Q30 E - Organic Chemistry (Mcmurry)

Question

Question: Propose structures for compounds that meet the following descriptions:

(a) ${\mathbf{C}}_{\mathbf{5}}{\mathbf{H}}_{\mathbf{8}}$with IR absorptions at 3300 and 2150$c{m}^{-1}$

(b) ${\mathbf{C}}_{\mathbf{4}}{\mathbf{H}}_{\mathbf{8}}\mathbf{O}$with a strong IR absorption at 3400 $c{m}^{-1}$

(c) ${\mathbf{C}}_{\mathbf{4}}{\mathbf{H}}_{\mathbf{8}}\mathbf{O}$with a strong IR absorption at 1715  $c{m}^{-1}$

(d) ${\mathbf{C}}_{\mathbf{8}}{\mathbf{H}}_{\mathbf{10}}$with IR absorptions at 1600 and 1500 $c{m}^{-1}$

Step-by-Step Solution

Answer

(a) Possible structures are$C{H}_{3}C{H}_{2}C{H}_{2}C\equiv CH$

  and ${\left(C{H}_3\right)}_{2}CHC\equiv CH$

 

(b) Possible structures are 

(c) Possible structures is

$C{H}_{3}C{H}_{2}COC{H}_3$

(d) Possible structures are 

Step-by-step Solution Step 1: IR (Infrared spectroscopy)

Infrared spectroscopy is the study of the interaction betweenthe sample andIR radiation which brings about change in vibrational-rotational level. Therefore, it is also called rotational-vibrational spectroscopy.

• It is a technique based on the vibrations of the atoms of a molecule.
• The waves are electromagnetic in nature and interact with polarity of the chemical bonds of the molecule.
• If there is no polarity in the molecule then the infrared interaction is inactive and the molecule does not produce any IR spectrum.

Step 2: Importance of Infrared spectroscopy in structure determination

Infrared spectroscopy is the most widely used tool for detection of functional groups in pure compounds, mixtures, and for compound comparison.

• Infrared spectra may be obtained from samples in all phases (solid, liquid, and gaseous).
• The spectrum is obtained in a minute.
• It provides a variety of information about structure, symmetry, purity structural, and geometrical isomers and H-bonding.

 

Step 3: Propose structures for the given compound

(a) Absorptions at 3300 and 2150 are due to terminal triple bond. The possible structures are given below.

$C{H}_{3}C{H}_{2}C{H}_{2}C\equiv CH$

(b) An IR absorption at 3400 is due to a hydroxyl group. Since no double bond absorption is present, the compound must be a cyclic alcohol.

An absorption at 1715 is due to a ketone. The only possible structure is shown below.

$C{H}_{3}C{H}_{2}COC{H}_3$

d) Absorptions at 1600and 1500 are due to aromatic ring. Possible structures are shown below.