An Atom Or Group Whose Presence Is Responsible For The Colour Of A Compound.docx

BS Hons. Chemistry (6th Semester)

Topic:Factors influencing the λmax value

Submitted to: Miss Shazia

Submitted by: Samra Ashiq (330) Asma Saeed (336)

Contents  Chromophore  Auxoxhrome  Conjugated system  Conjugated diene system  Woodward – Fieser’s rules for conjugated dienes, trienes, polyenes  Woodward-Fieser Rules for Calculating the λmax of Conjugated Carbonyl Compounds  Aromatic system

Spectroscopy “Spectroscopy is the branch of the science deals with the study of interaction of Electro Magnetic Radiation (EMR) with matter”

Absorption Spectroscopy: the type and amount of the radiation, which is absorbed depend upon the structure of the molecules and the numbers of molecules interacting with the radiation. The study of these dependencies is called absorption spectroscopy. (UV, IR, NMR, X-Ray

Factors influencing the λmax value:     

Chromophore Auxoxhrome Conjugated system Conjugated diene system Aromatic system

Chromophore An atom or group whose presence is responsible for the colour of a compound.The structural feature in a molecule , such as the carbonyl group in aldehyde and ketones, that is responsible for the absorption of electromagnetic radiation in the UV/VIS region, is called chromophore. The chromophore generally contain some degree of unsaturation. The wavelength of maximum absorption for a compound depends not only on the nature of the chromophore present in the molecule but also on the environment of the chromophore.

Types  

Dependent chromophore Independent chromophore

Independent chromophore When a single chromophore is sufficient to impart colour to the compound called independent chromophore.

Example Azo group (-N=N-) Nitro group (-NO)

Dependent chromophore When more than one chromophore is required to produce colour in the compound called dependent chromophores.

Example Carbonyl compounds, alkenes

Auxochrome The substituents which themselves do not show absorption i.e whish are not chromophores, but when attached to a chromophore they cause an increase in the intensity of the absorption, and possibly the wavelength , are called as Auxochrome.

Example The methoxy group in methyl vinyl ether CH2=CHOCH3 shift the π- π* absorption of the ethylenic double bond from 171 nm to 190 nm.

Effect of conjugation All auxochromic group contain non bonding electrons which are in chromophore. It is this conjugation which shifts the λmax to a longer wavelength.

Types    

Bathochromic effect or red shift Hypsochromic effect or blue shift Hyperchromic effect Hypochromic effect

Bathochromic effect or red shift The shift of the λmax to a longer wavelength.

Hypsochromic effect or blue shift A shift to a shorter wavelength.

Hyperchromic effect An increase in the intensity of absorption caused by a substituent.

Hypochromic effect Decrease in the intensity of absorption.

Conjugated systems The conjugation of an auxochrome with a chromophore shifts the π- π* absorption to a longer wavelength. The most dramatic effect is observed when a chromophore is in conjugation with another chromophore as, e.g. in 1,3-butadiene or in α,β-unsaturated carbonyl compounds. In fact the importance of UV/VIS spectroscopy lies in its ability to determine the presence,nature, and extent of conjugation.

Example 1 1,3-butadiene absorbs at 217 nm ,whereas ethylene absorbs at 171 nm. In 1,3-butadiene, the π orbital of the two ethylenic bonds combine to form four new π molecular orbitals, two bonding(π 1 and π2) and two anti bonding (π*3 and π*4)

Energy gap between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) in butadiene is smaller than in ethylene(ethene) Addition of successive conjugated double bonds progressively narrows the gap between HOMO and LUMO Thus longer wavelength (lower energy) radiation required for the transition

Example 2 In case of 1,3,5-hexatriene,the difference in energy between the highest occupied molecular orbital and the lowest unoccupied molecular orbital is further reduced and absorption occur at a still longer wavelength 1,3,5-hexatriene absorbs at 258 nm.

Thus progressively increasing conjugation moves the absorption maxima to longer wavelength and finally into visible region.

