IR spectra for hydrocarbons | Spectroscopy | Organic chemistry | Khan Academy

Summary

This video script delves into the analysis of carbon-hydrogen bonds in IR spectra, highlighting how bond wavenumbers vary with carbon hybridization states. It explains that sp-hybridized carbon-hydrogen bonds appear at 3300 wavenumbers, sp2 at 3100, and sp3 at 2900. The script guides viewers through interpreting IR spectra of hydrocarbons, including alkanes, alkynes, alkenes, and arenes, focusing on the diagnostic region and distinguishing features like bond strengths and vibration frequencies.

Takeaways

• 🔍 The wavenumber for a carbon-hydrogen bond stretch in an IR spectrum varies based on the hybridization state of the carbon atom involved.
• 🌉 For sp-hybridized carbon, the C-H bond stretch appears around 3300 wavenumbers, which is higher than for other hybridizations due to the bond's strength and shortness.
• 📏 The sp2-hybridized carbon's C-H bond stretch signal is found around 3100 wavenumbers, indicating a medium bond strength.
• 💠 With sp3-hybridized carbon, the C-H bond stretch is observed at approximately 2900 wavenumbers, reflecting the longest and weakest bond among the three hybridizations.
• 📊 Drawing a line at 3000 wavenumbers is a useful practice when analyzing IR spectra, as it helps to quickly identify the region where sp3-hybridized C-H bonds are expected to show their stretch.
• 🔬 The strength of a bond is directly related to the wavenumber at which its stretch appears in an IR spectrum; stronger bonds result in higher wavenumbers.
• 📚 In alkanes like decane, the IR spectrum is simple, with signals primarily below 3000 wavenumbers, indicating only sp3-hybridized C-H bonds.
• 🔎 In 1-octyne, the presence of an sp-hybridized C-H bond is indicated by a signal near 3300 wavenumbers, and a carbon-carbon triple bond stretch is seen at around 2100 wavenumbers.
• 📈 The diagnostic region of an IR spectrum (below 3000 wavenumbers) is particularly useful for identifying the type of carbon-hydrogen bonds present in a molecule.
• 🌐 Comparing IR spectra of different hydrocarbons, such as alkanes, alkenes, alkynes, and arenes, can reveal the presence of various hybridized carbons and functional groups like double and triple bonds.

Q & A

What is the approximate wavenumber for a carbon-hydrogen bond stretch in an IR spectrum?

The approximate wavenumber for a carbon-hydrogen bond stretch is a little bit over 3000 wavenumbers.

How does the hybridization state of carbon affect the wavenumber of a carbon-hydrogen bond stretch in an IR spectrum?

The wavenumber of a carbon-hydrogen bond stretch depends on the hybridization state of the carbon. For sp-hybridized carbon, the wavenumber is around 3300; for sp2-hybridized carbon, it's around 3100; and for sp3-hybridized carbon, it's around 2900.

What percentage of s character does an sp-hybridized orbital have?

An sp-hybridized orbital has 50% s character.

Why is the carbon-hydrogen bond where the carbon is sp-hybridized stronger than the one where the carbon is sp2- or sp3-hybridized?

The carbon-hydrogen bond where the carbon is sp-hybridized is stronger because the electron density is closest to the nucleus due to the higher percentage of s character in sp orbitals, making the bond shorter and stronger.

How does bond strength relate to the wavenumber in an IR spectrum?

A stronger bond has a higher force constant (k), which increases the frequency of bond vibrations, leading to a higher wavenumber in the IR spectrum.

What is the significance of drawing a line at 3000 wavenumbers when analyzing an IR spectrum of hydrocarbons?

Drawing a line at 3000 wavenumbers helps to distinguish between carbon-hydrogen bond stretches associated with sp3-hybridized carbons (below 3000) and those associated with sp2- or sp-hybridized carbons (above 3000).

What is the approximate wavenumber for the carbon-hydrogen bond stretch in decane, which is an alkane?

In decane, which only contains sp3-hybridized carbons, the carbon-hydrogen bond stretch appears under 3000 wavenumbers.

What is the approximate wavenumber for the carbon-hydrogen bond stretch where the carbon is sp-hybridized, as seen in 1-octyne?

The carbon-hydrogen bond stretch where the carbon is sp-hybridized, as in 1-octyne, appears around 3300 wavenumbers.

What is the characteristic wavenumber range for the carbon-carbon triple bond stretch in an alkyne?

The carbon-carbon triple bond stretch in an alkyne typically appears around 2100 wavenumbers.

How can the presence of an aromatic ring affect the wavenumber of a carbon-carbon double bond stretch in an IR spectrum?

In the presence of an aromatic ring, the carbon-carbon double bond stretch usually appears at a lower wavenumber than a typical aliphatic double bond, typically between 1600 to 1450 wavenumbers.

What is the difference in wavenumber between the carbon-hydrogen bond stretch of sp2-hybridized carbon and sp3-hybridized carbon in an IR spectrum?

The carbon-hydrogen bond stretch of sp2-hybridized carbon appears around 3100 wavenumbers, whereas for sp3-hybridized carbon, it appears around 2900 wavenumbers, showing a difference in wavenumber due to the different hybridization states.