CH 3 Cl + Cl 2 CH 2 Cl 2 + HCl

Transcription

1 Energetics 414 alogenation of Alkanes X 2 X X X 2 X X explosive for F 2 exothermic for l 2 and Br 2 endothermic for I 2 hlorination of Methane carried out at high temperature (400 ) 415 hlorination of Methane 4 l 2 3 l l 3 l l 2 2 l 2 l 2 l 2 l 2 l 3 l l 3 l 2 l 4 l Free adicals 416 Structure and Stability of Free adicals contain unpaired electrons Examples: O 2 :O O: NO : N O : l : l

2 Alkyl adicals Figure 417 Structure of Methyl adical 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 in the same way that carbocations are Methyl radical is planar, which suggests that carbon is sp 2 hybridized and that the unpaired electron is in a p orbital Alkyl adicals The order of stability of free radicals is the same as for carbocations Alkyl adicals Methyl radical less stable than 3 less stable than 3 Isopropyl radical (secondary) 3 Ethyl radical (primary) 3 less stable than 3 3 tert-butyl radical (tertiary) Alkyl adicals The order of stability of free radicals can be determined by measuring bond strengths By "bond strength" we mean the energy required to break a covalent bond A chemical bond can be broken in two different ways heterolytically or homolytically 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 omolytic eterolytic

3 omolytic The species formed by a homolytic bond cleavage of a neutral molecule are free radicals Therefore, measure enthalpy cost of homolytic bond cleavage to gain information about stability of free radicals The more stable the free-radical products, the weaker the bond, and the lower the bond-dissociation energy Measures of Free adical Stability Bond-dissociation enthalpy measurements tell us that isopropyl radical is 13 kj/mol more stable than propyl Measures of Free adical Stability Bond-dissociation enthalpy measurements tell us that tert-butyl radical is 30 kj/mol more stable than isobutyl ( 3 ) 2 2 ( 3 ) Mechanism of hlorination of Methane ( 3 ) 3 Mechanism of hlorination of Methane Free-radical chain mechanism Initiation step: (Light or eat is Necessary) : l : l: : l l : The initiation step "gets the reaction going" by producing free radicals chlorine chlorine atoms from chlorine molecules in this case Initiation step is followed by propagation steps Each propagation step consumes one free radical but generates another one First propagation step: 3 : Mechanism of hlorination of Methane l: 3 : l: Second propagation step: 3 : l : l : l: 3 : l:

4 First propagation step: 3 : Mechanism of hlorination of Methane l: 3 Second propagation step: 3 : l : l: 3 : l: : l: l: Almost all of the product is formed by repetitive cycles of the two propagation steps First propagation step: 3 : l: 3 Second propagation step: 3 : l : l: 3 : l: : l: l: 3 : : l : l : 3 : l: : l: 3 : : l : l : 3 : l: : l: Termination Steps Question 11 stop chain reaction by consuming free radicals 3 l: 3 : l: hardly any product is formed by termination step because concentration of free radicals at any instant is extremely low The step shown below is a step of the free-radical chlorination of chloromethane A) initiation B) propagation ) chain-terminating D) bond cleavage Question 16 For the free-radical reaction below, light is involved in which of the following reaction steps? 418 alogenation of igher Alkanes A) Initiation only B) Propagation only ) Termination only D) Initiation and propagation

5 hlorination of Alkanes can be used to prepare alkyl chlorides from alkanes in which all of the hydrogens are equivalent to one another l l l (78%) l 2 l l (73%) Major limitation: hlorination of Alkanes hlorination gives every possible monochloride derived from original carbon skeleton Not much difference in reactivity of different hydrogens in molecule Example hlorination of butane gives a mixture of 1-chlorobutane and 2-chlorobutane Percentage of Product that esults from Substitution of Indicated ydrogen if Every ollision with hlorine Atoms is Productive l 2 (28%) (72%) l l Percentage of Product that Actually esults from eplacement of Indicated ydrogen 46% 46% 46% 46% 46% 46% elative ates of ydrogen Atom Abstraction divide by = = 39 46% A secondary hydrogen is abstracted 39 times faster than a primary hydrogen by a chlorine atom

6 Similarly, chlorination of 2-methylbutane gives a mixture of isobutyl chloride and tert-butyl chloride Question l 2 (63%) l 3 ow many monochlorination products do you expect to obtain from the chlorination of 2-methylbutane? A) two B) three ) four (37%) 3 3 l D) five Answer 10 Percentage of Product that esults from eplacement of Indicated ydrogen ow many monochlorination products do you expect to obtain from the chlorination of 2-methylbutane? A) two B) three ) four D) five 70% 37% elative ates of ydrogen Atom Abstraction divide by = = 53 A tertiary hydrogen is abstracted 53 times faster than a primary hydrogen by a chlorine atom Selectivity of Free-radical alogenation 3 > 2 2 > 3 chlorination: bromination: hlorination of an alkane gives a mixture of every possible isomer having the same skeleton as the starting alkane Useful for synthesis only when all hydrogens in a molecule are equivalent Bromination is highly regioselective for substitution of tertiary hydrogens Major synthetic application is in synthesis of tertiary alkyl bromides

7 Synthetic Application of hlorination of an Alkane Question 20 l 2 (64%) l hlorination is useful for synthesis only when all of the hydrogens in a molecule are equivalent An alkane with a molecular formula of 8 18 reacts with l 2 in the presence of light and heat to give a single monochloride 8 17 l What is the most reasonable structure for the starting alkane? A) B) ( 3 2 ) ) ( 3 ) ( 3 ) 2 D) ( 3 ) ( 3 3 ) 3 Synthetic Application of Bromination of an Alkane (A) Barbamide, a cyanobacterial peptide containing a trichloromethyl group and (B) dysidenin, a barbamide-related compound isolated from a sponge-cyanobacterial association Br 2 Br (76%) Bromination is highly selective for substitution of tertiary hydrogens Major synthetic application is in synthesis of tertiary alkyl bromides Question 19 Which of the following best describes a mechanistic feature of the free-radical bromination (Br 2, light) of 2- methylpropane? A) The initiation step involves cleavage of a - bond B) The free-radical ( 3 ) 3 is produced in one propagation step and reacts with Br 2 in another ) The reaction is characterized by the homolytic cleavage of the -Br bond D) The reaction is concerted; ie, it occurs in a single step