Infrared (IR) spectroscopy is a powerful analytical technique used to identify and characterize different functional groups in organic molecules, including alcohols and phenols.Here's a detailed explanation of how IR spectroscopy can be utilized to examine these substances that helps in university level exams like NTA CSIR NET CHEMISTRY, GATE & IIT JAM CHEMISTRY:
Alcohols exhibit distinct IR absorptions attributed to both the stretching vibrations of O-H and C-O bonds. The C-O stretching vibration is observed in the region of 1260-1050 cm-1.
1. OH Stretching: The most prominent feature in the IR spectrum of alcohols is the broad peak due to the stretching vibration of the OH bond. When analyzed in a thin liquid film or in their pure form ("neat"), the O-H stretching vibration of alcohols typically occurs within the range of 3500-3200 cm-1, appearing as a highly intense and broad band.
It is intense due to larger change in the associated dipole moment w.r.t change in bond length during vibrations and broad due to hydrogen bonding.
2. CO Stretching: Another characteristic peak for alcohols is the strong CO stretching vibration, which typically appears in the range of 1260-1050 cm-1. The CO stretching peak is strong and intense. The stretches may be symmetric or asymmetric. This peak can provide information about the environment surrounding the hydroxyl group.
The exact position of this peak can vary depending on factors like the nature of the alcohol (primary, secondary, or tertiary).
| Type of alcohol | C-O stretch (cm-1) |
| Primary | 1070 - 1000 |
| Secondary | 1150 - 1075 |
| Tertiary | 1200 - 1100 |
| Phenols | 1260 - 1200 |
3. Hydrogen Bonding Effects: Due to the variable extent of proton sharing with neighboring oxygens, the covalent O-H bonds within an alcohol sample vibrate at slightly disparate frequencies, resulting in their appearance at slightly different positions in the IR spectrum. Instead of a single sharp peak, a broad array of overlapping peaks is observed. This broadness of the O-H peak facilitates its distinct identification within an IR spectrum.
The O-H bonds vibrate at slightly different frequencies due to varying extent of H-bonding. Unassociated alcohols show a fairly sharp absorption near 3600 cm–1, whereas hydrogen-bonded alcohols show a broader absorption in the 3300 to 3400 cm–1 range.
1. OH Stretching: Similar to alcohols, phenols also exhibit a broad peak due to OH stretching vibration. However, in phenols, this peak tends to be sharper compared to alcohols due to the absence of extensive hydrogen bonding. The OH stretching typically occurs around 3550-3450 cm-1.
2. C=C Stretching: Phenols contain an aromatic ring, so they exhibit C=C stretching vibrations in the range of 1500-1600 cm-1 (aromatic bands), which can help confirm the presence of the aromatic ring.
3. CO Stretching: As with alcohols, phenols also show CO stretching vibrations, usually in the same range as alcohols, around 1260-1050 cm-1.
Question-1) The infrared absorption, most affected by intra-molecular hydrogen bonding in 2-hydroxy acetophenone is: (UOHYD MSC 2012)
(A) methyl group C−H stretching.
(B) hydroxyl group O−H stretching.
(C) aromatic ring C−H bending.
(D) aromatic ring C−C stretching.
Answer: B
Explanation: Hydogen bonding is possible with O-H bond. Not with other bonds since their dipole moment is not suffice to make H-bonding.
Question-2) The most vibrational transitions for alcohols in IR spectrum appear in between…
A) 5000-4000 cm-1
B) 4000-400 cm-1
C) 900-400 cm-1
D) 400-100 cm-1
Answer: B
Explanation: Go through the peak positions of OH and CO stretching vibrations.
Question-3) The frequency of O-H stretch occurs at around 3600 cm-1, The O-D stretch frequency would be closest to…. (CSIR NET DEC 2019)
A) 3000 cm-1
B) 1800 cm-1
C) 900 cm-1
D) 2600 cm-1
solution: go through the video presentation on infrared spectroscopy of alcohols & phenols. Or Click below