Indian Journal of Chemistry Vol. 40B, January 2001, pp. 25-31

Cyclic voltammetric investigation of the 6-keto 9-17 mono methyl substitute of octadecanoic acids

Seyfettin Erturan*, Mustafa Yal

26 INDIAN J CHEM, SEC B, JANUARY 2001

c

(b) c

(a)

0 .5 1.0 1.5 2.0 2.5EN

0.5 1.0 1.5 2.0 2.5 EN

Figure l-Cyclic vo\tammograms for 5.10.4 M (a) of 6-keto -9-methyl octadeconoic acid at scan rates 25-50-75 and 100 mVs·1 ; (b) of 6-keto -14-methyl octadecohoic. acid at scan rates 50 and 100 mVs·1 in CHJCN+O.IMTBAFB .

electrode, to minimize ohmic drop and filled with the support ing electrolyte solution. The counter electrode is a Pt while in the proceding discussion it is mentioned pt foils. All cases except for the controlled potential coulometry, where it is a Pt foil in a glass tube, separated from the solution by a glass frit. The peak potentials and current are evaluated after subtraction of background current with the aid of a program running on a Illicrocomputer. Spectrophoto-metric measurement s are taken using a 1000 series Unicam-Mattson spectrophotometer.

Results and Discussion In acetonitrile solution, all 6-keto 9-17 mono-

methyl substituted octadeconoic acids studied undergo a two-electron anodic process at platinum

electrode between 0.55 and 2.3 V (V3. TBAFB). The CV wave splits into two waves, while on increasing the scan rate the first peak decreases.Each corresponding to one electron, whi le the first one shifts to more negative potentials,the CV wave tends to be reversible but the second peak is irreversible (Figure la,it» .The anodic counterpart develops the peak current ratio, Ip.lIpc,estimated according to Nicholson and Shain 12, increases with increasing scan rate and reaches unit at v =2.0 V.Furthermore, the peak current, Ipa, depends on the sq are root of the scan rate, V"2, with a correlation coefficient of 0.485.0n increasing the scan rate,the peak potential does shift to more negative values, the current functIon, Ip.lv ll2, decreases,and the peak width, Ep-Epl2, increases. These data suggest that 6-keto 9-17

ERTURAN el al. : CYCLIC VOLT AMMETRlC rNVESTIGA TION OF OCT ADECONOIC ACIDS 27

Table I-Voltammetric data for the anodic process for the nine different 6-keto 9-17 mono methyl substituted octadeconoic acids

Compd

6-Keto-9-methyl octadeconoic Acid

6-Keto-1 O-methyl octadeconoic Acid

6-Keto- ll-meth yl octadeconoic Acid

6-KelO-12-methyl octadeeonoi c Acid

6-Keto- 13-methyl octadeconoic Acid

6-Keto- 14-methyl octadeeonoie Acid

6-Keto- 15-methyl octadeconoic Acid

6-Keto-16-methyl octadeconoic Acid

6- Keto-17-methyl octadeconoic Acid

mono methyl oc tadeconoic acids give a two-electron diffusion-contro lled CV wave following an EEC mechanism in which the difference between the standard electrode potentials for two electron transfer steps . The anodic reaction ,which involves slightly more than one electron per mole of formate by coulometry, produces CO2 and RH.The reaction scheme outlined in steps 1-4 accounts for the products.

RCOOH ~ RCOO· + H;ad)

RCOO- -~.) RCOO(ad)

RCOO'(ad) ~ CO2 + R'(ad)

R'(ad) + H'(ad) ----7 RH

... (1)

... (2)

.. . (3)

... (4)

The separation of oxidation and reduction potential s, EPa and Epe" respective ly, was in the range of 15-108 mV for the scan rate of 50 mVs-t • The peak-current ratio erP.npe) ranging from 0.65 to 1.20 at v:50 mVs-l, indicates that radical formate are unstable in acetonitrile solution, undergoing probably subsequent chemical reactions. In addition, plot of peak current versus yll2 between 25 and 100 mVs·1

w~r~ lill ~ar iilJ icatillg a Jiffusioll -colltrollcd process .The peak potential separat ion, L\Ep. and the peak current ratios Ip.lIpc obtained from the cyclic voltammograms at 50 mV s· 1 are summ;,Jri zed In Tallie L

Ipiil A) Ep,(V) 6Ep(mV) Ip/lp,

0.65 1.05 28 0.6 0.70 1.35 45 0.8 0.65 1.05 29 0.6 0.72 1.40 50 0.9 0.70 1.1 8 28 0.7 0.80 1.75 85 1.0 0.68 1.18 28 0.6 0.75 1.45 65 0.9 0.71 1.1 8 28 0.7 0.85 1.78 68 1.0 0.75 1.20 30 0.8 0.90 1.85 95 1.1 0.77 1.22 32 0.8 0.93 1.90 100 1.1 0.80 1.25 35 0.9 0.95 1.98 108 1.2 0.67 1.20 30 0.7 0.73 1.73 83 0.8

In order to examine process in more detail , cyclic voltammograms were evaluated according to the theory of Nicholson and Shain . 13 Analysis of voltammograms showed that the studied 6-keto 9- I 7 methyl octadeconoic acid presented a "kinetic case" i.e. homogeneous chemical reaction was coupled with the electrode process. A most useful diagnos tic criterion for these kinds of mechani sms is reverse current and the investigation of the behavior of reverse transition times . The EEC can be represented is the same case, because in any event it shows the same qualitative characteristics. EEC-shows the same

behavior as the corresponding one- or two-step mechanism· i.e., E, and EC respective ly . By using these diognostic criteri a (Ipa! IPe ; V), the data obtained from Figure 2 are shown in Table II .

