Question

Question: How do the IR spectra of the isomers cyclopentane and pent-1-ene differ?

Short answer

Answer

IR spectra of 1-pentene will have an additional peak at $1650{\mathrm{cm}}^{-1}$.

1-pentene will show IR absorptions at $3150-3000{\mathrm{cm}}^{-1}$.

Step-by-step solution

IR absorption in hydrocarbons

IR analysis can be used for the identification of hydrocarbons.

Although all hydrocarbons have carbon-carbon single bonds, the peak due to absorption by a $\mathbf{C}\mathbf{=}\mathbf{C}$double bond or $\mathbf{C}\mathbf{\equiv }\mathbf{C}$triple bond can be used to distinguish between hydrocarbons.

Therefore, IR spectra of alkenes and alkynes will contain an extra peak for the double bond and the triple bond, respectively.

IR absorption of hybrid orbitals

The position of the peak due to the absorption of the carbon-hydrogen bond depends on the hybridization of the corresponding carbon atom.

${\mathrm{sp}}^3$hybridized carbon-hydrogen bonds cause absorption at less than $3000{\mathrm{cm}}^{-1}$. These absorptions appear as broad peaks in the spectrum.

${\mathrm{sp}}^2$and sp hybridized carbon produces peaks absorbed at higher frequencies.

Comparison of spectra of cyclopentane and pent-1-ene

Cyclopentane and pent-1-ene are examples of structural isomers. Therefore, both are composed of $\mathrm{C}-\mathrm{C}$single bonds and $\mathrm{C}-\mathrm{H}$bonds.

Pent-1-ene has a $\mathrm{C}=\mathrm{C}$double bond. This will give the IR spectrum of pent-1-ene an additional peak at $1650{\mathrm{cm}}^{-1}$. Since there are no double bonds in cyclopentane, this peak will be absent in the IR spectrum of cyclopentane.

Pent-1-ene will also show a $\mathrm{C}-\mathrm{H}$absorptions for ${\mathrm{sp}}^2$hybridized carbon at role="math" localid="1649146543231" $3150-3000{\mathrm{cm}}^{-1}$ because the double-bonded carbon atoms are hybridized.