University Chemistry: Infrared Spectroscopy Assignment and Analysis

This assignment delves into the principles and applications of infrared (IR) spectroscopy. It begins with an overview of IR spectroscopy, explaining how it is used to identify functional groups within molecules by detecting the absorption of infrared light at specific frequencies. The assignment explains the concept of the fingerprint region and how it is used to differentiate molecules. It explores the identification of functional groups in the non-fingerprint region and uses this to determine whether a spectrum belongs to butan-2-ol or butanone. The assignment also details the use of IR spectroscopy to distinguish between alcohols, carboxylic acids, and esters through five-zone analysis. Furthermore, the assignment provides solutions for deducing the structure of organic molecules using empirical formulas, mass spectrometry, and H-NMR spectroscopy, with detailed explanations of the analysis of the spectra to determine the structural formulas of the compounds.

Infrared Spectroscopy 1
INFRARED SPECTROSCOPY
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Infrared Spectroscopy 2
L02
1. Summarise the principles behind infrared spectroscopy.
The infra-red region of the spectrum has radiations with energy enough to stretch or
bend bonds in a molecule. In the IR region, each functional group of molecules has a
specific range of characteristic absorption frequencies. Infra-red absorption
spectroscopy involves detection of the frequencies of IR light absorbed by a given
molecule to determine the basic functional groups of molecules (Meislich 2014).
Atoms in a molecule are subject to different vibrations (stretching or bending) since they
are not fixed in the molecular structure. During IR absorption, a net change is caused in
the dipole moment of the molecule due to the vibrations or rotations within the
molecules of compounds with polar bonds. The IR radiation is absorbed when the
radiation’s frequency and the vibrational frequency of the molecules matches thus
causing a change in amplitudes of the vibrations. (Jones, Mulloy and Thomas
2013).The concept of absorption of specific frequencies of light by molecules is used to
deduce the corresponding molecular structure from the molecule’s characteristic
absorption. The absorbed frequencies of the infrared light correspond to the resonant
frequencies of vibration of the molecular bonds (Meislich 2014). The resonant
frequencies relate to the molecular bonds’ strength and the molecular atomic mass and
thus absorbed frequencies can be associated with a specific type of bond and functional
group.
2. IR Spectrum
i. Explain why the region between 1000cm-1 and 1550cm-1 is
described as the ‘fingerprint region’.
The region between 1000 cm-1 and 1550 cm-1 is known as the fingerprint region
because it consists of bands that are unique to each molecule. It is specific to each
molecule and is full of small peaks showing that IR absorption results from vibrations of
the whole molecule (Thornton 2009).
ii. Identify the functional groups that could be responsible for the peaks in
the non-fingerprint region of the spectrum.

Infrared Spectroscopy 3
a) alkyl C−H group
b) carboxylic acid C=O group
c) ketone C=O group
The non-fingerprint region of the IR spectrum generally has five zones namely Zone 1
(4000-3200 cm-1), zone 2 (3200-2600 cm-1), zone 3 (2400 -2000 cm-1), zone 4 (1850-
1650 cm-1) and zone 5 (1680-1550 cm-1). Each zone has distinct peaks corresponding
to the possibility of presence different molecular functional groups.
Performing the five-zone analysis, the peaks found in zone 2 only occur after 3000 cm-1
hence there are alkyl C−H functional group in this region.
The peak around 3000 cm-1 as well as the peak occurring at around 1720 cm-1 indicates
that carboxylic acid C=O group is present.
The strong peak occurring at around 1720 cm-1 indicates that ketone C=O group is
present.
iii. Is this the spectrum of butan-2-ol or butanone? Explain your answer.
This is butanone spectrum since butanone has ketone group which are present.
Butan-2-ol has alcohol group which is absent (Academics.eckerd.edu 2020).
3. Explain how IR spectroscopy can be used to distinguish between alcohols,
carboxylic acids and esters.
The five-zone analysis of the non-fingerprint regions of IR spectrum is used (Meislich
2014).
Alcohol ( O=H ) groups demonstrate strong and broad peaks between 3650-3200 cm-1.
Carboxylic acid (C=O) groups demonstrate broad peaks that are very strong between
3000-2500 cm-1 and also similar peaks between 1725- 1700 cm-1 (Ibarra, Valencia and
Pérez 2015).
Ester group demonstrates strong peaks between 1750-1735 cm-1.
L04

Infrared Spectroscopy 4
4.1: Propose the structure of an organic molecule with empirical formula C4H8O.
Solution
Using the mass spectrum, find the mass to charge ratios (m/z).
M= 72 (25%), M+1 = 73 (1%) and M+2 = 74 (0%). Rescaling the relative intensities to
obtain, M=72 (100%), M+1=73 (4%) and M+2=74 (0%).
No N, Cl, Br or S atoms exist in the compound from the formula (Chem.ucla.edu 2020).
Using the relative intensity at M+1 peak, the number of carbon atoms in the molecule is
obtained.
No . of carbons=( Relativeintensity at M +1 peak )/1.1 %
C= 4
1.1=3.6
From the given empirical formula, 4 carbons.
The molecular formula is determined by subtracting mass of carbon from total mass to
obtain the mass of remaining atoms.
Mass of remaing atoms=M −C3=72−4 ( 12 )=24
since H=1 and O=16
8 ( 1 ) +1(16)=24
The molecular formula is C4 H8 O.
Calculating DBE from the molecular formula as (Wishart and Madhava Rao 2010):
DBE=C – (H /2)+(N /2)+1
DBE=4− ( 8/2 )−0+1=1
A DBE of 1 means that there is only one pi bond or one ring.
Using the IR Spectrum to identify the functional groups.

