Infrared Aldehydes and Ketones PDF

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OBJECTIVES 1. To do comparison of carbonyl absorption frequencies of selected aldehydes and ketones, to illustrate the effect of substituents and conjugation. 2. To do the assignments of the main absorptions for individual aldehydes and ketones. 3. To analyze the spectra for bands resulting from the C=O groups and explain the absorptions

INTRODUCTION Firstly, infrared spectroscopy is the analysis of infrared light interacting with a molecule. This can be analysed in three ways by measuring absorption, emission and reflection. From IR spectroscopy, we can determine the functional groups as it can measures the vibrations of atoms. Generally, stronger bonds and light atoms will vibrate at a high stretching frequency. In this experiment, we will focus on the infrared spectra of aldehydes and ketones. Aldehydes and ketones are organic compounds which incorporate a carbonyl functional group, C=O. They will exhibit a strong C=O stretching band in the region from 1870 to 1540 cm-1 The stretching frequency for saturated aldehydes is at 1725 cm -1.

Next, aromatic

aldehydes absorb at lower frequencies compared to aliphatic aldehydes which absorb in the region from 1740 to 1720 cm-1. Aromatic aldehydes absorb at lower frequencies because internal hydrogen bonding shifts the absorption to lower frequencies. For carbonyl group of ketones, it will absorbs at slightly lower frequencies compared to aldehyde which at 1715 cm -1 for saturated ketones. Aldehydes tend to come at slightly higher frequencies than ketones because H is less electron-donating than an alkyl group.

APPARATUS AND MATERIAL 

Thin films, KBR pellet press, mortar and pestle



Benzaldehyde, cinnamaldehyde, n-butyraldehyde, benzophenone, cyclohexanone, and acetophenone.

HAZARD AND PRECAUTION

CHEMICAL

HAZARD

PRECAUTION

Benzaldehyde



Combustible liquid and vapor

Eliminate ignition sources (sparks, smoking, flames, hot surfaces) when working with flammable and combustible liquid and vapour.

Cinnamaldehyde



Harmful in contact with skin

Make sure wear protective gloves/protective clothing/eye protection/face protection before start the experiment.

Cyclohexanone



Causes severe eye irritation. May cause retinal damage that can lead to vision loss.

Immediately flush eyes with plenty of water for at least 15 minutes. Get medical aid immediately.

PROCEDURE 

Method

In sample preparation, thin films of about 0.015 to 0.025 nm thickness of the pure liquid for sample in liquid form were used, while KBr pellets for sample in solid form were prepared.

. The

infrared

spectra

of

benzaldehyde,

cinnamaldehyde,

n-butyraldehyde,

benzophenone, cyclohexanone, and acetophenone were analyzed.



Data analysis

The carbonyl absorption frequency was determined and the structure of the compound was written on each spectrum.

On the spectrum of benzaldehyde, assignments was made to the main bands found in the region around 3000 cm -1 and between 765 and 750 cm -1. The bonds or groups of bonds in the molecule responsible for these bands were indicated.

On the spectrum of cyclohexanone, similar assignments were made in the regions around 2900 and 1460cm-1.

The carbonyl frequencies of the aldehydes were compared; the effect of conjugation and aromaticity on the frequency of absorption of the carbonyl group for cinnamaldehyde and benzaldehyde as

compared with

n-butyraldehyde

were

Similar comparison of the carbonyl frequencies of the ketones regarding conjugation and aromaticity was made.

DISCUSSION

1. n-butyraldehyde

Spectrum shown above represents for n-butyraldehyde spectrum. This is because of C=O stretching observed at 1750 cm -1. The peak is strong and intense because of large C=O dipole moment. Also to confirm for an aldehyde compound, two weak peaks of C-H stretching will be observed at around 2750 cm -1 and 2850 cm-1 which also appeared at spectrum above. Lastly, C-H stretching for alkyl also observed at around 2900cm-1.