The compounds having a conjugated system of nine double bonds are usually yellow in color. The red colour of tomatoes and carrots arise from conjugated molecules of this type.

molecule

wavelength of maximum absorption (nm)

ethene

171

buta-1,3-diene

217

hexa-1,3,5-triene 258

The same principle applies when unlike chromophores are in conjugation with each other as, e.g, in α,βunsaturated carbonyl compounds. Methyl vinyl ketone, CH2=CHCOCH3, shows π- π* absorption at 215 nm, while neither the carbonyl group nor the isolated double bond shows π- π* absorption above 200 nm.

Conjugated diene system The effect of substituents on the wavelength of absorption especially in dienes, is additive so that the position of absorption is influenced in a systematic way. Certain empirical rules were formulated by Woodward, and later amplified by Fieser, to predict the position of absorption in open chain dienes and dienes in sex membered rings.( the rule show much better agreement of dienes in rings). The successful application of these rules lies in choosing the correct base value of the diene,and then adding the contribution made by each additional feature. It is therefore important to recognized the diene system correctly.

Types There are two distinct diene system

 

Heteroannular diene Homoannular diene

Homo-annular conjugated double bonds Are the conjugated double bond present in the same ring. it is also called as Homodiene. some examples of this type are:

Hetero-annular conjugated double bonds Are the double bonds which are present in different ring. for example.

Endocyclic double bond The double bond present in a ring only (inside).

Exocyclic double bond The double bond is a part of the ring (outside).

Woodward – Fieser’s rules for conjugated dienes, trienes, polyenes According to this rules each type of dienes or triens system is having a certain fixed value at which absorption takes place.

This constitutes the basic value or parent value.

This value of absorption maximum depends upon …… (I) The number of alkyl substitution or ring residues (II) The number of double bonds which extending conjugation (III) The presence s of polar groups such as Cl, Br, OR, are added to the basic value to obtain λ max for a particular compound. The parent values and increment for different substituents / groups (A) Parent value :-

1) Homo annular conjugated diene

λ 253 nm

2) Hetero annular conjugated diene Acyclic conjugated diene

λ 214 nm

3)Acyclic conjugated diene

λ 214 nm

4) Butadiene system open chain (Acyclic)

λ 217 nm

5) Acyclic triens

λ 245 nm

(B) Increment :1) Each alkyl substitution or ring residue

λ + 5 nm

2) Exocyclic double bond

λ +5 nm

3) Double bond extending conjugation

λ +30 nm

(C) Auxochromes :-O- alkyl

λ +6 nm

-S-alkyl

λ +30 nm

-Cl, -Br

λ +5 nm

-N(alkyl)2

λ +60 nm

-OCOCH3

λ

0 nm

EXAMPLES 1. Calculate absorption maximum in UV Spectrum of 2,4 Hexadiene. CH3 H3C

Butadiene system; basic value = 217nm 2 Alkyl substituent(2X5nm) = 10nm Calculated value Observed value

= 227 nm = 227 nm

2. Calculate λ max for 1,4- dimethylcyclohex-1,3-diene

Parent value for homoannular ring : Two alkyl substituents : Two ring residue :

= 253 nm 2 x 5 = 10 nm 2 x 5 = 10 nm

calculated value : observed value :

= 273 nm = 263 nm

3. Calculate λ max of following compound

Parent value for heteroannular diene : = 214 nm Four ring residue : 4 X5 = 20 nm Calculated value : Observed value :

= 234 nm = 236 nm

Parent value for heteroannular diene : = 214 nm 3 ring residue : 3 X5 = 15 nm Exocyclic double bond = 5 nm Thiomethyl substituent =30 nm Calculated value : Observed value :

= 264 nm = 268 nm

Acyclic butadiene = 217 nm Extended conjugation = +30 nm Calculated value

= 247 nm

Acyclic base 2 alkyl substitution Calculated value

= 217 nm = 10 nm = 227 nm

Parent value for heteroannular : Four ring residue : Exocyclic double bond Calculated value : Observed value :

Parent value for homoannular : Extended conjugation Alkyl substitution Calculated value :

= 214 nm 4 X5 = 20 nm =5 nm = 239 nm = 236 nm

= 253 nm 1 X30 = 30 nm 2x5 =10 nm = 293 nm

Parent value for homoannular (the highest value) : Extended conjugation 1 X30 Exocyclic double bond Alkyl substitution or ring residue 6x5

