2.Thin layer chromatography

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Evaluation Seminar on U-V Spectroscopy :

Evaluation Seminar on U-V Spectroscopy Presented by:- Gulfishan under the guidance of Mr Manzoor Ahmed

Slide 2:

Chromophore Auxochrome UV Shifts Effect of solvent Applications of UV Topics to be covered-

Slide 3:

Chromophore is defined as any isolated covalently bonded groups that shows a characteristic absorption in the ultravoilet or the visible region . Compound containing chromophore is chromogen Some of the important chromophores are ethylenic, acetylenic, carbonyls, acids , esters , nitrile group etc Chromophore (chroma- colour phoros - bearer)

Slide 4:

There are two types of chromophores a) Chromophores in which the group contains π electrons & they undergo n→ π * transitions eg: ethylenes, Acetylenes b) Chromophores which contain both π electrons & n- electrons(non bonding) such chromophores undergo two types of transitions ie π → π * & n→ π * eg: carbonyls,nitriles,azo,nitro

Slide 5:

Chromophore Transition λ max ε max solvent >C=C< -C=C- >C=O 0H І -C=O -N=N- -NO 2 π→π * π→ π * π→ π * n→ σ * n→ σ * n→ σ * n→ σ * n→ σ * 175 175 180 160 285 205 338 274 15,000 10,000 10,000 18,000 15 60 5 15 vapour Hexane Hexane Methanol Ethanol Methanol

Slide 6:

Auxochrome An auxochrome can be defined as any group which does not itself act as chromophore but whose presence brings about a shift of the absorption band towards the red end of the spectrum ( longer wavelength) Some common auxochromic groups are -OH ,-OR, -NH 2 , -NHR,-NR 2 , -SH

Slide 7:

Trans azo benzene λ max =320nm Trans p - ethoxy azo benzene λ max=385nm The presence of -OC 2 H 5 act as auxochrome increases the value of λ max

Slide 8:

Absorption & intensity shifts The position of absorption maxima & intensity of absorption can be modified in different ways by some structural changes or changes of solvents

Slide 9:

Wavelength( λ) Absorbance (Emax) Red shift Blue shift Hypochromic shift Hyperchromic shift

Slide 10:

Absorption & Intensity Shifts Bathocromic Shift ( Red Shift) : It involves the shift of absorption maximum towards the longer wavelength becoz of the presence of certain groups such as OH & NH 2 called auxochrome or by change of solvent

Slide 11:

Hypsochromic Shift (Blue shift) : It involves the shift of absorption maximum towards shorter wavelength . It may be caused by the removal of conjugation & also by changing the polarity of the solvent

Slide 12:

Incase of aniline, absorption maximum occurs at 280nm becoz the pair of electrons on nitrogen atom is in conjugation with the π bond system of the benzene ring . In its acidic solution , a blue shift is caused & absorption occurs at shorter wavelength + C 6 H 5 --N H 3 ion formed in acidic solution the electrons pair is no longer present & hence conjugation is removed

Slide 13:

Hyper chromic Shift : This effect involves an increase in the intensity of absorption & usually brought about by introduction of an auxochrome. For e g : Introduction of methyl group in position 2 of pyridine increases ε max ( λ max 262nm) from 2750 to 3560 ( λ max 262nm) for π→π * transition

Slide 14:

Hypo chromic Shift It involves a decrease in the intensity of absorption & is brought by groups which are able to distort the geometry of of the molecule for eg: when a methyl group is introduced in position 2 of biphenyls group hypo chromic effect occurs because of distortion caused by methyl groups

Slide 15:

Solvent effects A most suitable solvent is one which doesn’t itself absorb in the region under investigation. A dilute solution of the sample is always prepared for the spectral analysis Solvents Wavelength limit Ethanol 210 Hexane 210 Methanol 210 Cyclo hexane 210 Diethylether 210 Water 205 Benzene 280

Slide 16:

Benzene 280 Chloroform 245 Tetra hydro furan 220 Carbon tetra hydro chloride 265 Hexane & other hydrocarbons can be used as these are less polar & have least interactions with the molecule under investigation Thus in general the absorption maximum for non polar compounds is the same in alcohol (polar) as well as in hexane ( non polar )

Slide 17:

The absorption maxima for the polar compounds is usually shifted with the change in polarity of solvent. α, β –unsaturated carbonyl compounds show different shifts 1) n→ π * transition ( less intense ) The absorption band moves to a shorter wavelength by increasing the polarity of solvent 2) π→π * transition ( more intense ) The absorption band moves to longer wavelength by increasing the polarity of solvent

Slide 18:

