Question

Where would you expect each of the following compounds to absorb in the IR spectrum? (a) 4 -Penten-2-one (b) 3 -Penten-2-one (c) 2,2 -Dimethylcyclopentanone (d) \(m\) -Chlorobenzaldehyde (e) 3 -Cyclohexenone (f) 2 -Hexenal

Short answer

Expect C=O absorption for each at specific ranges: (a) 1710-1750 cm⁻¹, (b) similar (c) 1750-1780 cm⁻¹, (d) 1720-1740 cm⁻¹, (e) 1680-1700 cm⁻¹, (f) 1680-1700 cm⁻¹.

Step-by-step solution

Identification of Functional Groups

First, identify the key functional groups present in each compound, as these will influence where they absorb in the IR spectrum.

4-Penten-2-one Analysis

4-Penten-2-one contains a carbonyl group (C=O), which typically absorbs around 1710-1750 cm⁻¹, and a C=C group typical of alkenes, which absorbs at 1620-1680 cm⁻¹.

3-Penten-2-one Analysis

Similarly, 3-Penten-2-one also has a carbonyl group (C=O), absorbing around 1710-1750 cm⁻¹, and a C=C group absorbing around 1620-1680 cm⁻¹, similar to 4-Penten-2-one.

2,2-Dimethylcyclopentanone Analysis

For 2,2-Dimethylcyclopentanone, the key functional group is a cyclic carbonyl (C=O), expected to show absorption near 1750-1780 cm⁻¹.

m-Chlorobenzaldehyde Analysis

In m-Chlorobenzaldehyde, the aldehyde group absorbs at 1720-1740 cm⁻¹ for the carbonyl (C=O), and the aromatic C=C bonds show absorption around 1450-1600 cm⁻¹.

3-Cyclohexenone Analysis

3-Cyclohexenone contains a conjugated carbonyl group absorbing near 1680-1700 cm⁻¹, and the C=C bond of the cyclohexenyl ring absorbing at 1620-1680 cm⁻¹.

2-Hexenal Analysis

2-Hexenal, an α,β-unsaturated aldehyde, shows an absorption peak for the C=O at around 1680-1700 cm⁻¹, and the C=C bond is expected around 1620-1680 cm⁻¹.

Key concepts

Functional Groups

Functional groups are specific groups of atoms or bonds within molecules that have characteristic chemical behaviors. These are essential markers in organic chemistry that help in predicting how a molecule will react.
For instance, common functional groups include:

• Alcohols (-OH)
• Carbonyls (C=O)
• Amines (-NH₂)
• Alkenes (C=C)

Recognizing these groups allows chemists to determine infrared spectroscopy absorption bands, as different functional groups absorb light at different wavelengths due to bond vibrations. Understanding these functional groups helps identify peaks on an IR spectrum, aiding in the structural elucidation of the compound.

Carbonyl Group Absorption

The carbonyl group (C=O) is one of the most distinctive features observed in infrared (IR) spectroscopy. This group's strong dipole moment results in a very intense absorption band, typically appearing in the range of 1700 to 1750 cm⁻¹, depending on the molecular environment.
Key points include:

• Ketones and aldehydes have a sharp absorption between 1710 and 1750 cm⁻¹.
• If the carbonyl is part of a cyclic structure, the absorption can shift slightly higher, reaching up to 1780 cm⁻¹.
• Conjugation with other double bonds or aromatic rings can lower these frequencies to around 1680-1700 cm⁻¹.

The position of this absorption band is crucial for identifying and confirming the presence of a carbonyl group in a compound.

Alkene C=C Absorption

Alkenes are characterized by the carbon-carbon double bond (C=C), which shows up in the IR spectra as weaker bands due to a lesser change in dipole moment during vibration compared to carbonyls. Typically, the C=C absorption appears between 1620 and 1680 cm⁻¹.
Key insights for identifying C=C absorptions include:

• Isolated C=C bonds absorb closer to 1680 cm⁻¹ due to no conjugation effects.
• Conjugation with other double bonds or rings can often shift the absorption toward the lower end of the range.
• The peak is usually of medium intensity compared to a carbonyl peak.

The precise position helps chemists to distinguish between isolated and conjugated alkenes, aiding in better molecular characterization.

Aromatic C=C Absorption

Aromatic compounds have a unique structure that influences their IR absorption characteristics, particularly for the C=C bonds found in the aromatic ring. These C=C bonds typically absorb in the region of 1450-1600 cm⁻¹, which can distinguish them from non-aromatic alkene absorptions.
Important features include:

• The multiplicity of absorption bands due to the resonance-stabilized structure.
• These peaks are generally weaker and more complex due to the symmetrical and conjugated nature of aromatic rings.
• Identifying these bands requires analyzing the pattern rather than a single peak.

Recognizing this absorption pattern is crucial for confirming the presence of aromatic structures in a compound, playing a key role in molecular identification.