light

CH4 + xs Br2

CH3Br + HBr CH2Br2 + HBr

CHBr3 + HBr

CBr4 +HBr

Traditional statement of overall methane (CH4) + excess(xs) bromine (Br2) gas phase reaction

xs=> excess

Br2 is present

h1) CH4 + Br2 CH4-x Brx + xHBr x=1,2,3 or 4

2) A slightly more detailed statement

Product distribution depends on details of reaction conditions

Light source

t=0.0 s

CH4

Br2

0

0.01

t=0.01

trace CH3Br +HBr

t= 1.0

CH3Br +HBr

trace CH2Br2+ more HBr1.00

CHBr3

t=50

trace CBr4

50 s

0.0150s all in dark

Evacuated reaction bulb filled with

CH4 + Br2(xs) 250

CH3Br & CH2B r2

graphical views of CH4 + Br2 reaction

Quartz window

~Experimental set up

Molecular detector

CH4

CH3Br

CH2 Br2 CHBr3CBr4

Light on

Dark ------

Time

concentration

HBr

Time evolution of reaction species concentrations for CH4 + Br2

Observed Behavior Patterns Summarized:1) reaction will only go at >250oC in dark

or with input of ultraviolet (uv) light.2) uv wavelength required which is near

that for dissociation wavelength for X2 . 3) in excess X2 , a single photon (h) event

causes thousands of halogenations. (=> photo yield >>>>1, e.g. 1000-5000

halogenated methane molecules/1 photon of h.)4) O2 causes halogenation reaction to slow/stop.5) reactivity rate : F2 > Cl2 > Br2 > I2 .

http://www.youtube.com/watch?v=XAWWf86TJ0c

http://www.youtube.com/watch?v=K6c9qfa1xiM

Sort of Animation of free radical mechanism

Demonstration of radical reaction with alkane + Br2