page 35: Alkyl Halides (Haloalkanes)apsacwestridge.edu.pk/assets/admin/upload/notes/AlkylandHalides.pdf · 5 5 Alkyl Halides: R-X The carbon center is sp3 hybridized in alkyl halides and the

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Preparations of Alcohols

Page 36: Alkyl Halides (Haloalkanes)apsacwestridge.edu.pk/assets/admin/upload/notes/AlkylandHalides.pdf · 5 5 Alkyl Halides: R-X The carbon center is sp3 hybridized in alkyl halides and the

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Hydration of AlkenesHydroboration-oxidation

Oxymercuration-reduction

Page 37: Alkyl Halides (Haloalkanes)apsacwestridge.edu.pk/assets/admin/upload/notes/AlkylandHalides.pdf · 5 5 Alkyl Halides: R-X The carbon center is sp3 hybridized in alkyl halides and the

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Prepare 1,2-diols

Hydroxyration

Acid-catalyzed hydrolysis of epoxides

Page 38: Alkyl Halides (Haloalkanes)apsacwestridge.edu.pk/assets/admin/upload/notes/AlkylandHalides.pdf · 5 5 Alkyl Halides: R-X The carbon center is sp3 hybridized in alkyl halides and the

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Reduction of carbonyl compounds

Sodium borohydride, NaBH4, is ususlly chosen because of its safety and ease of handling.

Page 39: Alkyl Halides (Haloalkanes)apsacwestridge.edu.pk/assets/admin/upload/notes/AlkylandHalides.pdf · 5 5 Alkyl Halides: R-X The carbon center is sp3 hybridized in alkyl halides and the

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Reduction of carbonyl compoundsLithiun aluminum hydride, LiAlH4, is more reactive reducing agent than NaBH4. All carbonyl groups, including acids, esters, ketones, and aldehydes, are rapidly reduced by LiAlH4. Note that one hydrogen atom is delivered to the carbonyl carbon atom during ketone and aldehyde reductions but that two hydrogens become bonded to the former carbonyl carbon during carboxylic acid and ester reductions.

Page 40: Alkyl Halides (Haloalkanes)apsacwestridge.edu.pk/assets/admin/upload/notes/AlkylandHalides.pdf · 5 5 Alkyl Halides: R-X The carbon center is sp3 hybridized in alkyl halides and the

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Reaction of carbonyl compounds with Grignard reagents

Alkyl, aryl, and vinylic halides react with magnesium in ether or tetrahydrofuranto generate Grignard reagents, RMgX. These Grignard reagents react with carbonyl compounds to yield alcohols in much the same way that hydride reducing agents do.

Page 41: Alkyl Halides (Haloalkanes)apsacwestridge.edu.pk/assets/admin/upload/notes/AlkylandHalides.pdf · 5 5 Alkyl Halides: R-X The carbon center is sp3 hybridized in alkyl halides and the

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Ester reaction

Carboxylic acid don’t give addition products!

Page 42: Alkyl Halides (Haloalkanes)apsacwestridge.edu.pk/assets/admin/upload/notes/AlkylandHalides.pdf · 5 5 Alkyl Halides: R-X The carbon center is sp3 hybridized in alkyl halides and the

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Reactions of Alcohols

Page 43: Alkyl Halides (Haloalkanes)apsacwestridge.edu.pk/assets/admin/upload/notes/AlkylandHalides.pdf · 5 5 Alkyl Halides: R-X The carbon center is sp3 hybridized in alkyl halides and the

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Dehydration of Alcohol to AlkenesDehydration is a β elimination reaction in which the elements of OH and H areRemoved from the α and β carbon atoms, respectively.

Page 44: Alkyl Halides (Haloalkanes)apsacwestridge.edu.pk/assets/admin/upload/notes/AlkylandHalides.pdf · 5 5 Alkyl Halides: R-X The carbon center is sp3 hybridized in alkyl halides and the

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Dehydration in Acid

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Page 46: Alkyl Halides (Haloalkanes)apsacwestridge.edu.pk/assets/admin/upload/notes/AlkylandHalides.pdf · 5 5 Alkyl Halides: R-X The carbon center is sp3 hybridized in alkyl halides and the

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Page 47: Alkyl Halides (Haloalkanes)apsacwestridge.edu.pk/assets/admin/upload/notes/AlkylandHalides.pdf · 5 5 Alkyl Halides: R-X The carbon center is sp3 hybridized in alkyl halides and the

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Dehydration using POCl3 and Pyridine

Because some organic compounds decompose in the presence of strong acid. Another method to convert alcohols to alkenes has been developed by using phosphoorus oxychloride and pyridine.

