Photobromination of n-butyl bromide labelled with 82BR. on the intermediacy of bridged bromoalkyl radicals

TetrahedronLetter6 No. 4, pp 317 - 318, 1972.

PHOTOBROMINATJON ON

THE

OF

Laboratoire

N-BUTYL

INTERMEDIACY

Claude RONNEAU, Jean-Philippe

Per-n

BROMIDE

OF

BRIDGED

SOUMILLION,

de Chimie GBnerale

Preea.

Printed in Great Britain.

LABEL=JJ

WITH

BROMOALKYL

**BP..

RADICALS

Pierre DEJAIFVE and Albert BRUYLANTS rue de Namur,B-3000

et Organique,gS,

LOWAIN

(Belgium)

(Receivedin UK 11 November 19711 accepted for Publication28 December1971)

Several authors emphasiaed of bromoalkanes

the particular

of the observed

results have been given. According

first one, a bridged radical stabilizes of the odd electron

(

l-3,5-7)

the transition

. . CH,-CH2-CH

An alternative

explanation

is 'proposed by

amount of vicinal dibromide

Br . .. 1

mechanisms

TANNER et al., who showed that

increases as the reaction proceedst4).

by using n-butyl bromide

ferrate

in carbon tetrachloride

pical experiments

as have been used by

bromine. The brominaas a solvent, using TANNEF! (4).

(H-Br and Br2) are isolated by trapping on potassium

(II) at the head

the S-bro-

adds a bromine molecule.

labeled with radioactive

the same reaction conditions

inorganic products

is reversible,

to find criteria of choice between the two mentioned

tion of n-butyl bromide is performed essentially

state by a delocalisation

;1Hz

radical should form an olefinthatsubsequently

The present work,tempts

to the

.

Taking into account that the first step of the reaction moalkyl

of the photobromination

( lm4) which mainly leads to the vicinal dibromide.

Two explanations

the relative

selectivity

c8) of the chromatographic

The

hexacyano-

column. The results of a ty-

are given below.

The following table is especially nic species is considered

significant when the activity of the inorga-

: this fraction only contains

this low yield is maintained

even when the reaction

3% of the total activity:

is allowed to proceed to lar-

ge extents. A second series of experiments

aimed to determine

317

the location of radioactive

I?o. 4

RADIOACTIVITY REGISTERED AFTER PHOTOBROMINATION OF LABELED n-BUTYL BROMIDE ___________________________--~~-~~~~~~~~___-_--_---------_~~~~~-----~~~~~~~~~~~~ Temperature

: 20' C - Reaction mixture Inorg.species

Activity(::) 13563 (%)

C+HgBr

1,1-dibromo

1,2-

1,3-

Tribromo

(t)

269064

4295

108164

21736

20169

4491

60

1

25

5

5

1

3

(t)Activity

of products

retained

bromine in the 1,2 dibromobutane lid sodium hydroxyde measured;the

between

peaks.

(::)

In

counts

per

minute.

obtained. The latter is dehydrobrominated

and the activity

activity

: 20 ul C,H,Br* + 5 ul Br2 + 20 ul CC10

is distributed

in the produced as follows:lO%

on so-

l- and 2- bromobutenes

is

as 2-bromo- and 90% as l-bro-

mobutene. To conclude

,the particular

by the decomposition

of the reaction cannot be justified

of the i3-bromoalkyl radicals

On the contrary,it gy of the reaction,due intermediate

selectivity

into free olefin and bromine

is possibly explained by a lowering of the activation to a stabilisation

radical.However,the

of the transition

not explain the results obtained by tion includes a bridged radical,the ver,a recent paper makes it possible mechanism

proposed by

explanation:

and high selectivity,it

TANNER(4).Furthermore,even extent of the rearrangement to reconcile

does

if the reac-

must be low.Howe-

our findings with a modified

TANNER(').

(1) P.S.JUNEJA,E.M.HODNETT,J.~.Chem.Soc.,l967,~,5685. (2) P.S.SKELL

et alii,J.Am.Chem.Soc., 1961,8&504;1963,~,2849,2'350:1964,~,3334.

(3) W.THALER,J.Am.Chem.Soc.,1963,85,2607. (4) D.D.TANNER,D.D.DARWISH,M.W.MOSHER,N.J.BUNCE,J.Am.Chem.Soc.,I969,~,7398. (5) P.I.ABELL,L.H.PIETTE,J.Am.Chem.Soc.,l962,~,916. (6)J.G.TRAYNAM,W.G.HINES,J.Am.Chem.Soc.,1968,90,5208. (7) G.A.RUSSEL (8) C.RONNEAU (9) D.D.TANNER

ener-

state by bridging the

bridged radical is not a sufficient

while in agreement with the observed rearrangement

et alii,J.Am.Chem.Soc.,1963,85,2983,2988. et alii,Radiochem.Radioanal.LettPrs,l971,~,1,67. et alii,J.Am.Chem.Soc.,1971,~,4802,5846.

atoms.