INTERHALOGEN

Having studied (in the previous two chapters) the e ffe c t of electron releasin g su b stitu en ts in the phenyl ring of the arsine (YC^H^^gAs on the formation of halogen adducts i t was decided to look in to the e ffe c t of the presence of an electron withdrawing group on the pheryl ring. For th is purpose the tri(flu orop h en yl)arsin es were chosen and the re su lts are described below.

- 45 -

(a) Reaction with bromine

The r e su lt of the conductometric titr a tio n s of t r i (2- fluorophenyl)arsine, t r i (3-fluorophenyl)arsine and tri(4-flu orop h en yl)arsin e with bromine in a c e to n itr ile are illu str a te d graphically in Fig. 8 (curves A, B, and C r esp e c tiv e ly ).

Unlike the tri(m ethylphenyl)arsine-brom ine titr a tio n s , the tri(fluorophenyl)arsine-brom ine titr a tio n s do not give graphs of the same form. The reaction of tri(2-flu orop h en yl)arsin e with bromine gave only a s lig h t increase in conductivity; th is rose gently up to the 2 : 1 mole ra tio a fter which the conductivity remained steady thus giving a rather in d e fin ite 2:1 break in the graph. The so lu tio n assumed a lig h t yellow colour from the sta r t of the titr a tio n and the u ltr a v io le t spectrum showed that tribromide ion Xmax = 2 6 9 np) was present from the sta r t of the titr a tio n .

In the titr a tio n s of tri(3-flu orop h en yl)arsin e and t r i ( 4 -fluorophenyl)arsine with bromine the conductivity increased only slig h tly up to the 1 : 1 mole ra tio but thereafter the conductivity rose sharply and a d istin c t break was observed a t the 2:1 ra tio . Thus, in these two systems 1 : 1 and 2 : 1 breaks were observed suggesting the formation of 1 : 1 and 2 : 1 adducts (R^AsBr^ and R^AsBr^) in the solu tion . In conjunction with the conductivity changes, d is tin c t colour changes were observed during the titr a tio n s; the so lu tio n remained colou rless up to the 1 : 1 ratio but th ereafter became yellow , and the u ltr a v io le t spectrum of the so lu tion a t selected points during the titr a tio n s showed that Br^" ion was present a fter the 1 : 1 ra tio (strong broad peak with Xmax = at 2 6 9 up).

45(a)

s

10

8

6

4

2

1 :1

2: 1

Mole r a t i o , Br^: R^As

Fig.

8.

Conductometric titration of R^As w ith

bromi ne

( A ) (2- FC^H^)gAs-Bf^

I B ) (3-FC^H^)gAs-Br^

— 46 ~

Molar Gonductivity values a t the 1:1 and 2:1 ra tio s are given in Table 11.

Table 11

System Ajq (1 :1 )/S cm^ mol"^ _m (2 :1 )/S cm^ mol"^

(2-FC^Hy)gAs-Br2 — 6 6 . 2 (C = 0.0022358)*_m (3-FCgH^^2As-Br2 1 7 . 2 8 105.1 (C^ = 0.0075014) (C^ = 0.0066815) (4-FC^Hy)gAs-Br2 16.8 1 1 2 . 1 (C^ = 0.0 0 9 8 2 6 8) (C^ = 0.0083853) * C = mol dm~^_m

These values suggest that a ll the 1:1 adducts are weak e le c tr o ly te s and that the 2 : 1 adducts in the case of the 3-fluorophenyl and 4-fluorophenyl isomers are strong e le c tr o ly te s . The molar conductivity and shape of the conductometric titr a tio n graph for the (2-fluorophenyl)arsine compound i s suggesting an equilibrium .

From the above observation i t i s suggested that with t r i (2~fluorophenyl)arsine the reaction proceeds d irectly to the tetrabromide adduct thus,

0 : 1---► 2 : 1 ra tio

RgAs + 2Br2 RgAsBr^ (. R_AsBr Br_")._{3 "‘ S}

With tri(3-flu orop h en yl)arsin e and t r i ( 4-fluorophenyl)arsine the stepwise reaction may be represented by the follow ing equations.

0:1— ♦ 1 : 1 ra tio

1: 1'

RgAs + Br^- 2 : 1 ra tio

RgAsBrg (: RgAsBr Br") RgAsBrg + Br^- RgAsBr^ (' RgAsBr Br^").

— 47 —

(b) Reaction with iodine

The r e su lt of the conduotometric titr a tio n s of t r i (2-fluorophenyl)arsine, t r i (3-fluorophenyl)arsine and t r i ( 4-fluorophenyl)arsine with iodine in a c e to n itr ile are illu str a te d in Fig. 9 (curves A, B, and C r e sp e c tiv e ly ).

The conductivity did not r ise sig n ific a n tly when iodine was added to the triC 2-fluorophenyl)arsine, the colour of the so lu tion changed to brown from the sta r t of the iodine addition and a weak absorption due to Ig" ( Xmax = 292, and 362 ra^) was observed in the u ltr a v io le t spectrum of the so lu tio n . The presence mainly of molecular iodine is indicated and i t i s concluded from these r e su lts that no reaction occurs. In the other two titr a tio n s [tri(3-flu orop h en yl)arsin e and t r i ( 4-fluorophenyl)arsine] the conductivity rose steep ly givin g a break a t the 2:1 r a tio . The solu tion , (o r ig in a lly the so lu tio n was c o lo u rless), assumed a brown colour after the f ir s t add ition of iodine. The u ltr a v io le t spectrum at selected points showed that triio d id e ion ( Xmax = 292, 362 mp) was present from the sta r t of the reaction.

Molar conductivity values a t the 2:1 mole ra tio are given in Table 12.

System A ( 2 : 1)/S om^ mol"^_{m . ___} C = mol dm"3_m

(3-FCgH^)3-l3 68.8 0.0066815

74.2 0.0 0 5 85 85

These values show that the 2:1 adducts are medium e le c tr o ly te s in a c e to n itr ile . These molar conductivity values, considered in conjunction with the fa ct that the conductometric titr a tio n breaks are

471a )

6

2

1

1

2 :1