= 253 nm = 30 nm =5 nm =30 nm

Calculated value :

= 318 nm

Woodward-Fieser Rules for Calculating the λmax of Conjugated Carbonyl Compounds Woodward-Fieser rules can be extended to calculate the λmax of α,β-unsaturated carbonyl compounds. In a similar manner to Woodward rules for dienes discussed previously, there is base value to which the substituent effects can be added and the λmax can be calculated using the formula:

λmax = Base value + Σ Substituent Contributions + Σ Other Contributions

Core Chromophores With Base Values As Woodward and Fieser have listed, α,β-unsaturated carbonyl compounds have a range of influence on the λmax of the molecule depending upon: 1] The type of carbonyl functionality present. For example, α,β-unsaturated aldehyde contribute 210 nm while α,β-unsaturated ketones contribute 215 nm and α,β-unsaturated esters contribute 195 nm.

2] If the core is a part of a cyclic ring. For example, cyclopentenone contribution is 202 nm whilecyclohexenone is 215 nm.

3] If the conjugation is extended to γ,δ-positions to form dienes. For example, in such cases, a simple addition of 30 nm to the base value of the α,β-unsaturated carbonyl compound gives appropriate estimates to the observed influences.

Note- In cases of α,β-γ,δ-diene carbonyl compounds like those shown above, the extended conjugation at the α,β-γ,δ-positions is accounted for in the base value of the core chromophore and need not be added separately. If however there is another substituent at the α,β-γ,δpositions, then you must add an additional + 30 nm for each. Also the bond shown as β-γ is not counted as β substituent but as a part of the core chromophore and need not be added separately.

Substituent Effects

According to Woodward, in case of α,β-unsaturated carbonyl compounds, the location of the substituent is significant in determining the influence on the wavelength of maximum absorption. Substituents can be located on either α, β positions. If the conjugation is extended

to γ and δ positions, then substitutions at these position also play a vital role in determining the practical λmax.

1] Substituents at α-Position As we can see the from table 3 below the effect of different substituent when placed on the αposition. Effect of substituents on the α-position of α,β-unsaturated carbonyl compounds Substituent

Influence

-R (Alkyl group)

+ 10 nm

-OR (Alkoxy group)

+ 35 nm

-Cl (Chloro group)

+ 15 nm

-Br (Bromo group)

+ 25 nm

-OH (alcohol/hydroxyl)

+ 35 nm

-OC(O)R (Acyloxy/Ester)

+ 6 nm

2] Substituents at β-Position As we can see the from table 4 below the effect of different substituent when placed on the βposition. Effect of substituents on the β-position of α,β-unsaturated carbonyl compounds Substituent

Influence

-R (Alkyl group)

+ 12 nm

-OR (Alkoxy group)

+ 30 nm

-Cl (Chloro group)

+ 12 nm

-Br (Bromo group)

+ 30 nm

-OH (alcohol/hydroxyl)

+ 30 nm

-OC(O)R (Acyloxy/Ester)

+ 6 nm

-SR (Sulfide)

+ 85 nm

-NR2 (Amine)

+ 95 nm

3] Substituents at γ and δ-position As we can see the from table 5 below the effect of different substituent when placed on the γ or δ position. Effect of substituents on the γ or δ position of α,β-γ,δ-diene carbonyl compound. Substituent

Influence

-R (Alkyl group) (both γ and δ)

+ 18 nm

-OC(O)R (Acyloxy/Ester) (both γ and δ)

+ 6 nm

-Cl (Chloro) (both γ and δ)

+ 12 nm

-Br (Bromo) (both γ and δ)

+ 30 nm

-OH (alcohol/hydroxyl group) (only γ)

+ 50 nm

-OR (Alkoxy group) (only γ)

+ 30 nm

Other Contributors 1] Exocyclic Double Bonds In general exocyclic double bonds add an additional + 5 nm to the base value. In order to identify exocyclic double bonds we recommend you read the previous chapter on how to use Woodward-Fieser rules to calculate the λmax of conjugated dienes and polyenes. We have explained it extensively there. 2] Solvent Effects Since carbonyl functional groups have polarity, solvents play an important role in how the electronics of the structure play out. The rules are simple and straight forward: Solvent

