DAT Organic Chemistry: The C-H and Unsaturation Region

DAT High-Yield Concept

The $\pu{3000 cm^{-1}}$ Baseline: $\ce{C-H}$ stretching absorptions are diagnosed relative to $\pu{3000 cm^{-1}}$ : $\text{sp}^3$ $\ce{C-H}$ appears below $\pu{3000 cm^{-1}}$ , $\text{sp}^2$ $\ce{C-H}$ appears above ( $\pu{3000-3100 cm^{-1}}$ ), and $\text{sp}$ $\ce{C-H}$ appears near $\pu{3300 cm^{-1}}$ .
Double Bond Region ( $\pu{1600-1700 cm^{-1}}$ ): $\ce{C=C}$ (alkene) and $\ce{C=N}$ (imine) stretches occur here. $\ce{C=N}$ stretches are significantly stronger due to greater bond polarity.
Nitrile Region (near $\pu{2200 cm^{-1}}$ ): $\ce{C \equiv N}$ triple bonds produce a sharp, distinct signal near $\pu{2220-2260 cm^{-1}}$ .
Symmetry and IR Inactivity: Completely symmetric double or triple bonds produce no net change in dipole moment during stretching and yield no IR absorption signal.

1. C-H Stretching and Carbon Hybridization

Infrared (IR) absorption wavenumber ( $\tilde{\nu}$ ) is governed by Hooke's Law: higher bond force constants (stronger bonds) shift stretching vibrations to higher energy. Increasing s-character in carbon hybrid orbitals shortens and strengthens the carbon-hydrogen bond ( $\text{sp}$ with $50\%$ s-character > $\text{sp}^2$ with $33\%$ s-character > $\text{sp}^3$ with $25\%$ s-character).

Drawing a vertical reference line at $\pu{3000 cm^{-1}}$ on an IR spectrum enables immediate hybridization assignment:

$\text{sp}^3$ $\ce{C-H}$ (Alkanes): Absorbs just below $\pu{3000 cm^{-1}}$ , typically $\pu{2850-2960 cm^{-1}}$ .
$\text{sp}^2$ $\ce{C-H}$ (Alkenes / Aromatics): Absorbs just above $\pu{3000 cm^{-1}}$ , typically $\pu{3000-3100 cm^{-1}}$ .
$\text{sp}$ $\ce{C-H}$ (Terminal Alkynes): Absorbs as a prominent, sharp peak near $\pu{3300 cm^{-1}}$ .

C-H stretches: 3000 is the dividing line — More s character pulls the C-H bond shorter and stiffer, and a stiffer bond vibrates at higher energy, so the order runs sp above sp2 above sp3. That makes 3000 a hard line: anything to the left of it belongs to a carbon that is part of a double or triple bond, and anything to the right is an ordinary alkane C-H. The sp band near 3300 is narrow, which separates it from an alcohol O-H sitting in the same neighborhood.

Worked Example & DAT Trap: An unknown compound exhibits prominent IR absorptions at

\pu{2920 cm^{-1}}

\pu{3080 cm^{-1}}

, and

\pu{1650 cm^{-1}}

.
Reasoning: The absorption at

\pu{2920 cm^{-1}}

confirms

\text{sp}^3

\ce{C-H}

bonds. The peak at

\pu{3080 cm^{-1}}

(above

\pu{3000 cm^{-1}}

) paired with

\pu{1650 cm^{-1}}

confirms an alkene (containing

\text{sp}^2

\ce{C-H}

and

\ce{C=C}

).
Exam Trap: Do not mistake the sharp terminal alkyne

\text{sp}

\ce{C-H}

peak at

\pu{3300 cm^{-1}}

for a broad alcohol

\ce{O-H}

stretch near

\pu{3400 cm^{-1}}

(covered in depth in its own lesson). The

\text{sp}

\ce{C-H}

signal is characteristically sharp and narrow.

2. Double Bond Unsaturation: Alkenes and Imines ( $\pu{1600-1700 cm^{-1}}$ )

The region between $\pu{1600 cm^{-1}}$ and $\pu{1700 cm^{-1}}$ houses double bond stretching modes:

Alkene ( $\ce{C=C}$ ): Absorbs weakly to moderately between $\pu{1620 cm^{-1}}$ and $\pu{1680 cm^{-1}}$ .
Imine ( $\ce{C=N}$ ): Absorbs moderately to strongly between $\pu{1640 cm^{-1}}$ and $\pu{1690 cm^{-1}}$ .