Infrared Spectroscopy 5
From the given IR spectrum, the possible functional groups are the alkyl C−H group
(peaks in zone 2 occurring only after 3000 cm-1), the carboxylic acid C=O group (peak
around 3000 cm-1 in zone 2 as well as peak at around 1720 cm-1 in zone 4) and the
ketone C=O group (strong peak at around 1720 cm-1 of zone 4) (Webbook.nist.gov
2020).
Using the H-NMR Spectroscopy to determine structural formula (SETO 2012).
From the 1H-NMR Spectrum given, the following data are collected and tabulated.
Splitting Integral Factor Number of
hydrogen
Implications
Quartet 2 2 CH 2 in CH 2 CH 3
Triplet 3 3 CH 3 in CH 3 CH 2
Singlet 3 3 CH 3 in CH 3 CH 2
Total number of integrals:
3+2+3=10
Therefore for each integral, there is one hydrogen atom (Thornton 2009).
From the implications, the formula is C3 H8 which has 1 C and 1 O less than the
molecular formula. The carbon and oxygen form a carbonyl in the pi bond.
The structural formula deduced is.

Infrared Spectroscopy 6
4.2: The empirical formula of an unknown compound is C5H10O2 using information
provided to deduce the structure of the molecule.
Solution
Assuming 100% relative intensity for M peak=102 (100%), M+1=103 and M+2=104.
From the given empirical formula, the minimum number of carbon atoms are 5.To figure
out the molecular formula, subtract the mass of carbon from the compound’s mass to
obtain mass of remaining atoms (Wishart and Madhava Rao 2010)
M −C5=102−5 ( 12 ) =42.
Since H=1 and O=16
10(1)+2(16)=42
The molecular formula is C5 H10 O2.
The sharp absorption peak at 1750 cm-1 from IR spectrum indicated that esters or
ketones groups ( C=O ¿ could be present.
Calculating DBE from the molecular formula as:
DBE=C – (H /2)+(N /2)+1
DBE=5− ( 10/ 2 )−0+1=1
A DBE of 1 shows that there is one pi bond or one ring.
Using the data from H-NMR Spectroscopy, determine the structural formula (SETO
2012).
Chemical shift Splitting Integral factor Number of hydrogen Implications
0.8 Triplet 3 3 CH 3 in CH 3 CH 2
1.1 Sextet 2 2 CH 2 in CH 2 CH 3
2.3 Triplet 2 2 CH 2 in CH 2 CH 3
3.7 Singlet 3 3 CH 3 in CH 3 CH 2

Infrared Spectroscopy 7
Total number of integrals:
3+2+2+3=10
Therefore, for each integral, there is one hydrogen atom.
From the implications, the formula is C4 H10 which has 1 C and 2 O less than the
molecular formula. The carbon and two oxygen form a carbonyl in the pi bond
The structural formula deduced is.

Infrared Spectroscopy 8
Reference List
Ibarra, E., Valencia, O. and Pérez, H. (2015). Analysis of Betaines Using near Infrared
Spectroscopy. Journal of Near Infrared Spectroscopy, 13(3), pp.133-138.
Jones, C., Mulloy, B. and Thomas, A. (2013). Spectroscopic methods and analyses.
Totowa, N.J.: Humana Press, pp.88-122.
Meislich, H. (2014). Organic chemistry. New York, NY: McGraw-Hill Press.
SETO, H. (2012). The latest NMR measuring method - 1: Supersensitive NMR
measuring method subjected to HMBC. Kagaku to Seibutsu, 33(1), pp.42-49.
Thornton, D. (2009). Some Applications of Infrared Spectroscopy to Inorganic
Chemistry. Transactions of the Royal Society of South Africa, 47(2), pp.119-143.
Wishart, J. and Madhava Rao, B. (2010). Recent trends in radiation chemistry.
Singapore: World Scientific.
Webbook.nist.gov. (2020). 2-Butanone. [Online] Available at:
https://webbook.nist.gov/cgi/cbook.cgi?ID=C78933&Mask=200 [Accessed 5 Feb. 2020].
Chem.ucla.edu. (2020). [Online] Available at:
http://www.chem.ucla.edu/~harding/ec_tutorials/tutorial36.pdf [Accessed 4 Feb. 2020].
Academics.eckerd.edu. (2020). Eckerd College Academics. [Online] Available at:
http://academics.eckerd.edu/ [Accessed 4 Feb. 2020].