Influence

Water

– 8 nm

Methanol/Ethanol

– 1 nm

Ether

+ 6 nm

Hexane / Cyclohexane

+ 7 nm

3] Homoannular Cyclohexadiene In a special case where you have α,β-γ,δ-diene carbonyl compound and both the double bonds are present within one ring system you get a homoannular or homocyclic cyclohexadiene carbonyl compound. In such a case you must add an additional 35 nm to the system.

Aromatic system Benzene display three absorption bands, all originating from π- π* transitions. It absorbs at 184 nm (Ɛ, 60,000) and at 204 nm (Ɛ, 7,400) and has a fine structure (multiple peaks) in the region between 230 and 270 nm with λmax at 254 nm of relatively low intensity (Ɛ, 200) resulting from forbidden transition, due to the loss of symmetry caused by molecular vibrations. The first two bands (ethylenic) of benzene are generally designated as E1 and E2 bands, while the fine structure (benzenoid) is designated as B band. The fine structure is characteristic of almost all the simple aromatic compounds.

In fact, most aromatic compounds consist of three absorption bands, all due to π- π* transitions. However the position of those bands may be modified (generally move to higher wavelength) in substituted benzene and other aromatic compounds.

Unfortunately, prediction of the effects of various substituents is not always possible in the manner similar to that in the case of dienes and α,β-unsaturated carbonyl compounds; only some of the observed trends can be described. Substituents of simple alkyl groups on the benzene ring produces a small bathochromic effect with a slight increase in intensity, but does not destroy the structure. the red shift is attributed to hyperconjugation in which methyl group is more effective than other alkyl groups. Substituents of auxochromic groups (OH,NH2, etc) causes a pronounced red shift with large intensification, and loss of the fine structure, because of the conjugation between non bonding and the π electrons. Generally, the more extensive the conjugation, the less obvious is the vibrational fine structure. Altering the availability of the non bonding electron will alter the wavelength of maximum absorption. For example conversion of phenol to its anion results in a pronounced red shift and an increased intensity. Similarly with aniline is converted to the anilinium cation, it results in a blue shift and a spectrum almost identical to that of benzene is obtained , except that the fine structure is absent,

attachment of an unsaturated group such as ethylenic, acetylenic, nitrile,aldehydic, carboxylic and nitro, to the benzene ring produces a red shift often a great degree. The condensed rings aromatic hydrocarbons retain the absorption pattern, including the fine structure, typical of benzene. However, as the number of condensed rings increases, the absorption is shifted progressively to longer wavelength, until it appears in the visible region. E2 bands of naphthacene (λmax 480nm ;Ɛ, 11,000)and pentacene (λmax 580 nm; Ɛ, 12,600) occur in the visible region.

Reference 1. Woodward, R. B. Structure and Absoprtion Spectra. IV. Further Observations on α,βUnsaturated Ketones. J. Am. Chem. Soc. 1942. 64(1), 76-77. 2. Woodward, R. B. Clifford, A. F. Structure and Absoprtion Spectra. II. 3-Acetoxy-Δ5-(6)-norcholestene-7-carboxylic Acid. J. Am. Chem. Soc. 1941. 63(10), 2727-2729. 3. Spectroscopic Determination of Organic Compounds, 5th Edition, Silverstein, Bassler, Morrill 1991. 4. Kalsi, P. S. Spectroscopy of Organic Compounds. 6th Edition, New Age International Publishers, New Delhi, 2004. 5. Glagovich, N. Woodward’s rules for conjugated carbonyl compounds. [Online]http://www.chemistry.ccsu.edu/glagovich/teaching/316/uvvis/conjugated.html ( Accessed on July 27, 2012). 6. Author Unknown. Michigan State University – Department of Chemistry. Woodward-Fieser Rules for Calculating the π to π* λmax of Conjugated Carbonyl Compounds. [Online]http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/Spectrpy/UVVis/uvspec.htm (Accessed on July 27, 2012).