2. Benzophenone

Spectrum above is confirmed to be as a spectrum for benzophenone compound. First, a band for C=O stretching is appeared at around 1690 cm -1. The base value for ketone is 1715 cm -1 but conjugation of a carbonyl with a C=C bond shift the values to the lower frequencies. The carbonyl stretching absorption is one of the strongest IR absorptions, and is very useful in structure determination as one can determine both the number of carbonyl groups but also an estimation of which types; which for this spectrum is affirmed to be a ketone. Next, C-H stretching for aromatic (benzene ring) also observed at 3100 cm -1. Usually the absorption band for benzene ring looks like noise. Lastly, C=C stretching with multiple bands that appeared at range 1500 to 1600 cm-1 confirmed that this spectrum is for benzophenone.

3. Acetophenone

This spectrum is represent for acetophenone. From the spectrum, we can observed that an intense band appeared at around 1690 cm-1 which represent for C=O stretching for a ketone. C=O stretching of ketone supposed to appeared at 1715 cm

-1

but due to the delocalization

of electron in benzene ring, it shifted the value to lower frequency. Next, two bands at 1600 – 1610 cm-1 represent for C=C stretching. Usually for a C=C stretching, there will be multiple bands appeared. The C-H stretching for aromatic (benzene ring) also observed at 3100 cm-1. Last but not least, the band of C-H stretching for alkyl that observed at near 3000cm -1 is another indication that this compound is acetophenone and distinguish it from benzophenone.

CONCLUSION 1. Aliphatic aldehydes absorb in the region from 1740 to 1720 cm-1. Electronegative substitution on the -carbon increases the frequency of absorption of the C=O band. For instance, acetaldehyde absorbs at 1730cm -1, whereas trichloroacetaldehyde absorbs at 1768 cm -1. Conjugation of a double bond with the carbonyl group reduces the frequency of the carbonyl absorption. Aromatic aldehydes absorb at lower frequencies. Internal hydrogen bonding also shifts the absorption to lower frequencies. The carbonyl group of ketones absorbs at slightly lower frequencies than that of a corresponding aldehyde. A saturated aliphatic ketone has a carbonyl

absorption frequency of about 1715 cm-1; conjugation with a double bond causes an absorption shift to lower frequency. Intermolecular hydrogen bonding between a ketone and solvent such as methyl alcohol also decreases the carbonyl frequency. 2. The main absorption band for n-butyraldehyde is at 1750 cm -1 and for benzophenone and acetophenone is at 1690 cm-1 3. The spectra for bands resulting from the C=O groups are have a high intensity and

easily recognized in spectra analysis.

REFERENCES

1. Jacqueline, R. (2011). IR Spectroscopy Tutorial: Aldehydes. Retrieved from https://orgchemboulder.com/Spectroscopy/irtutor/aldehydesir.shtml#:~:text=IR %20Spectroscopy%20Tutorial%3A%20Aldehydes,1710%2D1685%20cm%2D1. 2. Chemistry LibreTexts. (2020). Spectroscopic Properties. Retrieved from https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Book %3A_Basic_Principles_of_Organic_Chemistry_(Roberts_and_Caserio)/16%3A_Carb onyl_Compounds_I_Aldehydes_and_Ketones._Addition_Reactions_of_the_Carbonyl_Group/16.04%3A _Spectroscopic_Properties 3. Venkatathri, N. (2015). Retrieved from https://www.researchgate.net/post/What_does_the_shift_of_IR_peak_to_higher_wav enumber_indicate 4. Characteristic IR Absorption Frequencies of Organic Functional Groups. Retrieved from http://www2.ups.edu/faculty/hanson/Spectroscopy/IR/IRfrequencies.html

QUESTION

1. Explain how a shift in the carbonyl frequency can occur if an alkyl group is replaced by a chlorine atom. A shift in the carbonyl frequency occurred when an alkyl group is replaced by a chlorine atom. This is because the chlorine atom is an electronegative species. When an electronegative species were introduced to the compound, the bond length may change due to the change in electronegativity of the neighbouring atom. This will result to the shift in absorption frequency.

2. At what frequency would you expect overtones for the C=O stretch of acetophenone? The overtones for the C=O stretch of acetophenone should be at around 3500 cm-1....