Effect of solvent polarity on various type of bands K- band : Due to conjugated dienes they are not effected by changing the polarity of the solvent while these bands due to enones shows a red shift by increasing the polarity R-band : The absorption shifts to shorter wavelength ( blue shift ) with the increase in polarity of the solvent B-band : The position as well as the intensity of the band is not shifted by increasing the polarity of solvent

Slide 19:

Effect of temperature & Solvent on the fineness of absorption band : As the temperature is decreased , vibrational & rotational energy state of the molecule are also lowered, when the absorption of light occurs at lower temperature , smaller distribution of excited states results. It produces the finer structure in the absorption band than higher temperature

Slide 20:

If the dielectric solvent of the solvent is high, there will be stronger solute- solvent interactions. Due to this, vibrational & rotational energy states of molecules increases & thus fineness of absorption band falls

Slide 21:

Conjugated dienes for eg: Ethylenes ( one double bond) absorbs at 170nm ( π→π * transitions ) while 1,3- butadiene ( two double bond in conjugation) absorbs at 217nm The bathochromic shift is more if the double bond are in conjugation as compared to isolated double bonds

Slide 22:

Applications of UV Spectroscopy Detection of functional group : If a spectrum is transparent above 200nm it shows the absence of 1) conjugation 2) carbonyl group ( aldehydes & ketones) 3) benzene or aromatic compounds 4) bromo or iodo compounds Extent of conjugation : Addition in un saturation with the increase in the number of double bonds( increase in value of n ) shifts the absorption to longer wavelength

Slide 23:

Distinction in conjugated & non conjugated compounds 0 0 ║ ║ ( CH 3 ) 2 = CH—C—CH 3 CH 2 === C —CH 2 —C—CH 3 │ CH 3 Identification of unknown compounds Examination of poly nuclear hydrocarbons

Slide 24:

Elucidation of the structure of vitamin A & K The absorption maxima for K 1 & K 2 are 243, 249, 260,269 & 330nm The vitamin A 1 absorbs at 325nm & absorption maxima for vitamin A 2 is 287 & 351nm The absorption maxima appear at longer wavelength for vitamin A 2 due to the presence of additional ethylenic bond

vitamin A2 vitamin A1 λmax = 351nm λmax = 325nm :

vitamin A 2 vitamin A 1 λ max = 351nm λ max = 325nm

Slide 26:

Preference over two tautomeric forms Consider 2- hydroxy pyridine which exists in equilibrium with its tautomeric form pyridone -2 → 2 - Hydroxy pyridine pyridone 2 The spectra of these two compounds were found to favour pyridone - 2 which is an α , β - unsaturated ketone & equilibrium is shifted towards the right ie pyridone-2

Slide 27:

Identification of compounds in different solvents Chloralhydrate shows an absorption maxima at 290nm in hexane while absorption disappears in aqueous solution Determination of configurations of geometrical isomers : Thus cis forms suffer distortion & absorption occurs at lower wavelength Cis –Stilbene - λ max - 283nm Trans – Stilbene - λ max - 295.5nm

Slide 28:

Cis Stilbene λ max = 283nm Tranz stillbene λ max= 295.5nm

Slide 29:

Distinguishes between Equatorial & Axial conformation : The n→ π * ( R- band ) which appears at longer wavelength in α , β - unsaturated ketones is influenced by the presence of polar groups in γ position. It has been noted that the effect of an axial substituent to displace the R band to longer wavelength is greater compared to that observed in its equitorial isomer

Slide 31:

Determination of strength of hydrogen bonding Solvents like water ethanol from hydrogen bonds with the n electrons of carbonyl oxygen. Due to this the energy of n electrons in the ground state is lowered depending upon the strength of hydrogen bonds. The n→ π * transitions of carbonyl compounds is shifted towards shorter wavelength

Slide 32:

Hindered rotation & conformational Analysis: Incase of 2,3- di- ter butyl - 1,3 butadiene there is little conjugation between two double bonds due to bulky ter butyl groups. It shows absorption at lower wavelength ( λ max 180nm) compared to 2,3 – dimethyl 1,3-butadiene ( λ max 225nm) in which there is no deviation from co planarity

Slide 33:

2,3 di - tert butyl 1,3 – butadiene λ max=180nm 2,3 dimethyl 1,3-butadiene λ max=225nm

Slide 34:

References Sharma B.K., Instrumental Methods for Chemical Analysis, 22th Edition, Krishna Prakashan Media Pvt. Ltd. Chatwal G.R., Instrumental Methods of Chemical Analysis, 5 th Edition, Himalaya Publishing House. Sharma Y.R., Elementry Organic Spectroscopy, 1 st Edition, S.Chand & Company Ltd.

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