Page 48: Alkyl Halides (Haloalkanes)apsacwestridge.edu.pk/assets/admin/upload/notes/AlkylandHalides.pdf · 5 5 Alkyl Halides: R-X The carbon center is sp3 hybridized in alkyl halides and the

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Conversion of Alcohol to Alkyl Halides with HX

Page 50: Alkyl Halides (Haloalkanes)apsacwestridge.edu.pk/assets/admin/upload/notes/AlkylandHalides.pdf · 5 5 Alkyl Halides: R-X The carbon center is sp3 hybridized in alkyl halides and the

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Mechanism

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Problem

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Solution

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Conversion of Alcohol to Alkyl Halides with SOCl2 and PBr3

Phosphorustribromide

1o and 2o alcohols can be converted to alkyl halides using SOCl2 and PBr3

Page 54: Alkyl Halides (Haloalkanes)apsacwestridge.edu.pk/assets/admin/upload/notes/AlkylandHalides.pdf · 5 5 Alkyl Halides: R-X The carbon center is sp3 hybridized in alkyl halides and the

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Mechanism

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Mechanism

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Oxidation of Alcohols

• Three important oxidation reagents– Potassium permanganate (KMnO4). Deep purple in color, such a solution

is a strong oxidant. In the course of reaction, the purple Mn(VII) is reduced to Mn(IV), which precipitates as brown manganese dioxide (MnO2).

– Chromic acid (H2CrO4). A strong oxidant usually used with alcohols, chromic acid can be produced in solution by two methods: (1) from sodium dichromate (Na2Cr2O7) and sulfuric acid, or (2) by dissolving chromic anhydride (CrO3) in concentrated sulfuric acid and water (this version is called Jones'reagent). During an oxidation reaction, the orange-colored Cr(VI) in this reagent forms greenish-blue Cr(III), which remains in solution.

– Pyridinium chlorochromate (C5H6NCrO3Cl, PCC). A mild oxidizing reagent, PCC is a soluble complex of chromic anhydride (CrO3) and pyridine in dilute HCl.

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• Oxidation of a 1o alcohol with a strong oxidant (KMnO4 or H2CrO4) gives a carboxylic acid.

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• An aldehyde (RCHO) can be obtained from a 1o alcohol if PCC (pyridinium chlorochromate) is used.

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• 2o Alcohols, which have only one hydrogen bonded to the carbon carrying the hydroxyl group, are oxidized by chromic acid or permanganate to ketones. The oxidation can proceed no further because the carbon double-bonded to the oxygen has no more hydrogens.

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Mechanism

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Summary Oxidation of Alcohols• Note that the overall change produced in the oxidation of 1o and 2o

alcohols is removal of a hydrogen from the hydroxyl group and from the carbon to which it is attached:

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Breathalyzer Tests

When the Breathalyzer test is used for suspected drunk drivers, the driver exhales a volume of breath into a solution containing the orange Cr6+ ion. If there is ethyl alcohol present in the exhaled air, the alcohol is oxidized, and the Cr6+ is reduced to give a green solution of Cr3+

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ETHERS

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Acyclic Ethers

OH H

104.5o

0.96 AoWater, H2OO

C C

111.7o

Methyl ether, CH3OCH3

HHH

1.43 Ao

HHH

109.5o1.10 Ao

CH3CH2 O CH2CH3

CH3CH2 O

H3C O

Ethoxy group

Methoxy group

Diethyl Ether

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Acyclic Ethers, R-O-R

1-Propoxypropane

Methoxybenzene “anisole”

Methoxycyclo-hexane

Cyclic Ethers

Non-Flammable Anaesthetics

ClCHF

CF

FO C H

F

FEnflurane

FCFF

CH

ClO C H

F

FIsoflurane

H3CO O

OCH3

Diethyl Ether

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Reaction of EtherWilliamson Ether Synthesis

Alkoxides needed in Williamson reaction are normally prepared by reactionof an alcohol with a strong base such as NaOH or NaH.

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Examples

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Acidic Cleavage of Ethers

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Cyclic Ethers

OTetrahydrofuran (THF) OO

Furan Pyran

Cyclic ethers are one of the main components of epoxy glues. Such a glue is strong,but also lightweight. It is used as a component of the Stealth bomber.

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Preparation of Epoxides

Mechanism

One stepWithout intermediates