Nitrogen is more electronegative than carbon, rendering the $\ce{C=N}$ bond far more polarized than $\ce{C=C}$ . Because IR signal intensity depends directly on the change in dipole moment during vibration ( $\frac{d\mu}{dx}$ ), imine stretches produce much stronger peaks than alkene stretches.

List of main IR spectroscopy bands (Source: Faverik · CC BY-SA 4.0 · Wikimedia Commons)

Worked Example (Symmetry & IR Inactivity): Contrast 1-hexene with

\ce{trans}

-3-hexene. 1-Hexene exhibits a clear

\ce{C=C}

absorption at

\pu{1642 cm^{-1}}

and an

\text{sp}^2

\ce{C-H}

stretch at

\pu{3080 cm^{-1}}

. Conversely,

\ce{trans}

-3-hexene shows no absorption at

\pu{1642 cm^{-1}}

; its symmetric structure undergoes zero dipole change during stretching, rendering the

\ce{C=C}

vibration IR-inactive.

3. Triple Bond Region: Nitriles near $\pu{2200 cm^{-1}}$

Triple bonds absorb in the diagnostic region between $\pu{2100 cm^{-1}}$ and $\pu{2300 cm^{-1}}$ :

Nitrile ( $\ce{C \equiv N}$ ): Absorbs sharply with medium-to-strong intensity near $\pu{2220-2260 cm^{-1}}$ .
Alkyne ( $\ce{C \equiv C}$ ): Absorbs weakly between $\pu{2100 cm^{-1}}$ and $\pu{2260 cm^{-1}}$ .

Nitriles are strongly polarized by the electronegative nitrogen atom, making the $\ce{C \equiv N}$ peak robust and easy to identify on DAT spectra.

DAT Mnemonic: "3000 is the Hybrid Line, 2200 for Triple, 1650 for Double Fine."
(Below 3000 =

\text{sp}^3

\ce{C-H}

, Above 3000 =

\text{sp}^2

\ce{C-H}

, 3300 =

\text{sp}

\ce{C-H}

; 2200 =

\ce{C\equiv N}

\ce{C\equiv C}

; 1600–1700 =

\ce{C=C}

\ce{C=N}

Summary Comparison of Unsaturation Signals

Functional Group / Bond	Hybridization / Bond Type	Wavenumber Range ( $\pu{cm^{-1}}$ )	Peak Intensity / Shape
Alkane $\ce{C-H}$	$\text{sp}^3$ $\ce{C-H}$	$2850 - 2960$	Strong to medium, sharp
Alkene / Aromatic $\ce{C-H}$	$\text{sp}^2$ $\ce{C-H}$	$3000 - 3100$	Medium, sharp
Terminal Alkyne $\ce{C-H}$	$\text{sp}$ $\ce{C-H}$	$\sim 3300$	Strong, sharp
Alkene $\ce{C=C}$	$\text{sp}^2-\text{sp}^2$ double bond	$1620 - 1680$	Weak to medium (inactive if symmetric)
Imine $\ce{C=N}$	$\text{sp}^2-\text{sp}^2$ double bond	$1640 - 1690$	Medium to strong
Nitrile $\ce{C \equiv N}$	$\text{sp}-\text{sp}$ triple bond	$2220 - 2260$	Medium to strong, sharp

Critical Thinking Questions

Why does an $\text{sp}$ $\ce{C-H}$ bond stretch at a higher wavenumber ( $\pu{3300 cm^{-1}}$ ) than an $\text{sp}^3$ $\ce{C-H}$ bond ( $\pu{2900 cm^{-1}}$ )?
A compound with molecular formula $\ce{C6H11N}$ exhibits a sharp IR absorption at $\pu{2240 cm^{-1}}$ , but no absorption above $\pu{3000 cm^{-1}}$ . What functional group is present, and what is the hybridization of all carbon-hydrogen bonds?
Why is the $\ce{C=N}$ stretch of an imine generally much more intense than the $\ce{C=C}$ stretch of an unfunctionalized alkene?

References

Sources returned by Google Search grounding during lesson verification.

IR spectra for hydrocarbons - Khan Academy — khanacademy.org
Interpreting IR Spectra - Chemistry Steps — chemistrysteps.com

The C-H and Unsaturation Region

Summary

Practice Questions

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