During the anodic oxidation after the first scan~ig peak (Figure I a, called peak A) at 1.05V di sappears and the peak potential depends on the scan rate. The half peak potential v is given by:

dEpl2 _ RT

d In v 2n a F

at 25°C a tenfold increase in V causes the oxidation peak to shift by 30/n mV in the positive direction as shown in:

Epl2 (v)=Eo -{0.07/n)- (O.029/n) log K +(O.029/n) log V (K=k l+k2)

28 INDIAN J CHEM, SEC B, JANUARY 2001

2.1

1.9

1.7

u 15 E-m E-

1.3

1.1

0.9

0]

0 100 150 200 250

V (rrW/s)

Figure 2--Ratio of anodic 'to cathodic peak currents as a function of rate of voltage scan for the 6-keto 9-methyl Octadeconoic acid

Table II-The diagnostic criteria of 6-keto-9:methyl octadeconoic acid

V (mV/s) Epa (V) Epc (V) Ipa/Ipc

50 1.05 0.88 1.00

lOG 1.12 0.90 1.40

150 1.20 0.95 1.65

200 1.35 1.00 1.78

250 1.40 1.10 1.85

300 1.45 1.15 1.90

The linear plots of peak current (Ip) vs" concentration are shown in F igu re 3. IPe increases with increasing scanning rate as shown in Figure 4 . The shift of Epn and Ip/lpe with dimensions K ')..In are also shown in Figures 5-6,and in Table I II .

Log Epa12 varies linearly with log v, the slope being 60/n mY. Finally the plot reaches a plateau whose height is dependent of the .potent ial-sweeping rate. The run was over before significant conversion of A to B can occur. The reaction proceeding had little effect on the electrochemical response.Here, the electroactive species (A.B) was generated by a reaction that precedes the electron transfer at the electrode in equilibrium with the reducible form of the electroactive species. A working curve showing the ratio of the diffusion-controlled current had been shown to fit the empirical equation \3 (Table I V. Figures 7-8).

6

'P. I

• eM

I

/

6

Figure 3-Plot of Ip vs. concentration for 6- 'eto-9-methyl octa-deconoic acid in acetonit rile, cl=2.0xlO-4 M, c2=5.0xlO·4 M, c3=7.5xlO·4 M, C4= 1.0x l 0.3 M.

The change in the oxidation potentia ls can be explained by the inductive and mesomeric effects of the keto (C=O) and methyl (-CH,) group contained in the molecu les . Substitll ted methyl group was a group expell ing electrons and that was why the inductive effect of substituted methyl group on carboxyl (-COOH) group was the unperturbed Nernstian behavior l4 •

ERTURAN et al.: CYCLIC VOLT AMMETRIC INVESTIGATION OF OCT ADECONOIC ACIDS 29

0.5 I I I I

-<

0.41 -( -

~0.46 -~ If

I 30 INDIAN J CHEM, SEC B, JANUARY 2001

0.7

0.6

O~

Figure 6-Variation of Epn with log KA. I12 for 6-keto-9-17 mono methyl subsituted octadeconoic acids

/

Ep2-' -0 -

- O.t,

Figure 8-Potential axis is in n(Ep/2- E I12)- RTIF InCKlI+K)

When the sweeping rate was increased the oxidation peaks for the reaction (1) and (2) were joining and giving only a ·single peak. The third peak which was appearing in the voltage range of 1.55-1.75Vwas representing particaJly adsobed hydrogen radical ;and the oxidation peak for adsorbed octadeconoic acid radical [see reaction (5), (6) and Figure Ib at point C] . The cations forming in this oxidation were unstable and therefore it could not be detected.For this reason, the reaction (3) was slow and on the other hand, the reaction (4) was too fast.

H' ~-~H+ (ad)

RCDO'(ad) ~ R + +C02

... (5)

.. . (6)

-' 7 I :O }() ' _\

\ '-

r 1) :'1. ., o.s ,- I . ,~

The anodic decarboxylation reaction had generally formed a mixture of products. This complexity results, in part, because both the initial electrode reaction product and subsequent intermediates may be capable of undergoing both electrochemical and chemical reactions which can lead to compounds characteristic of EEC reactions.Thi s situation is consistent with the information given in the literature Is. This would imply that the adsorbed species radical had a life time of at least a minute fraction of second, which was many orders of magnitude longer than the lifetime of free radicals . It was to be expected that when a mixtore of organic

molecules is involved,cathodic reactions will often have important role in determi ning product composition. !Furthermore, there had rarely been any attempt to control the anodic potential; any specificity which might be possible was likely to be lost without potential control.

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ERTURAN et al.: CYCLIC VOLT AMMETRIC INVESTIGA nON OF OCT ADECONOIC ACIDS 31

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