Chlorination of Higher Alkanes Chlorination of an alkane involves the substitution of a chlorine atom for a hydrogen atom on the alkane. All H’s are equivalent in CH4 and CH3CH3 so only one product forms. In many other alkanes, more than one type of hydrogen atom is present and more than one product can form. 1o 3o 2o

Chlorination of Higher Alkanes The presence of more than one type of hydrogen atom in an alkane often leads to a mixture of products.

Chlorination of Higher Alkanes The predicted (statistical) distribution and experimental results are different because 1-chloropropane forms from a 1o free radical

Chlorination of Higher Alkanes while 2-chloropropane forms from a 2o free radical

Chlorination of Higher Alkanes Example: Draw the mechanism for the chlorination of propane:

Chlorination of Higher Alkanes As a rule of thumb, the major product is often the one that is formed from the most stable free radical. There are EXCEPTIONS! Very large ratio of 1o H to 2o or 3o H may lead to 1o alkyl halide as the major product hu 62% 38%

Reactivity of Halogens All halogens react with methane via the same mechanism, but they have different relative reactivities. Relative reactivity refers to relative rates of reaction high reactivity: very fast rate low reactivity or unreactive: slow or zero rate F2 > Cl2 > Br2 > I2

Reactivity of Halogens The differences in reactivity of the halogens toward alkanes can be explained by differences in: Activation energy for the propagation steps DHrxn As Ea decreases, more collisions will have enough energy for the reaction to occur higher reactivity

Reactivity of Halogens Since DG ~ DHrxn for most organic reactions, as the reaction becomes more exothermic, the reaction becomes more spontaneous higher reactivity Ea Overall Relative Halogen (kcal/mol) DHrxn Reactivity F2 1.2 -102 high Cl2 3.8 -24.5 medium Br2 18.6 -7.5 low I2 34 +11 unreactive

Reactivity of Halogens F2 is the most reactive halogen low activation energy strongly exothermic reaction I2 is very unreactive. High activation energy endothermic reaction

Reactivity of Halogens The chlorination of propane leads to a broad mixture of products: 40% 1-chloropropane substitution of a 1o H 60% 2-chloropropane substitution of a 2o H Bromination of propane is more selective: 3% 1-bromopropane 97% 2-bromopropane

Reactivity of Halogens The RDS for chlorination of propane is exothermic. According to the Hammond Postulate, the transition state will more closely resemble the reactants. Little bond breakage has occurred. Small difference in Ea for formation of 1o vs. 2o radical. Stability of the free radical is less important in determining the structure of the final product.

Reactivity of Halogens

Reactivity of Halogens The RDS for the bromination of propane is endothermic. According to the Hammond Postulate, the transition state will resemble the products. More bond cleavage and new bond formation has occurred. Large difference in Ea for formation of 1o vs. 2o radical Stability of the free radical is much more important in determining structure of product.

Reactivity of Halogens

Bromination of Alkanes Example: Predict the major product of the following reactions.

Reactive Intermediates Reactive intermediates are highly reactive, short-lived species that are never present in high concentration. Four important reactive intermediates: carbonium ion (carbocation) free radical carbanion carbene

Reactive Intermediates Carbonium ion: a chemical species in which: carbon atom is bonded to 3 other atoms carbon atom has a positive charge planar, sp2 hybridized electrophile: “electron lover” an electron pair acceptor

Reactive Intermediates The stability of a carbonium ion increases as the number of alkyl groups attached to the positively charged carbon increases inductive effect donation of electron density through the sigma bonds partial overlap of filled orbitals with empty ones through resonance stabilization d+

Reactive Intermediates Stability of carbonium ion: 3o substituted allylic 2o allyl methyl vinyl > 1o > > Least stable Most stable

Reactive Intermediates Free radical: carbon atom with an unpaired electron The stability of a free radical increases as the number of alkyl groups attached to the positively charged carbon increases through resonance stabilization Stability: allyl > 3o > 2o > 1o > methyl > vinyl .

Reactive Intermediates Carbanion: a trivalent carbon atom that bears a negative charge a nucleophile electron pair donor Carbene: an uncharged, divalent carbon atom that has a nonbonding pair of electrons