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Reactions of Alkanes :Radicals

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Naturally Occurring Alkanes

Natural source is oil

Oil is distilled

Product is dependent Carbon number

Combustion

C1 C4

C15 C25

CnH2n+2O2 nCO2 + (2n+2) H2O

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Alkane Reactivity

Alkanes have strong bonds

C and H have similar electronegativities

Inert to nucleophiles and electrophiles Very Unreactive

Alkanes ~ Paraffins

Latin : Parum AffinusLittle Affinity

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Conversion of Petroleum: Pyrolysis

CH3CH2CH2CH2CH2CH3

CH3 + CH2CH2CH2CH2CH3

CH3 CH2 + CH2CH2CH2CH3

CH3 CH2 CH2 + CH2CH2CH3Various combination:

CH3 CH2 CH3+ CH3 CH2 CH3

CH2CH2CH2CH2CH3CH3 CH2 + CH3 (CH2 )5CH3

CH3 CH2 CH2 + CH2CH2CH2CH3

CH3 CH2 CH2

+

CH2CH2CH2CH2CH3 CH3 (CH2 )6CH3

1 2 3 4

C1,C2

C2,C3

C3,C4

propane

heptane

octane

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Radical Substitution : Cl and Br

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In a homolytic bond cleavage, the two electrons in

the bond are divided equally between the two atoms.

One electron goes with one atom, the second with

the other atom.

In a heterolytic cleavage, one atom retains both

electrons.

+

Homolytic

Heterolytic

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. The species formed by a homolytic bond

cleavage of a neutral molecule are free radicals.Therefore, measure energy cost of homolytic bond

cleavage to gain information about stability of free

radicals

Homolytic

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Multi-chlorination of Methane

Chlorination of methane does not stop cleanly at the

monochlorinated stage.

The reaction continues on to give a mixture of dichloro,

trichloro, and even tetrachloro products.

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Alkanes react with Cl2 and Br2 alkyl halide Need light (h ) or high temperatures

Why h or high T??

Homolytic cleavage of X2

Initiates radical formation

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Mechanism 3 stages of radical reactions

Initiation

Propagation chain reaction

Termination combination of two radicals

Free Radical atom with anunpaired electron

highly reactivereform octet

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Radical Chlorination of Methane

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Energy Diagramfor Alkane Chlorination

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Chlorination of Alkanes

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Alkane Bromination

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Structure and Stabilityof Free Radicals

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Most free radicals in which carbon bears the unpaired electron

are too unstable to be isolated.

Alkyl radicals are classified as primary, secondary, or tertiary

CR R

R

.

Alkyl Radicals

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The Geometry of Alkyl Radicals

Experimental evidence: indicates that most alkyradicalsistrigonal planarwith an unpaired electron

H3C

H3CCH3

sp2

C

hybridized

.

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Structure of methyl radical

Methyl radical is planar, which suggeststhat carbon is sp2 hybridized and that theunpaired electron is in a porbital.

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Hyperconjugation---structure of alkyl radicals

C

.C

H

H

H

H

H

CH2CH3---ethyl radical

C.

CH

HH

CH2

H H

CH2CH2CH3---propyl radical

C. CH

HH

CH2

C2H

H

CH2CH2CH3---isopropyl radical

CH2

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71%

Easier toBreak

Harder toBreak

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Reactivity of Alkanes

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Stability trend affects reactivity

Relative rates of alkyl radical formation by Cl

71%

29%

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Reaction of Methane with Other HalogensReactivity of halogens toward methane:

Fluorine: explode without precaution

Chlorine: to be control by heat an light

Bromine: less reactive than chlorine

Iodine: unreactive---in practice, no reaction occur

The order of activity:

F 2 > Cl2 > Br2 > I2

Reaction of Methane with Other Halogens

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Reactivity-Selectivity Principle

Chlorination

H < 0, exothermic

TS resembles reactant

Reaction is very fast

Bromination

H > 0, endothermic

TS resembles product

Reaction is very slow

Reactivity Selectivity Principle

The greater the reactivity of a species, the less selective it will be !!

79

21 98

2

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Radicals in Real Life : Biology

In vivo, radicals are formed via interactions withmetals:

Fe is in enzymes,

cytochrome P450 liver USED to detoxify???

CH2Cl2

P450

C

OH

ClCl

H

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Radicals in Real Life : Biology

Deoxyribonucleotide synthesis:

Monomer of DNA formed from the monomer of RNA

Radical Inhibitors : Antioxidants

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contain unpaired electrons

.. ..

. .O O::Examples: O2

NO N O:..

:.

Cl Cl .

..:

..

Free Radicals

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