From this IR spectum, we have determined that 3-methylbenzaldehyde contains at least one of each of the following functional groups:
Alkane, Aromatic, and Aldehyde.
Click the 'Interpreting an IR Spectrum' button for a general tutorial on interpreting an IR spectrum. Click on the "Example: 3-methylbut-3-en-1-ol" button for a walk-through with another compound. Click the 'Skip Tutorial' button to try some compounds on your own.
Skip Tutorial
Interpreting an IR Spectrum
Example: 3-methylbut-3-en-1-ol
There are no significant peaks from 3300-3650 cm-1, so this sample contains neither an amine nor an alcohol.
The twin peaks around 2900 cm-1 and 2700 cm-1, combined with the intense peak at 1697 cm-1, reflect the various stretching vibrations an aldehyde displays. Click on the highlighted peaks to see these stretching vibrations.
The final stage is to look for alcohols (O-H bonds) or amines (N-H bonds). If either of these are present, there will be a peak in the 3300-3650 cm-1 region. Are there any O-H or N-H bonds present in the compound?
There is a very intense peak in this region, which means there is a carbonyl group present in the function. Because of this, we need to identify the exact location of the peak. In this case, the peak is at 1697 cm-1.
This peak could represent two possible carbonyl groups: aldehydes (CHO) or carboxylic acids (COOH). An aldehyde is represented on the spectrum by two peaks appearing around 2700 cm-1 and 2900 cm-1. A carboxylic acid exhibits a very broad peak on the infrared spectrum around 2500-3000 cm-1. According to the above spectrum, what type of carbonyl group is present in the sample?
The next step is to look for carbonyl groups (C=O bonds). These are marked by intense peaks in the 1600-1800 cm-1 region. Are there any C=O bonds in this compound?
The compound contains an aromatic ring. Each highlighted peak represents a different stretching vibration or bending vibration between the bonds in the aromatic. Click on the peaks to see these bond movements.
The blue peak could either represent the C-H stretch of an alkene or an aromatic group. To determine which, we must look at secondary peaks in both the Functional Group and Fingerprint Regions. Alkenes will have additional peaks at 1600-1670 cm-1 and 675-990 cm-1, while aromatics have additional peaks at 1450-1600 cm-1 and 690-885 cm-1. Aromatics may also have small peaks in the 1600-2000 cm-1 region.
Based on this information, this sample contains an:
No absorptions appear at 3300 cm-1, so molecules of this compound do not have terminal alkyne functional groups. However there is a peak just below 3000 cm-1, which is a signature for the presence of an alkane C-H stretching vibration. Click on the purple peak to see this stretch.There is also a peak just above 3000 cm-1, which could correspond to either an alkene or aromatic functional group, although further information is required to distinguish between the two.
Not quite.
There are 7 absorption peaks, and each peak tells us something different.
1
2
3
4
5
6
7
First, consider any peaks near 3000 cm-1. These represent different C-H stretching vibrations, where the C atom in each case is a saturated C atom, part of a double bond (alkene) or aromatic ring, or part of a triple bond (alkyne).
Based on the above data, which type(s) of C-H bonds could be present in this sample? Check all that apply:
Chemists use infrared spectroscopy to determine the connectivity pattern of atoms (functional groups) in molecules of unknown substances. Many molecules absorb infrared radiation, causing the bonds in molecules to bend and stretch. The IR spectrum of a sample is a graph of how strongly a compound absorbs IR radiation of different wavenumbers or frequencies. Because functional groups in molecules always absorb IR radiation at specific wavenumbers, chemists can identify the functional groups in a molecule by analyzing the position and intensity of various absorption peaks in a molecule’s IR spectrum.
Consider the IR spectrum of 3-methylbenzaldehyde. When looking at the spectrum, chemists divide it into 2 regions. The first region (for wavenumbers greater than 1450 cm-1) is called the Functional Group Region and examines the primary functional groups and stretching vibrations. The second region (for wave numbers less than 1450 cm-1) looks at additional data about the compounds stretching and bending vibrations. This is called the Fingerprint Region. We will focus on the Functional Group Region.
Show Regions
Functional Group Region
Fingerprint Region
Look in the Functional Group Region. How many peaks are there?
Looking at the name and molecular structure of 3-methylbenzaldehyde, we identify several functional groups:
-benz indicates that there is a benzene ring, which is a type of aromatic ring.
-There are saturated carbon atoms (parts of the molecule with no double bonds, including the –CH3 methyl group) in each molecule, known as alkanes.
-Finally, aldehyde indicates that there is an aldehyde (CHO) group attached to the benzene ring.
Suppose that you did not know the name of this compound, but were only given an unknown substance. How could you figure out its functional groups?
3-methylbenzaldehyde
IR spectroscopy is all about identifying the functional groups of a chemical compound. Recall that a functional group is a particular group of atoms in a molecule with a specific connectivity pattern and is responsible for the characteristic reactions of that molecule. Common examples of functional groups are alkanes, alkenes, alkynes, and aromatic rings. If we know the name of the compound, we can identify its structure and functional groups. Which of the following structures identifies 3-methylbenzaldehyde?