Coupling constant

COUPLING CONSTANT and FACTORS AFFECTING THEM

1. COUPLING CONSTANT p. harshini Yada m. pharmacy 17p91s030 svec

2. DEFINITION: The distance between the centres of the two adjacent peaks in a multiplet is usually constant and is called as coupling constant the Value of coupling constant is independent of the external magnetic field. It is measured in hertz ( hz ) (or) in cycles per second (cps ).It is denoted by the letter (j )

3. Coupling constant From the value of J we can distinguish between two singlets and one doublet ; two doublets from one quartert . This can be done by simply recording the spectrum at two different radio frequencies. The value of J generally lies between 0 and 20 hertz.

4. examples Methane CH4 Ethyl bromide CH3-CH2-Br Here under the influence of two equivalent protons of a the signal of protons for b will appear as a quartert . According to (n+1) rule.

5. Jab = 6.8 cps Jbc = 7.3 cps The signal for b protons will be split into a sextet. If the Jab and Jbc are equal, a symmetrical sextet would have been observed. Since the values of J are close enough, the signal for b protons miiltiplet will resemble as sextet.

6. And by the help of coupling contant we can distinguish between cis and trans form. Eg : CH3CH2Cl ( ethyl chloride ) cis – J = 10 Hz trans – J = 15Hz It is clearly noted that the value of coupling constant depends partly on the number of covalent bonds through which protons may interact and also upon the structural relationships between the coupled protons.

7. They are: 1. Geminal coupling 2. Vicinal coupling 3. Substituent effects 4. Long range coupling 1.) Geminal coupling : in the case of geminal protons, the protons are attached to the same carbon atoms. Where the protons are in different environment. Here the value of J depends on the bond angle.

8. Eg : 2,2-dimethyl 3-trimethoxy silyl butane. Here the two protons are attached to the same carbon atom. And the double bond is present. Where the different groups are attached to this double bond. The protons are located in the different environment. And they are not chemically equivalent They couple with each other.

9. a = 1+1 =2 ( doublet ) b = 1+1 = 2 ( doublet ) J = 1.4 Hz bond angle is directly proportional to the coupling constant. If bond angle increases value of coupling constant increases. bond angle J value 105˚ 109˚ 125˚ ˃125˚ -25 cps -12 cps 0 cps + cps

10. Substituent effect : The increase in the electro negativity atom (or) group, which withdraws sigma electrons, that increases the value of coupling constant. eg : methyl chloride -10.8cps methyl fluoride -9.4cps

11. The value of J decreases if an electronegative substituent withdraws electron from the П bonds which decreases the coupling constant. Eg : ethene J= +2.3 cps vinyl fluoride J = -3.3 cps compound J value methane methyl chloride methyl fluoride ethene formaldehyde -12.4 cps -10.8 cps -9.4 cps +2.5 cps +41.0 cps

12. Vicinal coupling : In vicinal coupling, the protons are separated by three bonds. Here the value of J varies with dihedral angle. let us consider gauche and anti confermations . gauche 2 to 4 cps anti 5 to 12 cps dihedral angle J value 0˚ (or) 180˚ 90˚ (+)large negative

13. Eg : cyclohexane In cyclohexane , the absorption due to axial and equatroial protons are different. For the axial – axial interactions, the dihedral angle is (180˚) nearly. So the coupling constant is approximately 8cps. Where as for axial- axial and equatorial – equatorial interactions the dihedral angle is 60˚,the coupling constant is approximately 2cps.

14. The values of vicinal coupling constants for a few compounds are listed below: Long range coupling : Generally no coupling is absorved if the distance between the two absorbing nuclei is more than three covalent bonds is known as long range coupling compound J value propane ( gauche ) propane ( anti ) dibromomethane (gauche) (anti) 7.2 cps 6.5 cps 15.1 cps 3.4 cps

15. unsaturated (or) appreciable coupling fluoro compounds observed with the help of high resolution spectrometers In pi bond system, appreciable couplings are frequently observed between the protons separated by even four (or) five bonds.

16. Eg : 2,4-dichlorobenzaldehyde coupling takes place between the aldehydic proton and a ring proton compounds j value propene butene 2,3 dimethyl butanoic acid -1.7 cps -2.3 cps -1.2 cps

17. REFERENCES : Elementary organic spectroscopy ( principles and chemical applications ) – y.r . sharma J.B. Leane and R.E.Rechards , Trans . Faraday soc.55,518 (1959 )